draft-ietf-core-oscore-groupcomm-11.txt   draft-ietf-core-oscore-groupcomm-12.txt 
CoRE Working Group M. Tiloca CoRE Working Group M. Tiloca
Internet-Draft RISE AB Internet-Draft RISE AB
Intended status: Standards Track G. Selander Intended status: Standards Track G. Selander
Expires: August 26, 2021 F. Palombini Expires: 13 January 2022 F. Palombini
J. Mattsson J. Mattsson
Ericsson AB Ericsson AB
J. Park J. Park
Universitaet Duisburg-Essen Universitaet Duisburg-Essen
February 22, 2021 12 July 2021
Group OSCORE - Secure Group Communication for CoAP Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-11 draft-ietf-core-oscore-groupcomm-12
Abstract Abstract
This document defines Group Object Security for Constrained RESTful This document defines Group Object Security for Constrained RESTful
Environments (Group OSCORE), providing end-to-end security of CoAP Environments (Group OSCORE), providing end-to-end security of CoAP
messages exchanged between members of a group, e.g. sent over IP messages exchanged between members of a group, e.g., sent over IP
multicast. In particular, the described approach defines how OSCORE multicast. In particular, the described approach defines how OSCORE
is used in a group communication setting to provide source is used in a group communication setting to provide source
authentication for CoAP group requests, sent by a client to multiple authentication for CoAP group requests, sent by a client to multiple
servers, and for protection of the corresponding CoAP responses. servers, and for protection of the corresponding CoAP responses.
Group OSCORE also defines a pairwise mode where each member of the
group can efficiently derive a symmetric pairwise key with any other
member of the group for pairwise OSCORE communication.
Status of This Memo Status of This Memo
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Security Context . . . . . . . . . . . . . . . . . . . . . . 7 2. Security Context . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Common Context . . . . . . . . . . . . . . . . . . . . . 9 2.1. Common Context . . . . . . . . . . . . . . . . . . . . . 10
2.1.1. ID Context . . . . . . . . . . . . . . . . . . . . . 9 2.1.1. AEAD Algorithm . . . . . . . . . . . . . . . . . . . 10
2.1.2. Counter Signature Algorithm . . . . . . . . . . . . . 9 2.1.2. ID Context . . . . . . . . . . . . . . . . . . . . . 10
2.1.3. Counter Signature Parameters . . . . . . . . . . . . 9 2.1.3. Group Manager Public Key . . . . . . . . . . . . . . 10
2.1.4. Secret Derivation Algorithm . . . . . . . . . . . . . 10 2.1.4. Signature Encryption Algorithm . . . . . . . . . . . 10
2.1.5. Secret Derivation Parameters . . . . . . . . . . . . 11 2.1.5. Signature Algorithm . . . . . . . . . . . . . . . . . 11
2.2. Sender Context and Recipient Context . . . . . . . . . . 11 2.1.6. Group Encryption Key . . . . . . . . . . . . . . . . 11
2.3. Pairwise Keys . . . . . . . . . . . . . . . . . . . . . . 12 2.1.7. Pairwise Key Agreement Algorithm . . . . . . . . . . 11
2.3.1. Derivation of Pairwise Keys . . . . . . . . . . . . . 12 2.2. Sender Context and Recipient Context . . . . . . . . . . 12
2.3.2. Usage of Sequence Numbers . . . . . . . . . . . . . . 13 2.3. Format of Public Keys . . . . . . . . . . . . . . . . . . 13
2.3.3. Security Context for Pairwise Mode . . . . . . . . . 14 2.4. Pairwise Keys . . . . . . . . . . . . . . . . . . . . . . 14
2.4. Update of Security Context . . . . . . . . . . . . . . . 14 2.4.1. Derivation of Pairwise Keys . . . . . . . . . . . . . 14
2.4.1. Loss of Mutable Security Context . . . . . . . . . . 15 2.4.2. ECDH with Montgomery Coordinates . . . . . . . . . . 16
2.4.2. Exhaustion of Sender Sequence Number . . . . . . . . 16 2.4.3. Usage of Sequence Numbers . . . . . . . . . . . . . . 17
2.4.3. Retrieving New Security Context Parameters . . . . . 17 2.4.4. Security Context for Pairwise Mode . . . . . . . . . 17
3. The Group Manager . . . . . . . . . . . . . . . . . . . . . . 19 2.5. Update of Security Context . . . . . . . . . . . . . . . 18
3.1. Management of Group Keying Material . . . . . . . . . . . 20 2.5.1. Loss of Mutable Security Context . . . . . . . . . . 18
3.2. Responsibilities of the Group Manager . . . . . . . . . . 21 2.5.2. Exhaustion of Sender Sequence Number . . . . . . . . 19
4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 23 2.5.3. Retrieving New Security Context Parameters . . . . . 20
4.1. Counter Signature . . . . . . . . . . . . . . . . . . . . 23 3. The Group Manager . . . . . . . . . . . . . . . . . . . . . . 22
4.2. The 'kid' and 'kid context' parameters . . . . . . . . . 23 3.1. Support for Additional Principals . . . . . . . . . . . . 24
4.3. external_aad . . . . . . . . . . . . . . . . . . . . . . 23 3.2. Management of Group Keying Material . . . . . . . . . . . 24
5. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 25 3.2.1. Recycling of Identifiers . . . . . . . . . . . . . . 27
5.1. Examples of Compressed COSE Objects . . . . . . . . . . . 26 3.3. Responsibilities of the Group Manager . . . . . . . . . . 28
5.1.1. Examples in Group Mode . . . . . . . . . . . . . . . 26 4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 30
5.1.2. Examples in Pairwise Mode . . . . . . . . . . . . . . 27 4.1. Countersignature . . . . . . . . . . . . . . . . . . . . 30
4.1.1. Keystream Derivation . . . . . . . . . . . . . . . . 30
4.1.2. Clarifications on Using a Countersignature . . . . . 32
4.2. The 'kid' and 'kid context' parameters . . . . . . . . . 32
4.3. external_aad . . . . . . . . . . . . . . . . . . . . . . 32
5. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 35
5.1. Examples of Compressed COSE Objects . . . . . . . . . . . 36
5.1.1. Examples in Group Mode . . . . . . . . . . . . . . . 36
5.1.2. Examples in Pairwise Mode . . . . . . . . . . . . . . 37
6. Message Binding, Sequence Numbers, Freshness and Replay 6. Message Binding, Sequence Numbers, Freshness and Replay
Protection . . . . . . . . . . . . . . . . . . . . . . . . . 28 Protection . . . . . . . . . . . . . . . . . . . . . . . 38
6.1. Update of Replay Window . . . . . . . . . . . . . . . . . 28 6.1. Supporting Observe . . . . . . . . . . . . . . . . . . . 38
6.2. Message Freshness . . . . . . . . . . . . . . . . . . . . 29 6.2. Update of Replay Window . . . . . . . . . . . . . . . . . 38
7. Message Reception . . . . . . . . . . . . . . . . . . . . . . 29 6.3. Message Freshness . . . . . . . . . . . . . . . . . . . . 39
8. Message Processing in Group Mode . . . . . . . . . . . . . . 30 7. Message Reception . . . . . . . . . . . . . . . . . . . . . . 39
8.1. Protecting the Request . . . . . . . . . . . . . . . . . 31 8. Message Processing in Group Mode . . . . . . . . . . . . . . 40
8.1.1. Supporting Observe . . . . . . . . . . . . . . . . . 31 8.1. Protecting the Request . . . . . . . . . . . . . . . . . 41
8.2. Verifying the Request . . . . . . . . . . . . . . . . . . 32 8.1.1. Supporting Observe . . . . . . . . . . . . . . . . . 42
8.2.1. Supporting Observe . . . . . . . . . . . . . . . . . 34 8.2. Verifying the Request . . . . . . . . . . . . . . . . . . 43
8.3. Protecting the Response . . . . . . . . . . . . . . . . . 34 8.2.1. Supporting Observe . . . . . . . . . . . . . . . . . 44
8.3.1. Supporting Observe . . . . . . . . . . . . . . . . . 35 8.3. Protecting the Response . . . . . . . . . . . . . . . . . 45
8.4. Verifying the Response . . . . . . . . . . . . . . . . . 35 8.3.1. Supporting Observe . . . . . . . . . . . . . . . . . 46
8.4.1. Supporting Observe . . . . . . . . . . . . . . . . . 36 8.4. Verifying the Response . . . . . . . . . . . . . . . . . 46
9. Message Processing in Pairwise Mode . . . . . . . . . . . . . 37 8.4.1. Supporting Observe . . . . . . . . . . . . . . . . . 48
9.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 38 8.5. External Signature Checkers . . . . . . . . . . . . . . . 50
9.2. Main Differences from OSCORE . . . . . . . . . . . . . . 38 9. Message Processing in Pairwise Mode . . . . . . . . . . . . . 51
9.3. Protecting the Request . . . . . . . . . . . . . . . . . 39 9.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 52
9.4. Verifying the Request . . . . . . . . . . . . . . . . . . 39 9.2. Main Differences from OSCORE . . . . . . . . . . . . . . 52
9.5. Protecting the Response . . . . . . . . . . . . . . . . . 39 9.3. Protecting the Request . . . . . . . . . . . . . . . . . 52
9.6. Verifying the Response . . . . . . . . . . . . . . . . . 40 9.4. Verifying the Request . . . . . . . . . . . . . . . . . . 53
10. Security Considerations . . . . . . . . . . . . . . . . . . . 40 9.5. Protecting the Response . . . . . . . . . . . . . . . . . 53
10.1. Group-level Security . . . . . . . . . . . . . . . . . . 41 9.6. Verifying the Response . . . . . . . . . . . . . . . . . 54
10.2. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 42 10. Security Considerations . . . . . . . . . . . . . . . . . . . 55
10.3. Management of Group Keying Material . . . . . . . . . . 42 10.1. Security of the Group Mode . . . . . . . . . . . . . . . 56
10.4. Update of Security Context and Key Rotation . . . . . . 43 10.2. Security of the Pairwise Mode . . . . . . . . . . . . . 57
10.4.1. Late Update on the Sender . . . . . . . . . . . . . 43 10.3. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 58
10.4.2. Late Update on the Recipient . . . . . . . . . . . . 44 10.4. Management of Group Keying Material . . . . . . . . . . 58
10.5. Collision of Group Identifiers . . . . . . . . . . . . . 44 10.5. Update of Security Context and Key Rotation . . . . . . 59
10.6. Cross-group Message Injection . . . . . . . . . . . . . 45 10.5.1. Late Update on the Sender . . . . . . . . . . . . . 59
10.6.1. Attack Description . . . . . . . . . . . . . . . . . 45 10.5.2. Late Update on the Recipient . . . . . . . . . . . . 60
10.6.2. Attack Prevention in Group Mode . . . . . . . . . . 46 10.6. Collision of Group Identifiers . . . . . . . . . . . . . 60
10.7. Group OSCORE for Unicast Requests . . . . . . . . . . . 47 10.7. Cross-group Message Injection . . . . . . . . . . . . . 61
10.8. End-to-end Protection . . . . . . . . . . . . . . . . . 48 10.7.1. Attack Description . . . . . . . . . . . . . . . . . 61
10.9. Master Secret . . . . . . . . . . . . . . . . . . . . . 48 10.7.2. Attack Prevention in Group Mode . . . . . . . . . . 62
10.10. Replay Protection . . . . . . . . . . . . . . . . . . . 49 10.8. Prevention of Group Cloning Attack . . . . . . . . . . . 63
10.11. Message Freshness . . . . . . . . . . . . . . . . . . . 49 10.9. Group OSCORE for Unicast Requests . . . . . . . . . . . 63
10.12. Client Aliveness . . . . . . . . . . . . . . . . . . . . 50 10.10. End-to-end Protection . . . . . . . . . . . . . . . . . 65
10.13. Cryptographic Considerations . . . . . . . . . . . . . . 50 10.11. Master Secret . . . . . . . . . . . . . . . . . . . . . 65
10.14. Message Segmentation . . . . . . . . . . . . . . . . . . 51 10.12. Replay Protection . . . . . . . . . . . . . . . . . . . 65
10.15. Privacy Considerations . . . . . . . . . . . . . . . . . 51 10.13. Message Freshness . . . . . . . . . . . . . . . . . . . 66
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52 10.14. Client Aliveness . . . . . . . . . . . . . . . . . . . . 66
11.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . 52 10.15. Cryptographic Considerations . . . . . . . . . . . . . . 66
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 52 10.16. Message Segmentation . . . . . . . . . . . . . . . . . . 69
12.1. Normative References . . . . . . . . . . . . . . . . . . 52 10.17. Privacy Considerations . . . . . . . . . . . . . . . . . 69
12.2. Informative References . . . . . . . . . . . . . . . . . 54 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 70
Appendix A. Assumptions and Security Objectives . . . . . . . . 56 11.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . 70
A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 57 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 70
A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 58 12.1. Normative References . . . . . . . . . . . . . . . . . . 70
Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 59 12.2. Informative References . . . . . . . . . . . . . . . . . 72
Appendix C. Example of Group Identifier Format . . . . . . . . . 61 Appendix A. Assumptions and Security Objectives . . . . . . . . 76
Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 62 A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 76
Appendix E. Challenge-Response Synchronization . . . . . . . . . 63 A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 78
Appendix F. No Verification of Signatures in Group Mode . . . . 66 Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 79
Appendix G. Example Values with COSE Capabilities . . . . . . . 67 Appendix C. Example of Group Identifier Format . . . . . . . . . 81
Appendix H. Parameter Extensibility for Future COSE Algorithms . 68 Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 82
H.1. Counter Signature Parameters . . . . . . . . . . . . . . 68 Appendix E. Challenge-Response Synchronization . . . . . . . . . 83
H.2. Secret Derivation Parameters . . . . . . . . . . . . . . 69 Appendix F. Document Updates . . . . . . . . . . . . . . . . . . 86
H.3. 'par_countersign' in the external_aad . . . . . . . . . . 69 F.1. Version -11 to -12 . . . . . . . . . . . . . . . . . . . 86
Appendix I. Document Updates . . . . . . . . . . . . . . . . . . 71 F.2. Version -10 to -11 . . . . . . . . . . . . . . . . . . . 87
I.1. Version -10 to -11 . . . . . . . . . . . . . . . . . . . 71 F.3. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 88
I.2. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 72 F.4. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 89
I.3. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 72 F.5. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 90
I.4. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 73 F.6. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 91
I.5. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 75 F.7. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 92
I.6. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 75 F.8. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 92
I.7. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 76 F.9. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 93
I.8. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 76 F.10. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 93
I.9. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 77 F.11. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 94
I.10. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 78 F.12. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 95
I.11. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 79 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 96
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 96
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 80
1. Introduction 1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a web The Constrained Application Protocol (CoAP) [RFC7252] is a web
transfer protocol specifically designed for constrained devices and transfer protocol specifically designed for constrained devices and
networks [RFC7228]. Group communication for CoAP networks [RFC7228]. Group communication for CoAP
[I-D.ietf-core-groupcomm-bis] addresses use cases where deployed [I-D.ietf-core-groupcomm-bis] addresses use cases where deployed
devices benefit from a group communication model, for example to devices benefit from a group communication model, for example to
reduce latencies, improve performance and reduce bandwidth reduce latencies, improve performance, and reduce bandwidth
utilization. Use cases include lighting control, integrated building utilization. Use cases include lighting control, integrated building
control, software and firmware updates, parameter and configuration control, software and firmware updates, parameter and configuration
updates, commissioning of constrained networks, and emergency updates, commissioning of constrained networks, and emergency
multicast (see Appendix B). This specification defines the security multicast (see Appendix B). Group communication for CoAP
protocol for Group communication for CoAP [I-D.ietf-core-groupcomm-bis] mainly uses UDP/IP multicast as the
[I-D.ietf-core-groupcomm-bis]. underlying data transport.
Object Security for Constrained RESTful Environments (OSCORE) Object Security for Constrained RESTful Environments (OSCORE)
[RFC8613] describes a security protocol based on the exchange of [RFC8613] describes a security protocol based on the exchange of
protected CoAP messages. OSCORE builds on CBOR Object Signing and protected CoAP messages. OSCORE builds on CBOR Object Signing and
Encryption (COSE) Encryption (COSE)
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
provides end-to-end encryption, integrity, replay protection and provides end-to-end encryption, integrity, replay protection and
binding of response to request between a sender and a recipient, binding of response to request between a sender and a recipient,
independent of the transport layer also in the presence of independent of the transport layer also in the presence of
intermediaries. To this end, a CoAP message is protected by intermediaries. To this end, a CoAP message is protected by
including its payload (if any), certain options, and header fields in including its payload (if any), certain options, and header fields in
a COSE object, which replaces the authenticated and encrypted fields a COSE object, which replaces the authenticated and encrypted fields
in the protected message. in the protected message.
This document defines Group OSCORE, providing the same end-to-end This document defines Group OSCORE, a security protocol for Group
security properties as OSCORE in the case where CoAP requests have communication for CoAP [I-D.ietf-core-groupcomm-bis], providing the
multiple recipients. In particular, the described approach defines same end-to-end security properties as OSCORE in the case where CoAP
how OSCORE is used in a group communication setting to provide source requests have multiple recipients. In particular, the described
authentication for CoAP group requests, sent by a client to multiple approach defines how OSCORE is used in a group communication setting
servers, and for protection of the corresponding CoAP responses. to provide source authentication for CoAP group requests, sent by a
client to multiple servers, and for protection of the corresponding
CoAP responses. Group OSCORE also defines a pairwise mode where each
member of the group can efficiently derive a symmetric pairwise key
with any other member of the group for pairwise OSCORE communication.
Just like OSCORE, Group OSCORE is independent of the transport layer Just like OSCORE, Group OSCORE is independent of the transport layer
and works wherever CoAP does. Group communication for CoAP and works wherever CoAP does.
[I-D.ietf-core-groupcomm-bis] uses UDP/IP multicast as the underlying
data transport.
As with OSCORE, it is possible to combine Group OSCORE with As with OSCORE, it is possible to combine Group OSCORE with
communication security on other layers. One example is the use of communication security on other layers. One example is the use of
transport layer security, such as DTLS transport layer security, such as DTLS
[RFC6347][I-D.ietf-tls-dtls13], between one client and one proxy (and [RFC6347][I-D.ietf-tls-dtls13], between one client and one proxy (and
vice versa), or between one proxy and one server (and vice versa), in vice versa), or between one proxy and one server (and vice versa), in
order to protect the routing information of packets from observers. order to protect the routing information of packets from observers.
Note that DTLS does not define how to secure messages sent over IP Note that DTLS does not define how to secure messages sent over IP
multicast. multicast.
Group OSCORE defines two modes of operation: Group OSCORE defines two modes of operation, that can be used
independently or together:
o In the group mode, Group OSCORE requests and responses are * In the group mode, Group OSCORE requests and responses are
digitally signed with the private key of the sender and the digitally signed with the private key of the sender and the
signature is embedded in the protected CoAP message. The group signature is embedded in the protected CoAP message. The group
mode supports all COSE algorithms as well as signature mode supports all COSE signature algorithms as well as signature
verification by intermediaries. This mode is defined in Section 8 verification by intermediaries. This mode is defined in
and MUST be supported. Section 8.
o In the pairwise mode, two group members exchange Group OSCORE * In the pairwise mode, two group members exchange OSCORE requests
requests and responses over unicast, and the messages are and responses (typically) over unicast, and the messages are
protected with symmetric keys. These symmetric keys are derived protected with symmetric keys. These symmetric keys are derived
from Diffie-Hellman shared secrets, calculated with the asymmetric from Diffie-Hellman shared secrets, calculated with the asymmetric
keys of the sender and recipient, allowing for shorter integrity keys of the sender and recipient, allowing for shorter integrity
tags and therefore lower message overhead. This mode is defined tags and therefore lower message overhead. This mode is defined
in Section 9 and is OPTIONAL to support. in Section 9.
Both modes provide source authentication of CoAP messages. The Both modes provide source authentication of CoAP messages. The
application decides what mode to use, potentially on a per-message application decides what mode to use, potentially on a per-message
basis. Such decisions can be based, for instance, on pre-configured basis. Such decisions can be based, for instance, on pre-configured
policies or dynamic assessing of the target recipient and/or policies or dynamic assessing of the target recipient and/or
resource, among other things. One important case is when requests resource, among other things. One important case is when requests
are protected with the group mode, and responses with the pairwise are protected with the group mode, and responses with the pairwise
mode. Since such responses convey shorter integrity tags instead of mode. Since such responses convey shorter integrity tags instead of
bigger, full-fledged signatures, this significantly reduces the bigger, full-fledged signatures, this significantly reduces the
message overhead in case of many responses to one request. message overhead in case of many responses to one request.
skipping to change at page 6, line 32 skipping to change at page 6, line 42
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Readers are expected to be familiar with the terms and concepts Readers are expected to be familiar with the terms and concepts
described in CoAP [RFC7252] including "endpoint", "client", "server", described in CoAP [RFC7252] including "endpoint", "client", "server",
"sender" and "recipient"; group communication for CoAP "sender" and "recipient"; group communication for CoAP
[I-D.ietf-core-groupcomm-bis]; CBOR [RFC8949]; COSE [I-D.ietf-core-groupcomm-bis]; CBOR [RFC8949]; COSE
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
related counter signatures [I-D.ietf-cose-countersign]. related countersignatures [I-D.ietf-cose-countersign].
Readers are also expected to be familiar with the terms and concepts Readers are also expected to be familiar with the terms and concepts
for protection and processing of CoAP messages through OSCORE, such for protection and processing of CoAP messages through OSCORE, such
as "Security Context" and "Master Secret", defined in [RFC8613]. as "Security Context" and "Master Secret", defined in [RFC8613].
Terminology for constrained environments, such as "constrained Terminology for constrained environments, such as "constrained
device" and "constrained-node network", is defined in [RFC7228]. device" and "constrained-node network", is defined in [RFC7228].
This document refers also to the following terminology. This document refers also to the following terminology.
o Keying material: data that is necessary to establish and maintain * Keying material: data that is necessary to establish and maintain
secure communication among endpoints. This includes, for secure communication among endpoints. This includes, for
instance, keys and IVs [RFC4949]. instance, keys and IVs [RFC4949].
o Group: a set of endpoints that share group keying material and * Group: a set of endpoints that share group keying material and
security parameters (Common Context, see Section 2). That is, security parameters (Common Context, see Section 2). That is,
unless otherwise specified, the term group used in this unless otherwise specified, the term group used in this document
specification refers to a "security group" (see Section 2.1 of refers to a "security group" (see Section 2.1 of
[I-D.ietf-core-groupcomm-bis]), not to be confused with "CoAP [I-D.ietf-core-groupcomm-bis]), not to be confused with "CoAP
group" or "application group". group" or "application group".
o Group Manager: entity responsible for a group. Each endpoint in a * Group Manager: entity responsible for a group. Each endpoint in a
group communicates securely with the respective Group Manager, group communicates securely with the respective Group Manager,
which is neither required to be an actual group member nor to take which is neither required to be an actual group member nor to take
part in the group communication. The full list of part in the group communication. The full list of
responsibilities of the Group Manager is provided in Section 3.2. responsibilities of the Group Manager is provided in Section 3.3.
o Silent server: member of a group that never sends protected * Silent server: member of a group that never sends protected
responses in reply to requests. For CoAP group communications, responses in reply to requests. For CoAP group communications,
requests are normally sent without necessarily expecting a requests are normally sent without necessarily expecting a
response. A silent server may send unprotected responses, as response. A silent server may send unprotected responses, as
error responses reporting an OSCORE error. Note that an endpoint error responses reporting an OSCORE error. Note that an endpoint
can implement both a silent server and a client, i.e. the two can implement both a silent server and a client, i.e., the two
roles are independent. An endpoint acting only as a silent server roles are independent. An endpoint acting only as a silent server
performs only Group OSCORE processing on incoming requests. performs only Group OSCORE processing on incoming requests.
Silent servers maintain less keying material and in particular do Silent servers maintain less keying material and in particular do
not have a Sender Context for the group. Since silent servers do not have a Sender Context for the group. Since silent servers do
not have a Sender ID, they cannot support the pairwise mode. not have a Sender ID, they cannot support the pairwise mode.
o Group Identifier (Gid): identifier assigned to the group, unique * Group Identifier (Gid): identifier assigned to the group, unique
within the set of groups of a given Group Manager. within the set of groups of a given Group Manager.
o Group request: CoAP request message sent by a client in the group * Birth Gid: with respect to a group member, the Gid obtained by
that group member upon (re-)joining the group.
* Group request: CoAP request message sent by a client in the group
to all servers in that group. to all servers in that group.
o Source authentication: evidence that a received message in the * Key Generation Number: an integer value identifying the current
version of the keying material used in a group.
* Source authentication: evidence that a received message in the
group originated from a specific identified group member. This group originated from a specific identified group member. This
also provides assurance that the message was not tampered with by also provides assurance that the message was not tampered with by
anyone, be it a different legitimate group member or an endpoint anyone, be it a different legitimate group member or an endpoint
which is not a group member. which is not a group member.
2. Security Context 2. Security Context
This specification refers to a group as a set of endpoints sharing This document refers to a group as a set of endpoints sharing keying
keying material and security parameters for executing the Group material and security parameters for executing the Group OSCORE
OSCORE protocol (see Section 1.1). Each endpoint which is member of protocol (see Section 1.1). Regardless of what it actually supports,
a group maintains a Security Context as defined in Section 3 of each endpoint of a group is aware of whether the group uses the group
[RFC8613], extended as follows (see Figure 1): mode, or the pairwise mode, or both.
o One Common Context, shared by all the endpoints in the group. Two All members of a group maintain a Security Context as defined in
new parameters are included in the Common Context, namely Counter Section 3 of [RFC8613] and extended as defined in this section. How
Signature Algorithm and Counter Signature Parameters. These the Security Context is established by the group members is out of
relate to the computation of counter signatures, when messages are scope for this document, but if there is more than one Security
protected using the group mode (see Section 8). Context applicable to a message, then the endpoints MUST be able to
tell which Security Context was latest established.
If the pairwise mode is supported, the Common Context is further The default setting for how to manage information about the group,
extended with two new parameters, namely Secret Derivation including the Security Context, is described in terms of a Group
Algorithm and Secret Derivation Parameters. These relate to the Manager (see Section 3). In particular, the Group Manager indicates
derivation of a static-static Diffie-Hellman shared secret, from whether the group uses the group mode, the pairwise mode, or both of
which pairwise keys are derived (see Section 2.3.1) to protect them, as part of the group data provided to candidate group members
when joining the group.
The remainder of this section provides further details about the
Security Context of Group OSCORE. In particular, each endpoint which
is member of a group maintains a Security Context as defined in
Section 3 of [RFC8613], extended as follows (see Figure 1).
* One Common Context, shared by all the endpoints in the group.
Several new parameters are included in the Common Context.
If a Group Manager is used for maintaining the group, the Common
Context is extended with the public key of the Group Manager.
When processing a message, the public key of the Group Manager is
included in the external additional authenticated data.
If the group uses the group mode, the Common context is extended
with the following new parameters.
- Signature Encryption Algorithm and Signature Algorithm. These
relate to the encryption/decryption operations and to the
computation/verification of countersignatures, respectively,
when a message is protected with the group mode (see
Section 8).
- Group Encryption Key, used to perform encryption/decryption of
countersignatures, when a message is protected with the group
mode (see Section 8).
If the group uses the pairwise mode, the Common Context is
extended with a Pairwise Key Agreement Algorithm used for
agreement on a static-static Diffie-Hellman shared secret, from
which pairwise keys are derived (see Section 2.4.1) to protect
messages with the pairwise mode (see Section 9). messages with the pairwise mode (see Section 9).
o One Sender Context, extended with the endpoint's private key. The * One Sender Context, extended with the endpoint's public and
private key is used to sign the message in group mode, and for private key pair. The private key is used to sign messages in
deriving the pairwise keys in pairwise mode (see Section 2.3). If group mode, or for deriving pairwise keys in pairwise mode (see
the pairwise mode is supported, the Sender Context is also Section 2.4). When processing a message, the public key is
extended with the Pairwise Sender Keys associated to the other included in the external additional authenticated data.
endpoints (see Section 2.3). The Sender Context is omitted if the
endpoint is configured exclusively as silent server.
o One Recipient Context for each endpoint from which messages are If the endpoint supports the pairwise mode, the Sender Context is
also extended with the Pairwise Sender Keys associated to the
other endpoints (see Section 2.4).
The Sender Context is omitted if the endpoint is configured
exclusively as silent server.
* One Recipient Context for each endpoint from which messages are
received. It is not necessary to maintain Recipient Contexts received. It is not necessary to maintain Recipient Contexts
associated to endpoints from which messages are not (expected to associated to endpoints from which messages are not (expected to
be) received. The Recipient Context is extended with the public be) received. The Recipient Context is extended with the public
key of the associated endpoint, used to verify the signature in key of the associated endpoint, used to verify the signature in
group mode and for deriving the pairwise keys in pairwise mode group mode and for deriving the pairwise keys in pairwise mode
(see Section 2.3). If the pairwise mode is supported, then the (see Section 2.4). If the endpoint supports the pairwise mode,
Recipient Context is also extended with the Pairwise Recipient Key then the Recipient Context is also extended with the Pairwise
associated to the other endpoint (see Section 2.3). Recipient Key associated to the other endpoint (see Section 2.4).
+-------------------+-----------------------------------------------+
| Context Component | New Information Elements |
+-------------------+-----------------------------------------------+
| Common Context | Counter Signature Algorithm |
| | Counter Signature Parameters |
| | *Secret Derivation Algorithm |
| | *Secret Derivation Parameters |
+-------------------+-----------------------------------------------+
| Sender Context | Endpoint's own private key |
| | *Pairwise Sender Keys for the other endpoints |
+-------------------+-----------------------------------------------+
| Each | Public key of the other endpoint |
| Recipient Context | *Pairwise Recipient Key of the other endpoint |
+-------------------+-----------------------------------------------+
Figure 1: Additions to the OSCORE Security Context. Optional
additions are labeled with an asterisk.
Further details about the Security Context of Group OSCORE are +-------------------+------------------------------------------------+
provided in the remainder of this section. How the Security Context | Context Component | New Information Elements |
is established by the group members is out of scope for this +-------------------+------------------------------------------------+
specification, but if there is more than one Security Context | Common Context | Group Manager Public Key |
applicable to a message, then the endpoints MUST be able to tell | | * Signature Encryption Algorithm |
which Security Context was latest established. | | * Signature Algorithm |
| | * Group Encryption Key |
| | ^ Pairwise Key Agreement Algorithm |
+-------------------+------------------------------------------------+
| Sender Context | Endpoint's own public and private key pair |
| | ^ Pairwise Sender Keys for the other endpoints |
+-------------------+------------------------------------------------+
| Each | Public key of the other endpoint |
| Recipient Context | ^ Pairwise Recipient Key of the other endpoint |
+-------------------+------------------------------------------------+
The default setting for how to manage information about the group is Figure 1: Additions to the OSCORE Security Context. The optional
described in terms of a Group Manager (see Section 3). elements labeled with * (with ^) are present only if the group
uses the group mode (the pairwise mode).
2.1. Common Context 2.1. Common Context
The Common Context may be acquired from the Group Manager (see The Common Context may be acquired from the Group Manager (see
Section 3). The following sections define how the Common Context is Section 3). The following sections define how the Common Context is
extended, compared to [RFC8613]. extended, compared to [RFC8613].
2.1.1. ID Context 2.1.1. AEAD Algorithm
The ID Context parameter (see Sections 3.3 and 5.1 of [RFC8613]) in AEAD Algorithm identifies the COSE AEAD algorithm to use for
encryption, when messages are protected using the pairwise mode (see
Section 9). This algorithm MUST provide integrity protection. This
parameter is immutable once the Common Context is established, and it
is not relevant if the group uses only the group mode.
For endpoints that support the pairwise mode, the AEAD algorithm AES-
CCM-16-64-128 defined in Section 4.2 of
[I-D.ietf-cose-rfc8152bis-algs] is mandatory to implement.
2.1.2. ID Context
The ID Context parameter (see Sections 3.1 and 3.3 of [RFC8613]) in
the Common Context SHALL contain the Group Identifier (Gid) of the the Common Context SHALL contain the Group Identifier (Gid) of the
group. The choice of the Gid format is application specific. An group. The choice of the Gid format is application specific. An
example of specific formatting of the Gid is given in Appendix C. example of specific formatting of the Gid is given in Appendix C.
The application needs to specify how to handle potential collisions The application needs to specify how to handle potential collisions
between Gids (see Section 10.5). between Gids (see Section 10.6).
2.1.2. Counter Signature Algorithm 2.1.3. Group Manager Public Key
Counter Signature Algorithm identifies the digital signature Group Manager Public Key specifies the public key of the Group
algorithm used to compute a counter signature on the COSE object (see Manager. This is included in the external additional authenticated
Sections 3.2 and 3.3 of [I-D.ietf-cose-countersign]), when messages data (see Section 4.3).
are protected using the group mode (see Section 8).
This parameter is immutable once the Common Context is established. Each group member MUST obtain the public key of the Group Manager
Counter Signature Algorithm MUST take value from the "Value" column with a valid proof-of-possession of the corresponding private key,
of the "COSE Algorithms" Registry [COSE.Algorithms]. The value is for instance from the Group Manager itself when joining the group.
associated to a COSE key type, as specified in the "Capabilities" Further details on the provisioning of the Group Manager's public key
column of the "COSE Algorithms" Registry [COSE.Algorithms]. COSE to the group members are out of the scope of this document.
capabilities for algorithms are defined in Section 8 of
[I-D.ietf-cose-rfc8152bis-algs].
The EdDSA signature algorithm and the elliptic curve Ed25519 2.1.4. Signature Encryption Algorithm
[RFC8032] are mandatory to implement. If elliptic curve signatures
are used, it is RECOMMENDED to implement deterministic signatures
with additional randomness as specified in
[I-D.mattsson-cfrg-det-sigs-with-noise].
2.1.3. Counter Signature Parameters Signature Encryption Algorithm identifies the algorithm to use for
enryption, when messages are protected using the group mode (see
Section 8). This algorithm MAY provide integrity protection. This
parameter is immutable once the Common Context is established.
Counter Signature Parameters identifies the parameters associated to For endpoints that support the group mode and use authenticated
the digital signature algorithm specified in Counter Signature encryption, the AEAD algorithm AES-CCM-16-64-128 defined in
Algorithm. This parameter is immutable once the Common Context is Section 4.2 of [I-D.ietf-cose-rfc8152bis-algs] is mandatory to
established. implement.
This parameter is a CBOR array including the following two elements, 2.1.5. Signature Algorithm
whose exact structure and value depend on the value of Counter
Signature Algorithm:
o The first element is the array of COSE capabilities for Counter Signature Algorithm identifies the digital signature algorithm used
Signature Algorithm, as specified for that algorithm in the to compute a countersignature on the COSE object (see Sections 3.2
"Capabilities" column of the "COSE Algorithms" Registry and 3.3 of [I-D.ietf-cose-countersign]), when messages are protected
[COSE.Algorithms] (see Section 8.1 of using the group mode (see Section 8). This parameter is immutable
[I-D.ietf-cose-rfc8152bis-algs]). once the Common Context is established.
o The second element is the array of COSE capabilities for the COSE For endpoints that support the group mode, the EdDSA signature
key type associated to Counter Signature Algorithm, as specified algorithm and the elliptic curve Ed25519 [RFC8032] are mandatory to
for that key type in the "Capabilities" column of the "COSE Key implement. If elliptic curve signatures are used, it is RECOMMENDED
Types" Registry [COSE.Key.Types] (see Section 8.2 of to implement deterministic signatures with additional randomness as
[I-D.ietf-cose-rfc8152bis-algs]). specified in [I-D.mattsson-cfrg-det-sigs-with-noise].
Examples of Counter Signature Parameters are in Appendix G. 2.1.6. Group Encryption Key
This format is consistent with every counter signature algorithm Group Encryption Key specifies the encryption key for deriving a
currently considered in [I-D.ietf-cose-rfc8152bis-algs], i.e. with keystream to encrypt/decrypt a countersignature, when a message is
algorithms that have only the COSE key type as their COSE capability. protected with the group mode (see Section 8).
Appendix H describes how Counter Signature Parameters can be
generalized for possible future registered algorithms having a
different set of COSE capabilities.
2.1.4. Secret Derivation Algorithm The Group Encryption Key is derived as defined for Sender/Recipient
Keys in Section 3.2.1 of [RFC8613], with the following differences.
Secret Derivation Algorithm identifies the elliptic curve Diffie- * The 'alg_aead' element of the 'info' array takes the value of
Hellman algorithm used to derive a static-static Diffie-Hellman Signature Encryption Algorithm from the Common Context (see
shared secret, from which pairwise keys are derived (see Section 2.1.5).
Section 2.3.1) to protect messages with the pairwise mode (see
Section 9).
This parameter is immutable once the Common Context is established. * The 'type' element of the 'info' array is "Group Encryption Key".
Secret Derivation Algorithm MUST take value from the "Value" column The label is an ASCII string and does not include a trailing NUL
of the "COSE Algorithms" Registry [COSE.Algorithms]. The value is byte.
associated to a COSE key type, as specified in the "Capabilities"
column of the "COSE Algorithms" Registry [COSE.Algorithms]. COSE * L and the 'L' element of the 'info' array are the size of the
capabilities for algorithms are defined in Section 8 of output of the HKDF Algorithm from the Common Context (see
[I-D.ietf-cose-rfc8152bis-algs]. Section 3.2 of [RFC8613]), in bytes.
2.1.7. Pairwise Key Agreement Algorithm
Pairwise Key Agreement Algorithm identifies the elliptic curve
Diffie-Hellman algorithm used to derive a static-static Diffie-
Hellman shared secret, from which pairwise keys are derived (see
Section 2.4.1) to protect messages with the pairwise mode (see
Section 9). This parameter is immutable once the Common Context is
established.
For endpoints that support the pairwise mode, the ECDH-SS + HKDF-256 For endpoints that support the pairwise mode, the ECDH-SS + HKDF-256
algorithm specified in Section 6.3.1 of algorithm specified in Section 6.3.1 of
[I-D.ietf-cose-rfc8152bis-algs] and the X25519 curve [RFC7748] are [I-D.ietf-cose-rfc8152bis-algs] and the X25519 curve [RFC7748] are
mandatory to implement. mandatory to implement.
2.1.5. Secret Derivation Parameters 2.2. Sender Context and Recipient Context
Secret Derivation Parameters identifies the parameters associated to
the elliptic curve Diffie-Hellman algorithm specified in Secret
Derivation Algorithm. This parameter is immutable once the Common
Context is established.
This parameter is a CBOR array including the following two elements,
whose exact structure and value depend on the value of Secret
Derivation Algorithm:
o The first element is the array of COSE capabilities for Secret
Derivation Algorithm, as specified for that algorithm in the
"Capabilities" column of the "COSE Algorithms" Registry
[COSE.Algorithms] (see Section 8.1 of
[I-D.ietf-cose-rfc8152bis-algs]).
o The second element is the array of COSE capabilities for the COSE OSCORE specifies the derivation of Sender Context and Recipient
key type associated to Secret Derivation Algorithm, as specified Context, specifically of Sender/Recipient Keys and Common IV, from a
for that key type in the "Capabilities" column of the "COSE Key set of input parameters (see Section 3.2 of [RFC8613]). Like in
Types" Registry [COSE.Key.Types] (see Section 8.2 of [RFC8613], HKDF SHA-256 is the mandatory to implement HKDF.
[I-D.ietf-cose-rfc8152bis-algs]).
Examples of Secret Derivation Parameters are in Appendix G. The derivation of Sender/Recipient Keys and Common IV defined in
OSCORE applies also to Group OSCORE, with the following extensions
compared to Section 3.2.1 of [RFC8613].
This format is consistent with every elliptic curve Diffie-Hellman * If the group uses (also) the group mode, the 'alg_aead' element of
algorithm currently considered in [I-D.ietf-cose-rfc8152bis-algs], the 'info' array takes the value of Signature Encryption Algorithm
i.e. with algorithms that have only the COSE key type as their COSE from the Common Context (see Section 2.1.5).
capability. Appendix H describes how Secret Derivation Parameters
can be generalized for possible future registered algorithms having a
different set of COSE capabilities.
2.2. Sender Context and Recipient Context * If the group uses only the pairwise mode, the 'alg_aead' element
of the 'info' array takes the value of AEAD Algorithm from the
Common Context (see Section 2.1.1).
OSCORE specifies the derivation of Sender Context and Recipient The Sender ID SHALL be unique for each endpoint in a group with a
Context, specifically of Sender/Recipient Keys and Common IV, from a certain tuple (Master Secret, Master Salt, Group Identifier), see
set of input parameters (see Section 3.2 of [RFC8613]). This Section 3.3 of [RFC8613].
derivation applies also to Group OSCORE, and the mandatory-to-
implement HKDF and AEAD algorithms are the same as in [RFC8613]. The
Sender ID SHALL be unique for each endpoint in a group with a fixed
Master Secret, Master Salt and Group Identifier (see Section 3.3 of
[RFC8613]).
For Group OSCORE, the Sender Context and Recipient Context For Group OSCORE, the Sender Context and Recipient Context
additionally contain asymmetric keys, as described previously in additionally contain asymmetric keys, as described previously in
Section 2. The private/public key pair of the sender can, for Section 2. The private/public key pair of the sender can, for
example, be generated by the endpoint or provisioned during example, be generated by the endpoint or provisioned during
manufacturing. manufacturing.
With the exception of the public key of the sender endpoint and the With the exception of the public key of the sender endpoint and the
possibly associated pairwise keys, a receiver endpoint can derive a possibly associated pairwise keys, a receiver endpoint can derive a
complete Security Context from a received Group OSCORE message and complete Security Context from a received Group OSCORE message and
skipping to change at page 12, line 24 skipping to change at page 13, line 9
For severely constrained devices, it may be not feasible to For severely constrained devices, it may be not feasible to
simultaneously handle the ongoing processing of a recently received simultaneously handle the ongoing processing of a recently received
message in parallel with the retrieval of the sender endpoint's message in parallel with the retrieval of the sender endpoint's
public key. Such devices can be configured to drop a received public key. Such devices can be configured to drop a received
message for which there is no (complete) Recipient Context, and message for which there is no (complete) Recipient Context, and
retrieve the sender endpoint's public key in order to have it retrieve the sender endpoint's public key in order to have it
available to verify subsequent messages from that endpoint. available to verify subsequent messages from that endpoint.
An endpoint admits a maximum amount of Recipient Contexts for a same An endpoint admits a maximum amount of Recipient Contexts for a same
Security Context, e.g. due to memory limitations. After reaching Security Context, e.g., due to memory limitations. After reaching
that limit, the creation of a new Recipient Context results in an that limit, the creation of a new Recipient Context results in an
overflow. When this happens, the endpoint has to delete a current overflow. When this happens, the endpoint has to delete a current
Recipient Context to install the new one. It is up to the Recipient Context to install the new one. It is up to the
application to define policies for selecting the current Recipient application to define policies for selecting the current Recipient
Context to delete. A newly installed Recipient Context that has Context to delete. A newly installed Recipient Context that has
required to delete another Recipient Context is initialized with an required to delete another Recipient Context is initialized with an
invalid Replay Window, and accordingly requires the endpoint to take invalid Replay Window, and accordingly requires the endpoint to take
appropriate actions (see Section 2.4.1.2). appropriate actions (see Section 2.5.1.2).
2.3. Pairwise Keys 2.3. Format of Public Keys
In a group, the following MUST hold for the public key of each
endpoint as well as for the public key of the Group Manager.
* All public keys MUST be encoded according to the same format used
in the group. The format MUST provide the full set of information
related to the public key algorithm, including, e.g., the used
elliptic curve (when applicable).
* All public keys MUST be for the public key algorithm used in the
group and aligned with the possible associated parameters used in
the group, e.g., the used elliptic curve (when applicable).
If the group uses (also) the group mode, the public key algorithm is
the Signature Algorithm used in the group. If the group uses only
the pairwise mode, the public key algorithm is the Pairwise Key
Agreement Algorithm used in the group.
If CWTs [RFC8392] or unprotected CWT claim sets [I-D.ietf-rats-uccs]
are used as public key format, the public key algorithm is fully
described by a COSE key type and its "kty" and "crv" parameters.
If X.509 certificates [RFC7925] or C509 certificates
[I-D.ietf-cose-cbor-encoded-cert] are used as public key format, the
public key algorithm is fully described by the "algorithm" field of
the "SubjectPublicKeyInfo" structure, and by the
"subjectPublicKeyAlgorithm" element, respectively.
Public keys are also used to derive pairwise keys (see Section 2.4.1)
and are included in the external additional authenticated data (see
Section 4.3). In both of these cases, an endpoint in a group MUST
treat public keys as opaque data, i.e., by considering the same
binary representation made available to other endpoints in the group,
possibly through a designated trusted source (e.g., the Group
Manager).
For example, an X.509 certificate is provided as its direct binary
serialization. If C509 certificates or CWTs are used as credential
format, they are provided as the binary serialization of a (possibly
tagged) CBOR array. If a CWT claim set is used as credential format,
it is provided as the binary serialization of a CBOR map.
2.4. Pairwise Keys
Certain signature schemes, such as EdDSA and ECDSA, support a secure Certain signature schemes, such as EdDSA and ECDSA, support a secure
combined signature and encryption scheme. This section specifies the combined signature and encryption scheme. This section specifies the
derivation of "pairwise keys", for use in the pairwise mode defined derivation of "pairwise keys", for use in the pairwise mode defined
in Section 9. in Section 9. Group OSCORE keys used for both signature and
encryption MUST NOT be used for any other purposes than Group OSCORE.
2.3.1. Derivation of Pairwise Keys 2.4.1. Derivation of Pairwise Keys
Using the Group OSCORE Security Context (see Section 2), a group Using the Group OSCORE Security Context (see Section 2), a group
member can derive AEAD keys to protect point-to-point communication member can derive AEAD keys, to protect point-to-point communication
between itself and any other endpoint in the group. The same AEAD between itself and any other endpoint in the group by means of the
algorithm as in the group mode is used. The key derivation of these AEAD Algorithm from the Common Context (see Section 2.1.1). The key
so-called pairwise keys follows the same construction as in derivation of these so-called pairwise keys follows the same
Section 3.2.1 of [RFC8613]: construction as in Section 3.2.1 of [RFC8613]:
Pairwise Sender Key = HKDF(Sender Key, IKM-Sender, info, L)
Pairwise Recipient Key = HKDF(Recipient Key, IKM-Recipient, info, L)
with
IKM-Sender = Sender Pub Key | Recipient Pub Key | Shared Secret
IKM-Recipient = Recipient Pub Key | Sender Pub Key | Shared Secret
Pairwise Sender Key = HKDF(Sender Key, Shared Secret, info, L)
Pairwise Recipient Key = HKDF(Recipient Key, Shared Secret, info, L)
where: where:
o The Pairwise Sender Key is the AEAD key for processing outgoing * The Pairwise Sender Key is the AEAD key for processing outgoing
messages addressed to endpoint X. messages addressed to endpoint X.
o The Pairwise Recipient Key is the AEAD key for processing incoming * The Pairwise Recipient Key is the AEAD key for processing incoming
messages from endpoint X. messages from endpoint X.
o HKDF is the HKDF algorithm specified by Secret Derivation * HKDF is the OSCORE HKDF algorithm [RFC8613] from the Common
Algorithm from the Common Context (see Section 2.1.4). Context.
o The Sender Key and private key are from the Sender Context. The * The Sender Key from the Sender Context is used as salt in the
Sender Key is used as salt in the HKDF, when deriving the Pairwise HKDF, when deriving the Pairwise Sender Key.
Sender Key.
o The Recipient Key and the public key are from the Recipient * The Recipient Key from the Recipient Context associated to
Context associated to endpoint X. The Recipient Key is used as endpoint X is used as salt in the HKDF, when deriving the Pairwise
salt in the HKDF, when deriving the Pairwise Recipient Key. Recipient Key.
o The Shared Secret is computed as a static-static Diffie-Hellman * IKM-Sender is the Input Keying Material (IKM) used in the HKDF for
shared secret [NIST-800-56A], where the endpoint uses its private the derivation of the Pairwise Sender Key. IKM-Sender is the byte
key and the public key of the other endpoint X. The Shared Secret string concatenation of the endpoint's own (signature) public key,
is used as Input Keying Material (IKM) in the HKDF. the endpoint X's (signature) public key from the Recipient
Context, and the Shared Secret. The two (signature) public keys
are binary encoded as defined in Section 2.3.
o info and L are as defined in Section 3.2.1 of [RFC8613]. * IKM-Recipient is the Input Keying Material (IKM) used in the HKDF
for the derivation of the Recipient Sender Key. IKM-Recipient is
the byte string concatenation of the endpoint X's (signature)
public key from the Recipient Context, the endpoint's own
(signature) public key, and the Shared Secret. The two
(signature) public keys are binary encoded as defined in
Section 2.3.
* The Shared Secret is computed as a cofactor Diffie-Hellman shared
secret, see Section 5.7.1.2 of [NIST-800-56A], using the Pairwise
Key Agreement Algorithm. The endpoint uses its private key from
the Sender Context and the public key of the other endpoint X from
the associated Recipient Context. Note the requirement of
validation of public keys in Section 10.15. For X25519 and X448,
the procedure is described in Section 5 of [RFC7748] using public
keys mapped to Montgomery coordinates, see Section 2.4.2.
* info and L are as defined in Section 3.2.1 of [RFC8613]. That is:
- The 'alg_aead' element of the 'info' array takes the value of
AEAD Algorithm from the Common Context (see Section 2.1.1).
- L and the 'L' element of the 'info' array are the size of the
key for the AEAD Algorithm from the Common Context (see
Section 2.1.1), in bytes.
If EdDSA asymmetric keys are used, the Edward coordinates are mapped If EdDSA asymmetric keys are used, the Edward coordinates are mapped
to Montgomery coordinates using the maps defined in Sections 4.1 and to Montgomery coordinates using the maps defined in Sections 4.1 and
4.2 of [RFC7748], before using the X25519 and X448 functions defined 4.2 of [RFC7748], before using the X25519 and X448 functions defined
in Section 5 of [RFC7748]. in Section 5 of [RFC7748]. For further details, see Section 2.4.2.
ECC asymmetric keys in Montgomery or Weirstrass form are used
directly in the key agreement algorithm without coordinate mapping.
After establishing a partially or completely new Security Context After establishing a partially or completely new Security Context
(see Section 2.4 and Section 3.1), the old pairwise keys MUST be (see Section 2.5 and Section 3.2), the old pairwise keys MUST be
deleted. Since new Sender/Recipient Keys are derived from the new deleted. Since new Sender/Recipient Keys are derived from the new
group keying material (see Section 2.2), every group member MUST use group keying material (see Section 2.2), every group member MUST use
the new Sender/Recipient Keys when deriving new pairwise keys. the new Sender/Recipient Keys when deriving new pairwise keys.
As long as any two group members preserve the same asymmetric keys, As long as any two group members preserve the same asymmetric keys,
their Diffie-Hellman shared secret does not change across updates of their Diffie-Hellman shared secret does not change across updates of
the group keying material. the group keying material.
2.3.2. Usage of Sequence Numbers 2.4.2. ECDH with Montgomery Coordinates
2.4.2.1. Curve25519
The y-coordinate of the other endpoint's Ed25519 public key is
decoded as specified in Section 5.1.3 of [RFC8032]. The Curve25519
u-coordinate is recovered as u = (1 + y) / (1 - y) (mod p) following
the map in Section 4.1 of [RFC7748]. Note that the mapping is not
defined for y = 1, and that y = -1 maps to u = 0 which corresponds to
the neutral group element and thus will result in a degenerate shared
secret. Therefore implementations MUST abort if the y-coordinate of
the other endpoint's Ed25519 public key is 1 or -1 (mod p).
The private signing key byte strings (= the lower 32 bytes used for
generating the public key, see step 1 of Section 5.1.5 of [RFC8032])
are decoded the same way for signing in Ed25519 and scalar
multiplication in X25519. Hence, to compute the shared secret the
endpoint applies the X25519 function to the Ed25519 private signing
key byte string and the encoded u-coordinate byte string as specified
in Section 5 of [RFC7748].
2.4.2.2. Curve448
The y-coordinate of the other endpoint's Ed448 public key is decoded
as specified in Section 5.2.3. of [RFC8032]. The Curve448
u-coordinate is recovered as u = y^2 * (d * y^2 - 1) / (y^2 - 1) (mod
p) following the map from "edwards448" in Section 4.2 of [RFC7748],
and also using the relation x^2 = (y^2 - 1)/(d * y^2 - 1) from the
curve equation. Note that the mapping is not defined for y = 1 or
-1. Therefore implementations MUST abort if the y-coordinate of the
peer endpoint's Ed448 public key is 1 or -1 (mod p).
The private signing key byte strings (= the lower 57 bytes used for
generating the public key, see step 1 of Section 5.2.5 of [RFC8032])
are decoded the same way for signing in Ed448 and scalar
multiplication in X448. Hence, to compute the shared secret the
endpoint applies the X448 function to the Ed448 private signing key
byte string and the encoded u-coordinate byte string as specified in
Section 5 of [RFC7748].
2.4.3. Usage of Sequence Numbers
When using any of its Pairwise Sender Keys, a sender endpoint When using any of its Pairwise Sender Keys, a sender endpoint
including the 'Partial IV' parameter in the protected message MUST including the 'Partial IV' parameter in the protected message MUST
use the current fresh value of the Sender Sequence Number from its use the current fresh value of the Sender Sequence Number from its
Sender Context (see Section 2.2). That is, the same Sender Sequence Sender Context (see Section 2.2). That is, the same Sender Sequence
Number space is used for all outgoing messages protected with Group Number space is used for all outgoing messages protected with Group
OSCORE, thus limiting both storage and complexity. OSCORE, thus limiting both storage and complexity.
On the other hand, when combining group and pairwise communication On the other hand, when combining group and pairwise communication
modes, this may result in the Partial IV values moving forward more modes, this may result in the Partial IV values moving forward more
often. This can happen when a client engages in frequent or long often. This can happen when a client engages in frequent or long
sequences of one-to-one exchanges with servers in the group, by sequences of one-to-one exchanges with servers in the group, by
sending requests over unicast. sending requests over unicast.
2.3.3. Security Context for Pairwise Mode 2.4.4. Security Context for Pairwise Mode
If the pairwise mode is supported, the Security Context additionally If the pairwise mode is supported, the Security Context additionally
includes Secret Derivation Algorithm, Secret Derivation Parameters includes Pairwise Key Agreement Algorithm and the pairwise keys, as
and the pairwise keys, as described at the beginning of Section 2. described at the beginning of Section 2.
The pairwise keys as well as the shared secrets used in their The pairwise keys as well as the shared secrets used in their
derivation (see Section 2.3.1) may be stored in memory or recomputed derivation (see Section 2.4.1) may be stored in memory or recomputed
every time they are needed. The shared secret changes only when a every time they are needed. The shared secret changes only when a
public/private key pair used for its derivation changes, which public/private key pair used for its derivation changes, which
results in the pairwise keys also changing. Additionally, the results in the pairwise keys also changing. Additionally, the
pairwise keys change if the Sender ID changes or if a new Security pairwise keys change if the Sender ID changes or if a new Security
Context is established for the group (see Section 2.4.3). In order Context is established for the group (see Section 2.5.3). In order
to optimize protocol performance, an endpoint may store the derived to optimize protocol performance, an endpoint may store the derived
pairwise keys for easy retrieval. pairwise keys for easy retrieval.
In the pairwise mode, the Sender Context includes the Pairwise Sender In the pairwise mode, the Sender Context includes the Pairwise Sender
Keys to use with the other endpoints (see Figure 1). In order to Keys to use with the other endpoints (see Figure 1). In order to
identify the right key to use, the Pairwise Sender Key for endpoint X identify the right key to use, the Pairwise Sender Key for endpoint X
may be associated to the Recipient ID of endpoint X, as defined in may be associated to the Recipient ID of endpoint X, as defined in
the Recipient Context (i.e. the Sender ID from the point of view of the Recipient Context (i.e., the Sender ID from the point of view of
endpoint X). In this way, the Recipient ID can be used to lookup for endpoint X). In this way, the Recipient ID can be used to lookup for
the right Pairwise Sender Key. This association may be implemented in the right Pairwise Sender Key. This association may be implemented in
different ways, e.g. by storing the pair (Recipient ID, Pairwise different ways, e.g., by storing the pair (Recipient ID, Pairwise
Sender Key) or linking a Pairwise Sender Key to a Recipient Context. Sender Key) or linking a Pairwise Sender Key to a Recipient Context.
2.4. Update of Security Context 2.5. Update of Security Context
It is RECOMMENDED that the immutable part of the Security Context is It is RECOMMENDED that the immutable part of the Security Context is
stored in non-volatile memory, or that it can otherwise be reliably stored in non-volatile memory, or that it can otherwise be reliably
accessed throughout the operation of the group, e.g. after a device accessed throughout the operation of the group, e.g., after a device
reboots. However, also immutable parts of the Security Context may reboots. However, also immutable parts of the Security Context may
need to be updated, for example due to scheduled key renewal, new or need to be updated, for example due to scheduled key renewal, new or
re-joining members in the group, or the fact that the endpoint re-joining members in the group, or the fact that the endpoint
changes Sender ID (see Section 2.4.3). changes Sender ID (see Section 2.5.3).
On the other hand, the mutable parts of the Security Context are On the other hand, the mutable parts of the Security Context are
updated by the endpoint when executing the security protocol, but may updated by the endpoint when executing the security protocol, but may
nevertheless become outdated, e.g. due to loss of the mutable nevertheless become outdated, e.g., due to loss of the mutable
Security Context (see Section 2.4.1) or exhaustion of Sender Sequence Security Context (see Section 2.5.1) or exhaustion of Sender Sequence
Numbers (see Section 2.4.2). Numbers (see Section 2.5.2).
If it is not feasible or practically possible to store and maintain If it is not feasible or practically possible to store and maintain
up-to-date the mutable part in non-volatile memory (e.g., due to up-to-date the mutable part in non-volatile memory (e.g., due to
limited number of write operations), the endpoint MUST be able to limited number of write operations), the endpoint MUST be able to
detect a loss of the mutable Security Context and MUST accordingly detect a loss of the mutable Security Context and MUST accordingly
take the actions defined in Section 2.4.1. take the actions defined in Section 2.5.1.
2.4.1. Loss of Mutable Security Context 2.5.1. Loss of Mutable Security Context
An endpoint may lose its mutable Security Context, e.g. due to a An endpoint may lose its mutable Security Context, e.g., due to a
reboot (see Section 2.4.1.1) or to an overflow of Recipient Contexts reboot (see Section 2.5.1.1) or to an overflow of Recipient Contexts
(see Section 2.4.1.2). (see Section 2.5.1.2).
In such a case, the endpoint needs to prevent the re-use of a nonce In such a case, the endpoint needs to prevent the re-use of a nonce
with the same AEAD key, and to handle incoming replayed messages. with the same AEAD key, and to handle incoming replayed messages.
2.4.1.1. Reboot and Total Loss 2.5.1.1. Reboot and Total Loss
In case a loss of the Sender Context and/or of the Recipient Contexts In case a loss of the Sender Context and/or of the Recipient Contexts
is detected (e.g. following a reboot), the endpoint MUST NOT protect is detected (e.g., following a reboot), the endpoint MUST NOT protect
further messages using this Security Context to avoid reusing an AEAD further messages using this Security Context to avoid reusing an AEAD
nonce with the same AEAD key. nonce with the same AEAD key.
In particular, before resuming its operations in the group, the In particular, before resuming its operations in the group, the
endpoint MUST retrieve new Security Context parameters from the Group endpoint MUST retrieve new Security Context parameters from the Group
Manager (see Section 2.4.3) and use them to derive a new Sender Manager (see Section 2.5.3) and use them to derive a new Sender
Context (see Section 2.2). Since this includes a newly derived Context (see Section 2.2). Since this includes a newly derived
Sender Key, the server will not reuse the same pair (key, nonce), Sender Key, a server will not reuse the same pair (key, nonce), even
even when using the Partial IV of (old re-injected) requests to build when using the Partial IV of (old re-injected) requests to build the
the AEAD nonce for protecting the corresponding responses. AEAD nonce for protecting the corresponding responses.
From then on, the endpoint MUST use the latest installed Sender From then on, the endpoint MUST use the latest installed Sender
Context to protect outgoing messages. Also, newly created Recipient Context to protect outgoing messages. Also, newly created Recipient
Contexts will have a Replay Window which is initialized as valid. Contexts will have a Replay Window which is initialized as valid.
If not able to establish an updated Sender Context, e.g. because of If not able to establish an updated Sender Context, e.g., because of
lack of connectivity with the Group Manager, the endpoint MUST NOT lack of connectivity with the Group Manager, the endpoint MUST NOT
protect further messages using the current Security Context and MUST protect further messages using the current Security Context and MUST
NOT accept incoming messages from other group members, as currently NOT accept incoming messages from other group members, as currently
unable to detect possible replays. unable to detect possible replays.
2.4.1.2. Overflow of Recipient Contexts 2.5.1.2. Overflow of Recipient Contexts
After reaching the maximum amount of Recipient Contexts, an endpoint After reaching the maximum amount of Recipient Contexts, an endpoint
will experience an overflow when installing a new Recipient Context, will experience an overflow when installing a new Recipient Context,
as it requires to first delete an existing one (see Section 2.2). as it requires to first delete an existing one (see Section 2.2).
Every time this happens, the Replay Window of the new Recipient Every time this happens, the Replay Window of the new Recipient
Context is initialized as not valid. Therefore, the endpoint MUST Context is initialized as not valid. Therefore, the endpoint MUST
take the following actions, before accepting request messages from take the following actions, before accepting request messages from
the client associated to the new Recipient Context. the client associated to the new Recipient Context.
If it is not configured as silent server, the endpoint MUST either: If it is not configured as silent server, the endpoint MUST either:
o Retrieve new Security Context parameters from the Group Manager * Retrieve new Security Context parameters from the Group Manager
and derive a new Sender Context, as defined in Section 2.4.1.1; or and derive a new Sender Context, as defined in Section 2.5.1.1; or
o When receiving a first request to process with the new Recipient * When receiving a first request to process with the new Recipient
Context, use the approach specified in Appendix E and based on the Context, use the approach specified in Appendix E and based on the
Echo Option for CoAP [I-D.ietf-core-echo-request-tag], if Echo Option for CoAP [I-D.ietf-core-echo-request-tag], if
supported. In particular, the endpoint MUST use its Partial IV supported. In particular, the endpoint MUST use its Partial IV
when generating the AEAD nonce and MUST include the Partial IV in when generating the AEAD nonce and MUST include the Partial IV in
the response message conveying the Echo Option. If the endpoint the response message conveying the Echo Option. If the endpoint
supports the CoAP Echo Option, it is RECOMMENDED to take this supports the CoAP Echo Option, it is RECOMMENDED to take this
approach. approach.
If it is configured exclusively as silent server, the endpoint MUST If it is configured exclusively as silent server, the endpoint MUST
wait for the next group rekeying to occur, in order to derive a new wait for the next group rekeying to occur, in order to derive a new
Security Context and re-initialize the Replay Window of each Security Context and re-initialize the Replay Window of each
Recipient Contexts as valid. Recipient Contexts as valid.
2.4.2. Exhaustion of Sender Sequence Number 2.5.2. Exhaustion of Sender Sequence Number
An endpoint can eventually exhaust the Sender Sequence Number, which An endpoint can eventually exhaust the Sender Sequence Number, which
is incremented for each new outgoing message including a Partial IV. is incremented for each new outgoing message including a Partial IV.
This is the case for group requests, Observe notifications [RFC7641] This is the case for group requests, Observe notifications [RFC7641]
and, optionally, any other response. and, optionally, any other response.
Implementations MUST be able to detect an exhaustion of Sender Implementations MUST be able to detect an exhaustion of Sender
Sequence Number, after the endpoint has consumed the largest usable Sequence Number, after the endpoint has consumed the largest usable
value. If an implementation's integers support wrapping addition, value. If an implementation's integers support wrapping addition,
the implementation MUST treat Sender Sequence Number as exhausted the implementation MUST treat Sender Sequence Number as exhausted
when a wrap-around is detected. when a wrap-around is detected.
Upon exhausting the Sender Sequence Numbers, the endpoint MUST NOT Upon exhausting the Sender Sequence Numbers, the endpoint MUST NOT
use this Security Context to protect further messages including a use this Security Context to protect further messages including a
Partial IV. Partial IV.
The endpoint SHOULD inform the Group Manager, retrieve new Security The endpoint SHOULD inform the Group Manager, retrieve new Security
Context parameters from the Group Manager (see Section 2.4.3), and Context parameters from the Group Manager (see Section 2.5.3), and
use them to derive a new Sender Context (see Section 2.2). use them to derive a new Sender Context (see Section 2.2).
From then on, the endpoint MUST use its latest installed Sender From then on, the endpoint MUST use its latest installed Sender
Context to protect outgoing messages. Context to protect outgoing messages.
2.4.3. Retrieving New Security Context Parameters 2.5.3. Retrieving New Security Context Parameters
The Group Manager can assist an endpoint with an incomplete Sender The Group Manager can assist an endpoint with an incomplete Sender
Context to retrieve missing data of the Security Context and thereby Context to retrieve missing data of the Security Context and thereby
become fully operational in the group again. The two main options become fully operational in the group again. The two main options
for the Group Manager are described in this section: i) assignment of for the Group Manager are described in this section: i) assignment of
a new Sender ID to the endpoint (see Section 2.4.3.1); and ii) a new Sender ID to the endpoint (see Section 2.5.3.1); and ii)
establishment of a new Security Context for the group (see establishment of a new Security Context for the group (see
Section 2.4.3.2). The update of the Replay Window in each of the Section 2.5.3.2). The update of the Replay Window in each of the
Recipient Contexts is discussed in Section 6.1. Recipient Contexts is discussed in Section 6.2.
As group membership changes, or as group members get new Sender IDs As group membership changes, or as group members get new Sender IDs
(see Section 2.4.3.1) so do the relevant Recipient IDs that the other (see Section 2.5.3.1) so do the relevant Recipient IDs that the other
endpoints need to keep track of. As a consequence, group members may endpoints need to keep track of. As a consequence, group members may
end up retaining stale Recipient Contexts, that are no longer useful end up retaining stale Recipient Contexts, that are no longer useful
to verify incoming secure messages. to verify incoming secure messages.
The Recipient ID ('kid') SHOULD NOT be considered as a persistent and The Recipient ID ('kid') SHOULD NOT be considered as a persistent and
reliable indicator of a group member. Such an indication can be reliable indicator of a group member. Such an indication can be
achieved only by using that member's public key, when verifying achieved only by using that member's public key, when verifying
countersignatures of received messages (in group mode), or when countersignatures of received messages (in group mode), or when
verifying messages integrity-protected with pairwise keying material verifying messages integrity-protected with pairwise keying material
derived from asymmetric keys (in pairwise mode). derived from asymmetric keys (in pairwise mode).
Furthermore, applications MAY define policies to: i) delete Furthermore, applications MAY define policies to: i) delete
(long-)unused Recipient Contexts and reduce the impact on storage (long-)unused Recipient Contexts and reduce the impact on storage
space; as well as ii) check with the Group Manager that a public key space; as well as ii) check with the Group Manager that a public key
is currently the one associated to a 'kid' value, after a number of is currently the one associated to a 'kid' value, after a number of
consecutive failed verifications. consecutive failed verifications.
2.4.3.1. New Sender ID for the Endpoint 2.5.3.1. New Sender ID for the Endpoint
The Group Manager may assign a new Sender ID to an endpoint, while The Group Manager may assign a new Sender ID to an endpoint, while
leaving the Gid, Master Secret and Master Salt unchanged in the leaving the Gid, Master Secret and Master Salt unchanged in the
group. In this case, the Group Manager MUST assign a Sender ID that group. In this case, the Group Manager MUST assign a Sender ID that
has never been assigned before in the group under the current Gid has not been used in the group since the latest time when the current
value. Gid value was assigned to the group (see Section 3.2).
Having retrieved the new Sender ID, and potentially other missing Having retrieved the new Sender ID, and potentially other missing
data of the immutable Security Context, the endpoint can derive a new data of the immutable Security Context, the endpoint can derive a new
Sender Context (see Section 2.2). When doing so, the endpoint resets Sender Context (see Section 2.2). When doing so, the endpoint resets
the Sender Sequence Number in its Sender Context to 0, and derives a the Sender Sequence Number in its Sender Context to 0, and derives a
new Sender Key. This is in turn used to possibly derive new Pairwise new Sender Key. This is in turn used to possibly derive new Pairwise
Sender Keys. Sender Keys.
From then on, the endpoint MUST use its latest installed Sender From then on, the endpoint MUST use its latest installed Sender
Context to protect outgoing messages. Context to protect outgoing messages.
The assignment of a new Sender ID may be the result of different The assignment of a new Sender ID may be the result of different
processes. The endpoint may request a new Sender ID, e.g. because of processes. The endpoint may request a new Sender ID, e.g., because
exhaustion of Sender Sequence Numbers (see Section 2.4.2). An of exhaustion of Sender Sequence Numbers (see Section 2.5.2). An
endpoint may request to re-join the group, e.g. because of losing its endpoint may request to re-join the group, e.g., because of losing
mutable Security Context (see Section 2.4.1), and is provided with a its mutable Security Context (see Section 2.5.1), and is provided
new Sender ID together with the latest immutable Security Context. with a new Sender ID together with the latest immutable Security
Context.
For the other group members, the Recipient Context corresponding to For the other group members, the Recipient Context corresponding to
the old Sender ID becomes stale (see Section 3.1). the old Sender ID becomes stale (see Section 3.2).
2.4.3.2. New Security Context for the Group 2.5.3.2. New Security Context for the Group
The Group Manager may establish a new Security Context for the group The Group Manager may establish a new Security Context for the group
(see Section 3.1). The Group Manager does not necessarily establish (see Section 3.2). The Group Manager does not necessarily establish
a new Security Context for the group if one member has an outdated a new Security Context for the group if one member has an outdated
Security Context (see Section 2.4.3.1), unless that was already Security Context (see Section 2.5.3.1), unless that was already
planned or required for other reasons. planned or required for other reasons.
All the group members need to acquire new Security Context parameters All the group members need to acquire new Security Context parameters
from the Group Manager. Once having acquired new Security Context from the Group Manager. Once having acquired new Security Context
parameters, each group member performs the following actions. parameters, each group member performs the following actions.
o From then on, it MUST NOT use the current Security Context to * From then on, it MUST NOT use the current Security Context to
start processing new messages for the considered group. start processing new messages for the considered group.
o It completes any ongoing message processing for the considered * It completes any ongoing message processing for the considered
group. group.
o It derives and install a new Security Context. In particular: * It derives and install a new Security Context. In particular:
* It re-derives the keying material stored in its Sender Context - It re-derives the keying material stored in its Sender Context
and Recipient Contexts (see Section 2.2). The Master Salt used and Recipient Contexts (see Section 2.2). The Master Salt used
for the re-derivations is the updated Master Salt parameter if for the re-derivations is the updated Master Salt parameter if
provided by the Group Manager, or the empty byte string provided by the Group Manager, or the empty byte string
otherwise. otherwise.
* It resets to 0 its Sender Sequence Number in its Sender - It resets to 0 its Sender Sequence Number in its Sender
Context. Context.
* It re-initializes the Replay Window of each Recipient Context. - It re-initializes the Replay Window of each Recipient Context.
* It resets to 0 the sequence number of each ongoing observation - For each ongoing observation where it is an observer client and
where it is an observer client and that it wants to keep that it wants to keep active, it resets to 0 the Notification
active. Number of each associated server (see Section 6.1).
From then on, it can resume processing new messages for the From then on, it can resume processing new messages for the
considered group. In particular: considered group. In particular:
o It MUST use its latest installed Sender Context to protect * It MUST use its latest installed Sender Context to protect
outgoing messages. outgoing messages.
o It SHOULD use its latest installed Recipient Contexts to process * It SHOULD use its latest installed Recipient Contexts to process
incoming messages, unless application policies admit to incoming messages, unless application policies admit to
temporarily retain and use the old, recent, Security Context (see temporarily retain and use the old, recent, Security Context (see
Section 10.4.1). Section 10.5.1).
The distribution of a new Gid and Master Secret may result in The distribution of a new Gid and Master Secret may result in
temporarily misaligned Security Contexts among group members. In temporarily misaligned Security Contexts among group members. In
particular, this may result in a group member not being able to particular, this may result in a group member not being able to
process messages received right after a new Gid and Master Secret process messages received right after a new Gid and Master Secret
have been distributed. A discussion on practical consequences and have been distributed. A discussion on practical consequences and
possible ways to address them, as well as on how to handle the old possible ways to address them, as well as on how to handle the old
Security Context, is provided in Section 10.4. Security Context, is provided in Section 10.5.
3. The Group Manager 3. The Group Manager
As with OSCORE, endpoints communicating with Group OSCORE need to As with OSCORE, endpoints communicating with Group OSCORE need to
establish the relevant Security Context. Group OSCORE endpoints need establish the relevant Security Context. Group OSCORE endpoints need
to acquire OSCORE input parameters, information about the group(s) to acquire OSCORE input parameters, information about the group(s)
and about other endpoints in the group(s). This specification is and about other endpoints in the group(s). This document is based on
based on the existence of an entity called Group Manager which is the existence of an entity called Group Manager and responsible for
responsible for the group, but does not mandate how the Group Manager the group, but it does not mandate how the Group Manager interacts
interacts with the group members. The responsibilities of the Group with the group members. The responsibilities of the Group Manager
Manager are compiled in Section 3.2. are compiled together in Section 3.3.
It is RECOMMENDED to use a Group Manager as described in It is RECOMMENDED to use a Group Manager as described in
[I-D.ietf-ace-key-groupcomm-oscore], where the join process is based [I-D.ietf-ace-key-groupcomm-oscore], where the join process is based
on the ACE framework for authentication and authorization in on the ACE framework for authentication and authorization in
constrained environments [I-D.ietf-ace-oauth-authz]. constrained environments [I-D.ietf-ace-oauth-authz].
The Group Manager assigns unique Group Identifiers (Gids) to The Group Manager assigns an integer Key Generation Number to each of
different groups under its control, as well as unique Sender IDs (and its groups, identifying the current version of the keying material
thereby Recipient IDs) to the members of those groups. According to used in that group. The first Key Generation Number assigned to
a hierarchical approach, the Gid value assigned to a group is every group MUST be 0. Separately for each group, the value of the
associated to a dedicated space for the values of Sender ID and Key Generation Number increases strictly monotonically, each time the
Recipient ID of the members of that group. Group Manager distributes new keying material to that group (see
Section 3.2). That is, if the current Key Generation Number for a
group is X, then X+1 will denote the keying material distributed and
used in that group immediately after the current one.
The Group Manager MUST NOT reassign a Gid value to the same group, The Group Manager assigns unique Group Identifiers (Gids) to the
and MUST NOT reassign a Sender ID within the same group under the groups under its control. Also, for each group, the Group Manager
same Gid value. assigns unique Sender IDs (and thus Recipient IDs) to the respective
group members. According to a hierarchical approach, the Gid value
assigned to a group is associated to a dedicated space for the values
of Sender ID and Recipient ID of the members of that group.
In addition, the Group Manager maintains records of the public keys When a node (re-)joins a group, it is provided also with the current
of endpoints in a group, and provides information about the group and Gid to use in the group, namely the Birth Gid of that node for that
its members to other group members and selected roles. Upon nodes' group. For each group member, the Group Manager MUST store the
joining, the Group Manager collects such public keys and MUST verify latest corresponding Birth Gid until that member leaves the group.
proof-of-possession of the respective private key. In case the node has in fact re-joined the group, the newly
determined Birth Gid overwrites the one currently stored.
The Group Manager maintains records of the public keys of endpoints
in a group, and provides information about the group and its members
to other group members and to external principals with selected roles
(see Section 3.1). Upon nodes' joining, the Group Manager collects
such public keys and MUST verify proof-of-possession of the
respective private key.
An endpoint acquires group data such as the Gid and OSCORE input An endpoint acquires group data such as the Gid and OSCORE input
parameters including its own Sender ID from the Group Manager, and parameters including its own Sender ID from the Group Manager, and
provides information about its public key to the Group Manager, for provides information about its public key to the Group Manager, for
example upon joining the group. example upon joining the group.
A group member can retrieve from the Group Manager the public key and Furthermore, when joining the group or later on as a group member, an
other information associated to another member of the group, with endpoint can retrieve from the Group Manager the public key of the
which it can generate the corresponding Recipient Context. In Group Manager as well as the public key and other information
particular, the requested public key is provided together with the associated to other members of the group, with which it can derive
Sender ID of the associated group member. An application can the corresponding Recipient Context. Together with the requested
configure a group member to asynchronously retrieve information about public keys, the Group Manager MUST provide the Sender ID of the
Recipient Contexts, e.g. by Observing [RFC7641] a resource at the associated group members and the current Key Generation Number in the
Group Manager to get updates on the group membership. group. An application can configure a group member to asynchronously
retrieve information about Recipient Contexts, e.g., by Observing
[RFC7641] a resource at the Group Manager to get updates on the group
membership.
The Group Manager MAY serve additional entities acting as signature 3.1. Support for Additional Principals
checkers, e.g. intermediary gateways. These entities do not join a
group as members, but can retrieve public keys of group members from
the Group Manager, in order to verify counter signatures of group
messages. A signature checker MUST be authorized for retrieving
public keys of members in a specific group from the Group Manager.
To this end, the same method mentioned above based on the ACE
framework [I-D.ietf-ace-oauth-authz] can be used.
3.1. Management of Group Keying Material The Group Manager MAY serve additional principals acting as signature
checkers, e.g., intermediary gateways. These principals do not join
a group as members, but can retrieve public keys of group members and
other selected group data from the Group Manager, in order to solely
verify countersignatures of messages protected in group mode (see
Section 8.5).
In order to establish a new Security Context for a group, a new Group In order to verify countersignatures of messages in a group, a
Identifier (Gid) for that group and a new value for the Master Secret signature checker needs to retrieve the following information about
parameter MUST be generated. When distributing the new Gid and that group from the Group Manager.
Master Secret, the Group Manager MAY distribute also a new value for
the Master Salt parameter, and should preserve the current value of
the Sender ID of each group member.
The Group Manager MUST NOT reassign a Gid value to the same group. * The current ID Context (Gid) used in the group.
That is, every group can have a given Gid at most once during its
lifetime. An example of Gid format supporting this operation is
provided in Appendix C.
The Group Manager MUST NOT reassign a previously used Sender ID * The public keys of the group members and the public key of the
('kid') with the same Gid, Master Secret and Master Salt. That is, Group Manager.
the Group Manager MUST NOT reassign a Sender ID value within a same
group under the same Gid value (see Section 2.4.3.1). Within this * The current Group Encryption Key (see Section 2.1.6).
restriction, the Group Manager can assign a Sender ID used under an
old Gid value, thus avoiding Sender ID values to irrecoverably grow * The identifiers of the algorithms used in the group (see
in size. Section 2), i.e.: i) Signature Encryption Algorithm and Signature
Algorithm; and ii) AEAD Algorithm and Pairwise Key Agreement
Algorithm, if the group uses also the pairwise mode.
A signature checker MUST be authorized before it can retrieve such
information. To this end, the same method mentioned above based on
the ACE framework [I-D.ietf-ace-oauth-authz] can be used.
3.2. Management of Group Keying Material
In order to establish a new Security Context for a group, the Group
Manager MUST generate and assign to the group a new Group Identifier
(Gid) and a new value for the Master Secret parameter. When doing
so, a new value for the Master Salt parameter MAY also be generated
and assigned to the group. When establishing the new Security
Context, the Group Manager should preserve the current value of the
Sender ID of each group member.
The specific group key management scheme used to distribute new
keying material, is out of the scope of this document. However, it
is RECOMMENDED that the Group Manager supports the Group Rekeying
Process described in [I-D.ietf-ace-key-groupcomm-oscore]. When
possible, the delivery of rekeying messages should use a reliable
transport, in order to reduce chances of group members missing a
rekeying instance.
The set of group members should not be assumed as fixed, i.e., the
group membership is subject to changes, possibly on a frequent basis.
The Group Manager MUST rekey the group when one or more currently
present endpoints leave the group, or in order to evict them as
compromised or suspected so. In either case, this excludes such
nodes from future communications in the group, and thus preserves
forward security. If required by the application, the Group Manager
MUST rekey the group also before one or more new joining endpoints
are added to the group, thus preserving backward security.
The establishment of the new Security Context for the group takes the
following steps.
1. The Group Manager MUST increment by 1 the Key Generation Number
for the group.
2. The Group Manager MUST check if the new Gid to be distributed
coincides with the Birth Gid of any of the current group members.
If any of such "elder members" is found in the group, then:
* The Group Manager MUST evict the elder members from the group.
That is, the Group Manager MUST terminate their membership and
MUST rekey the group in such a way that the new keying
material is not provided to those evicted elder members. This
ensures that an Observe notification [RFC7641] can never
successfully match against the Observe requests of two
different observations.
* Until a further following group rekeying, the Group Manager
MUST store the list of those latest-evicted elder members. If
any of those endpoints re-joins the group before a further
following group rekeying occurs, the Group Manager MUST NOT
rekey the group upon their re-joining. When one of those
endpoints re-joins the group, the Group Manager can rely,
e.g., on the ongoing secure communication association to
recognize the endpoint as included in the stored list.
3. The Group Manager MUST build a set of stale Sender IDs including:
* The Sender IDs that, during the current Gid, were both
assigned to an endpoint and subsequently relinquished (see
Section 2.5.3.1).
* The current Sender IDs of the group members that the upcoming
group rekeying aims to exclude from future group
communications, if any.
4. The Group Manager rekeys the group, by distributing:
* The new keying material, i.e., the new Master Secret, the new
Gid and (optionally) the new Master Salt.
* The new Key Generation Number from step 1.
* The set of stale Sender IDs from step 3.
Further information may be distributed, depending on the specific
group key management scheme used in the group.
When receiving the new group keying materal, a group member considers
the received stale Sender IDs and performs the following actions.
* The group member MUST remove every public key associated to a
stale Sender ID from its list of group members' public keys used
in the group.
* The group member MUST delete each of its Recipient Contexts used
in the group whose corresponding Recipient ID is a stale Sender
ID.
After that, the group member installs the new keying material and
derives the corresponding new Security Context.
A group member might miss one group rekeying or more consecutive
instances. As a result, the group member will retain old group
keying material with Key Generation Number GEN_OLD. Eventually, the
group member can notice the discrepancy, e.g., by repeatedly failing
to verify incoming messages, or by explicitly querying the Group
Manager for the current Key Generation Number. Once the group member
gains knowledge of having missed a group rekeying, it MUST delete the
old keying material it owns.
Then, the group member proceeds according to the following steps.
1. The group member retrieves from the Group Manager the current
group keying material, together with the current Key Generation
Number GEN_NEW. The group member MUST NOT install the obtained
group keying material yet.
2. The group member asks the Group Manager for the set of stale
Sender IDs.
3. If no exact indication can be obtained from the Group Manager,
the group member MUST remove all the public keys from its list of
group members' public keys used in the group and MUST delete all
its Recipient Contexts used in the group.
Otherwise, the group member MUST remove every public key
associated to a stale Sender ID from its list of group members'
public keys used in the group, and MUST delete each of its
Recipient Contexts used in the group whose corresponding
Recipient ID is a stale Sender ID.
4. The group member installs the current group keying material, and
derives the corresponding new Security Context.
Alternatively, the group member can re-join the group. In such a
case, the group member MUST take one of the following two actions.
* The group member performs steps 2 and 3 above. Then, the group
member re-joins the group.
* The group member re-joins the group with the same roles it
currently has in the group, and, during the re-joining process, it
asks the Group Manager for the public keys of all the current
group members.
Then, given Z the set of public keys received from the Group
Manager, the group member removes every public key which is not in
Z from its list of group members' public keys used in the group,
and deletes each of its Recipient Contexts used in the group that
does not include any of the public keys in Z.
By removing public keys and deleting Recipient Contexts associated to
stale Sender IDs, it is ensured that a recipient endpoint owning the
latest group keying material does not store the public keys of sender
endpoints that are not current group members. This in turn allows
group members to rely on owned public keys to confidently assert the
group membership of sender endpoints, when receiving incoming
messages protected in group mode (see Section 8).
3.2.1. Recycling of Identifiers
Although the Gid value changes every time a group is rekeyed, the
Group Manager can reassign a Gid to the same group over that group's
lifetime. This would happen, for instance, once the whole space of
Gid values has been used for the group in question.
From the moment when a Gid is assigned to a group until the moment a
new Gid is assigned to that same group, the Group Manager MUST NOT
reassign a Sender ID within the group. This prevents to reuse a
Sender ID ('kid') with the same Gid, Master Secret and Master Salt.
Within this restriction, the Group Manager can assign a Sender ID
used under an old Gid value (including under a same, recycled Gid
value), thus avoiding Sender ID values to irrecoverably grow in size.
Even when an endpoint joining a group is recognized as a current Even when an endpoint joining a group is recognized as a current
member of that group, e.g. through the ongoing secure communication member of that group, e.g., through the ongoing secure communication
association, the Group Manager MUST assign a new Sender ID different association, the Group Manager MUST assign a new Sender ID different
than the one currently used by the endpoint in the group, unless the than the one currently used by the endpoint in the group, unless the
group is rekeyed first and a new Gid value is established. group is rekeyed first and a new Gid value is established.
Figure 2 overviews the different keying material components, Figure 2 overviews the different keying material components,
considering their relation and possible reuse across group rekeying. considering their relation and possible reuse across group rekeying.
Components changed in lockstep * Changing a kid does not Components changed in lockstep
upon a group rekeying need changing the Group ID upon a group rekeying
+----------------------------+ +----------------------------+ * Changing a kid does not
| | * A kid is not reassigned | | need changing the Group ID
| Master Group |<--> kid1 under the same Group ID | Master Group |<--> kid1
| Secret <---> o <---> ID | | Secret <---> o <---> ID | * A kid is not reassigned
| ^ |<--> kid2 * Upon changing the Group ID, | ^ |<--> kid2 under the ongoing usage of
| | | every current kid should | | | the current Group ID
| | |<--> kid3 be preserved for efficient | | |<--> kid3
| v | key rollover | v | * Upon changing the Group ID,
| Master Salt | ... ... | Master Salt | ... ... every current kid should
| (optional) | * After changing Group ID, an | (optional) | be preserved for efficient
| | key rollover
| The Key Generation Number |
| is incremented by 1 | * After changing Group ID, an
| | unused kid can be assigned | | unused kid can be assigned
+----------------------------+ +----------------------------+
Figure 2: Relations among keying material components. Figure 2: Relations among keying material components.
If required by the application (see Appendix A.1), it is RECOMMENDED
to adopt a group key management scheme, and securely distribute a new
value for the Gid and for the Master Secret parameter of the group's
Security Context, before a new joining endpoint is added to the group
or after a currently present endpoint leaves the group. This is
necessary to preserve backward security and forward security in the
group, if the application requires it.
The specific approach used to distribute new group data is out of the
scope of this document. However, it is RECOMMENDED that the Group
Manager supports the distribution of the new Gid and Master Secret
parameter to the group according to the Group Rekeying Process
described in [I-D.ietf-ace-key-groupcomm-oscore].
3.2. Responsibilities of the Group Manager 3.3. Responsibilities of the Group Manager
The Group Manager is responsible for performing the following tasks: The Group Manager is responsible for performing the following tasks:
1. Creating and managing OSCORE groups. This includes the 1. Creating and managing OSCORE groups. This includes the
assignment of a Gid to every newly created group, as well as assignment of a Gid to every newly created group, ensuring
ensuring uniqueness of Gids within the set of its OSCORE groups. uniqueness of Gids within the set of its OSCORE groups, and
tracking the Birth Gids of current group members in each group.
2. Defining policies for authorizing the joining of its OSCORE 2. Defining policies for authorizing the joining of its OSCORE
groups. groups.
3. Handling the join process to add new endpoints as group members. 3. Handling the join process to add new endpoints as group members.
4. Establishing the Common Context part of the Security Context, 4. Establishing the Common Context part of the Security Context,
and providing it to authorized group members during the join and providing it to authorized group members during the join
process, together with the corresponding Sender Context. process, together with the corresponding Sender Context.
5. Updating the Gid of its OSCORE groups, upon renewing the 5. Updating the Key Generation Number and the Gid of its OSCORE
respective Security Context. This includes ensuring that the groups, upon renewing the respective Security Context.
same Gid value is not reassigned to the same group.
6. Generating and managing Sender IDs within its OSCORE groups, as 6. Generating and managing Sender IDs within its OSCORE groups, as
well as assigning and providing them to new endpoints during the well as assigning and providing them to new endpoints during the
join process, or to current group members upon request of join process, or to current group members upon request of
renewal or re-joining. renewal or re-joining. This includes ensuring that:
This includes ensuring that each Sender ID: is unique within * Each Sender ID is unique within each of the OSCORE groups;
each of the OSCORE groups; and is not reassigned within the same
group under the same Gid value, i.e. not even to a current group * Each Sender ID is not reassigned within the same group since
member re-joining the same group without a rekeying happening the latest time when the current Gid value was assigned to
first. the group. That is, the Sender ID is not reassigned even to
a current group member re-joining the same group, without a
rekeying happening first.
7. Defining communication policies for each of its OSCORE groups, 7. Defining communication policies for each of its OSCORE groups,
and signaling them to new endpoints during the join process. and signaling them to new endpoints during the join process.
8. Renewing the Security Context of an OSCORE group upon membership 8. Renewing the Security Context of an OSCORE group upon membership
change, by revoking and renewing common security parameters and change, by revoking and renewing common security parameters and
keying material (rekeying). keying material (rekeying).
9. Providing the management keying material that a new endpoint 9. Providing the management keying material that a new endpoint
requires to participate in the rekeying process, consistently requires to participate in the rekeying process, consistently
with the key management scheme used in the group joined by the with the key management scheme used in the group joined by the
new endpoint. new endpoint.
10. Acting as key repository, in order to handle the public keys of 10. Assisting a group member that has missed a group rekeying
instance to understand which public keys and Recipient Contexts
to delete, as associated to former group members.
11. Acting as key repository, in order to handle the public keys of
the members of its OSCORE groups, and providing such public keys the members of its OSCORE groups, and providing such public keys
to other members of the same group upon request. The actual to other members of the same group upon request. The actual
storage of public keys may be entrusted to a separate secure storage of public keys may be entrusted to a separate secure
storage device or service. storage device or service.
11. Validating that the format and parameters of public keys of 12. Validating that the format and parameters of public keys of
group members are consistent with the countersignature algorithm group members are consistent with the public key algorithm and
and related parameters used in the respective OSCORE group. related parameters used in the respective OSCORE group.
The Group Manager described in [I-D.ietf-ace-key-groupcomm-oscore] The Group Manager described in [I-D.ietf-ace-key-groupcomm-oscore]
provides these functionalities. provides these functionalities.
4. The COSE Object 4. The COSE Object
Building on Section 5 of [RFC8613], this section defines how to use Building on Section 5 of [RFC8613], this section defines how to use
COSE [I-D.ietf-cose-rfc8152bis-struct] to wrap and protect data in COSE [I-D.ietf-cose-rfc8152bis-struct] to wrap and protect data in
the original message. OSCORE uses the untagged COSE_Encrypt0 the original message. OSCORE uses the untagged COSE_Encrypt0
structure with an Authenticated Encryption with Associated Data structure with an Authenticated Encryption with Associated Data
(AEAD) algorithm. Unless otherwise specified, the following (AEAD) algorithm. Unless otherwise specified, the following
modifications apply for both the group mode and the pairwise mode of modifications apply for both the group mode and the pairwise mode of
Group OSCORE. Group OSCORE.
4.1. Counter Signature 4.1. Countersignature
When protecting a message in group mode, the 'unprotected' field MUST When protecting a message in group mode, the 'unprotected' field MUST
additionally include the following parameter: additionally include the following parameter:
o COSE_CounterSignature0: its value is set to the counter signature * COSE_CounterSignature0: its value is set to the encrypted
of the COSE object, computed by the sender as described in countersignature of the COSE object, namely ENC_SIGNATURE. That
Sections 3.2 and 3.3 of [I-D.ietf-cose-countersign], by using its is:
private key and according to the Counter Signature Algorithm and
Counter Signature Parameters in the Security Context.
In particular, the Countersign_structure contains the context text - The countersignature of the COSE object, namely SIGNATURE, is
string "CounterSignature0", the external_aad as defined in computed by the sender as described in Sections 3.2 and 3.3 of
Section 4.3 of this specification, and the ciphertext of the COSE [I-D.ietf-cose-countersign], by using its private key and
object as payload. according to the Signature Algorithm in the Security Context.
In particular, the Countersign_structure contains the context
text string "CounterSignature0", the external_aad as defined in
Section 4.3 of this document, and the ciphertext of the COSE
object as payload.
- The encrypted countersignature, namely ENC_SIGNATURE, is
computed as
ENC_SIGNATURE = SIGNATURE XOR KEYSTREAM
where KEYSTREAM is derived as per Section 4.1.1.
4.1.1. Keystream Derivation
The following defines how an endpoint derives the keystream
KEYSTREAM, used to encrypt/decrypt the countersignature of an
outgoing/incoming message M protected in group mode.
The keystream SHALL be derived as follows, by using the HKDF
Algorithm from the Common Context (see Section 3.2 of [RFC8613]),
which consists of composing the HKDF-Extract and HKDF-Expand steps
[RFC5869].
KEYSTREAM = HKDF(salt, IKM, info, L)
The input parameters of HKDF are as follows.
* salt takes as value the Partial IV (PIV) used to protect M. Note
that, if M is a response, salt takes as value either: i) the fresh
Partial IV generated by the server and included in the response;
or ii) the same Partial IV of the request generated by the client
and not included in the response.
* IKM is the Group Encryption Key from the Common Context (see
Section 2.1.6).
* info is the serialization of a CBOR array consisting of (the
notation follows [RFC8610]):
info = [
id : bstr,
id_context : bstr,
type : bool,
L: uint
]
where:
* id is the Sender ID of the endpoint that generated PIV.
* id_context is the ID Context (Gid) used when protecting M.
Note that, in case of group rekeying, a server might use a
different Gid when protecting a response, compared to the Gid that
it used to verify (that the client used to protect) the request,
see Section 8.3.
* type is the CBOR simple value True (0xf5) if M is a request, or
the CBOR simple value False (0xf4) otherwise.
* L is the size of the countersignature, as per Signature Algorithm
from the Common Context (see Section 2.1.5), in bytes.
4.1.2. Clarifications on Using a Countersignature
Note that the literature commonly refers to a countersignature as a
signature computed by a principal A over a document already protected
by a different principal B.
However, the COSE_Countersignature0 structure belongs to the set of
abbreviated countersignatures defined in Sections 3.2 and 3.3 of
[I-D.ietf-cose-countersign], which were designed primarily to deal
with the problem of encrypted group messaging, but where it is
required to know who originated the message.
Since the parameters for computing or verifying the abbreviated
countersignature generated by A are provided by the same context used
to describe the security processing performed by B and to be
countersigned, these structures are applicable also when the two
principals A and B are actually the same one, like the sender of a
Group OSCORE message protected in group mode.
4.2. The 'kid' and 'kid context' parameters 4.2. The 'kid' and 'kid context' parameters
The value of the 'kid' parameter in the 'unprotected' field of The value of the 'kid' parameter in the 'unprotected' field of
response messages MUST be set to the Sender ID of the endpoint response messages MUST be set to the Sender ID of the endpoint
transmitting the message, if the request was protected in group mode. transmitting the message, if the request was protected in group mode.
That is, unlike in [RFC8613], the 'kid' parameter is always present That is, unlike in [RFC8613], the 'kid' parameter is always present
in responses to a request that was protected in group mode. in responses to a request that was protected in group mode.
The value of the 'kid context' parameter in the 'unprotected' field The value of the 'kid context' parameter in the 'unprotected' field
of requests messages MUST be set to the ID Context, i.e. the Group of requests messages MUST be set to the ID Context, i.e., the Group
Identifier value (Gid) of the group. That is, unlike in [RFC8613], Identifier value (Gid) of the group. That is, unlike in [RFC8613],
the 'kid context' parameter is always present in requests. the 'kid context' parameter is always present in requests.
4.3. external_aad 4.3. external_aad
The external_aad of the Additional Authenticated Data (AAD) is The external_aad of the Additional Authenticated Data (AAD) is
different compared to OSCORE, and is defined in this section. different compared to OSCORE [RFC8613], and is defined in this
section.
The same external_aad structure is used in group mode and pairwise The same external_aad structure is used in group mode and pairwise
mode for encryption (see Section 5.3 of mode for authenticated encryption/decryption (see Section 5.3 of
[I-D.ietf-cose-rfc8152bis-struct]), as well as in group mode for [I-D.ietf-cose-rfc8152bis-struct]), as well as in group mode for
signing (see Section 4.4 of [I-D.ietf-cose-rfc8152bis-struct]). computing and verifying the countersignature (see Section 4.4 of
[I-D.ietf-cose-rfc8152bis-struct]).
In particular, the external_aad includes also the counter signature In particular, the external_aad includes also the Signature
algorithm and related signature parameters, the value of the 'kid Algorithm, the Signature Encryption Algorithm, the Pairwise Key
context' in the COSE object of the request, and the OSCORE option of Agreement Algorithm, the value of the 'kid context' in the COSE
the protected message. object of the request, the OSCORE option of the protected message,
the sender's public key, and the Group Manager's public key.
The external_aad SHALL be a CBOR array wrapped in a bstr object as
defined below, following the notation of [RFC8610]:
external_aad = bstr .cbor aad_array external_aad = bstr .cbor aad_array
aad_array = [ aad_array = [
oscore_version : uint, oscore_version : uint,
algorithms : [alg_aead : int / tstr, algorithms : [alg_aead : int / tstr / null,
alg_countersign : int / tstr, alg_signature_enc : int / tstr / null,
par_countersign : [countersign_alg_capab, alg_signature : int / tstr / null,
countersign_key_type_capab]], alg_pairwise_key_agreement : int / tstr / null],
request_kid : bstr, request_kid : bstr,
request_piv : bstr, request_piv : bstr,
options : bstr, options : bstr,
request_kid_context : bstr, request_kid_context : bstr,
OSCORE_option: bstr OSCORE_option: bstr,
sender_public_key: bstr,
gm_public_key: bstr / null
] ]
Figure 3: external_aad Figure 3: external_aad
Compared with Section 5.4 of [RFC8613], the aad_array has the Compared with Section 5.4 of [RFC8613], the aad_array has the
following differences. following differences.
o The 'algorithms' array additionally includes: * The 'algorithms' array is extended as follows.
* 'alg_countersign', which specifies Counter Signature Algorithm The parameter 'alg_aead' MUST be set to the CBOR simple value Null
from the Common Context (see Section 2.1.2). This parameter if the group does not use the pairwise mode, regardless whether
MUST encode the value of Counter Signature Algorithm as a CBOR the endpoint supports the pairwise mode or not. Otherwise, this
integer or text string, consistently with the "Value" field in parameter MUST encode the value of AEAD Algorithm from the Common
the "COSE Algorithms" Registry for this counter signature Context (see Section 2.1.1), as per Section 5.4 of [RFC8613].
algorithm.
* 'par_countersign', which specifies the CBOR array Counter Furthermore, the 'algorithms' array additionally includes:
Signature Parameters from the Common Context (see
Section 2.1.3). In particular:
+ 'countersign_alg_capab' is the array of COSE capabilities - 'alg_signature_enc', which specifies Signature Encryption
for the countersignature algorithm indicated in Algorithm from the Common Context (see Section 2.1.5). This
'alg_countersign'. This is the first element of the CBOR parameter MUST be set to the CBOR simple value Null if the
array Counter Signature Parameters from the Common Context. group does not use the group mode, regardless whether the
endpoint supports the group mode or not. Otherwise, this
parameter MUST encode the value of Signature Encryption
Algorithm as a CBOR integer or text string, consistently with
the "Value" field in the "COSE Algorithms" Registry for this
AEAD algorithm.
+ 'countersign_key_type_capab' is the array of COSE - 'alg_signature', which specifies Signature Algorithm from the
capabilities for the COSE key type used by the Common Context (see Section 2.1.5). This parameter MUST be set
countersignature algorithm indicated in 'alg_countersign'. to the CBOR simple value Null if the group does not use the
This is the second element of the CBOR array Counter group mode, regardless whether the endpoint supports the group
Signature Parameters from the Common Context. mode or not. Otherwise, this parameter MUST encode the value
of Signature Algorithm as a CBOR integer or text string,
consistently with the "Value" field in the "COSE Algorithms"
Registry for this signature algorithm.
This format is consistent with every counter signature - 'alg_pairwise_key_agreement', which specifies Pairwise Key
algorithm currently considered in Agreement Algorithm from the Common Context (see
[I-D.ietf-cose-rfc8152bis-algs], i.e. with algorithms that have Section 2.1.5). This parameter MUST be set to the CBOR simple
only the COSE key type as their COSE capability. Appendix H value Null if the group does not use the pairwise mode,
describes how 'par_countersign' can be generalized for possible regardless whether the endpoint supports the pairwise mode or
future registered algorithms having a different set of COSE not. Otherwise, this parameter MUST encode the value of
capabilities. Pairwise Key Agreement Algorithm as a CBOR integer or text
string, consistently with the "Value" field in the "COSE
Algorithms" Registry for this HKDF algorithm.
o The new element 'request_kid_context' contains the value of the * The new element 'request_kid_context' contains the value of the
'kid context' in the COSE object of the request (see Section 4.2). 'kid context' in the COSE object of the request (see Section 4.2).
In case Observe [RFC7641] is used, this enables endpoints to In case Observe [RFC7641] is used, this enables endpoints to
safely keep an observation active beyond a possible change of Gid, safely keep an observation active beyond a possible change of Gid
i.e. of ID Context, following a group rekeying (see Section 3.1). (i.e., of ID Context), following a group rekeying (see
In fact, it ensures that every notification cryptographically Section 3.2). In fact, it ensures that every notification
matches with only one observation request, rather than with cryptographically matches with only one observation request,
multiple ones that were protected with different keying material rather than with multiple ones that were protected with different
but share the same 'request_kid' and 'request_piv' values. keying material but share the same 'request_kid' and 'request_piv'
values.
o The new element 'OSCORE_option', containing the value of the * The new element 'OSCORE_option', containing the value of the
OSCORE Option present in the protected message, encoded as a OSCORE Option present in the protected message, encoded as a
binary string. This prevents the attack described in Section 10.6 binary string. This prevents the attack described in Section 10.7
when using the group mode, as further explained in Section 10.6.2. when using the group mode, as further explained in Section 10.7.2.
Note for implementation: this construction requires the OSCORE Note for implementation: this construction requires the OSCORE
option of the message to be generated and finalized before option of the message to be generated and finalized before
computing the ciphertext of the COSE_Encrypt0 object (when using computing the ciphertext of the COSE_Encrypt0 object (when using
the group mode or the pairwise mode) and before calculating the the group mode or the pairwise mode) and before calculating the
counter signature (when using the group mode). Also, the countersignature (when using the group mode). Also, the aad_array
aad_array needs to be large enough to contain the largest possible needs to be large enough to contain the largest possible OSCORE
OSCORE option. option.
* The new element 'sender_public_key', containing the sender's
public key. This parameter MUST be set to a CBOR byte string,
which encodes the sender's public key in its original binary
representation made available to other endpoints in the group (see
Section 2.3).
* The new element 'gm_public_key', containing the Group Manager's
public key. If no Group Manager maintains the group, this
parameter MUST encode the CBOR simple value Null. Otherwise, this
parameter MUST be set to a CBOR byte string, which encodes the
Group Manager's public key in its original binary representation
made available to other endpoints in the group (see Section 2.3).
This prevents the attack described in Section 10.8.
5. OSCORE Header Compression 5. OSCORE Header Compression
The OSCORE header compression defined in Section 6 of [RFC8613] is The OSCORE header compression defined in Section 6 of [RFC8613] is
used, with the following differences. used, with the following differences.
o The payload of the OSCORE message SHALL encode the ciphertext of * The payload of the OSCORE message SHALL encode the ciphertext of
the COSE_Encrypt0 object. In the group mode, the ciphertext above the COSE_Encrypt0 object. In the group mode, the ciphertext above
is concatenated with the value of the COSE_CounterSignature0 of is concatenated with the value of the COSE_CounterSignature0 of
the COSE object, computed as described in Section 4.1. the COSE object, computed as described in Section 4.1.
o This specification defines the usage of the sixth least * This document defines the usage of the sixth least significant
significant bit, called "Group Flag", in the first byte of the bit, called "Group Flag", in the first byte of the OSCORE option
OSCORE option containing the OSCORE flag bits. This flag bit is containing the OSCORE flag bits. This flag bit is specified in
specified in Section 11.1. Section 11.1.
o The Group Flag MUST be set to 1 if the OSCORE message is protected * The Group Flag MUST be set to 1 if the OSCORE message is protected
using the group mode (see Section 8). using the group mode (see Section 8).
o The Group Flag MUST be set to 0 if the OSCORE message is protected * The Group Flag MUST be set to 0 if the OSCORE message is protected
using the pairwise mode (see Section 9). The Group Flag MUST also using the pairwise mode (see Section 9). The Group Flag MUST also
be set to 0 for ordinary OSCORE messages processed according to be set to 0 for ordinary OSCORE messages processed according to
[RFC8613]. [RFC8613].
5.1. Examples of Compressed COSE Objects 5.1. Examples of Compressed COSE Objects
This section covers a list of OSCORE Header Compression examples of This section covers a list of OSCORE Header Compression examples of
Group OSCORE used in group mode (see Section 5.1.1) or in pairwise Group OSCORE used in group mode (see Section 5.1.1) or in pairwise
mode (see Section 5.1.2). mode (see Section 5.1.2).
The examples assume that the COSE_Encrypt0 object is set (which means The examples assume that the COSE_Encrypt0 object is set (which means
the CoAP message and cryptographic material is known). Note that the the CoAP message and cryptographic material is known). Note that the
examples do not include the full CoAP unprotected message or the full examples do not include the full CoAP unprotected message or the full
Security Context, but only the input necessary to the compression Security Context, but only the input necessary to the compression
mechanism, i.e. the COSE_Encrypt0 object. The output is the mechanism, i.e., the COSE_Encrypt0 object. The output is the
compressed COSE object as defined in Section 5 and divided into two compressed COSE object as defined in Section 5 and divided into two
parts, since the object is transported in two CoAP fields: OSCORE parts, since the object is transported in two CoAP fields: OSCORE
option and payload. option and payload.
The examples assume that the plaintext (see Section 5.3 of [RFC8613]) The examples assume that the plaintext (see Section 5.3 of [RFC8613])
is 6 bytes long, and that the AEAD tag is 8 bytes long, hence is 6 bytes long, and that the AEAD tag is 8 bytes long, hence
resulting in a ciphertext which is 14 bytes long. When using the resulting in a ciphertext which is 14 bytes long. When using the
group mode, the COSE_CounterSignature0 byte string as described in group mode, the COSE_CounterSignature0 byte string as described in
Section 4 is assumed to be 64 bytes long. Section 4 is assumed to be 64 bytes long.
5.1.1. Examples in Group Mode 5.1.1. Examples in Group Mode
o Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid = * Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
0x25, Partial IV = 5 and kid context = 0x44616c. 0x25, Partial IV = 5 and kid context = 0x44616c.
* Before compression (96 bytes): * Before compression (96 bytes):
[ [
h'', h'',
{ 4:h'25', 6:h'05', 10:h'44616c', 11:h'de9e ... f1' }, { 4:h'25', 6:h'05', 10:h'44616c', 11:h'de9e ... f1' },
h'aea0155667924dff8a24e4cb35b9' h'aea0155667924dff8a24e4cb35b9'
] ]
* After compression (85 bytes): * After compression (85 bytes):
Flag byte: 0b00111001 = 0x39 (1 byte) Flag byte: 0b00111001 = 0x39 (1 byte)
Option Value: 0x39 05 03 44 61 6c 25 (7 bytes) Option Value: 0x39 05 03 44 61 6c 25 (7 bytes)
Payload: 0xaea0155667924dff8a24e4cb35b9 de9e ... f1 Payload: 0xaea0155667924dff8a24e4cb35b9 de9e ... f1
(14 bytes + size of the counter signature) (14 bytes + size of the encrypted countersignature)
o Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid = * Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid =
0x52 and no Partial IV. 0x52 and no Partial IV.
* Before compression (88 bytes): * Before compression (88 bytes):
[ [
h'', h'',
{ 4:h'52', 11:h'ca1e ... b3' }, { 4:h'52', 11:h'ca1e ... b3' },
h'60b035059d9ef5667c5a0710823b' h'60b035059d9ef5667c5a0710823b'
] ]
* After compression (80 bytes): * After compression (80 bytes):
Flag byte: 0b00101000 = 0x28 (1 byte) Flag byte: 0b00101000 = 0x28 (1 byte)
Option Value: 0x28 52 (2 bytes) Option Value: 0x28 52 (2 bytes)
Payload: 0x60b035059d9ef5667c5a0710823b ca1e ... b3 Payload: 0x60b035059d9ef5667c5a0710823b ca1e ... b3
(14 bytes + size of the counter signature) (14 bytes + size of the encrypted countersignature)
5.1.2. Examples in Pairwise Mode 5.1.2. Examples in Pairwise Mode
o Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid = * Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
0x25, Partial IV = 5 and kid context = 0x44616c. 0x25, Partial IV = 5 and kid context = 0x44616c.
* Before compression (29 bytes): * Before compression (29 bytes):
[ [
h'', h'',
{ 4:h'25', 6:h'05', 10:h'44616c' }, { 4:h'25', 6:h'05', 10:h'44616c' },
h'aea0155667924dff8a24e4cb35b9' h'aea0155667924dff8a24e4cb35b9'
] ]
* After compression (21 bytes): * After compression (21 bytes):
Flag byte: 0b00011001 = 0x19 (1 byte) Flag byte: 0b00011001 = 0x19 (1 byte)
Option Value: 0x19 05 03 44 61 6c 25 (7 bytes) Option Value: 0x19 05 03 44 61 6c 25 (7 bytes)
Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes) Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)
o Response with ciphertext = 0x60b035059d9ef5667c5a0710823b and no * Response with ciphertext = 0x60b035059d9ef5667c5a0710823b and no
Partial IV. Partial IV.
* Before compression (18 bytes): * Before compression (18 bytes):
[ [
h'', h'',
{}, {},
h'60b035059d9ef5667c5a0710823b' h'60b035059d9ef5667c5a0710823b'
] ]
skipping to change at page 28, line 41 skipping to change at page 38, line 22
6. Message Binding, Sequence Numbers, Freshness and Replay Protection 6. Message Binding, Sequence Numbers, Freshness and Replay Protection
The requirements and properties described in Section 7 of [RFC8613] The requirements and properties described in Section 7 of [RFC8613]
also apply to Group OSCORE. In particular, Group OSCORE provides also apply to Group OSCORE. In particular, Group OSCORE provides
message binding of responses to requests, which enables absolute message binding of responses to requests, which enables absolute
freshness of responses that are not notifications, relative freshness freshness of responses that are not notifications, relative freshness
of requests and notification responses, and replay protection of of requests and notification responses, and replay protection of
requests. In addition, the following holds for Group OSCORE. requests. In addition, the following holds for Group OSCORE.
6.1. Update of Replay Window 6.1. Supporting Observe
When Observe [RFC7641] is used, a client maintains for each ongoing
observation one Notification Number for each different server. Then,
separately for each server, the client uses the associated
Notification Number to perform ordering and replay protection of
notifications received from that server (see Section 8.4.1).
Group OSCORE allows to preserve an observation active indefinitely,
even in case the group is rekeyed, with consequent change of ID
Context, or in case the observer client obtains a new Sender ID.
As defined in Section 8 when discussing support for Observe, this is
achieved by the client and server(s) storing the 'kid' and 'kid
context' used in the original Observe request, throughout the whole
duration of the observation.
Upon leaving the group or before re-joining the group, a group member
MUST terminate all the ongoing observations that it has started in
the group as observer client.
6.2. Update of Replay Window
Sender Sequence Numbers seen by a server as Partial IV values in Sender Sequence Numbers seen by a server as Partial IV values in
request messages can spontaneously increase at a fast pace, for request messages can spontaneously increase at a fast pace, for
example when a client exchanges unicast messages with other servers example when a client exchanges unicast messages with other servers
using the Group OSCORE Security Context. As in OSCORE [RFC8613], a using the Group OSCORE Security Context. As in OSCORE [RFC8613], a
server always needs to accept such increases and accordingly updates server always needs to accept such increases and accordingly updates
the Replay Window in each of its Recipient Contexts. the Replay Window in each of its Recipient Contexts.
As discussed in Section 2.4.1, a newly created Recipient Context As discussed in Section 2.5.1, a newly created Recipient Context
would have an invalid Replay Window, if its installation has required would have an invalid Replay Window, if its installation has required
to delete another Recipient Context. Hence, the server is not able to delete another Recipient Context. Hence, the server is not able
to verify if a request from the client associated to the new to verify if a request from the client associated to the new
Recipient Context is a replay. When this happens, the server MUST Recipient Context is a replay. When this happens, the server MUST
validate the Replay Window of the new Recipient Context, before validate the Replay Window of the new Recipient Context, before
accepting messages from the associated client (see Section 2.4.1). accepting messages from the associated client (see Section 2.5.1).
Furthermore, when the Group Manager establishes a new Security Furthermore, when the Group Manager establishes a new Security
Context for the group (see Section 2.4.3.2), every server re- Context for the group (see Section 2.5.3.2), every server re-
initializes the Replay Window in each of its Recipient Contexts. initializes the Replay Window in each of its Recipient Contexts.
6.2. Message Freshness 6.3. Message Freshness
When receiving a request from a client for the first time, the server When receiving a request from a client for the first time, the server
is not synchronized with the client's Sender Sequence Number, i.e. it is not synchronized with the client's Sender Sequence Number, i.e.,
is not able to verify if that request is fresh. This applies to a it is not able to verify if that request is fresh. This applies to a
server that has just joined the group, with respect to already server that has just joined the group, with respect to already
present clients, and recurs as new clients are added as group present clients, and recurs as new clients are added as group
members. members.
During its operations in the group, the server may also lose During its operations in the group, the server may also lose
synchronization with a client's Sender Sequence Number. This can synchronization with a client's Sender Sequence Number. This can
happen, for instance, if the server has rebooted or has deleted its happen, for instance, if the server has rebooted or has deleted its
previously synchronized version of the Recipient Context for that previously synchronized version of the Recipient Context for that
client (see Section 2.4.1). client (see Section 2.5.1).
If the application requires message freshness, e.g. according to If the application requires message freshness, e.g., according to
time- or event-based policies, the server has to (re-)synchronize time- or event-based policies, the server has to (re-)synchronize
with a client's Sender Sequence Number before delivering request with a client's Sender Sequence Number before delivering request
messages from that client to the application. To this end, the messages from that client to the application. To this end, the
server can use the approach in Appendix E based on the Echo Option server can use the approach in Appendix E based on the Echo Option
for CoAP [I-D.ietf-core-echo-request-tag], as a variant of the for CoAP [I-D.ietf-core-echo-request-tag], as a variant of the
approach defined in Appendix B.1.2 of [RFC8613] applicable to Group approach defined in Appendix B.1.2 of [RFC8613] applicable to Group
OSCORE. OSCORE.
7. Message Reception 7. Message Reception
Upon receiving a protected message, a recipient endpoint retrieves a Upon receiving a protected message, a recipient endpoint retrieves a
Security Context as in [RFC8613]. An endpoint MUST be able to Security Context as in [RFC8613]. An endpoint MUST be able to
distinguish between a Security Context to process OSCORE messages as distinguish between a Security Context to process OSCORE messages as
in [RFC8613] and a Group OSCORE Security Context to process Group in [RFC8613] and a Group OSCORE Security Context to process Group
OSCORE messages as defined in this specification. OSCORE messages as defined in this document.
To this end, an endpoint can take into account the different To this end, an endpoint can take into account the different
structure of the Security Context defined in Section 2, for example structure of the Security Context defined in Section 2, for example
based on the presence of Counter Signature Algorithm in the Common based on the presence of Signature Algorithm and/or Pairwise Key
Context. Alternatively implementations can use an additional Agreement Algorithm in the Common Context. Alternatively
parameter in the Security Context, to explicitly signal that it is implementations can use an additional parameter in the Security
intended for processing Group OSCORE messages. Context, to explicitly signal that it is intended for processing
Group OSCORE messages.
If either of the following two conditions holds, a recipient endpoint If either of the following conditions holds, a recipient endpoint
MUST discard the incoming protected message: MUST discard the incoming protected message:
o The Group Flag is set to 0, and the recipient endpoint retrieves a * The Group Flag is set to 0, and the recipient endpoint retrieves a
Security Context which is both valid to process the message and Security Context which is both valid to process the message and
also associated to an OSCORE group, but the endpoint does not also associated to an OSCORE group, but the endpoint does not
support the pairwise mode. support the pairwise mode.
o The Group Flag is set to 1, and the recipient endpoint can not * The Group Flag is set to 1, and the recipient endpoint retrieves a
Security Context which is both valid to process the message and
also associated to an OSCORE group, but the endpoint does not
support the group mode.
* The Group Flag is set to 1, and the recipient endpoint can not
retrieve a Security Context which is both valid to process the retrieve a Security Context which is both valid to process the
message and also associated to an OSCORE group. message and also associated to an OSCORE group.
As per Section 6.1 of [RFC8613], this holds also when retrieving a As per Section 6.1 of [RFC8613], this holds also when retrieving a
Security Context which is valid but not associated to an OSCORE Security Context which is valid but not associated to an OSCORE
group. Future specifications may define how to process incoming group. Future specifications may define how to process incoming
messages protected with a Security Contexts as in [RFC8613], when messages protected with a Security Contexts as in [RFC8613], when
the Group Flag bit is set to 1. the Group Flag bit is set to 1.
Otherwise, if a Security Context associated to an OSCORE group and Otherwise, if a Security Context associated to an OSCORE group and
skipping to change at page 30, line 39 skipping to change at page 40, line 47
Note that, if the Group Flag is set to 0, and the recipient endpoint Note that, if the Group Flag is set to 0, and the recipient endpoint
retrieves a Security Context which is valid to process the message retrieves a Security Context which is valid to process the message
but is not associated to an OSCORE group, then the message is but is not associated to an OSCORE group, then the message is
processed according to [RFC8613]. processed according to [RFC8613].
8. Message Processing in Group Mode 8. Message Processing in Group Mode
When using the group mode, messages are protected and processed as When using the group mode, messages are protected and processed as
specified in [RFC8613], with the modifications described in this specified in [RFC8613], with the modifications described in this
section. The security objectives of the group mode are discussed in section. The security objectives of the group mode are discussed in
Appendix A.2. The group mode MUST be supported. Appendix A.2.
The Group Manager indicates that the group uses (also) the group
mode, as part of the group data provided to candidate group members
when joining the group.
During all the steps of the message processing, an endpoint MUST use During all the steps of the message processing, an endpoint MUST use
the same Security Context for the considered group. That is, an the same Security Context for the considered group. That is, an
endpoint MUST NOT install a new Security Context for that group (see endpoint MUST NOT install a new Security Context for that group (see
Section 2.4.3.2) until the message processing is completed. Section 2.5.3.2) until the message processing is completed.
The group mode MUST be used to protect group requests intended for The group mode MUST be used to protect group requests intended for
multiple recipients or for the whole group. This includes both multiple recipients or for the whole group. This includes both
requests directly addressed to multiple recipients, e.g. sent by the requests directly addressed to multiple recipients, e.g., sent by the
client over multicast, as well as requests sent by the client over client over multicast, as well as requests sent by the client over
unicast to a proxy, that forwards them to the intended recipients unicast to a proxy, that forwards them to the intended recipients
over multicast [I-D.ietf-core-groupcomm-bis]. over multicast [I-D.ietf-core-groupcomm-bis]. For encryption and
decryption operations, the Signature Encryption Algorithm from the
Common Context is used.
As per [RFC7252][I-D.ietf-core-groupcomm-bis], group requests sent As per [RFC7252][I-D.ietf-core-groupcomm-bis], group requests sent
over multicast MUST be Non-Confirmable, and thus are not over multicast MUST be Non-Confirmable, and thus are not
retransmitted by the CoAP messaging layer. Instead, applications retransmitted by the CoAP messaging layer. Instead, applications
should store such outgoing messages for a predefined, sufficient should store such outgoing messages for a predefined, sufficient
amount of time, in order to correctly perform possible amount of time, in order to correctly perform possible
retransmissions at the application layer. According to Section 5.2.3 retransmissions at the application layer. According to Section 5.2.3
of [RFC7252], responses to Non-Confirmable group requests SHOULD also of [RFC7252], responses to Non-Confirmable group requests SHOULD also
be Non-Confirmable, but endpoints MUST be prepared to receive be Non-Confirmable, but endpoints MUST be prepared to receive
Confirmable responses in reply to a Non-Confirmable group request. Confirmable responses in reply to a Non-Confirmable group request.
Confirmable group requests are acknowledged in non-multicast Confirmable group requests are acknowledged in non-multicast
environments, as specified in [RFC7252]. environments, as specified in [RFC7252].
Furthermore, endpoints in the group locally perform error handling Furthermore, endpoints in the group locally perform error handling
and processing of invalid messages according to the same principles and processing of invalid messages according to the same principles
adopted in [RFC8613]. However, a recipient MUST stop processing and adopted in [RFC8613]. However, a recipient MUST stop processing and
silently reject any message which is malformed and does not follow silently reject any message which is malformed and does not follow
the format specified in Section 4 of this specification, or which is the format specified in Section 4 of this document, or which is not
not cryptographically validated in a successful way. In either case, cryptographically validated in a successful way. In either case, it
it is RECOMMENDED that the recipient does not send back any error is RECOMMENDED that the recipient does not send back any error
message. This prevents servers from replying with multiple error message. This prevents servers from replying with multiple error
messages to a client sending a group request, so avoiding the risk of messages to a client sending a group request, so avoiding the risk of
flooding and possibly congesting the network. flooding and possibly congesting the network.
8.1. Protecting the Request 8.1. Protecting the Request
A client transmits a secure group request as described in Section 8.1 A client transmits a secure group request as described in Section 8.1
of [RFC8613], with the following modifications. of [RFC8613], with the following modifications.
o In step 2, the Additional Authenticated Data is modified as * In step 2, the Additional Authenticated Data is modified as
described in Section 4 of this document. described in Section 4 of this document.
o In step 4, the encryption of the COSE object is modified as * In step 4, the encryption of the COSE object is modified as
described in Section 4 of this document. The encoding of the described in Section 4 of this document. The encoding of the
compressed COSE object is modified as described in Section 5 of compressed COSE object is modified as described in Section 5 of
this document. In particular, the Group Flag MUST be set to 1. this document. In particular, the Group Flag MUST be set to 1.
The Signature Encryption Algorithm from the Common Context MUST be
used.
o In step 5, the counter signature is computed and the format of the * In step 5, the countersignature is computed and the format of the
OSCORE message is modified as described in Section 4 and Section 5 OSCORE message is modified as described in Section 4 and Section 5
of this document. In particular, the payload of the OSCORE of this document. In particular the payload of the OSCORE message
message includes also the counter signature. includes also the encrypted countersignature (see Section 4.1).
8.1.1. Supporting Observe 8.1.1. Supporting Observe
If Observe [RFC7641] is supported, the following holds for each newly If Observe [RFC7641] is supported, the following holds for each newly
started observation. started observation.
o If the client intends to keep the observation active beyond a * If the client intends to keep the observation active beyond a
possible change of Sender ID, the client MUST store the value of possible change of Sender ID, the client MUST store the value of
the 'kid' parameter from the original Observe request, and retain the 'kid' parameter from the original Observe request, and retain
it for the whole duration of the observation. Even in case the it for the whole duration of the observation. Even in case the
client is individually rekeyed and receives a new Sender ID from client is individually rekeyed and receives a new Sender ID from
the Group Manager (see Section 2.4.3.1), the client MUST NOT the Group Manager (see Section 2.5.3.1), the client MUST NOT
update the stored value associated to a particular Observe update the stored value associated to a particular Observe
request. request.
o If the client intends to keep the observation active beyond a * If the client intends to keep the observation active beyond a
possible change of ID Context following a group rekeying (see possible change of ID Context following a group rekeying (see
Section 3.1), then the following applies. Section 3.2), then the following applies.
* The client MUST store the value of the 'kid context' parameter - The client MUST store the value of the 'kid context' parameter
from the original Observe request, and retain it for the whole from the original Observe request, and retain it for the whole
duration of the observation. Upon establishing a new Security duration of the observation. Upon establishing a new Security
Context with a new Gid as ID Context (see Section 2.4.3.2), the Context with a new Gid as ID Context (see Section 2.5.3.2), the
client MUST NOT update the stored value associated to a client MUST NOT update the stored value associated to a
particular Observe request. particular Observe request.
* The client MUST store an invariant identifier of the group, - The client MUST store an invariant identifier of the group,
which is immutable even in case the Security Context of the which is immutable even in case the Security Context of the
group is re-established. For example, this invariant group is re-established. For example, this invariant
identifier can be the "group name" in identifier can be the "group name" in
[I-D.ietf-ace-key-groupcomm-oscore], where it is used for [I-D.ietf-ace-key-groupcomm-oscore], where it is used for
joining the group and retrieving the current group keying joining the group and retrieving the current group keying
material from the Group Manager. material from the Group Manager.
After a group rekeying, such an invariant information makes it After a group rekeying, such an invariant information makes it
simpler for the observer client to retrieve the current group simpler for the observer client to retrieve the current group
keying material from the Group Manager, in case the client has keying material from the Group Manager, in case the client has
missed both the rekeying messages and the first observe missed both the rekeying messages and the first observe
notification protected with the new Security Context (see notification protected with the new Security Context (see
Section 8.3.1). Section 8.3.1).
8.2. Verifying the Request 8.2. Verifying the Request
Upon receiving a secure group request with the Group Flag set to 1, Upon receiving a secure group request with the Group Flag set to 1,
following the procedure in Section 7, a server proceeds as described following the procedure in Section 7, a server proceeds as described
in Section 8.2 of [RFC8613], with the following modifications. in Section 8.2 of [RFC8613], with the following modifications.
o In step 2, the decoding of the compressed COSE object follows * In step 2, the decoding of the compressed COSE object follows
Section 5 of this document. In particular: Section 5 of this document. In particular:
* If the server discards the request due to not retrieving a - If the server discards the request due to not retrieving a
Security Context associated to the OSCORE group, the server MAY Security Context associated to the OSCORE group, the server MAY
respond with a 4.01 (Unauthorized) error message. When doing respond with a 4.01 (Unauthorized) error message. When doing
so, the server MAY set an Outer Max-Age option with value zero, so, the server MAY set an Outer Max-Age option with value zero,
and MAY include a descriptive string as diagnostic payload. and MAY include a descriptive string as diagnostic payload.
* If the received 'kid context' matches an existing ID Context - If the received 'kid context' matches an existing ID Context
(Gid) but the received 'kid' does not match any Recipient ID in (Gid) but the received 'kid' does not match any Recipient ID in
this Security Context, then the server MAY create a new this Security Context, then the server MAY create a new
Recipient Context for this Recipient ID and initialize it Recipient Context for this Recipient ID and initialize it
according to Section 3 of [RFC8613], and also retrieve the according to Section 3 of [RFC8613], and also retrieve the
associated public key. Such a configuration is application associated public key. Such a configuration is application
specific. If the application does not specify dynamic specific. If the application does not specify dynamic
derivation of new Recipient Contexts, then the server SHALL derivation of new Recipient Contexts, then the server SHALL
stop processing the request. stop processing the request.
o In step 4, the Additional Authenticated Data is modified as * In step 4, the Additional Authenticated Data is modified as
described in Section 4 of this document. described in Section 4 of this document.
o In step 6, the server also verifies the counter signature using * In step 6, the server also verifies the countersignature using the
the public key of the client from the associated Recipient public key of the client from the associated Recipient Context.
Context. In particular: In particular:
* If the server does not have the public key of the client yet, - If the server does not have the public key of the client yet,
the server MUST stop processing the request and MAY respond the server MUST stop processing the request and MAY respond
with a 5.03 (Service Unavailable) response. The response MAY with a 5.03 (Service Unavailable) response. The response MAY
include a Max-Age Option, indicating to the client the number include a Max-Age Option, indicating to the client the number
of seconds after which to retry. If the Max-Age Option is not of seconds after which to retry. If the Max-Age Option is not
present, a retry time of 60 seconds will be assumed by the present, a retry time of 60 seconds will be assumed by the
client, as default value defined in Section 5.10.5 of client, as default value defined in Section 5.10.5 of
[RFC7252]. [RFC7252].
* If the signature verification fails, the server SHALL stop - The server retrieves the encrypted countersignature
processing the request and MAY respond with a 4.00 (Bad ENC_SIGNATURE from the message payload, and computes the
Request) response. The server MAY set an Outer Max-Age option original countersignature SIGNATURE as
with value zero. The diagnostic payload MAY contain a string,
which, if present, MUST be "Decryption failed" as if the
decryption had failed. Furthermore, the Replay Window MUST be
updated only if both the signature verification and the
decryption succeed.
o Additionally, if the used Recipient Context was created upon SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM
where KEYSTREAM is derived as per Section 4.1.1.
The following verification applies to the original
countersignature SIGNATURE.
- The server MUST perform signature verification before
decrypting the COSE object. Implementations that cannot
perform the two steps in this order MUST ensure that no access
to the plaintext is possible before a successful signature
verification and MUST prevent any possible leak of time-related
information that can yield side-channel attacks.
- If the signature verification fails, the server SHALL stop
processing the request, SHALL NOT update the Replay Window, and
MAY respond with a 4.00 (Bad Request) response. The server MAY
set an Outer Max-Age option with value zero. The diagnostic
payload MAY contain a string, which, if present, MUST be
"Decryption failed" as if the decryption had failed.
- When decrypting the COSE object using the Recipient Key, the
Signature Encryption Algorithm from the Common Context MUST be
used.
* Additionally, if the used Recipient Context was created upon
receiving this group request and the message is not verified receiving this group request and the message is not verified
successfully, the server MAY delete that Recipient Context. Such successfully, the server MAY delete that Recipient Context. Such
a configuration, which is specified by the application, mitigates a configuration, which is specified by the application, mitigates
attacks that aim at overloading the server's storage. attacks that aim at overloading the server's storage.
A server SHOULD NOT process a request if the received Recipient ID A server SHOULD NOT process a request if the received Recipient ID
('kid') is equal to its own Sender ID in its own Sender Context. For ('kid') is equal to its own Sender ID in its own Sender Context. For
an example where this is not fulfilled, see Section 7.2.1 in an example where this is not fulfilled, see Sections 7.2.1 and 7.2.4
[I-D.tiloca-core-observe-multicast-notifications]. of [I-D.ietf-core-observe-multicast-notifications].
8.2.1. Supporting Observe 8.2.1. Supporting Observe
If Observe [RFC7641] is supported, the following holds for each newly If Observe [RFC7641] is supported, the following holds for each newly
started observation. started observation.
o The server MUST store the value of the 'kid' parameter from the * The server MUST store the value of the 'kid' parameter from the
original Observe request, and retain it for the whole duration of original Observe request, and retain it for the whole duration of
the observation. The server MUST NOT update the stored value of a the observation. The server MUST NOT update the stored value of a
'kid' parameter associated to a particular Observe request, even 'kid' parameter associated to a particular Observe request, even
in case the observer client is individually rekeyed and starts in case the observer client is individually rekeyed and starts
using a new Sender ID received from the Group Manager (see using a new Sender ID received from the Group Manager (see
Section 2.4.3.1). Section 2.5.3.1).
o The server MUST store the value of the 'kid context' parameter * The server MUST store the value of the 'kid context' parameter
from the original Observe request, and retain it for the whole from the original Observe request, and retain it for the whole
duration of the observation, beyond a possible change of ID duration of the observation, beyond a possible change of ID
Context following a group rekeying (see Section 3.1). That is, Context following a group rekeying (see Section 3.2). That is,
upon establishing a new Security Context with a new Gid as ID upon establishing a new Security Context with a new Gid as ID
Context (see Section 2.4.3.2), the server MUST NOT update the Context (see Section 2.5.3.2), the server MUST NOT update the
stored value associated to the ongoing observation. stored value associated to the ongoing observation.
8.3. Protecting the Response 8.3. Protecting the Response
If a server generates a CoAP message in response to a Group OSCORE If a server generates a CoAP message in response to a Group OSCORE
request, then the server SHALL follow the description in Section 8.3 request, then the server SHALL follow the description in Section 8.3
of [RFC8613], with the modifications described in this section. of [RFC8613], with the modifications described in this section.
Note that the server always protects a response with the Sender Note that the server always protects a response with the Sender
Context from its latest Security Context, and that establishing a new Context from its latest Security Context, and that establishing a new
Security Context resets the Sender Sequence Number to 0 (see Security Context resets the Sender Sequence Number to 0 (see
Section 3.1). Section 3.2).
o In step 2, the Additional Authenticated Data is modified as * In step 2, the Additional Authenticated Data is modified as
described in Section 4 of this document. described in Section 4 of this document.
o In step 3, if the server is using a different Security Context for * In step 3, if the server is using a different Security Context for
the response compared to what was used to verify the request (see the response compared to what was used to verify the request (see
Section 3.1), then the server MUST include its Sender Sequence Section 3.2), then the server MUST include its Sender Sequence
Number as Partial IV in the response and use it to build the AEAD Number as Partial IV in the response and use it to build the AEAD
nonce to protect the response. This prevents the AEAD nonce from nonce to protect the response. This prevents the AEAD nonce from
the request from being reused. the request from being reused.
o In step 4, the encryption of the COSE object is modified as * In step 4, the encryption of the COSE object is modified as
described in Section 4 of this document. The encoding of the described in Section 4 of this document. The encoding of the
compressed COSE object is modified as described in Section 5 of compressed COSE object is modified as described in Section 5 of
this document. In particular, the Group Flag MUST be set to 1. this document. In particular, the Group Flag MUST be set to 1.
The Signature Encryption Algorithm from the Common Context MUST be
used.
If the server is using a different ID Context (Gid) for the If the server is using a different ID Context (Gid) for the
response compared to what was used to verify the request (see response compared to what was used to verify the request (see
Section 3.1), then the new ID Context MUST be included in the 'kid Section 3.2), then the new ID Context MUST be included in the 'kid
context' parameter of the response. context' parameter of the response.
o In step 5, the counter signature is computed and the format of the The server can obtain a new Sender ID from the Group Manager, when
OSCORE message is modified as described in Section 5 of this individually rekeyed (see Section 2.5.3.1) or when re-joining the
document. In particular, the payload of the OSCORE message group. In such a case, the server can help the client to
includes also the counter signature. synchronize, by including the 'kid' parameter in a response
protected in group mode, even when the request was protected in
pairwise mode (see Section 9.3).
That is, when responding to a request protected in pairwise mode,
the server SHOULD include the 'kid' parameter in a response
protected in group mode, if it is replying to that client for the
first time since the assignment of its new Sender ID.
* In step 5, the countersignature is computed and the format of the
OSCORE message is modified as described in Section 4 and Section 5
of this document. In particular the payload of the OSCORE message
includes also the encrypted countersignature (see Section 4.1).
8.3.1. Supporting Observe 8.3.1. Supporting Observe
If Observe [RFC7641] is supported, the following holds when If Observe [RFC7641] is supported, the following holds when
protecting notifications for an ongoing observation. protecting notifications for an ongoing observation.
o The server MUST use the stored value of the 'kid' parameter from * The server MUST use the stored value of the 'kid' parameter from
the original Observe request (see Section 8.2.1), as value for the the original Observe request (see Section 8.2.1), as value for the
'request_kid' parameter in the external_aad structure (see 'request_kid' parameter in the external_aad structure (see
Section 4.3). Section 4.3).
o The server MUST use the stored value of the 'kid context' * The server MUST use the stored value of the 'kid context'
parameter from the original Observe request (see Section 8.2.1), parameter from the original Observe request (see Section 8.2.1),
as value for the 'request_kid_context' parameter in the as value for the 'request_kid_context' parameter in the
external_aad structure (see Section 4.3). external_aad structure (see Section 4.3).
Furthermore, the server may have ongoing observations started by Furthermore, the server may have ongoing observations started by
Observe requests protected with an old Security Context. After Observe requests protected with an old Security Context. After
completing the establishment of a new Security Context, the server completing the establishment of a new Security Context, the server
MUST protect the following notifications with the Sender Context of MUST protect the following notifications with the Sender Context of
the new Security Context. the new Security Context.
skipping to change at page 36, line 7 skipping to change at page 47, line 7
8.4. Verifying the Response 8.4. Verifying the Response
Upon receiving a secure response message with the Group Flag set to Upon receiving a secure response message with the Group Flag set to
1, following the procedure in Section 7, the client proceeds as 1, following the procedure in Section 7, the client proceeds as
described in Section 8.4 of [RFC8613], with the following described in Section 8.4 of [RFC8613], with the following
modifications. modifications.
Note that a client may receive a response protected with a Security Note that a client may receive a response protected with a Security
Context different from the one used to protect the corresponding Context different from the one used to protect the corresponding
group request, and that, upon the establishment of a new Security request, and that, upon the establishment of a new Security Context,
Context, the client re-initializes its Replay Windows in its the client re-initializes its Replay Windows in its Recipient
Recipient Contexts (see Section 3.1). Contexts (see Section 3.2).
o In step 2, the decoding of the compressed COSE object is modified * In step 2, the decoding of the compressed COSE object is modified
as described in Section 5 of this document. In particular, a as described in Section 5 of this document. In particular, a
'kid' may not be present, if the response is a reply to a request 'kid' may not be present, if the response is a reply to a request
protected in pairwise mode. In such a case, the client assumes protected in pairwise mode. In such a case, the client assumes
the response 'kid' to be exactly the one of the server to which the response 'kid' to be the Recipient ID for the server to which
the request protected in pairwise mode was intended for. the request protected in pairwise mode was intended for.
If the response 'kid context' matches an existing ID Context (Gid) If the response 'kid context' matches an existing ID Context (Gid)
but the received/assumed 'kid' does not match any Recipient ID in but the received/assumed 'kid' does not match any Recipient ID in
this Security Context, then the client MAY create a new Recipient this Security Context, then the client MAY create a new Recipient
Context for this Recipient ID and initialize it according to Context for this Recipient ID and initialize it according to
Section 3 of [RFC8613], and also retrieve the associated public Section 3 of [RFC8613], and also retrieve the associated public
key. If the application does not specify dynamic derivation of key. If the application does not specify dynamic derivation of
new Recipient Contexts, then the client SHALL stop processing the new Recipient Contexts, then the client SHALL stop processing the
response. response.
o In step 3, the Additional Authenticated Data is modified as * In step 3, the Additional Authenticated Data is modified as
described in Section 4 of this document. described in Section 4 of this document.
o In step 5, the client also verifies the counter signature using * In step 5, the client also verifies the countersignature using the
the public key of the server from the associated Recipient public key of the server from the associated Recipient Context.
Context. If the verification fails, the same steps are taken as In particular:
if the decryption had failed.
o Additionally, if the used Recipient Context was created upon - The client MUST perform signature verification before
decrypting the COSE object. Implementations that cannot
perform the two steps in this order MUST ensure that no access
to the plaintext is possible before a successful signature
verification and MUST prevent any possible leak of time-related
information that can yield side-channel attacks.
- The client retrieves the encrypted countersignature
ENC_SIGNATURE from the message payload, and computes the
original countersignature SIGNATURE as
SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM
where KEYSTREAM is derived as per Section 4.1.1.
The following verification applies to the original
countersignature SIGNATURE.
- If the verification of the countersignature fails, the server
SHALL stop processing the response, and SHALL NOT update the
Notification Number associated to the server if the response is
an Observe notification [RFC7641].
- After a successful verification of the countersignature, the
client performs also the following actions if the response is
not an Observe notification.
o In case the request was protected in pairwise mode and the
'kid' parameter is present in the response, the client
checks whether this received 'kid' is equal to the expected
'kid', i.e., the known Recipient ID for the server to which
the request was intended for.
o If this is not the case, the client checks whether the
server that has sent the response is the same one to which
the request was intended for. This can be done by checking
that the public key used to verify the countersignature of
the response is equal to the Recipient Public Key taken as
input to derive the Pairwise Sender Key used for protecting
the request (see Section 2.4.1).
o If the client determines that the response has come from a
different server than the expected one, then the client
SHALL discard the response and SHALL NOT deliver it to the
application. Otherwise, the client hereafter considers the
received 'kid' as the current Recipient ID for the server.
- When decrypting the COSE object using the Recipient Key, the
Signature Encryption Algorithm from the Common Context MUST be
used.
* Additionally, if the used Recipient Context was created upon
receiving this response and the message is not verified receiving this response and the message is not verified
successfully, the client MAY delete that Recipient Context. Such successfully, the client MAY delete that Recipient Context. Such
a configuration, which is specified by the application, mitigates a configuration, which is specified by the application, mitigates
attacks that aim at overloading the client's storage. attacks that aim at overloading the client's storage.
8.4.1. Supporting Observe 8.4.1. Supporting Observe
If Observe [RFC7641] is supported, the following holds when verifying If Observe [RFC7641] is supported, the following holds when verifying
notifications for an ongoing observation. notifications for an ongoing observation.
o The client MUST use the stored value of the 'kid' parameter from * The client MUST use the stored value of the 'kid' parameter from
the original Observe request (see Section 8.1.1), as value for the the original Observe request (see Section 8.1.1), as value for the
'request_kid' parameter in the external_aad structure (see 'request_kid' parameter in the external_aad structure (see
Section 4.3). Section 4.3).
o The client MUST use the stored value of the 'kid context' * The client MUST use the stored value of the 'kid context'
parameter from the original Observe request (see Section 8.1.1), parameter from the original Observe request (see Section 8.1.1),
as value for the 'request_kid_context' parameter in the as value for the 'request_kid_context' parameter in the
external_aad structure (see Section 4.3). external_aad structure (see Section 4.3).
This ensures that the client can correctly verify notifications, even This ensures that the client can correctly verify notifications, even
in case it is individually rekeyed and starts using a new Sender ID in case it is individually rekeyed and starts using a new Sender ID
received from the Group Manager (see Section 2.4.3.1), as well as received from the Group Manager (see Section 2.5.3.1), as well as
when it installs a new Security Context with a new ID Context (Gid) when it installs a new Security Context with a new ID Context (Gid)
following a group rekeying (see Section 3.1). following a group rekeying (see Section 3.2).
* The ordering and the replay protection of notifications received
from a server are performed as per Sections 4.1.3.5.2 and 7.4.1 of
[RFC8613], by using the Notification Number associated to that
server for the observation in question. In addition, the client
performs the following actions for each ongoing observation.
- When receiving the first valid notification from a server, the
client MUST store the current kid "kid1" of that server for the
observation in question. If the 'kid' field is included in the
OSCORE option of the notification, its value specifies "kid1".
If the Observe request was protected in pairwise mode (see
Section 9.3), the 'kid' field may not be present in the OSCORE
option of the notification (see Section 4.2). In this case,
the client assumes "kid1" to be the Recipient ID for the server
to which the Observe request was intended for.
- When receiving another valid notification from the same server
- which can be identified and recognized through the same
public key used to verify the countersignature - the client
determines the current kid "kid2" of the server as above for
"kid1", and MUST check whether "kid2" is equal to the stored
"kid1". If "kid1" and "kid2" are different, the client MUST
cancel or re-register the observation in question.
Note that, if "kid2" is different from "kid1" and the 'kid'
field is omitted from the notification - which is possible if
the Observe request was protected in pairwise mode - then the
client will compute a wrong keystream to decrypt the
countersignature (i.e., by using "kid1" rather than "kid2" in
the 'id' field of the 'info' array in Section 4.1.1), thus
subsequently failing to verify the countersignature and
discarding the notification.
This ensures that the client remains able to correctly perform the
ordering and replay protection of notifications, even in case a
server legitimately starts using a new Sender ID, as received from
the Group Manager when individually rekeyed (see Section 2.5.3.1) or
when re-joining the group.
8.5. External Signature Checkers
When receiving a message protected in group mode, a signature checker
(see Section 3.1) proceeds as follows.
* The signature checker retrieves the encrypted countersignature
ENC_SIGNATURE from the message payload, and computes the original
countersignature SIGNATURE as
SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM
where KEYSTREAM is derived as per Section 4.1.1.
* The signature checker verifies the original countersignature
SIGNATURE, by using the public key of the sender endpoint. The
signature checker determines the public key to use based on the ID
Context (Gid) and the Sender ID of the sender endpoint.
Note that the following applies when attempting to verify the
countersignature of a response message.
* The response may not include a Partial IV and/or an ID Context.
In such a case, the signature checker considers the same values
from the corresponding request, i.e., the request matching with
the response by CoAP Token value.
* The response may not include a Sender ID. This can happen when
the response protected in group mode matches a request protected
in pairwise mode (see Section 9.1), with a case in point provided
by [I-D.amsuess-core-cachable-oscore]. In such a case, the
signature checker needs to use other means (e.g., source
addressing information of the server endpoint) to identify the
correct public key to use for verifying the countersignature of
the response.
The particular actions following a successful or unsuccessful
verification of the countersignature are application specific and out
of the scope of this document.
9. Message Processing in Pairwise Mode 9. Message Processing in Pairwise Mode
When using the pairwise mode of Group OSCORE, messages are protected When using the pairwise mode of Group OSCORE, messages are protected
and processed as in [RFC8613], with the modifications described in and processed as in [RFC8613], with the modifications described in
this section. The security objectives of the pairwise mode are this section. The security objectives of the pairwise mode are
discussed in Appendix A.2. discussed in Appendix A.2.
The pairwise mode takes advantage of an existing Security Context for The pairwise mode takes advantage of an existing Security Context for
the group mode to establish a Security Context shared exclusively the group mode to establish a Security Context shared exclusively
with any other member. In order to use the pairwise mode, the with any other member. In order to use the pairwise mode in a group
signature scheme of the group mode MUST support a combined signature that uses also the group mode, the signature scheme of the group mode
and encryption scheme. This can be, for example, signature using MUST support a combined signature and encryption scheme. This can
ECDSA, and encryption using AES-CCM with a key derived with ECDH. be, for example, signature using ECDSA, and encryption using AES-CCM
with a key derived with ECDH. For encryption and decryption
operations, the AEAD Algorithm from the Common Context is used (see
Section 2.1.1).
The pairwise mode does not support the use of additional entities The pairwise mode does not support the use of additional entities
acting as verifiers of source authentication and integrity of group acting as verifiers of source authentication and integrity of group
messages, such as intermediary gateways (see Section 3). messages, such as intermediary gateways (see Section 3).
The pairwise mode MAY be supported. An endpoint implementing only a An endpoint implementing only a silent server does not support the
silent server does not support the pairwise mode. pairwise mode.
If the signature algorithm used in the group supports ECDH (e.g., If the signature algorithm used in the group supports ECDH (e.g.,
ECDSA, EdDSA), the pairwise mode MUST be supported by endpoints that ECDSA, EdDSA), the pairwise mode MUST be supported by endpoints that
use the CoAP Echo Option [I-D.ietf-core-echo-request-tag] and/or use the CoAP Echo Option [I-D.ietf-core-echo-request-tag] and/or
block-wise transfers [RFC7959], for instance for responses after the block-wise transfers [RFC7959], for instance for responses after the
first block-wise request, which possibly targets all servers in the first block-wise request, which possibly targets all servers in the
group and includes the CoAP Block2 option (see Section 3.7 of group and includes the CoAP Block2 option (see Section 3.8 of
[I-D.ietf-core-groupcomm-bis]). This prevents the attack described [I-D.ietf-core-groupcomm-bis]). This prevents the attack described
in Section 10.7, which leverages requests sent over unicast to a in Section 10.9, which leverages requests sent over unicast to a
single group member and protected with the group mode. single group member and protected with the group mode.
Senders cannot use the pairwise mode to protect a message intended Senders cannot use the pairwise mode to protect a message intended
for multiple recipients. In fact, the pairwise mode is defined only for multiple recipients. In fact, the pairwise mode is defined only
between two endpoints and the keying material is thus only available between two endpoints and the keying material is thus only available
to one recipient. to one recipient.
However, a sender can use the pairwise mode to protect a message sent However, a sender can use the pairwise mode to protect a message sent
to (but not intended for) multiple recipients, if interested in a to (but not intended for) multiple recipients, if interested in a
response from only one of them. For instance, this is useful to response from only one of them. For instance, this is useful to
support the address discovery service defined in Section 9.1, when a support the address discovery service defined in Section 9.1, when a
single 'kid' value is indicated in the payload of a request sent to single 'kid' value is indicated in the payload of a request sent to
multiple recipients, e.g. over multicast. multiple recipients, e.g., over multicast.
The Group Manager MAY indicate that the group uses also the pairwise The Group Manager indicates that the group uses (also) the pairwise
mode, as part of the group data provided to candidate group members mode, as part of the group data provided to candidate group members
when joining the group. when joining the group.
9.1. Pre-Conditions 9.1. Pre-Conditions
In order to protect an outgoing message in pairwise mode, the sender In order to protect an outgoing message in pairwise mode, the sender
needs to know the public key and the Recipient ID for the recipient needs to know the public key and the Recipient ID for the recipient
endpoint, as stored in the Recipient Context associated to that endpoint, as stored in the Recipient Context associated to that
endpoint (see Section 2.3.3). endpoint (see Section 2.4.4).
Furthermore, the sender needs to know the individual address of the Furthermore, the sender needs to know the individual address of the
recipient endpoint. This information may not be known at any given recipient endpoint. This information may not be known at any given
point in time. For instance, right after having joined the group, a point in time. For instance, right after having joined the group, a
client may know the public key and Recipient ID for a given server, client may know the public key and Recipient ID for a given server,
but not the addressing information required to reach it with an but not the addressing information required to reach it with an
individual, one-to-one request. individual, one-to-one request.
To make addressing information of individual endpoints available, To make addressing information of individual endpoints available,
servers in the group MAY expose a resource to which a client can send servers in the group MAY expose a resource to which a client can send
skipping to change at page 38, line 39 skipping to change at page 52, line 36
values specified in the request payload. The specified set may be values specified in the request payload. The specified set may be
empty, hence identifying all the servers in the group. Further empty, hence identifying all the servers in the group. Further
details of such an interface are out of scope for this document. details of such an interface are out of scope for this document.
9.2. Main Differences from OSCORE 9.2. Main Differences from OSCORE
The pairwise mode protects messages between two members of a group, The pairwise mode protects messages between two members of a group,
essentially following [RFC8613], but with the following notable essentially following [RFC8613], but with the following notable
differences. differences.
o The 'kid' and 'kid context' parameters of the COSE object are used * The 'kid' and 'kid context' parameters of the COSE object are used
as defined in Section 4.2 of this document. as defined in Section 4.2 of this document.
o The external_aad defined in Section 4.3 of this document is used * The external_aad defined in Section 4.3 of this document is used
for the encryption process. for the encryption process.
o The Pairwise Sender/Recipient Keys used as Sender/Recipient keys * The Pairwise Sender/Recipient Keys used as Sender/Recipient keys
are derived as defined in Section 2.3 of this document. are derived as defined in Section 2.4 of this document.
9.3. Protecting the Request 9.3. Protecting the Request
When using the pairwise mode, the request is protected as defined in When using the pairwise mode, the request is protected as defined in
Section 8.1 of [RFC8613], with the differences summarized in Section 8.1 of [RFC8613], with the differences summarized in
Section 9.2 of this document. The following difference also applies. Section 9.2 of this document. The following difference also applies.
o If Observe [RFC7641] is supported, what defined in Section 8.1.1 * If Observe [RFC7641] is supported, what defined in Section 8.1.1
of this document holds. of this document holds.
9.4. Verifying the Request 9.4. Verifying the Request
Upon receiving a request with the Group Flag set to 0, following the Upon receiving a request with the Group Flag set to 0, following the
procedure in Section 7, the server MUST process it as defined in procedure in Section 7, the server MUST process it as defined in
Section 8.2 of [RFC8613], with the differences summarized in Section 8.2 of [RFC8613], with the differences summarized in
Section 9.2 of this document. The following differences also apply. Section 9.2 of this document. The following differences also apply.
o If the server discards the request due to not retrieving a * If the server discards the request due to not retrieving a
Security Context associated to the OSCORE group or to not Security Context associated to the OSCORE group or to not
supporting the pairwise mode, the server MAY respond with a 4.01 supporting the pairwise mode, the server MAY respond with a 4.01
(Unauthorized) error message or a 4.02 (Bad Option) error message, (Unauthorized) error message or a 4.02 (Bad Option) error message,
respectively. When doing so, the server MAY set an Outer Max-Age respectively. When doing so, the server MAY set an Outer Max-Age
option with value zero, and MAY include a descriptive string as option with value zero, and MAY include a descriptive string as
diagnostic payload. diagnostic payload.
o If a new Recipient Context is created for this Recipient ID, new * If a new Recipient Context is created for this Recipient ID, new
Pairwise Sender/Recipient Keys are also derived (see Pairwise Sender/Recipient Keys are also derived (see
Section 2.3.1). The new Pairwise Sender/Recipient Keys are Section 2.4.1). The new Pairwise Sender/Recipient Keys are
deleted if the Recipient Context is deleted as a result of the deleted if the Recipient Context is deleted as a result of the
message not being successfully verified. message not being successfully verified.
o If Observe [RFC7641] is supported, what defined in Section 8.2.1 * If Observe [RFC7641] is supported, what defined in Section 8.2.1
of this document holds. of this document holds.
9.5. Protecting the Response 9.5. Protecting the Response
When using the pairwise mode, a response is protected as defined in When using the pairwise mode, a response is protected as defined in
Section 8.3 of [RFC8613], with the differences summarized in Section 8.3 of [RFC8613], with the differences summarized in
Section 9.2 of this document. The following differences also apply. Section 9.2 of this document. The following differences also apply.
o As discussed in Section 2.4.3.1, the server can obtain a new * If the server is using a different Security Context for the
Sender ID from the Group Manager. In such a case, the server can response compared to what was used to verify the request (see
help the client to synchronize, by including the 'kid' parameter Section 3.2), then the server MUST include its Sender Sequence
in a response protected in pairwise mode, even when the request Number as Partial IV in the response and use it to build the AEAD
was also protected in pairwise mode. nonce to protect the response. This prevents the AEAD nonce from
the request from being reused.
* If the server is using a different ID Context (Gid) for the
response compared to what was used to verify the request (see
Section 3.2), then the new ID Context MUST be included in the 'kid
context' parameter of the response.
* The server can obtain a new Sender ID from the Group Manager, when
individually rekeyed (see Section 2.5.3.1) or when re-joining the
group. In such a case, the server can help the client to
synchronize, by including the 'kid' parameter in a response
protected in pairwise mode, even when the request was also
protected in pairwise mode.
That is, when responding to a request protected in pairwise mode, That is, when responding to a request protected in pairwise mode,
the server SHOULD include the 'kid' parameter in a response the server SHOULD include the 'kid' parameter in a response
protected in pairwise mode, if it is replying to that client for protected in pairwise mode, if it is replying to that client for
the first time since the assignment of its new Sender ID. the first time since the assignment of its new Sender ID.
o If Observe [RFC7641] is supported, what defined in Section 8.3.1 * If Observe [RFC7641] is supported, what defined in Section 8.3.1
of this document holds. of this document holds.
9.6. Verifying the Response 9.6. Verifying the Response
Upon receiving a response with the Group Flag set to 0, following the Upon receiving a response with the Group Flag set to 0, following the
procedure in Section 7, the client MUST process it as defined in procedure in Section 7, the client MUST process it as defined in
Section 8.4 of [RFC8613], with the differences summarized in Section 8.4 of [RFC8613], with the differences summarized in
Section 9.2 of this document. The following differences also apply. Section 9.2 of this document. The following differences also apply.
o If a new Recipient Context is created for this Recipient ID, new * The client may receive a response protected with a Security
Context different from the one used to protect the corresponding
request. Also, upon the establishment of a new Security Context,
the client re-initializes its Replay Windows in its Recipient
Contexts (see Section 3.2).
* The same as described in Section 8.4 holds with respect to
handling the 'kid' parameter of the response, when received as a
reply to a request protected in pairwise mode. The client can
also in this case check whether the replying server is the
expected one, by relying on the server's public key. However,
since the response is protected in pairwise mode, the public key
is not used for verifying a countersignature as in Section 8.4,
but rather as input to derive the Pairwise Recipient Key used to
decrypt and verify the response (see Section 2.4.1).
* If a new Recipient Context is created for this Recipient ID, new
Pairwise Sender/Recipient Keys are also derived (see Pairwise Sender/Recipient Keys are also derived (see
Section 2.3.1). The new Pairwise Sender/Recipient Keys are Section 2.4.1). The new Pairwise Sender/Recipient Keys are
deleted if the Recipient Context is deleted as a result of the deleted if the Recipient Context is deleted as a result of the
message not being successfully verified. message not being successfully verified.
o If Observe [RFC7641] is supported, what defined in Section 8.4.1 * If Observe [RFC7641] is supported, what defined in Section 8.4.1
of this document holds. of this document holds. The client can also in this case identify
a server to be the same one across a change of Sender ID, by
relying on the server's public key. However, since the
notification is protected in pairwise mode, the public key is not
used for verifying a countersignature as in Section 8.4, but
rather as input to derive the Pairwise Recipient Key used to
decrypt and verify the notification (see Section 2.4.1).
10. Security Considerations 10. Security Considerations
The same threat model discussed for OSCORE in Appendix D.1 of The same threat model discussed for OSCORE in Appendix D.1 of
[RFC8613] holds for Group OSCORE. In addition, when using the group [RFC8613] holds for Group OSCORE. In addition, when using the group
mode, source authentication of messages is explicitly ensured by mode, source authentication of messages is explicitly ensured by
means of counter signatures, as discussed in Section 10.1. means of countersignatures, as discussed in Section 10.1.
Note that, even if an endpoint is authorized to be a group member and
to take part in group communications, there is a risk that it behaves
inappropriately. For instance, it can forward the content of
messages in the group to unauthorized entities. However, in many use
cases, the devices in the group belong to a common authority and are
configured by a commissioner (see Appendix B), which results in a
practically limited risk and enables a prompt detection/reaction in
case of misbehaving.
The same considerations on supporting Proxy operations discussed for The same considerations on supporting Proxy operations discussed for
OSCORE in Appendix D.2 of [RFC8613] hold for Group OSCORE. OSCORE in Appendix D.2 of [RFC8613] hold for Group OSCORE.
The same considerations on protected message fields for OSCORE The same considerations on protected message fields for OSCORE
discussed in Appendix D.3 of [RFC8613] hold for Group OSCORE. discussed in Appendix D.3 of [RFC8613] hold for Group OSCORE.
The same considerations on uniqueness of (key, nonce) pairs for The same considerations on uniqueness of (key, nonce) pairs for
OSCORE discussed in Appendix D.4 of [RFC8613] hold for Group OSCORE. OSCORE discussed in Appendix D.4 of [RFC8613] hold for Group OSCORE.
This is further discussed in Section 10.2 of this document. This is further discussed in Section 10.3 of this document.
The same considerations on unprotected message fields for OSCORE The same considerations on unprotected message fields for OSCORE
discussed in Appendix D.5 of [RFC8613] hold for Group OSCORE, with discussed in Appendix D.5 of [RFC8613] hold for Group OSCORE, with
the following differences. First, the 'kid context' of request the following differences. First, the 'kid context' of request
messages is part of the Additional Authenticated Data, thus safely messages is part of the Additional Authenticated Data, thus safely
enabling to keep observations active beyond a possible change of ID enabling to keep observations active beyond a possible change of ID
Context (Gid), following a group rekeying (see Section 4.3). Second, Context (Gid), following a group rekeying (see Section 4.3). Second,
the counter signature included in a Group OSCORE message protected in the countersignature included in a Group OSCORE message protected in
group mode is computed also over the value of the OSCORE option, group mode is computed also over the value of the OSCORE option,
which is also part of the Additional Authenticated Data used in the which is also part of the Additional Authenticated Data used in the
signing process. This is further discussed in Section 10.6 of this signing process. This is further discussed in Section 10.7 of this
document. document.
As discussed in Section 6.2.3 of [I-D.ietf-core-groupcomm-bis], Group As discussed in Section 6.2.3 of [I-D.ietf-core-groupcomm-bis], Group
OSCORE addresses security attacks against CoAP listed in Sections OSCORE addresses security attacks against CoAP listed in Sections
11.2-11.6 of [RFC7252], especially when run over IP multicast. 11.2-11.6 of [RFC7252], especially when run over IP multicast.
The rest of this section first discusses security aspects to be taken The rest of this section first discusses security aspects to be taken
into account when using Group OSCORE. Then it goes through aspects into account when using Group OSCORE. Then it goes through aspects
covered in the security considerations of OSCORE (see Section 12 of covered in the security considerations of OSCORE (see Section 12 of
[RFC8613]), and discusses how they hold when Group OSCORE is used. [RFC8613]), and discusses how they hold when Group OSCORE is used.
10.1. Group-level Security 10.1. Security of the Group Mode
The group mode described in Section 8 relies on commonly shared group The group mode defined in Section 8 relies on commonly shared group
keying material to protect communication within a group. This has keying material to protect communication within a group. Using the
the following implications. group mode has the implications discussed below. The following
refers to group members as the endpoints in the group owning the
latest version of the group keying material.
o Messages are encrypted at a group level (group-level data * Messages are encrypted at a group level (group-level data
confidentiality), i.e. they can be decrypted by any member of the confidentiality), i.e., they can be decrypted by any member of the
group, but not by an external adversary or other external group, but not by an external adversary or other external
entities. entities.
o The AEAD algorithm provides only group authentication, i.e. it * If the used encryption algorithm provides integrity protection,
ensures that a message sent to a group has been sent by a member then it also ensures group authentication and proof of group
of that group, but not necessarily by the alleged sender. This is membership, but not source authentication. That is, it ensures
why source authentication of messages sent to a group is ensured that a message sent to a group has been sent by a member of that
through a counter signature, which is computed by the sender using group, but not necessarily by the alleged sender. In fact, any
group member is able to derive the Sender Key used by the actual
sender endpoint, and thus can compute a valid authentication tag.
Therefore, the message content could originate from any of the
current group members.
Furthermore, if the used encryption algorithm does not provide
integrity protection, then it does not ensure any level of message
authentication or proof of group membership.
On the other hand, proof of group membership is always ensured by
construction through the strict management of the group keying
material (see Section 3.2). That is, the group is rekeyed in case
of nodes' leaving, and the current group members are informed of
former group members. Thus, a current group member owning the
latest group keying material does not own the public key of any
former group member.
This allows a recipient endpoint to rely on the owned public keys,
in order to always confidently assert the group membership of a
sender endpoint when processing an incoming message, i.e., to
assert that the sender endpoint was a group member when it signed
the message. In turn, this prevents a former group member to
possibly re-sign and inject in the group a stored message that was
protected with old keying material.
* Source authentication of messages sent to a group is ensured
through a countersignature, which is computed by the sender using
its own private key and then appended to the message payload. its own private key and then appended to the message payload.
Also, the countersignature is encrypted by using a keystream
derived from the group keying material (see Section 4.1). This
ensures group privacy, i.e., an attacker cannot track an endpoint
over two groups by linking messages between the two groups, unless
being also a member of those groups.
Instead, the pairwise mode described in Section 9 protects messages The security properties of the group mode are summarized below.
by using pairwise symmetric keys, derived from the static-static
Diffie-Hellman shared secret computed from the asymmetric keys of the 1. Asymmetric source authentication, by means of a countersignature.
sender and recipient endpoint (see Section 2.3). Therefore, in the
pairwise mode, the AEAD algorithm provides both pairwise data- 2. Symmetric group authentication, by means of an authentication tag
(only for encryption algorithms providing integrity protection).
3. Symmetric group confidentiality, by means of symmetric
encryption.
4. Proof of group membership, by strictly managing the group keying
material, as well as by means of integrity tags when using an
encryption algorithm that provides also integrity protection.
5. Group privacy, by encrypting the countersignature.
The group mode fulfills the security properties above while also
displaying the following benefits. First, the use of encryption
algorithm that does not provide integrity protection results in a
minimal communication overhead, by limiting the message payload to
the ciphertext and the encrypted countersignature. Second, it is
possible to deploy semi-trusted principals such as signature checkers
(see Section 3.1), which can break property 5, but cannot break
properties 1, 2 and 3.
10.2. Security of the Pairwise Mode
The pairwise mode defined in Section 9 protects messages by using
pairwise symmetric keys, derived from the static-static Diffie-
Hellman shared secret computed from the asymmetric keys of the sender
and recipient endpoint (see Section 2.4).
The used encryption algorithm MUST provide integrity protection.
Therefore, the pairwise mode ensures both pairwise data-
confidentiality and source authentication of messages, without using confidentiality and source authentication of messages, without using
counter signatures. countersignatures. Furthermore, the recipient endpoint achieves
proof of group membership for the sender endpoint, since only current
group members have the required keying material to derive a valid
Pairwise Sender/Recipient Key.
The long-term storing of the Diffie-Hellman shared secret is a The long-term storing of the Diffie-Hellman shared secret is a
potential security issue. In fact, if the shared secret of two group potential security issue. In fact, if the shared secret of two group
members is leaked, a third group member can exploit it to impersonate members is leaked, a third group member can exploit it to impersonate
any of those two group members, by deriving and using their pairwise any of those two group members, by deriving and using their pairwise
key. The possibility of such leakage should be contemplated, as more key. The possibility of such leakage should be contemplated, as more
likely to happen than the leakage of a private key, which could be likely to happen than the leakage of a private key, which could be
rather protected at a significantly higher level than generic memory, rather protected at a significantly higher level than generic memory,
e.g. by using a Trusted Platform Module. Therefore, there is a e.g., by using a Trusted Platform Module. Therefore, there is a
trade-off between the maximum amount of time a same shared secret is trade-off between the maximum amount of time a same shared secret is
stored and the frequency of its re-computing. stored and the frequency of its re-computing.
Note that, even if an endpoint is authorized to be a group member and 10.3. Uniqueness of (key, nonce)
to take part in group communications, there is a risk that it behaves
inappropriately. For instance, it can forward the content of
messages in the group to unauthorized entities. However, in many use
cases, the devices in the group belong to a common authority and are
configured by a commissioner (see Appendix B), which results in a
practically limited risk and enables a prompt detection/reaction in
case of misbehaving.
10.2. Uniqueness of (key, nonce)
The proof for uniqueness of (key, nonce) pairs in Appendix D.4 of The proof for uniqueness of (key, nonce) pairs in Appendix D.4 of
[RFC8613] is also valid in group communication scenarios. That is, [RFC8613] is also valid in group communication scenarios. That is,
given an OSCORE group: given an OSCORE group:
o Uniqueness of Sender IDs within the group is enforced by the Group * Uniqueness of Sender IDs within the group is enforced by the Group
Manager, which never reassigns the same Sender ID within the same Manager. In fact, from the moment when a Gid is assigned to a
group under the same Gid value. group until the moment a new Gid is assigned to that same group,
the Group Manager does not reassign a Sender ID within the group
(see Section 3.2).
o The case A in Appendix D.4 of [RFC8613] concerns all group * The case A in Appendix D.4 of [RFC8613] concerns all group
requests and responses including a Partial IV (e.g. Observe requests and responses including a Partial IV (e.g., Observe
notifications). In this case, same considerations from [RFC8613] notifications). In this case, same considerations from [RFC8613]
apply here as well. apply here as well.
o The case B in Appendix D.4 of [RFC8613] concerns responses not * The case B in Appendix D.4 of [RFC8613] concerns responses not
including a Partial IV (e.g. single response to a group request). including a Partial IV (e.g., single response to a group request).
In this case, same considerations from [RFC8613] apply here as In this case, same considerations from [RFC8613] apply here as
well. well.
As a consequence, each message encrypted/decrypted with the same As a consequence, each message encrypted/decrypted with the same
Sender Key is processed by using a different (ID_PIV, PIV) pair. Sender Key is processed by using a different (ID_PIV, PIV) pair.
This means that nonces used by any fixed encrypting endpoint are This means that nonces used by any fixed encrypting endpoint are
unique. Thus, each message is processed with a different (key, unique. Thus, each message is processed with a different (key,
nonce) pair. nonce) pair.
10.3. Management of Group Keying Material 10.4. Management of Group Keying Material
The approach described in this specification should take into account The approach described in this document should take into account the
the risk of compromise of group members. In particular, this risk of compromise of group members. In particular, this document
document specifies that a key management scheme for secure revocation specifies that a key management scheme for secure revocation and
and renewal of Security Contexts and group keying material should be renewal of Security Contexts and group keying material MUST be
adopted. adopted.
[I-D.ietf-ace-key-groupcomm-oscore] provides a simple rekeying scheme [I-D.ietf-ace-key-groupcomm-oscore] provides a simple rekeying scheme
for renewing the Security Context in a group. for renewing the Security Context in a group.
Alternative rekeying schemes which are more scalable with the group Alternative rekeying schemes which are more scalable with the group
size may be needed in dynamic, large-scale groups where endpoints can size may be needed in dynamic, large-scale groups where endpoints can
join and leave at any time, in order to limit the impact on join and leave at any time, in order to limit the impact on
performance due to the Security Context and keying material update. performance due to the Security Context and keying material update.
10.4. Update of Security Context and Key Rotation 10.5. Update of Security Context and Key Rotation
A group member can receive a message shortly after the group has been A group member can receive a message shortly after the group has been
rekeyed, and new security parameters and keying material have been rekeyed, and new security parameters and keying material have been
distributed by the Group Manager. distributed by the Group Manager.
This may result in a client using an old Security Context to protect This may result in a client using an old Security Context to protect
a request, and a server using a different new Security Context to a request, and a server using a different new Security Context to
protect a corresponding response. As a consequence, clients may protect a corresponding response. As a consequence, clients may
receive a response protected with a Security Context different from receive a response protected with a Security Context different from
the one used to protect the corresponding request. the one used to protect the corresponding request.
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server MUST include its Sender Sequence Number as Partial IV in the server MUST include its Sender Sequence Number as Partial IV in the
response and use it to build the AEAD nonce to protect the response. response and use it to build the AEAD nonce to protect the response.
This prevents the AEAD nonce from the request from being reused with This prevents the AEAD nonce from the request from being reused with
the new Security Context. the new Security Context.
The client will process that response using the new Security Context, The client will process that response using the new Security Context,
provided that it has installed the new security parameters and keying provided that it has installed the new security parameters and keying
material before the message processing. material before the message processing.
In case block-wise transfer [RFC7959] is used, the same In case block-wise transfer [RFC7959] is used, the same
considerations from Section 7.2 of [I-D.ietf-ace-key-groupcomm] hold. considerations from Section 9.2 of [I-D.ietf-ace-key-groupcomm] hold.
Furthermore, as described below, a group rekeying may temporarily Furthermore, as described below, a group rekeying may temporarily
result in misaligned Security Contexts between the sender and result in misaligned Security Contexts between the sender and
recipient of a same message. recipient of a same message.
10.4.1. Late Update on the Sender 10.5.1. Late Update on the Sender
In this case, the sender protects a message using the old Security In this case, the sender protects a message using the old Security
Context, i.e. before having installed the new Security Context. Context, i.e., before having installed the new Security Context.
However, the recipient receives the message after having installed However, the recipient receives the message after having installed
the new Security Context, and is thus unable to correctly process it. the new Security Context, and is thus unable to correctly process it.
A possible way to ameliorate this issue is to preserve the old, A possible way to ameliorate this issue is to preserve the old,
recent, Security Context for a maximum amount of time defined by the recent, Security Context for a maximum amount of time defined by the
application. By doing so, the recipient can still try to process the application. By doing so, the recipient can still try to process the
received message using the old retained Security Context as a second received message using the old retained Security Context as a second
attempt. This makes particular sense when the recipient is a client, attempt. This makes particular sense when the recipient is a client,
that would hence be able to process incoming responses protected with that would hence be able to process incoming responses protected with
the old, recent, Security Context used to protect the associated the old, recent, Security Context used to protect the associated
skipping to change at page 44, line 18 skipping to change at page 60, line 20
processed with the new Security Context. processed with the new Security Context.
This tolerance preserves the processing of secure messages throughout This tolerance preserves the processing of secure messages throughout
a long-lasting key rotation, as group rekeying processes may likely a long-lasting key rotation, as group rekeying processes may likely
take a long time to complete, especially in large scale groups. On take a long time to complete, especially in large scale groups. On
the other hand, a former (compromised) group member can abusively the other hand, a former (compromised) group member can abusively
take advantage of this, and send messages protected with the old take advantage of this, and send messages protected with the old
retained Security Context. Therefore, a conservative application retained Security Context. Therefore, a conservative application
policy should not admit the retention of old Security Contexts. policy should not admit the retention of old Security Contexts.
10.4.2. Late Update on the Recipient 10.5.2. Late Update on the Recipient
In this case, the sender protects a message using the new Security In this case, the sender protects a message using the new Security
Context, but the recipient receives that message before having Context, but the recipient receives that message before having
installed the new Security Context. Therefore, the recipient would installed the new Security Context. Therefore, the recipient would
not be able to correctly process the message and hence discards it. not be able to correctly process the message and hence discards it.
If the recipient installs the new Security Context shortly after that If the recipient installs the new Security Context shortly after that
and the sender endpoint retransmits the message, the former will and the sender endpoint retransmits the message, the former will
still be able to receive and correctly process the message. still be able to receive and correctly process the message.
In any case, the recipient should actively ask the Group Manager for In any case, the recipient should actively ask the Group Manager for
an updated Security Context according to an application-defined an updated Security Context according to an application-defined
policy, for instance after a given number of unsuccessfully decrypted policy, for instance after a given number of unsuccessfully decrypted
incoming messages. incoming messages.
10.5. Collision of Group Identifiers 10.6. Collision of Group Identifiers
In case endpoints are deployed in multiple groups managed by In case endpoints are deployed in multiple groups managed by
different non-synchronized Group Managers, it is possible for Group different non-synchronized Group Managers, it is possible for Group
Identifiers of different groups to coincide. Identifiers of different groups to coincide.
This does not impair the security of the AEAD algorithm. In fact, as This does not impair the security of the AEAD algorithm. In fact, as
long as the Master Secret is different for different groups and this long as the Master Secret is different for different groups and this
condition holds over time, AEAD keys are different among different condition holds over time, AEAD keys are different among different
groups. groups.
The entity assigning an IP multicast address may help limiting the The entity assigning an IP multicast address may help limiting the
chances to experience such collisions of Group Identifiers. In chances to experience such collisions of Group Identifiers. In
particular, it may allow the Group Managers of groups using the same particular, it may allow the Group Managers of groups using the same
IP multicast address to share their respective list of assigned Group IP multicast address to share their respective list of assigned Group
Identifiers currently in use. Identifiers currently in use.
10.6. Cross-group Message Injection 10.7. Cross-group Message Injection
A same endpoint is allowed to and would likely use the same public/ A same endpoint is allowed to and would likely use the same public/
private key pair in multiple OSCORE groups, possibly administered by private key pair in multiple OSCORE groups, possibly administered by
different Group Managers. different Group Managers.
When a sender endpoint sends a message protected in pairwise mode to When a sender endpoint sends a message protected in pairwise mode to
a recipient endpoint in an OSCORE group, a malicious group member may a recipient endpoint in an OSCORE group, a malicious group member may
attempt to inject the message to a different OSCORE group also attempt to inject the message to a different OSCORE group also
including the same endpoints (see Section 10.6.1). including the same endpoints (see Section 10.7.1).
This practically relies on altering the content of the OSCORE option, This practically relies on altering the content of the OSCORE option,
and having the same MAC in the ciphertext still correctly validating, and having the same MAC in the ciphertext still correctly validating,
which has a success probability depending on the size of the MAC. which has a success probability depending on the size of the MAC.
As discussed in Section 10.6.2, the attack is practically infeasible As discussed in Section 10.7.2, the attack is practically infeasible
if the message is protected in group mode, thanks to the counter if the message is protected in group mode, thanks to the
signature also bound to the OSCORE option through the Additional countersignature also bound to the OSCORE option through the
Authenticated Data used in the signing process (see Section 4.3). Additional Authenticated Data used in the signing process (see
Section 4.3).
10.6.1. Attack Description 10.7.1. Attack Description
Let us consider: Let us consider:
o Two OSCORE groups G1 and G2, with ID Context (Group ID) Gid1 and * Two OSCORE groups G1 and G2, with ID Context (Group ID) Gid1 and
Gid2, respectively. Both G1 and G2 use the AEAD cipher AES-CCM- Gid2, respectively. Both G1 and G2 use the AEAD cipher AES-CCM-
16-64-128, i.e. the MAC of the ciphertext is 8 bytes in size. 16-64-128, i.e., the MAC of the ciphertext is 8 bytes in size.
o A sender endpoint X which is member of both G1 and G2, and uses * A sender endpoint X which is member of both G1 and G2, and uses
the same public/private key pair in both groups. The endpoint X the same public/private key pair in both groups. The endpoint X
has Sender ID Sid1 in G1 and Sender ID Sid2 in G2. The pairs has Sender ID Sid1 in G1 and Sender ID Sid2 in G2. The pairs
(Sid1, Gid1) and (Sid2, Gid2) identify the same public key of X in (Sid1, Gid1) and (Sid2, Gid2) identify the same public key of X in
G1 and G2, respectively. G1 and G2, respectively.
o A recipient endpoint Y which is member of both G1 and G2, and uses * A recipient endpoint Y which is member of both G1 and G2, and uses
the same public/private key pair in both groups. The endpoint Y the same public/private key pair in both groups. The endpoint Y
has Sender ID Sid3 in G1 and Sender ID Sid4 in G2. The pairs has Sender ID Sid3 in G1 and Sender ID Sid4 in G2. The pairs
(Sid3, Gid1) and (Sid4, Gid2) identify the same public key of Y in (Sid3, Gid1) and (Sid4, Gid2) identify the same public key of Y in
G1 and G2, respectively. G1 and G2, respectively.
o A malicious endpoint Z is also member of both G1 and G2. Hence, Z * A malicious endpoint Z is also member of both G1 and G2. Hence, Z
is able to derive the Sender Keys used by X in G1 and G2. is able to derive the Sender Keys used by X in G1 and G2.
When X sends a message M1 addressed to Y in G1 and protected in When X sends a message M1 addressed to Y in G1 and protected in
pairwise mode, Z can intercept M1, and attempt to forge a valid pairwise mode, Z can intercept M1, and attempt to forge a valid
message M2 to be injected in G2, making it appear as still sent by X message M2 to be injected in G2, making it appear as still sent by X
to Y and valid to be accepted. to Y and valid to be accepted.
More in detail, Z intercepts and stops message M1, and forges a More in detail, Z intercepts and stops message M1, and forges a
message M2 by changing the value of the OSCORE option from M1 as message M2 by changing the value of the OSCORE option from M1 as
follows: the 'kid context' is set to G2 (rather than G1); and the follows: the 'kid context' is set to G2 (rather than G1); and the
skipping to change at page 46, line 20 skipping to change at page 62, line 20
Upon receiving M2, there is a probability equal to 2^-64 that Y Upon receiving M2, there is a probability equal to 2^-64 that Y
successfully verifies the same unchanged MAC by using the Pairwise successfully verifies the same unchanged MAC by using the Pairwise
Recipient Key associated to X in G2. Recipient Key associated to X in G2.
Note that Z does not know the pairwise keys of X and Y, since it does Note that Z does not know the pairwise keys of X and Y, since it does
not know and is not able to compute their shared Diffie-Hellman not know and is not able to compute their shared Diffie-Hellman
secret. Therefore, Z is not able to check offline if a performed secret. Therefore, Z is not able to check offline if a performed
forgery is actually valid, before sending the forged message to G2. forgery is actually valid, before sending the forged message to G2.
10.6.2. Attack Prevention in Group Mode 10.7.2. Attack Prevention in Group Mode
When a Group OSCORE message is protected with the group mode, the When a Group OSCORE message is protected with the group mode, the
counter signature is computed also over the value of the OSCORE countersignature is computed also over the value of the OSCORE
option, which is part of the Additional Authenticated Data used in option, which is part of the Additional Authenticated Data used in
the signing process (see Section 4.3). the signing process (see Section 4.3).
That is, other than over the ciphertext, the countersignature is That is, other than over the ciphertext, the countersignature is
computed over: the ID Context (Gid) and the Partial IV, which are computed over: the ID Context (Gid) and the Partial IV, which are
always present in group requests; as well as the Sender ID of the always present in group requests; as well as the Sender ID of the
message originator, which is always present in group requests as well message originator, which is always present in group requests as well
as in responses to requests protected in group mode. as in responses to requests protected in group mode.
Since the signing process takes as input also the ciphertext of the Since the signing process takes as input also the ciphertext of the
COSE_Encrypt0 object, the countersignature is bound not only to the COSE_Encrypt0 object, the countersignature is bound not only to the
intended OSCORE group, hence to the triplet (Master Secret, Master intended OSCORE group, hence to the triplet (Master Secret, Master
Salt, ID Context), but also to a specific Sender ID in that group and Salt, ID Context), but also to a specific Sender ID in that group and
to its specific symmetric key used for AEAD encryption, hence to the to its specific symmetric key used for AEAD encryption, hence to the
quartet (Master Secret, Master Salt, ID Context, Sender ID). quartet (Master Secret, Master Salt, ID Context, Sender ID).
This makes it practically infeasible to perform the attack described This makes it practically infeasible to perform the attack described
in Section 10.6.1, since it would require the adversary to in Section 10.7.1, since it would require the adversary to
additionally forge a valid countersignature that replaces the additionally forge a valid countersignature that replaces the
original one in the forged message M2. original one in the forged message M2.
If the countersignature did not cover the OSCORE option, the attack If the countersignature did not cover the OSCORE option, the attack
would still be possible against response messages protected in group would still be possible against response messages protected in group
mode, since the same unchanged countersignature from message M1 would mode, since the same unchanged countersignature from message M1 would
be also valid in message M2. be also valid in message M2.
Also, the following attack simplifications would hold, since Z is Also, the following attack simplifications would hold, since Z is
able to derive the Sender/Recipient Keys of X and Y in G1 and G2. able to derive the Sender/Recipient Keys of X and Y in G1 and G2.
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operations to test all the Partial IVs, which can be done in real- operations to test all the Partial IVs, which can be done in real-
time. The probability that a single given message M1 can be used to time. The probability that a single given message M1 can be used to
forge a response M2 for a given request would be equal to 2^-24, forge a response M2 for a given request would be equal to 2^-24,
since there are more MAC values (8 bytes in size) than Partial IV since there are more MAC values (8 bytes in size) than Partial IV
values (5 bytes in size). values (5 bytes in size).
Note that, by changing the Partial IV as discussed above, any member Note that, by changing the Partial IV as discussed above, any member
of G1 would also be able to forge a valid signed response message M2 of G1 would also be able to forge a valid signed response message M2
to be injected in the same group G1. to be injected in the same group G1.
10.7. Group OSCORE for Unicast Requests 10.8. Prevention of Group Cloning Attack
Both when using the group mode and the pairwise mode, the message
protection covers also the Group Manager's public key. This public
key is included in the Additional Authenticated Data used in the
signing process and/or in the integrity-protected encryption process
(see Section 4.3).
By doing so, an endpoint X member of a group G1 cannot perform the
following attack.
1. X sets up a group G2 where it acts as Group Manager.
2. X makes G2 a "clone" of G1, i.e., G1 and G2 use the same
algorithms and have the same Master Secret, Master Salt and ID
Context.
3. X collects a message M sent to G1 and injects it in G2.
4. Members of G2 accept M and believe it to be originated in G2.
The attack above is effectively prevented, since message M is
protected by including the public key of G1's Group Manager in the
Additional Authenticated Data. Therefore, members of G2 do not
successfully verify and decrypt M, since they correctly use the
public key of X as Group Manager of G2 when attempting to.
10.9. Group OSCORE for Unicast Requests
If a request is intended to be sent over unicast as addressed to a If a request is intended to be sent over unicast as addressed to a
single group member, it is NOT RECOMMENDED for the client to protect single group member, it is NOT RECOMMENDED for the client to protect
the request by using the group mode as defined in Section 8.1. the request by using the group mode as defined in Section 8.1.
This does not include the case where the client sends a request over This does not include the case where the client sends a request over
unicast to a proxy, to be forwarded to multiple intended recipients unicast to a proxy, to be forwarded to multiple intended recipients
over multicast [I-D.ietf-core-groupcomm-bis]. In this case, the over multicast [I-D.ietf-core-groupcomm-bis]. In this case, the
client MUST protect the request with the group mode, even though it client MUST protect the request with the group mode, even though it
is sent to the proxy over unicast (see Section 8). is sent to the proxy over unicast (see Section 8).
If the client uses the group mode with its own Sender Key to protect If the client uses the group mode with its own Sender Key to protect
a unicast request to a group member, an on-path adversary can, right a unicast request to a group member, an on-path adversary can, right
then or later on, redirect that request to one/many different group then or later on, redirect that request to one/many different group
member(s) over unicast, or to the whole OSCORE group over multicast. member(s) over unicast, or to the whole OSCORE group over multicast.
By doing so, the adversary can induce the target group member(s) to By doing so, the adversary can induce the target group member(s) to
perform actions intended for one group member only. Note that the perform actions intended for one group member only. Note that the
adversary can be external, i.e. (s)he does not need to also be a adversary can be external, i.e., (s)he does not need to also be a
member of the OSCORE group. member of the OSCORE group.
This is due to the fact that the client is not able to indicate the This is due to the fact that the client is not able to indicate the
single intended recipient in a way which is secure and possible to single intended recipient in a way which is secure and possible to
process for Group OSCORE on the server side. In particular, Group process for Group OSCORE on the server side. In particular, Group
OSCORE does not protect network addressing information such as the IP OSCORE does not protect network addressing information such as the IP
address of the intended recipient server. It follows that the address of the intended recipient server. It follows that the
server(s) receiving the redirected request cannot assert whether that server(s) receiving the redirected request cannot assert whether that
was the original intention of the client, and would thus simply was the original intention of the client, and would thus simply
assume so. assume so.
The impact of such an attack depends especially on the REST method of The impact of such an attack depends especially on the REST method of
the request, i.e. the Inner CoAP Code of the OSCORE request message. the request, i.e., the Inner CoAP Code of the OSCORE request message.
In particular, safe methods such as GET and FETCH would trigger In particular, safe methods such as GET and FETCH would trigger
(several) unintended responses from the targeted server(s), while not (several) unintended responses from the targeted server(s), while not
resulting in destructive behavior. On the other hand, non safe resulting in destructive behavior. On the other hand, non safe
methods such as PUT, POST and PATCH/iPATCH would result in the target methods such as PUT, POST and PATCH/iPATCH would result in the target
server(s) taking active actions on their resources and possible server(s) taking active actions on their resources and possible
cyber-physical environment, with the risk of destructive consequences cyber-physical environment, with the risk of destructive consequences
and possible implications for safety. and possible implications for safety.
A client can instead use the pairwise mode as defined in Section 9.3, A client can instead use the pairwise mode as defined in Section 9.3,
in order to protect a request sent to a single group member by using in order to protect a request sent to a single group member by using
pairwise keying material (see Section 2.3). This prevents the attack pairwise keying material (see Section 2.4). This prevents the attack
discussed above by construction, as only the intended server is able discussed above by construction, as only the intended server is able
to derive the pairwise keying material used by the client to protect to derive the pairwise keying material used by the client to protect
the request. A client supporting the pairwise mode SHOULD use it to the request. A client supporting the pairwise mode SHOULD use it to
protect requests sent to a single group member over unicast, instead protect requests sent to a single group member over unicast, instead
of using the group mode. For an example where this is not fulfilled, of using the group mode. For an example where this is not fulfilled,
see Section 7.2.1 in see Sections 7.2.1 and 7.2.4 of
[I-D.tiloca-core-observe-multicast-notifications]. [I-D.ietf-core-observe-multicast-notifications].
With particular reference to block-wise transfers [RFC7959], With particular reference to block-wise transfers [RFC7959],
Section 3.7 of [I-D.ietf-core-groupcomm-bis] points out that, while Section 3.8 of [I-D.ietf-core-groupcomm-bis] points out that, while
an initial request including the CoAP Block2 option can be sent over an initial request including the CoAP Block2 option can be sent over
multicast, any other request in a transfer has to occur over unicast, multicast, any other request in a transfer has to occur over unicast,
individually addressing the servers in the group. individually addressing the servers in the group.
Additional considerations are discussed in Appendix E, with respect Additional considerations are discussed in Appendix E, with respect
to requests including a CoAP Echo Option to requests including a CoAP Echo Option
[I-D.ietf-core-echo-request-tag] that has to be sent over unicast, as [I-D.ietf-core-echo-request-tag] that has to be sent over unicast, as
a challenge-response method for servers to achieve synchronization of a challenge-response method for servers to achieve synchronization of
clients' Sender Sequence Number. clients' Sender Sequence Number.
10.8. End-to-end Protection 10.10. End-to-end Protection
The same considerations from Section 12.1 of [RFC8613] hold for Group The same considerations from Section 12.1 of [RFC8613] hold for Group
OSCORE. OSCORE.
Additionally, (D)TLS and Group OSCORE can be combined for protecting Additionally, (D)TLS and Group OSCORE can be combined for protecting
message exchanges occurring over unicast. However, it is not message exchanges occurring over unicast. However, it is not
possible to combine (D)TLS and Group OSCORE for protecting message possible to combine (D)TLS and Group OSCORE for protecting message
exchanges where messages are (also) sent over multicast. exchanges where messages are (also) sent over multicast.
10.9. Master Secret 10.11. Master Secret
Group OSCORE derives the Security Context using the same construction Group OSCORE derives the Security Context using the same construction
as OSCORE, and by using the Group Identifier of a group as the as OSCORE, and by using the Group Identifier of a group as the
related ID Context. Hence, the same required properties of the related ID Context. Hence, the same required properties of the
Security Context parameters discussed in Section 3.3 of [RFC8613] Security Context parameters discussed in Section 3.3 of [RFC8613]
hold for this document. hold for this document.
With particular reference to the OSCORE Master Secret, it has to be With particular reference to the OSCORE Master Secret, it has to be
kept secret among the members of the respective OSCORE group and the kept secret among the members of the respective OSCORE group and the
Group Manager responsible for that group. Also, the Master Secret Group Manager responsible for that group. Also, the Master Secret
must have a good amount of randomness, and the Group Manager can must have a good amount of randomness, and the Group Manager can
generate it offline using a good random number generator. This generate it offline using a good random number generator. This
includes the case where the Group Manager rekeys the group by includes the case where the Group Manager rekeys the group by
generating and distributing a new Master Secret. Randomness generating and distributing a new Master Secret. Randomness
requirements for security are described in [RFC4086]. requirements for security are described in [RFC4086].
10.10. Replay Protection 10.12. Replay Protection
As in OSCORE [RFC8613], also Group OSCORE relies on Sender Sequence As in OSCORE [RFC8613], also Group OSCORE relies on Sender Sequence
Numbers included in the COSE message field 'Partial IV' and used to Numbers included in the COSE message field 'Partial IV' and used to
build AEAD nonces. build AEAD nonces.
Note that the Partial IV of an endpoint does not necessarily grow Note that the Partial IV of an endpoint does not necessarily grow
monotonically. For instance, upon exhaustion of the endpoint Sender monotonically. For instance, upon exhaustion of the endpoint Sender
Sequence Number, the Partial IV also gets exhausted. As discussed in Sequence Number, the Partial IV also gets exhausted. As discussed in
Section 2.4.3, this results either in the endpoint being individually Section 2.5.3, this results either in the endpoint being individually
rekeyed and getting a new Sender ID, or in the establishment of a new rekeyed and getting a new Sender ID, or in the establishment of a new
Security Context in the group. Therefore, uniqueness of (key, nonce) Security Context in the group. Therefore, uniqueness of (key, nonce)
pairs (see Section 10.2) is preserved also when a new Security pairs (see Section 10.3) is preserved also when a new Security
Context is established. Context is established.
Since one-to-many communication such as multicast usually involves Since one-to-many communication such as multicast usually involves
unreliable transports, the simplification of the Replay Window to a unreliable transports, the simplification of the Replay Window to a
size of 1 suggested in Section 7.4 of [RFC8613] is not viable with size of 1 suggested in Section 7.4 of [RFC8613] is not viable with
Group OSCORE, unless exchanges in the group rely only on unicast Group OSCORE, unless exchanges in the group rely only on unicast
messages. messages.
As discussed in Section 6.1, a Replay Window may be initialized as As discussed in Section 6.2, a Replay Window may be initialized as
not valid, following the loss of mutable Security Context not valid, following the loss of mutable Security Context
Section 2.4.1. In particular, Section 2.4.1.1 and Section 2.4.1.2 Section 2.5.1. In particular, Section 2.5.1.1 and Section 2.5.1.2
define measures that endpoints need to take in such a situation, define measures that endpoints need to take in such a situation,
before resuming to accept incoming messages from other group members. before resuming to accept incoming messages from other group members.
10.11. Message Freshness 10.13. Message Freshness
As discussed in Section 6.2, a server may not be able to assert As discussed in Section 6.3, a server may not be able to assert
whether an incoming request is fresh, in case it does not have or has whether an incoming request is fresh, in case it does not have or has
lost synchronization with the client's Sender Sequence Number. lost synchronization with the client's Sender Sequence Number.
If freshness is relevant for the application, the server may If freshness is relevant for the application, the server may
(re-)synchronize with the client's Sender Sequence Number at any (re-)synchronize with the client's Sender Sequence Number at any
time, by using the approach described in Appendix E and based on the time, by using the approach described in Appendix E and based on the
CoAP Echo Option [I-D.ietf-core-echo-request-tag], as a variant of CoAP Echo Option [I-D.ietf-core-echo-request-tag], as a variant of
the approach defined in Appendix B.1.2 of [RFC8613] applicable to the approach defined in Appendix B.1.2 of [RFC8613] applicable to
Group OSCORE. Group OSCORE.
10.12. Client Aliveness 10.14. Client Aliveness
Building on Section 12.5 of [RFC8613], a server may use the CoAP Echo Building on Section 12.5 of [RFC8613], a server may use the CoAP Echo
Option [I-D.ietf-core-echo-request-tag] to verify the aliveness of Option [I-D.ietf-core-echo-request-tag] to verify the aliveness of
the client that originated a received request, by using the approach the client that originated a received request, by using the approach
described in Appendix E of this specification. described in Appendix E of this document.
10.13. Cryptographic Considerations 10.15. Cryptographic Considerations
The same considerations from Section 12.6 of [RFC8613] about the The same considerations from Section 12.6 of [RFC8613] about the
maximum Sender Sequence Number hold for Group OSCORE. maximum Sender Sequence Number hold for Group OSCORE.
As discussed in Section 2.4.2, an endpoint that experiences an As discussed in Section 2.5.2, an endpoint that experiences an
exhaustion of its own Sender Sequence Numbers MUST NOT protect exhaustion of its own Sender Sequence Numbers MUST NOT protect
further messages including a Partial IV, until it has derived a new further messages including a Partial IV, until it has derived a new
Sender Context. This prevents the endpoint to reuse the same AEAD Sender Context. This prevents the endpoint to reuse the same AEAD
nonces with the same Sender Key. nonces with the same Sender Key.
In order to renew its own Sender Context, the endpoint SHOULD inform In order to renew its own Sender Context, the endpoint SHOULD inform
the Group Manager, which can either renew the whole Security Context the Group Manager, which can either renew the whole Security Context
by means of group rekeying, or provide only that endpoint with a new by means of group rekeying, or provide only that endpoint with a new
Sender ID value. In either case, the endpoint derives a new Sender Sender ID value. In either case, the endpoint derives a new Sender
Context, and in particular a new Sender Key. Context, and in particular a new Sender Key.
Additionally, the same considerations from Section 12.6 of [RFC8613] Additionally, the same considerations from Section 12.6 of [RFC8613]
hold for Group OSCORE, about building the AEAD nonce and the secrecy hold for Group OSCORE, about building the AEAD nonce and the secrecy
of the Security Context parameters. of the Security Context parameters.
The EdDSA signature algorithm and the elliptic curve Ed25519 For endpoints that support the group mode, the EdDSA signature
[RFC8032] are mandatory to implement. For endpoints that support the algorithm Ed25519 [RFC8032] is mandatory to implement. The group
pairwise mode, the ECDH-SS + HKDF-256 algorithm specified in mode uses the "encrypt-then-sign" construction, i.e., the
Section 6.3.1 of [I-D.ietf-cose-rfc8152bis-algs] and the X25519 curve countersignature is computed over the COSE_Encrypt0 object (see
[RFC7748] are also mandatory to implement. Section 4.1). This is motivated by enabling additional principals
acting as signature checkers (see Section 3.1), which do not join a
group as members but are allowed to verify countersignatures of
messages protected in group mode without being able to decrypt them
(see Section 8.5).
If the encryption algorithm used in group mode provides integrity
protection, countersignatures of COSE_Encrypt0 with short
authentication tags do not provide the security properties associated
with the same algorithm used in COSE_Sign (see Section 6 of
[I-D.ietf-cose-countersign]). To provide 128-bit security against
collision attacks, the tag length MUST be at least 256-bits. A
countersignature of a COSE_Encrypt0 with AES-CCM-16-64-128 provides
at most 32 bits of integrity protection.
For endpoints that support the pairwise mode, the ECDH-SS + HKDF-256
algorithm specified in Section 6.3.1 of
[I-D.ietf-cose-rfc8152bis-algs] and the X25519 algorithm [RFC7748]
are also mandatory to implement.
Constrained IoT devices may alternatively represent Montgomery curves Constrained IoT devices may alternatively represent Montgomery curves
and (twisted) Edwards curves [RFC7748] in the short-Weierstrass form and (twisted) Edwards curves [RFC7748] in the short-Weierstrass form
Wei25519, with which the algorithms ECDSA25519 and ECDH25519 can be Wei25519, with which the algorithms ECDSA25519 and ECDH25519 can be
used for signature operations and Diffie-Hellman secret calculation, used for signature operations and Diffie-Hellman secret calculation,
respectively [I-D.ietf-lwig-curve-representations]. respectively [I-D.ietf-lwig-curve-representations].
For many constrained IoT devices, it is problematic to support more For many constrained IoT devices, it is problematic to support more
than one signature algorithm or multiple whole cipher suites. As a than one signature algorithm or multiple whole cipher suites. As a
consequence, some deployments using, for instance, ECDSA with NIST consequence, some deployments using, for instance, ECDSA with NIST
P-256 may not support the mandatory signature algorithm but that P-256 may not support the mandatory signature algorithm but that
should not be an issue for local deployments. should not be an issue for local deployments.
The derivation of pairwise keys defined in Section 2.3.1 is The derivation of pairwise keys defined in Section 2.4.1 is
compatible with ECDSA and EdDSA asymmetric keys, but is not compatible with ECDSA and EdDSA asymmetric keys, but is not
compatible with RSA asymmetric keys. The security of using the same compatible with RSA asymmetric keys.
key pair for Diffie-Hellman and for signing is demonstrated in
[Degabriele].
10.14. Message Segmentation For the public key translation from Ed25519 (Ed448) to X25519 (X448)
specified in Section 2.4.1, variable time methods can be used since
the translation operates on public information. Any byte string of
appropriate length is accepted as a public key for X25519 (X448) in
[RFC7748]. It is therefore not necessary for security to validate
the translated public key (assuming the translation was successful).
The security of using the same key pair for Diffie-Hellman and for
signing (by considering the ECDH procedure in Section 2.4 as a Key
Encapsulation Mechanism (KEM)) is demonstrated in [Degabriele] and
[Thormarker].
Applications using ECDH (except X25519 and X448) based KEM in
Section 2.4 are assumed to verify that a peer endpoint's public key
is on the expected curve and that the shared secret is not the point
at infinity. The KEM in [Degabriele] checks that the shared secret
is different from the point at infinity, as does the procedure in
Section 5.7.1.2 of [NIST-800-56A] which is referenced in Section 2.4.
Extending Theorem 2 of [Degabriele], [Thormarker] shows that the same
key pair can be used with X25519 and Ed25519 (X448 and Ed448) for the
KEM specified in Section 2.4. By symmetry in the KEM used in this
document, both endpoints can consider themselves to have the
recipient role in the KEM - as discussed in Section 7 of [Thormarker]
- and rely on the mentioned proofs for the security of their key
pairs.
Theorem 3 in [Degabriele] shows that the same key pair can be used
for an ECDH based KEM and ECDSA. The KEM uses a different KDF than
in Section 2.4, but the proof only depends on that the KDF has
certain required properties, which are the typical assumptions about
HKDF, e.g., that output keys are pseudorandom. In order to comply
with the assumptions of Theorem 3, received public keys MUST be
successfully validated, see Section 5.6.2.3.4 of [NIST-800-56A]. The
validation MAY be performed by a trusted Group Manager. For
[Degabriele] to apply as it is written, public keys need to be in the
expected subgroup. For this we rely on cofactor DH, Section 5.7.1.2
of [NIST-800-56A] which is referenced in Section 2.4.
HashEdDSA variants of Ed25519 and Ed448 are not used by COSE, see
Section 2.2 of [I-D.ietf-cose-rfc8152bis-algs], and are not covered
by the analysis in [Thormarker], and hence MUST NOT be used with the
public keys used with pairwise keys as specified in this document.
10.16. Message Segmentation
The same considerations from Section 12.7 of [RFC8613] hold for Group The same considerations from Section 12.7 of [RFC8613] hold for Group
OSCORE. OSCORE.
10.15. Privacy Considerations 10.17. Privacy Considerations
Group OSCORE ensures end-to-end integrity protection and encryption Group OSCORE ensures end-to-end integrity protection and encryption
of the message payload and all options that are not used for proxy of the message payload and all options that are not used for proxy
operations. In particular, options are processed according to the operations. In particular, options are processed according to the
same class U/I/E that they have for OSCORE. Therefore, the same same class U/I/E that they have for OSCORE. Therefore, the same
privacy considerations from Section 12.8 of [RFC8613] hold for Group privacy considerations from Section 12.8 of [RFC8613] hold for Group
OSCORE. OSCORE, with the following addition.
* When protecting a message in group mode, the countersignature is
encrypted by using a keystream derived from the group keying
material (see Section 4.1 and Section 4.1.1). This ensures group
privacy. That is, an attacker cannot track an endpoint over two
groups by linking messages between the two groups, unless being
also a member of those groups.
Furthermore, the following privacy considerations hold about the Furthermore, the following privacy considerations hold about the
OSCORE option, which may reveal information on the communicating OSCORE option, which may reveal information on the communicating
endpoints. endpoints.
o The 'kid' parameter, which is intended to help a recipient * The 'kid' parameter, which is intended to help a recipient
endpoint to find the right Recipient Context, may reveal endpoint to find the right Recipient Context, may reveal
information about the Sender Endpoint. When both a request and information about the Sender Endpoint. When both a request and
the corresponding responses include the 'kid' parameter, this may the corresponding responses include the 'kid' parameter, this may
reveal information about both a client sending a request and all reveal information about both a client sending a request and all
the possibly replying servers sending their own individual the possibly replying servers sending their own individual
response. response.
o The 'kid context' parameter, which is intended to help a recipient * The 'kid context' parameter, which is intended to help a recipient
endpoint to find the right Security Context, reveals information endpoint to find the right Security Context, reveals information
about the sender endpoint. In particular, it reveals that the about the sender endpoint. In particular, it reveals that the
sender endpoint is a member of a particular OSCORE group, whose sender endpoint is a member of a particular OSCORE group, whose
current Group ID is indicated in the 'kid context' parameter. current Group ID is indicated in the 'kid context' parameter.
When receiving a group request, each of the recipient endpoints can When receiving a group request, each of the recipient endpoints can
reply with a response that includes its Sender ID as 'kid' parameter. reply with a response that includes its Sender ID as 'kid' parameter.
All these responses will be matchable with the request through the All these responses will be matchable with the request through the
Token. Thus, even if these responses do not include a 'kid context' Token. Thus, even if these responses do not include a 'kid context'
parameter, it becomes possible to understand that the responder parameter, it becomes possible to understand that the responder
endpoints are in the same group of the requester endpoint. endpoints are in the same group of the requester endpoint.
Furthermore, using the mechanisms described in Appendix E to achieve Furthermore, using the mechanisms described in Appendix E to achieve
Sender Sequence Number synchronization with a client may reveal when Sender Sequence Number synchronization with a client may reveal when
a server device goes through a reboot. This can be mitigated by the a server device goes through a reboot. This can be mitigated by the
server device storing the precise state of the Replay Window of each server device storing the precise state of the Replay Window of each
known client on a clean shutdown. known client on a clean shutdown.
Finally, the mechanism described in Section 10.5 to prevent Finally, the mechanism described in Section 10.6 to prevent
collisions of Group Identifiers from different Group Managers may collisions of Group Identifiers from different Group Managers may
reveal information about events in the respective OSCORE groups. In reveal information about events in the respective OSCORE groups. In
particular, a Group Identifier changes when the corresponding group particular, a Group Identifier changes when the corresponding group
is rekeyed. Thus, Group Managers might use the shared list of Group is rekeyed. Thus, Group Managers might use the shared list of Group
Identifiers to infer the rate and patterns of group membership Identifiers to infer the rate and patterns of group membership
changes triggering a group rekeying, e.g. due to newly joined members changes triggering a group rekeying, e.g., due to newly joined
or evicted (compromised) members. In order to alleviate this privacy members or evicted (compromised) members. In order to alleviate this
concern, it should be hidden from the Group Managers which exact privacy concern, it should be hidden from the Group Managers which
Group Manager has currently assigned which Group Identifiers in its exact Group Manager has currently assigned which Group Identifiers in
OSCORE groups. its OSCORE groups.
11. IANA Considerations 11. IANA Considerations
Note to RFC Editor: Please replace "[This Document]" with the RFC Note to RFC Editor: Please replace "[This Document]" with the RFC
number of this specification and delete this paragraph. number of this document and delete this paragraph.
This document has the following actions for IANA. This document has the following actions for IANA.
11.1. OSCORE Flag Bits Registry 11.1. OSCORE Flag Bits Registry
IANA is asked to add the following value entry to the "OSCORE Flag IANA is asked to add the following value entry to the "OSCORE Flag
Bits" subregistry defined in Section 13.7 of [RFC8613] as part of the Bits" subregistry defined in Section 13.7 of [RFC8613] as part of the
"CoRE Parameters" registry. "CoRE Parameters" registry.
+--------------+------------+-----------------------------+-----------+ +--------------+------------+-----------------------------+-----------+
skipping to change at page 52, line 43 skipping to change at page 71, line 5
| 2 | Group Flag | For using a Group OSCORE | [This | | 2 | Group Flag | For using a Group OSCORE | [This |
| | | Security Context, set to 1 | Document] | | | | Security Context, set to 1 | Document] |
| | | if the message is protected | | | | | if the message is protected | |
| | | with the group mode | | | | | with the group mode | |
+--------------+------------+-----------------------------+-----------+ +--------------+------------+-----------------------------+-----------+
12. References 12. References
12.1. Normative References 12.1. Normative References
[COSE.Algorithms]
IANA, "COSE Algorithms",
<https://www.iana.org/assignments/cose/
cose.xhtml#algorithms>.
[COSE.Key.Types]
IANA, "COSE Key Types",
<https://www.iana.org/assignments/cose/cose.xhtml#key-
type>.
[I-D.ietf-core-groupcomm-bis] [I-D.ietf-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", draft- for the Constrained Application Protocol (CoAP)", Work in
ietf-core-groupcomm-bis-03 (work in progress), February Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
2021. 04, 12 July 2021, <https://www.ietf.org/archive/id/draft-
ietf-core-groupcomm-bis-04.txt>.
[I-D.ietf-cose-countersign] [I-D.ietf-cose-countersign]
Schaad, J. and R. Housley, "CBOR Object Signing and Schaad, J. and R. Housley, "CBOR Object Signing and
Encryption (COSE): Countersignatures", draft-ietf-cose- Encryption (COSE): Countersignatures", Work in Progress,
countersign-02 (work in progress), December 2020. Internet-Draft, draft-ietf-cose-countersign-05, 23 June
2021, <https://www.ietf.org/archive/id/draft-ietf-cose-
countersign-05.txt>.
[I-D.ietf-cose-rfc8152bis-algs] [I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-12 Initial Algorithms", Work in Progress, Internet-Draft,
(work in progress), September 2020. draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-algs-12.txt>.
[I-D.ietf-cose-rfc8152bis-struct] [I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", draft-ietf-cose-rfc8152bis- Structures and Process", Work in Progress, Internet-Draft,
struct-15 (work in progress), February 2021. draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-struct-15.txt>.
[NIST-800-56A] [NIST-800-56A]
Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R. Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
Davis, "Recommendation for Pair-Wise Key-Establishment Davis, "Recommendation for Pair-Wise Key-Establishment
Schemes Using Discrete Logarithm Cryptography - NIST Schemes Using Discrete Logarithm Cryptography - NIST
Special Publication 800-56A, Revision 3", April 2018, Special Publication 800-56A, Revision 3", April 2018,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar3.pdf>. NIST.SP.800-56Ar3.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
skipping to change at page 54, line 14 skipping to change at page 72, line 23
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032, Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017, DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>. <https://www.rfc-editor.org/info/rfc8032>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments "Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>. <https://www.rfc-editor.org/info/rfc8613>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949, Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020, DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>. <https://www.rfc-editor.org/info/rfc8949>.
12.2. Informative References 12.2. Informative References
[Degabriele] [Degabriele]
Degabriele, J., Lehmann, A., Paterson, K., Smart, N., and Degabriele, J.P., Lehmann, A., Paterson, K.G., Smart,
M. Strefler, "On the Joint Security of Encryption and N.P., and M. Strefler, "On the Joint Security of
Signature in EMV", December 2011, Encryption and Signature in EMV", December 2011,
<https://eprint.iacr.org/2011/615>. <https://eprint.iacr.org/2011/615>.
[I-D.amsuess-core-cachable-oscore]
Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
Progress, Internet-Draft, draft-amsuess-core-cachable-
oscore-01, 22 February 2021,
<https://www.ietf.org/archive/id/draft-amsuess-core-
cachable-oscore-01.txt>.
[I-D.ietf-ace-key-groupcomm] [I-D.ietf-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", draft-ietf-ace-key-groupcomm-11 Communication using ACE", Work in Progress, Internet-
(work in progress), February 2021. Draft, draft-ietf-ace-key-groupcomm-13, 12 July 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-key-
groupcomm-13.txt>.
[I-D.ietf-ace-key-groupcomm-oscore] [I-D.ietf-ace-key-groupcomm-oscore]
Tiloca, M., Park, J., and F. Palombini, "Key Management Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm- for OSCORE Groups in ACE", Work in Progress, Internet-
oscore-10 (work in progress), February 2021. Draft, draft-ietf-ace-key-groupcomm-oscore-11, 12 July
2021, <https://www.ietf.org/archive/id/draft-ietf-ace-key-
groupcomm-oscore-11.txt>.
[I-D.ietf-ace-oauth-authz] [I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0 Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-37 Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
(work in progress), February 2021. draft-ietf-ace-oauth-authz-43, 10 July 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
authz-43.txt>.
[I-D.ietf-core-echo-request-tag] [I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo, Amsüss, C., Mattsson, J. P., and G. Selander, "CoAP: Echo,
Request-Tag, and Token Processing", draft-ietf-core-echo- Request-Tag, and Token Processing", Work in Progress,
request-tag-12 (work in progress), January 2021. Internet-Draft, draft-ietf-core-echo-request-tag-12, 1
February 2021, <https://www.ietf.org/archive/id/draft-
ietf-core-echo-request-tag-12.txt>.
[I-D.ietf-core-observe-multicast-notifications]
Tiloca, M., Hoeglund, R., Amsuess, C., and F. Palombini,
"Observe Notifications as CoAP Multicast Responses", Work
in Progress, Internet-Draft, draft-ietf-core-observe-
multicast-notifications-01, 12 July 2021,
<https://www.ietf.org/archive/id/draft-ietf-core-observe-
multicast-notifications-01.txt>.
[I-D.ietf-cose-cbor-encoded-cert]
Raza, S., Höglund, J., Selander, G., Mattsson, J. P., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
ietf-cose-cbor-encoded-cert-01, 25 May 2021,
<https://www.ietf.org/archive/id/draft-ietf-cose-cbor-
encoded-cert-01.txt>.
[I-D.ietf-lwig-curve-representations] [I-D.ietf-lwig-curve-representations]
Struik, R., "Alternative Elliptic Curve Representations", Struik, R., "Alternative Elliptic Curve Representations",
draft-ietf-lwig-curve-representations-20 (work in Work in Progress, Internet-Draft, draft-ietf-lwig-curve-
progress), February 2021. representations-21, 9 June 2021,
<https://www.ietf.org/archive/id/draft-ietf-lwig-curve-
representations-21.txt>.
[I-D.ietf-lwig-security-protocol-comparison] [I-D.ietf-lwig-security-protocol-comparison]
Mattsson, J., Palombini, F., and M. Vucinic, "Comparison Mattsson, J. P., Palombini, F., and M. Vucinic,
of CoAP Security Protocols", draft-ietf-lwig-security- "Comparison of CoAP Security Protocols", Work in Progress,
protocol-comparison-05 (work in progress), November 2020. Internet-Draft, draft-ietf-lwig-security-protocol-
comparison-05, 2 November 2020,
<https://www.ietf.org/archive/id/draft-ietf-lwig-security-
protocol-comparison-05.txt>.
[I-D.ietf-rats-uccs]
Birkholz, H., O'Donoghue, J., Cam-Winget, N., and C.
Bormann, "A CBOR Tag for Unprotected CWT Claims Sets",
Work in Progress, Internet-Draft, draft-ietf-rats-uccs-00,
19 May 2021, <https://www.ietf.org/archive/id/draft-ietf-
rats-uccs-00.txt>.
[I-D.ietf-tls-dtls13] [I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-41 (work in progress), 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
February 2021. dtls13-43, 30 April 2021, <https://www.ietf.org/internet-
drafts/draft-ietf-tls-dtls13-43.txt>.
[I-D.mattsson-cfrg-det-sigs-with-noise] [I-D.mattsson-cfrg-det-sigs-with-noise]
Mattsson, J., Thormarker, E., and S. Ruohomaa, Mattsson, J. P., Thormarker, E., and S. Ruohomaa,
"Deterministic ECDSA and EdDSA Signatures with Additional "Deterministic ECDSA and EdDSA Signatures with Additional
Randomness", draft-mattsson-cfrg-det-sigs-with-noise-02 Randomness", Work in Progress, Internet-Draft, draft-
(work in progress), March 2020. mattsson-cfrg-det-sigs-with-noise-02, 11 March 2020,
<https://www.ietf.org/archive/id/draft-mattsson-cfrg-det-
[I-D.somaraju-ace-multicast] sigs-with-noise-02.txt>.
Somaraju, A., Kumar, S., Tschofenig, H., and W. Werner,
"Security for Low-Latency Group Communication", draft-
somaraju-ace-multicast-02 (work in progress), October
2016.
[I-D.tiloca-core-observe-multicast-notifications]
Tiloca, M., Hoeglund, R., Amsuess, C., and F. Palombini,
"Observe Notifications as CoAP Multicast Responses",
draft-tiloca-core-observe-multicast-notifications-05 (work
in progress), February 2021.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 "Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>. <https://www.rfc-editor.org/info/rfc4944>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", [RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>. <https://www.rfc-editor.org/info/rfc4949>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011, DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>. <https://www.rfc-editor.org/info/rfc6282>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>. January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014, DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>. <https://www.rfc-editor.org/info/rfc7228>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained [RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641, Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015, DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>. <https://www.rfc-editor.org/info/rfc7641>.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959, the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016, DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>. <https://www.rfc-editor.org/info/rfc7959>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[Thormarker]
Thormarker, E., "On using the same key pair for Ed25519
and an X25519 based KEM", April 2021,
<https://eprint.iacr.org/2021/509>.
Appendix A. Assumptions and Security Objectives Appendix A. Assumptions and Security Objectives
This section presents a set of assumptions and security objectives This section presents a set of assumptions and security objectives
for the approach described in this document. The rest of this for the approach described in this document. The rest of this
section refers to three types of groups: section refers to three types of groups:
o Application group, i.e. a set of CoAP endpoints that share a * Application group, i.e., a set of CoAP endpoints that share a
common pool of resources. common pool of resources.
o Security group, as defined in Section 1.1 of this specification. * Security group, as defined in Section 1.1 of this document. There
There can be a one-to-one or a one-to-many relation between can be a one-to-one or a one-to-many relation between security
security groups and application groups, and vice versa. groups and application groups, and vice versa.
o CoAP group, i.e. a set of CoAP endpoints where each endpoint is * CoAP group, i.e., a set of CoAP endpoints where each endpoint is
configured to receive one-to-many CoAP requests, e.g. sent to the configured to receive one-to-many CoAP requests, e.g., sent to the
group's associated IP multicast address and UDP port as defined in group's associated IP multicast address and UDP port as defined in
[I-D.ietf-core-groupcomm-bis]. An endpoint may be a member of [I-D.ietf-core-groupcomm-bis]. An endpoint may be a member of
multiple CoAP groups. There can be a one-to-one or a one-to-many multiple CoAP groups. There can be a one-to-one or a one-to-many
relation between application groups and CoAP groups. Note that a relation between application groups and CoAP groups. Note that a
device sending a CoAP request to a CoAP group is not necessarily device sending a CoAP request to a CoAP group is not necessarily
itself a member of that group: it is a member only if it also has itself a member of that group: it is a member only if it also has
a CoAP server endpoint listening to requests for this CoAP group, a CoAP server endpoint listening to requests for this CoAP group,
sent to the associated IP multicast address and port. In order to sent to the associated IP multicast address and port. In order to
provide secure group communication, all members of a CoAP group as provide secure group communication, all members of a CoAP group as
well as all further endpoints configured only as clients sending well as all further endpoints configured only as clients sending
CoAP (multicast) requests to the CoAP group have to be member of a CoAP (multicast) requests to the CoAP group have to be member of a
security group. There can be a one-to-one or a one-to-many security group. There can be a one-to-one or a one-to-many
relation between security groups and CoAP groups, and vice versa. relation between security groups and CoAP groups, and vice versa.
A.1. Assumptions A.1. Assumptions
The following points are assumed to be already addressed and are out The following points are assumed to be already addressed and are out
of the scope of this document. of the scope of this document.
o Multicast communication topology: this document considers both * Multicast communication topology: this document considers both
1-to-N (one sender and multiple recipients) and M-to-N (multiple 1-to-N (one sender and multiple recipients) and M-to-N (multiple
senders and multiple recipients) communication topologies. The senders and multiple recipients) communication topologies. The
1-to-N communication topology is the simplest group communication 1-to-N communication topology is the simplest group communication
scenario that would serve the needs of a typical Low-power and scenario that would serve the needs of a typical Low-power and
Lossy Network (LLN). Examples of use cases that benefit from Lossy Network (LLN). Examples of use cases that benefit from
secure group communication are provided in Appendix B. secure group communication are provided in Appendix B.
In a 1-to-N communication model, only a single client transmits In a 1-to-N communication model, only a single client transmits
data to the CoAP group, in the form of request messages; in an data to the CoAP group, in the form of request messages; in an
M-to-N communication model (where M and N do not necessarily have M-to-N communication model (where M and N do not necessarily have
the same value), M clients transmit data to the CoAP group. the same value), M clients transmit data to the CoAP group.
According to [I-D.ietf-core-groupcomm-bis], any possible proxy According to [I-D.ietf-core-groupcomm-bis], any possible proxy
entity is supposed to know about the clients. Also, every client entity is supposed to know about the clients. Also, every client
expects and is able to handle multiple response messages expects and is able to handle multiple response messages
associated to a same request sent to the CoAP group. associated to a same request sent to the CoAP group.
o Group size: security solutions for group communication should be * Group size: security solutions for group communication should be
able to adequately support different and possibly large security able to adequately support different and possibly large security
groups. The group size is the current number of members in a groups. The group size is the current number of members in a
security group. In the use cases mentioned in this document, the security group. In the use cases mentioned in this document, the
number of clients (normally the controlling devices) is expected number of clients (normally the controlling devices) is expected
to be much smaller than the number of servers (i.e. the controlled to be much smaller than the number of servers (i.e., the
devices). A security solution for group communication that controlled devices). A security solution for group communication
supports 1 to 50 clients would be able to properly cover the group that supports 1 to 50 clients would be able to properly cover the
sizes required for most use cases that are relevant for this group sizes required for most use cases that are relevant for this
document. The maximum group size is expected to be in the range document. The maximum group size is expected to be in the range
of 2 to 100 devices. Security groups larger than that should be of 2 to 100 devices. Security groups larger than that should be
divided into smaller independent groups. divided into smaller independent groups. One should not assume
that the set of members of a security group remains fixed. That
is, the group membership is subject to changes, possibly on a
frequent basis.
o Communication with the Group Manager: an endpoint must use a * Communication with the Group Manager: an endpoint must use a
secure dedicated channel when communicating with the Group secure dedicated channel when communicating with the Group
Manager, also when not registered as a member of the security Manager, also when not registered as a member of the security
group. group.
o Provisioning and management of Security Contexts: a Security * Provisioning and management of Security Contexts: a Security
Context must be established among the members of the security Context must be established among the members of the security
group. A secure mechanism must be used to generate, revoke and group. A secure mechanism must be used to generate, revoke and
(re-)distribute keying material, communication policies and (re-)distribute keying material, communication policies and
security parameters in the security group. The actual security parameters in the security group. The actual
provisioning and management of the Security Context is out of the provisioning and management of the Security Context is out of the
scope of this document. scope of this document.
o Multicast data security ciphersuite: all members of a security * Multicast data security ciphersuite: all members of a security
group must agree on a ciphersuite to provide authenticity, group must agree on a ciphersuite to provide authenticity,
integrity and confidentiality of messages in the group. The integrity and confidentiality of messages in the group. The
ciphersuite is specified as part of the Security Context. ciphersuite is specified as part of the Security Context.
o Backward security: a new device joining the security group should * Backward security: a new device joining the security group should
not have access to any old Security Contexts used before its not have access to any old Security Contexts used before its
joining. This ensures that a new member of the security group is joining. This ensures that a new member of the security group is
not able to decrypt confidential data sent before it has joined not able to decrypt confidential data sent before it has joined
the security group. The adopted key management scheme should the security group. The adopted key management scheme should
ensure that the Security Context is updated to ensure backward ensure that the Security Context is updated to ensure backward
confidentiality. The actual mechanism to update the Security confidentiality. The actual mechanism to update the Security
Context and renew the group keying material in the security group Context and renew the group keying material in the security group
upon a new member's joining has to be defined as part of the group upon a new member's joining has to be defined as part of the group
key management scheme. key management scheme.
o Forward security: entities that leave the security group should * Forward security: entities that leave the security group should
not have access to any future Security Contexts or message not have access to any future Security Contexts or message
exchanged within the security group after their leaving. This exchanged within the security group after their leaving. This
ensures that a former member of the security group is not able to ensures that a former member of the security group is not able to
decrypt confidential data sent within the security group anymore. decrypt confidential data sent within the security group anymore.
Also, it ensures that a former member is not able to send Also, it ensures that a former member is not able to send
protected messages to the security group anymore. The actual protected messages to the security group anymore. The actual
mechanism to update the Security Context and renew the group mechanism to update the Security Context and renew the group
keying material in the security group upon a member's leaving has keying material in the security group upon a member's leaving has
to be defined as part of the group key management scheme. to be defined as part of the group key management scheme.
A.2. Security Objectives A.2. Security Objectives
The approach described in this document aims at fulfilling the The approach described in this document aims at fulfilling the
following security objectives: following security objectives:
o Data replay protection: group request messages or response * Data replay protection: group request messages or response
messages replayed within the security group must be detected. messages replayed within the security group must be detected.
o Data confidentiality: messages sent within the security group * Data confidentiality: messages sent within the security group
shall be encrypted. shall be encrypted.
o Group-level data confidentiality: the group mode provides group- * Group-level data confidentiality: the group mode provides group-
level data confidentiality since messages are encrypted at a group level data confidentiality since messages are encrypted at a group
level, i.e. in such a way that they can be decrypted by any member level, i.e., in such a way that they can be decrypted by any
of the security group, but not by an external adversary or other member of the security group, but not by an external adversary or
external entities. other external entities.
o Pairwise data confidentiality: the pairwise mode especially * Pairwise data confidentiality: the pairwise mode especially
provides pairwise data confidentiality, since messages are provides pairwise data confidentiality, since messages are
encrypted using pairwise keying material shared between any two encrypted using pairwise keying material shared between any two
group members, hence they can be decrypted only by the intended group members, hence they can be decrypted only by the intended
single recipient. single recipient.
o Source message authentication: messages sent within the security * Source message authentication: messages sent within the security
group shall be authenticated. That is, it is essential to ensure group shall be authenticated. That is, it is essential to ensure
that a message is originated by a member of the security group in that a message is originated by a member of the security group in
the first place, and in particular by a specific, identifiable the first place, and in particular by a specific, identifiable
member of the security group. member of the security group.
o Message integrity: messages sent within the security group shall * Message integrity: messages sent within the security group shall
be integrity protected. That is, it is essential to ensure that a be integrity protected. That is, it is essential to ensure that a
message has not been tampered with, either by a group member, or message has not been tampered with, either by a group member, or
by an external adversary or other external entities which are not by an external adversary or other external entities which are not
members of the security group. members of the security group.
o Message ordering: it must be possible to determine the ordering of * Message ordering: it must be possible to determine the ordering of
messages coming from a single sender. In accordance with OSCORE messages coming from a single sender. In accordance with OSCORE
[RFC8613], this results in providing absolute freshness of [RFC8613], this results in providing absolute freshness of
responses that are not notifications, as well as relative responses that are not notifications, as well as relative
freshness of group requests and notification responses. It is not freshness of group requests and notification responses. It is not
required to determine ordering of messages from different senders. required to determine ordering of messages from different senders.
Appendix B. List of Use Cases Appendix B. List of Use Cases
Group Communication for CoAP [I-D.ietf-core-groupcomm-bis] provides Group Communication for CoAP [I-D.ietf-core-groupcomm-bis] provides
the necessary background for multicast-based CoAP communication, with the necessary background for multicast-based CoAP communication, with
particular reference to low-power and lossy networks (LLNs) and particular reference to low-power and lossy networks (LLNs) and
resource constrained environments. The interested reader is resource constrained environments. The interested reader is
encouraged to first read [I-D.ietf-core-groupcomm-bis] to understand encouraged to first read [I-D.ietf-core-groupcomm-bis] to understand
the non-security related details. This section discusses a number of the non-security related details. This section discusses a number of
use cases that benefit from secure group communication, and refers to use cases that benefit from secure group communication, and refers to
the three types of groups from Appendix A. Specific security the three types of groups from Appendix A. Specific security
requirements for these use cases are discussed in Appendix A. requirements for these use cases are discussed in Appendix A.
o Lighting control: consider a building equipped with IP-connected * Lighting control: consider a building equipped with IP-connected
lighting devices, switches, and border routers. The lighting lighting devices, switches, and border routers. The lighting
devices acting as servers are organized into application groups devices acting as servers are organized into application groups
and CoAP groups, according to their physical location in the and CoAP groups, according to their physical location in the
building. For instance, lighting devices in a room or corridor building. For instance, lighting devices in a room or corridor
can be configured as members of a single application group and can be configured as members of a single application group and
corresponding CoAP group. Those lighting devices together with corresponding CoAP group. Those lighting devices together with
the switches acting as clients in the same room or corridor can be the switches acting as clients in the same room or corridor can be
configured as members of the corresponding security group. configured as members of the corresponding security group.
Switches are then used to control the lighting devices by sending Switches are then used to control the lighting devices by sending
on/off/dimming commands to all lighting devices in the CoAP group, on/off/dimming commands to all lighting devices in the CoAP group,
while border routers connected to an IP network backbone (which is while border routers connected to an IP network backbone (which is
also multicast-enabled) can be used to interconnect routers in the also multicast-enabled) can be used to interconnect routers in the
building. Consequently, this would also enable logical groups to building. Consequently, this would also enable logical groups to
be formed even if devices with a role in the lighting application be formed even if devices with a role in the lighting application
may be physically in different subnets (e.g. on wired and wireless may be physically in different subnets (e.g., on wired and
networks). Connectivity between lighting devices may be realized, wireless networks). Connectivity between lighting devices may be
for instance, by means of IPv6 and (border) routers supporting realized, for instance, by means of IPv6 and (border) routers
6LoWPAN [RFC4944][RFC6282]. Group communication enables supporting 6LoWPAN [RFC4944][RFC6282]. Group communication
synchronous operation of a set of connected lights, ensuring that enables synchronous operation of a set of connected lights,
the light preset (e.g. dimming level or color) of a large set of ensuring that the light preset (e.g., dimming level or color) of a
luminaires are changed at the same perceived time. This is large set of luminaires are changed at the same perceived time.
especially useful for providing a visual synchronicity of light This is especially useful for providing a visual synchronicity of
effects to the user. As a practical guideline, events within a light effects to the user. As a practical guideline, events
200 ms interval are perceived as simultaneous by humans, which is within a 200 ms interval are perceived as simultaneous by humans,
necessary to ensure in many setups. Devices may reply back to the which is necessary to ensure in many setups. Devices may reply
switches that issue on/off/dimming commands, in order to report back to the switches that issue on/off/dimming commands, in order
about the execution of the requested operation (e.g. OK, failure, to report about the execution of the requested operation (e.g.,
error) and their current operational status. In a typical OK, failure, error) and their current operational status. In a
lighting control scenario, a single switch is the only entity typical lighting control scenario, a single switch is the only
responsible for sending commands to a set of lighting devices. In entity responsible for sending commands to a set of lighting
more advanced lighting control use cases, a M-to-N communication devices. In more advanced lighting control use cases, a M-to-N
topology would be required, for instance in case multiple sensors communication topology would be required, for instance in case
(presence or day-light) are responsible to trigger events to a set multiple sensors (presence or day-light) are responsible to
of lighting devices. Especially in professional lighting trigger events to a set of lighting devices. Especially in
scenarios, the roles of client and server are configured by the professional lighting scenarios, the roles of client and server
lighting commissioner, and devices strictly follow those roles. are configured by the lighting commissioner, and devices strictly
follow those roles.
o Integrated building control: enabling Building Automation and * Integrated building control: enabling Building Automation and
Control Systems (BACSs) to control multiple heating, ventilation Control Systems (BACSs) to control multiple heating, ventilation
and air-conditioning units to predefined presets. Controlled and air-conditioning units to predefined presets. Controlled
units can be organized into application groups and CoAP groups in units can be organized into application groups and CoAP groups in
order to reflect their physical position in the building, e.g. order to reflect their physical position in the building, e.g.,
devices in the same room can be configured as members of a single devices in the same room can be configured as members of a single
application group and corresponding CoAP group. As a practical application group and corresponding CoAP group. As a practical
guideline, events within intervals of seconds are typically guideline, events within intervals of seconds are typically
acceptable. Controlled units are expected to possibly reply back acceptable. Controlled units are expected to possibly reply back
to the BACS issuing control commands, in order to report about the to the BACS issuing control commands, in order to report about the
execution of the requested operation (e.g. OK, failure, error) execution of the requested operation (e.g., OK, failure, error)
and their current operational status. and their current operational status.
o Software and firmware updates: software and firmware updates often * Software and firmware updates: software and firmware updates often
comprise quite a large amount of data. This can overload a Low- comprise quite a large amount of data. This can overload a Low-
power and Lossy Network (LLN) that is otherwise typically used to power and Lossy Network (LLN) that is otherwise typically used to
deal with only small amounts of data, on an infrequent base. deal with only small amounts of data, on an infrequent base.
Rather than sending software and firmware updates as unicast Rather than sending software and firmware updates as unicast
messages to each individual device, multicasting such updated data messages to each individual device, multicasting such updated data
to a larger set of devices at once displays a number of benefits. to a larger set of devices at once displays a number of benefits.
For instance, it can significantly reduce the network load and For instance, it can significantly reduce the network load and
decrease the overall time latency for propagating this data to all decrease the overall time latency for propagating this data to all
devices. Even if the complete whole update process itself is devices. Even if the complete whole update process itself is
secured, securing the individual messages is important, in case secured, securing the individual messages is important, in case
updates consist of relatively large amounts of data. In fact, updates consist of relatively large amounts of data. In fact,
checking individual received data piecemeal for tampering avoids checking individual received data piecemeal for tampering avoids
that devices store large amounts of partially corrupted data and that devices store large amounts of partially corrupted data and
that they detect tampering hereof only after all data has been that they detect tampering hereof only after all data has been
received. Devices receiving software and firmware updates are received. Devices receiving software and firmware updates are
expected to possibly reply back, in order to provide a feedback expected to possibly reply back, in order to provide a feedback
about the execution of the update operation (e.g. OK, failure, about the execution of the update operation (e.g., OK, failure,
error) and their current operational status. error) and their current operational status.
o Parameter and configuration update: by means of multicast * Parameter and configuration update: by means of multicast
communication, it is possible to update the settings of a set of communication, it is possible to update the settings of a set of
similar devices, both simultaneously and efficiently. Possible similar devices, both simultaneously and efficiently. Possible
parameters are related, for instance, to network load management parameters are related, for instance, to network load management
or network access controls. Devices receiving parameter and or network access controls. Devices receiving parameter and
configuration updates are expected to possibly reply back, to configuration updates are expected to possibly reply back, to
provide a feedback about the execution of the update operation provide a feedback about the execution of the update operation
(e.g. OK, failure, error) and their current operational status. (e.g., OK, failure, error) and their current operational status.
o Commissioning of Low-power and Lossy Network (LLN) systems: a * Commissioning of Low-power and Lossy Network (LLN) systems: a
commissioning device is responsible for querying all devices in commissioning device is responsible for querying all devices in
the local network or a selected subset of them, in order to the local network or a selected subset of them, in order to
discover their presence, and be aware of their capabilities, discover their presence, and be aware of their capabilities,
default configuration, and operating conditions. Queried devices default configuration, and operating conditions. Queried devices
displaying similarities in their capabilities and features, or displaying similarities in their capabilities and features, or
sharing a common physical location can be configured as members of sharing a common physical location can be configured as members of
a single application group and corresponding CoAP group. Queried a single application group and corresponding CoAP group. Queried
devices are expected to reply back to the commissioning device, in devices are expected to reply back to the commissioning device, in
order to notify their presence, and provide the requested order to notify their presence, and provide the requested
information and their current operational status. information and their current operational status.
o Emergency multicast: a particular emergency related information * Emergency multicast: a particular emergency related information
(e.g. natural disaster) is generated and multicast by an emergency (e.g., natural disaster) is generated and multicast by an
notifier, and relayed to multiple devices. The latter may reply emergency notifier, and relayed to multiple devices. The latter
back to the emergency notifier, in order to provide their feedback may reply back to the emergency notifier, in order to provide
and local information related to the ongoing emergency. This kind their feedback and local information related to the ongoing
of setups should additionally rely on a fault tolerance multicast emergency. This kind of setups should additionally rely on a
algorithm, such as Multicast Protocol for Low-Power and Lossy fault tolerance multicast algorithm, such as Multicast Protocol
Networks (MPL). for Low-Power and Lossy Networks (MPL).
Appendix C. Example of Group Identifier Format Appendix C. Example of Group Identifier Format
This section provides an example of how the Group Identifier (Gid) This section provides an example of how the Group Identifier (Gid)
can be specifically formatted. That is, the Gid can be composed of can be specifically formatted. That is, the Gid can be composed of
two parts, namely a Group Prefix and a Group Epoch. two parts, namely a Group Prefix and a Group Epoch.
For each group, the Group Prefix is constant over time and is For each group, the Group Prefix is constant over time and is
uniquely defined in the set of all the groups associated to the same uniquely defined in the set of all the groups associated to the same
Group Manager. The choice of the Group Prefix for a given group's Group Manager. The choice of the Group Prefix for a given group's
Security Context is application specific. The size of the Group Security Context is application specific. The size of the Group
Prefix directly impact on the maximum number of distinct groups under Prefix directly impact on the maximum number of distinct groups under
the same Group Manager. the same Group Manager.
The Group Epoch is set to 0 upon the group's initialization, and is The Group Epoch is set to 0 upon the group's initialization, and is
incremented by 1 each time new keying material, together with a new incremented by 1 each time new keying material, together with a new
Gid, is distributed to the group in order to establish a new Security Gid, is distributed to the group in order to establish a new Security
Context (see Section 3.1). Context (see Section 3.2).
As an example, a 3-byte Gid can be composed of: i) a 1-byte Group As an example, a 3-byte Gid can be composed of: i) a 1-byte Group
Prefix '0xb1' interpreted as a raw byte string; and ii) a 2-byte Prefix '0xb1' interpreted as a raw byte string; and ii) a 2-byte
Group Epoch interpreted as an unsigned integer ranging from 0 to Group Epoch interpreted as an unsigned integer ranging from 0 to
65535. Then, after having established the Common Context 61532 times 65535. Then, after having established the Common Context 61532 times
in the group, its Gid will assume value '0xb1f05c'. in the group, its Gid will assume value '0xb1f05c'.
Using an immutable Group Prefix for a group assumes that enough time Using an immutable Group Prefix for a group assumes that enough time
elapses before all possible Group Epoch values are used, since the elapses before all possible Group Epoch values are used, i.e., before
Group Manager never reassigns the same Gid to the same group. Thus, the Group Manager starts reassigning Gid values to the same group
the expected highest rate for addition/removal of group members and (see Section 3.2). Thus, the expected highest rate for addition/
consequent group rekeying should be taken into account for a proper removal of group members and consequent group rekeying should be
dimensioning of the Group Epoch size. taken into account for a proper dimensioning of the Group Epoch size.
As discussed in Section 10.5, if endpoints are deployed in multiple As discussed in Section 10.6, if endpoints are deployed in multiple
groups managed by different non-synchronized Group Managers, it is groups managed by different non-synchronized Group Managers, it is
possible that Group Identifiers of different groups coincide at some possible that Group Identifiers of different groups coincide at some
point in time. In this case, a recipient has to handle coinciding point in time. In this case, a recipient has to handle coinciding
Group Identifiers, and has to try using different Security Contexts Group Identifiers, and has to try using different Security Contexts
to process an incoming message, until the right one is found and the to process an incoming message, until the right one is found and the
message is correctly verified. Therefore, it is favorable that Group message is correctly verified. Therefore, it is favorable that Group
Identifiers from different Group Managers have a size that result in Identifiers from different Group Managers have a size that result in
a small probability of collision. How small this probability should a small probability of collision. How small this probability should
be is up to system designers. be is up to system designers.
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The Group Manager must verify that the joining endpoint is authorized The Group Manager must verify that the joining endpoint is authorized
to join the group. To this end, the Group Manager can directly to join the group. To this end, the Group Manager can directly
authorize the joining endpoint, or expect it to provide authorization authorize the joining endpoint, or expect it to provide authorization
evidence previously obtained from a trusted entity. Further details evidence previously obtained from a trusted entity. Further details
about the authorization of joining endpoints are out of scope. about the authorization of joining endpoints are out of scope.
In case of successful authorization check, the Group Manager In case of successful authorization check, the Group Manager
generates a Sender ID assigned to the joining endpoint, before generates a Sender ID assigned to the joining endpoint, before
proceeding with the rest of the join process. That is, the Group proceeding with the rest of the join process. That is, the Group
Manager provides the joining endpoint with the keying material and Manager provides the joining endpoint with the keying material and
parameters to initialize the Security Context (see Section 2). The parameters to initialize the Security Context, including its own
actual provisioning of keying material and parameters to the joining public key (see Section 2). The actual provisioning of keying
endpoint is out of the scope of this document. material and parameters to the joining endpoint is out of the scope
of this document.
It is RECOMMENDED that the join process adopts the approach described It is RECOMMENDED that the join process adopts the approach described
in [I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework in [I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework
for Authentication and Authorization in constrained environments for Authentication and Authorization in constrained environments
[I-D.ietf-ace-oauth-authz]. [I-D.ietf-ace-oauth-authz].
Appendix E. Challenge-Response Synchronization Appendix E. Challenge-Response Synchronization
This section describes a possible approach that a server endpoint can This section describes a possible approach that a server endpoint can
use to synchronize with Sender Sequence Numbers of client endpoints use to synchronize with Sender Sequence Numbers of client endpoints
in the group. In particular, the server performs a challenge- in the group. In particular, the server performs a challenge-
response exchange with a client, by using the Echo Option for CoAP response exchange with a client, by using the Echo Option for CoAP
described in Section 2 of [I-D.ietf-core-echo-request-tag] and described in Section 2 of [I-D.ietf-core-echo-request-tag] and
according to Appendix B.1.2 of [RFC8613]. according to Appendix B.1.2 of [RFC8613].
That is, upon receiving a request from a particular client for the That is, upon receiving a request from a particular client for the
first time, the server processes the message as described in this first time, the server processes the message as described in this
specification, but, even if valid, does not deliver it to the document, but, even if valid, does not deliver it to the application.
application. Instead, the server replies to the client with an Instead, the server replies to the client with an OSCORE protected
OSCORE protected 4.01 (Unauthorized) response message, including only 4.01 (Unauthorized) response message, including only the Echo Option
the Echo Option and no diagnostic payload. The Echo option value and no diagnostic payload. The Echo option value SHOULD NOT be
SHOULD NOT be reused; when it is reused, it MUST be highly unlikely reused; when it is reused, it MUST be highly unlikely to have been
to have been used with this client recently. Since this response is used with this client recently. Since this response is protected
protected with the Security Context used in the group, the client with the Security Context used in the group, the client will consider
will consider the response valid upon successfully decrypting and the response valid upon successfully decrypting and verifying it.
verifying it.
The server stores the Echo Option value included therein, together The server stores the Echo Option value included therein, together
with the pair (gid,kid), where 'gid' is the Group Identifier of the with the pair (gid,kid), where 'gid' is the Group Identifier of the
OSCORE group and 'kid' is the Sender ID of the client in the group, OSCORE group and 'kid' is the Sender ID of the client in the group,
as specified in the 'kid context' and 'kid' fields of the OSCORE as specified in the 'kid context' and 'kid' fields of the OSCORE
Option of the request, respectively. After a group rekeying has been Option of the request, respectively. After a group rekeying has been
completed and a new Security Context has been established in the completed and a new Security Context has been established in the
group, which results also in a new Group Identifier (see group, which results also in a new Group Identifier (see
Section 3.1), the server MUST delete all the stored Echo values Section 3.2), the server MUST delete all the stored Echo values
associated to members of that group. associated to members of that group.
Upon receiving a 4.01 (Unauthorized) response that includes an Echo Upon receiving a 4.01 (Unauthorized) response that includes an Echo
Option and originates from a verified group member, the client sends Option and originates from a verified group member, the client sends
a request as a unicast message addressed to the same server, echoing a request as a unicast message addressed to the same server, echoing
the Echo Option value. The client MUST NOT send the request the Echo Option value. The client MUST NOT send the request
including the Echo Option over multicast. including the Echo Option over multicast.
If the signature algorithm used in the group supports ECDH (e.g. If the group uses also the group mode and the used Signature
ECDSA, EdDSA), the client MUST use the pairwise mode of Group OSCORE Algorithm supports ECDH (e.g., ECDSA, EdDSA), the client MUST use the
to protect the request, as described in Section 9.3. Note that, as pairwise mode of Group OSCORE to protect the request, as described in
defined in Section 9, members of such a group and that use the Echo Section 9.3. Note that, as defined in Section 9, members of such a
Option MUST support the pairwise mode. group and that use the Echo Option MUST support the pairwise mode.
The client does not necessarily resend the same group request, but The client does not necessarily resend the same group request, but
can instead send a more recent one, if the application permits it. can instead send a more recent one, if the application permits it.
This makes it possible for the client to not retain previously sent This makes it possible for the client to not retain previously sent
group requests for full retransmission, unless the application group requests for full retransmission, unless the application
explicitly requires otherwise. In either case, the client uses a explicitly requires otherwise. In either case, the client uses a
fresh Sender Sequence Number value from its own Sender Context. If fresh Sender Sequence Number value from its own Sender Context. If
the client stores group requests for possible retransmission with the the client stores group requests for possible retransmission with the
Echo Option, it should not store a given request for longer than a Echo Option, it should not store a given request for longer than a
preconfigured time interval. Note that the unicast request echoing preconfigured time interval. Note that the unicast request echoing
the Echo Option is correctly treated and processed as a message, the Echo Option is correctly treated and processed as a message,
since the 'kid context' field including the Group Identifier of the since the 'kid context' field including the Group Identifier of the
OSCORE group is still present in the OSCORE Option as part of the OSCORE group is still present in the OSCORE Option as part of the
COSE object (see Section 4). COSE object (see Section 4).
Upon receiving the unicast request including the Echo Option, the Upon receiving the unicast request including the Echo Option, the
server performs the following verifications. server performs the following verifications.
o If the server does not store an Echo Option value for the pair * If the server does not store an Echo Option value for the pair
(gid,kid), it considers: i) the time t1 when it has established (gid,kid), it considers: i) the time t1 when it has established
the Security Context used to protect the received request; and ii) the Security Context used to protect the received request; and ii)
the time t2 when the request has been received. Since a valid the time t2 when the request has been received. Since a valid
request cannot be older than the Security Context used to protect request cannot be older than the Security Context used to protect
it, the server verifies that (t2 - t1) is less than the largest it, the server verifies that (t2 - t1) is less than the largest
amount of time acceptable to consider the request fresh. amount of time acceptable to consider the request fresh.
o If the server stores an Echo Option value for the pair (gid,kid) * If the server stores an Echo Option value for the pair (gid,kid)
associated to that same client in the same group, the server associated to that same client in the same group, the server
verifies that the option value equals that same stored value verifies that the option value equals that same stored value
previously sent to that client. previously sent to that client.
If the verifications above fail, the server MUST NOT process the If the verifications above fail, the server MUST NOT process the
request further and MAY send a 4.01 (Unauthorized) response including request further and MAY send a 4.01 (Unauthorized) response including
an Echo Option. an Echo Option.
If the verifications above are successful and the Replay Window has If the verifications above are successful and the Replay Window has
not been set yet, the server updates its Replay Window to mark the not been set yet, the server updates its Replay Window to mark the
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The challenge-response approach described in this appendix provides The challenge-response approach described in this appendix provides
an assurance of absolute message freshness. However, it can result an assurance of absolute message freshness. However, it can result
in an impact on performance which is undesirable or unbearable, in an impact on performance which is undesirable or unbearable,
especially in large groups where many endpoints at the same time especially in large groups where many endpoints at the same time
might join as new members or lose synchronization. might join as new members or lose synchronization.
Note that endpoints configured as silent servers are not able to Note that endpoints configured as silent servers are not able to
perform the challenge-response described above, as they do not store perform the challenge-response described above, as they do not store
a Sender Context to secure the 4.01 (Unauthorized) response to the a Sender Context to secure the 4.01 (Unauthorized) response to the
client. Therefore, silent servers should adopt alternative client. Therefore, silent servers should adopt alternative
approaches to achieve and maintain synchronization with sender approaches to achieve and maintain synchronization with Sender
sequence numbers of clients. Sequence Numbers of clients.
Since requests including the Echo Option are sent over unicast, a Since requests including the Echo Option are sent over unicast, a
server can be a victim of the attack discussed in Section 10.7, when server can be a victim of the attack discussed in Section 10.9, when
such requests are protected with the group mode of Group OSCORE, as such requests are protected with the group mode of Group OSCORE, as
described in Section 8.1. described in Section 8.1.
Instead, protecting requests with the Echo Option by using the Instead, protecting requests with the Echo Option by using the
pairwise mode of Group OSCORE as described in Section 9.3 prevents pairwise mode of Group OSCORE as described in Section 9.3 prevents
the attack in Section 10.7. In fact, only the exact server involved the attack in Section 10.9. In fact, only the exact server involved
in the Echo exchange is able to derive the correct pairwise key used in the Echo exchange is able to derive the correct pairwise key used
by the client to protect the request including the Echo Option. by the client to protect the request including the Echo Option.
In either case, an internal on-path adversary would not be able to In either case, an internal on-path adversary would not be able to
mix up the Echo Option value of two different unicast requests, sent mix up the Echo Option value of two different unicast requests, sent
by a same client to any two different servers in the group. In fact, by a same client to any two different servers in the group. In fact,
if the group mode was used, this would require the adversary to forge if the group mode was used, this would require the adversary to forge
the client's countersignature in both such requests. As a the client's countersignature in both such requests. As a
consequence, each of the two servers remains able to selectively consequence, each of the two servers remains able to selectively
accept a request with the Echo Option only if it is waiting for that accept a request with the Echo Option only if it is waiting for that
exact integrity-protected Echo Option value, and is thus the intended exact integrity-protected Echo Option value, and is thus the intended
recipient. recipient.
Appendix F. No Verification of Signatures in Group Mode Appendix F. Document Updates
There are some application scenarios using group communication that
have particularly strict requirements. One example of this is the
requirement of low message latency in non-emergency lighting
applications [I-D.somaraju-ace-multicast]. For those applications
which have tight performance constraints and relaxed security
requirements, it can be inconvenient for some endpoints to verify
digital signatures in order to assert source authenticity of received
messages protected with the group mode. In other cases, the
signature verification can be deferred or only checked for specific
actions. For instance, a command to turn a bulb on where the bulb is
already on does not need the signature to be checked. In such
situations, the counter signature needs to be included anyway as part
of a message protected with the group mode, so that an endpoint that
needs to validate the signature for any reason has the ability to do
so.
In this specification, it is NOT RECOMMENDED that endpoints do not
verify the counter signature of received messages protected with the
group mode. However, it is recognized that there may be situations
where it is not always required. The consequence of not doing the
signature validation in messages protected with the group mode is
that security in the group is based only on the group-authenticity of
the shared keying material used for encryption. That is, endpoints
in the group would have evidence that the received message has been
originated by a group member, although not specifically identifiable
in a secure way. This can violate a number of security requirements,
as the compromise of any element in the group means that the attacker
has the ability to control the entire group. Even worse, the group
may not be limited in scope, and hence the same keying material might
be used not only for light bulbs but for locks as well. Therefore,
extreme care must be taken in situations where the security
requirements are relaxed, so that deployment of the system will
always be done safely.
Appendix G. Example Values with COSE Capabilities
The table below provides examples of values for Counter Signature
Parameters in the Common Context (see Section 2.1.3), for different
values of Counter Signature Algorithm.
+-------------------+---------------------------------------------+
| Counter Signature | Example Values for Counter |
| Algorithm | Signature Parameters |
+-------------------+---------------------------------------------+
| (-8) // EdDSA | [1], [1, 6] // 1: OKP ; 1: OKP, 6: Ed25519 |
| (-8) // EdDSA | [1], [1, 7] // 1: OKP ; 1: OKP, 7: Ed448 |
| (-7) // ES256 | [2], [2, 1] // 2: EC2 ; 2: EC2, 1: P-256 |
| (-35) // ES384 | [2], [2, 2] // 2: EC2 ; 2: EC2, 2: P-384 |
| (-36) // ES512 | [2], [2, 3] // 2: EC2 ; 2: EC2, 3: P-521 |
| (-37) // PS256 | [3], [3] // 3: RSA ; 3: RSA |
| (-38) // PS384 | [3], [3] // 3: RSA ; 3: RSA |
| (-39) // PS512 | [3], [3] // 3: RSA ; 3: RSA |
+-------------------+---------------------------------------------+
Figure 4: Examples of Counter Signature Parameters
The table below provides examples of values for Secret Derivation
Parameters in the Common Context (see Section 2.1.5), for different
values of Secret Derivation Algorithm.
+-----------------------+--------------------------------------------+
| Secret Derivation | Example Values for Secret |
| Algorithm | Derivation Parameters |
+-----------------------+--------------------------------------------+
| (-27) // ECDH-SS | [1], [1, 4] // 1: OKP ; 1: OKP, 4: X25519 |
| // + HKDF-256 | |
| (-27) // ECDH-SS | [1], [1, 5] // 1: OKP ; 1: OKP, 5: X448 |
| // + HKDF-256 | |
| (-27) // ECDH-SS | [2], [2, 1] // 2: EC2 ; 2: EC2, 1: P-256 |
| // + HKDF-256 | |
| (-27) // ECDH-SS | [2], [2, 2] // 2: EC2 ; 2: EC2, 2: P-384 |
| // + HKDF-256 | |
| (-27) // ECDH-SS | [2], [2, 3] // 2: EC2 ; 2: EC2, 3: P-512 |
| // + HKDF-256 | |
+-----------------------+--------------------------------------------+
Figure 5: Examples of Secret Derivation Parameters
Appendix H. Parameter Extensibility for Future COSE Algorithms
As defined in Section 8.1 of [I-D.ietf-cose-rfc8152bis-algs], future
algorithms can be registered in the "COSE Algorithms" Registry
[COSE.Algorithms] as specifying none or multiple COSE capabilities.
To enable the seamless use of such future registered algorithms, this
section defines a general, agile format for parameters of the
Security Context (see Section 2.1.3 and Section 2.1.5) and for
related elements of the external_aad structure (see Section 4.3).
If any of the currently registered COSE algorithms is considered,
using this general format yields the same structure defined in this
document for the items above, thus ensuring retro-compatibility.
H.1. Counter Signature Parameters
The definition of Counter Signature Parameters in the Common Context
(see Section 2.1.3) is generalized as follows.
Counter Signature Parameters is a CBOR array CS_PARAMS including N+1
elements, whose exact structure and value depend on the value of
Counter Signature Algorithm.
o The first element, i.e. CS_PARAMS[0], is the array of the N COSE
capabilities for Counter Signature Algorithm, as specified for
that algorithm in the "Capabilities" column of the "COSE
Algorithms" Registry [COSE.Algorithms] (see Section 8.1 of
[I-D.ietf-cose-rfc8152bis-algs]).
o Each following element CS_PARAMS[i], i.e. with index i > 0, is the
array of COSE capabilities for the algorithm capability specified
in CS_PARAMS[0][i-1].
For example, if CS_PARAMS[0][0] specifies the key type as
capability of the algorithm, then CS_PARAMS[1] is the array of
COSE capabilities for the COSE key type associated to Counter
Signature Algorithm, as specified for that key type in the
"Capabilities" column of the "COSE Key Types" Registry
[COSE.Key.Types] (see Section 8.2 of
[I-D.ietf-cose-rfc8152bis-algs]).
H.2. Secret Derivation Parameters
The definition of Secret Derivation Parameters in the Common Context RFC EDITOR: PLEASE REMOVE THIS SECTION.
(see Section 2.1.5) is generalized as follows.
Secret Derivation Parameters is a CBOR array SD_PARAMS including N+1 F.1. Version -11 to -12
elements, whose exact structure and value depend on the value of
Secret Derivation Algorithm.
o The first element, i.e. SD_PARAMS[0], is the array of the N COSE * No mode of operation is mandatory to support.
capabilities for Secret Derivation Algorithm, as specified for
that algorithm in the "Capabilities" column of the "COSE
Algorithms" Registry [COSE.Algorithms] (see Section 8.1 of
[I-D.ietf-cose-rfc8152bis-algs]).
o Each following element SD_PARAMS[i], i.e. with index i > 0, is the * Revised parameters of the Security Context, COSE object and
array of COSE capabilities for the algorithm capability specified external_aad.
in SD_PARAMS[0][i-1].
For example, if SD_PARAMS[0][0] specifies the key type as * Revised management of keying material for the Group Manager.
capability of the algorithm, then SD_PARAMS[1] is the array of
COSE capabilities for the COSE key type associated to Secret
Derivation Algorithm, as specified for that key type in the
"Capabilities" column of the "COSE Key Types" Registry
[COSE.Key.Types] (see Section 8.2 of
[I-D.ietf-cose-rfc8152bis-algs]).
H.3. 'par_countersign' in the external_aad * Informing of former members when rekeying the group.
The definition of the 'par_countersign' element in the 'algorithms' * Admit encryption-only algorithms in group mode.
array of the external_aad structure (see Section 4.3) is generalized
as follows.
The 'par_countersign' element takes the CBOR array CS_PARAMS * Encrypted countersignature through a keystream.
specified by Counter Signature Parameters in the Common Context (see
Section 2.1.3), considering the format generalization in Appendix H.
In particular:
o The first element 'countersign_alg_capab' is the array of COSE * Added public key of the Group Manager as key material and
capabilities for the countersignature algorithm indicated in protected data.
'alg_countersign'. This is CS_PARAMS[0], i.e. the first element
of the CBOR array CS_PARAMS specified by Counter Signature
Parameters in the Common Context.
o Each following element 'countersign_capab_i' (i = 1, ..., N) is * Clarifications about message processing, especially notifications.
the array of COSE capabilities for the algorithm capability
specified in 'countersign_alg_capab'[i-1]. This algorithm
capability is the element CS_PARAMS[0][i-1] of the CBOR array
CS_PARAMS specified by Counter Signature Parameters in the Common
Context.
For example, if 'countersign_alg_capab'[i-1] specifies the key * Guidance for message processing of external signature checkers.
type as capability of the algorithm, then 'countersign_capab_i' is
the array of COSE capabilities for the COSE key type associated to
Counter Signature Algorithm, as specified for that key type in the
"Capabilities" column of the "COSE Key Types" Registry
[COSE.Key.Types] (see Section 8.2 of
[I-D.ietf-cose-rfc8152bis-algs]).
external_aad = bstr .cbor aad_array * Updated derivation of pairwise keys, with more security
considerations.
aad_array = [ * Termination of ongoing observations as client, upon leaving or
oscore_version : uint, before re-joining the group.
algorithms : [alg_aead : int / tstr,
alg_countersign : int / tstr,
par_countersign : [countersign_alg_capab,
countersign_capab_1,
countersign_capab_2,
...,
countersign__capab_N]],
request_kid : bstr,
request_piv : bstr,
options : bstr,
request_kid_context : bstr,
OSCORE_option: bstr
]
countersign_alg_capab : [c_1 : any, c_2 : any, ..., c_N : any] * Recycling Group IDs by tracking the "Birth Gid" of each group
member.
Figure 6: external_aad with general 'par_countersign' * Expanded security and privacy considerations about the group mode.
Appendix I. Document Updates * Removed appendices on skipping signature verification and on COSE
capabilities.
RFC EDITOR: PLEASE REMOVE THIS SECTION. * Fixes and editorial improvements.
I.1. Version -10 to -11 F.2. Version -10 to -11
o Loss of Recipient Contexts due to their overflow. * Loss of Recipient Contexts due to their overflow.
o Added diagram on keying material components and their relation. * Added diagram on keying material components and their relation.
o Distinction between anti-replay and freshness. * Distinction between anti-replay and freshness.
o Preservation of Sender IDs over rekeying. * Preservation of Sender IDs over rekeying.
o Clearer cause-effect about reset of SSN. * Clearer cause-effect about reset of SSN.
o The GM provides public keys of group members with associated * The GM provides public keys of group members with associated
Sender IDs. Sender IDs.
o Removed 'par_countersign_key' from the external_aad. * Removed 'par_countersign_key' from the external_aad.
o One single format for the external_aad, both for encryption and * One single format for the external_aad, both for encryption and
signing. signing.
o Presence of 'kid' in responses to requests protected with the * Presence of 'kid' in responses to requests protected with the
pairwise mode. pairwise mode.
o Inclusion of 'kid_context' in notifications following a group * Inclusion of 'kid_context' in notifications following a group
rekeying. rekeying.
o Pairwise mode presented with OSCORE as baseline. * Pairwise mode presented with OSCORE as baseline.
o Revised examples with signature values. * Revised examples with signature values.
o Decoupled growth of clients' Sender Sequence Numbers and loss of * Decoupled growth of clients' Sender Sequence Numbers and loss of
synchronization for server. synchronization for server.
o Sender IDs not recycled in the group under the same Gid. * Sender IDs not recycled in the group under the same Gid.
o Processing and description of the Group Flag bit in the OSCORE * Processing and description of the Group Flag bit in the OSCORE
option. option.
o Usage of the pairwise mode for multicast requests. * Usage of the pairwise mode for multicast requests.
o Clarifications on synchronization using the Echo option. * Clarifications on synchronization using the Echo option.
o General format of context parameters and external_aad elements, * General format of context parameters and external_aad elements,
supporting future registered COSE algorithms (new Appendix). supporting future registered COSE algorithms (new Appendix).
o Fixes and editorial improvements. * Fixes and editorial improvements.
I.2. Version -09 to -10 F.3. Version -09 to -10
o Removed 'Counter Signature Key Parameters' from the Common * Removed 'Counter Signature Key Parameters' from the Common
Context. Context.
o New parameters in the Common Context covering the DH secret * New parameters in the Common Context covering the DH secret
derivation. derivation.
o New counter signature header parameter from draft-ietf-cose- * New countersignature header parameter from draft-ietf-cose-
countersign. countersign.
o Stronger policies non non-recycling of Sender IDs and Gid. * Stronger policies non non-recycling of Sender IDs and Gid.
o The Sender Sequence Number is reset when establishing a new * The Sender Sequence Number is reset when establishing a new
Security Context. Security Context.
o Added 'request_kid_context' in the aad_array. * Added 'request_kid_context' in the aad_array.
o The server can respond with 5.03 if the client's public key is not * The server can respond with 5.03 if the client's public key is not
available. available.
o The observer client stores an invariant identifier of the group. * The observer client stores an invariant identifier of the group.
o Relaxed storing of original 'kid' for observer clients. * Relaxed storing of original 'kid' for observer clients.
o Both client and server store the 'kid_context' of the original * Both client and server store the 'kid_context' of the original
observation request. observation request.
o The server uses a fresh PIV if protecting the response with a * The server uses a fresh PIV if protecting the response with a
Security Context different from the one used to protect the Security Context different from the one used to protect the
request. request.
o Clarifications on MTI algorithms and curves. * Clarifications on MTI algorithms and curves.
o Removed optimized requests. * Removed optimized requests.
o Overall clarifications and editorial revision. * Overall clarifications and editorial revision.
I.3. Version -08 to -09 F.4. Version -08 to -09
o Pairwise keys are discarded after group rekeying. * Pairwise keys are discarded after group rekeying.
o Signature mode renamed to group mode. * Signature mode renamed to group mode.
o The parameters for countersignatures use the updated COSE * The parameters for countersignatures use the updated COSE
registries. Newly defined IANA registries have been removed. registries. Newly defined IANA registries have been removed.
o Pairwise Flag bit renamed as Group Flag bit, set to 1 in group * Pairwise Flag bit renamed as Group Flag bit, set to 1 in group
mode and set to 0 in pairwise mode. mode and set to 0 in pairwise mode.
o Dedicated section on updating the Security Context. * Dedicated section on updating the Security Context.
o By default, sender sequence numbers and replay windows are not * By default, sender sequence numbers and replay windows are not
reset upon group rekeying. reset upon group rekeying.
o An endpoint implementing only a silent server does not support the * An endpoint implementing only a silent server does not support the
pairwise mode. pairwise mode.
o Separate section on general message reception. * Separate section on general message reception.
o Pairwise mode moved to the document body. * Pairwise mode moved to the document body.
o Considerations on using the pairwise mode in non-multicast * Considerations on using the pairwise mode in non-multicast
settings. settings.
o Optimized requests are moved as an appendix. * Optimized requests are moved as an appendix.
o Normative support for the signature and pairwise mode. * Normative support for the signature and pairwise mode.
o Revised methods for synchronization with clients' sender sequence * Revised methods for synchronization with clients' sender sequence
number. number.
o Appendix with example values of parameters for countersignatures. * Appendix with example values of parameters for countersignatures.
o Clarifications and editorial improvements. * Clarifications and editorial improvements.
I.4. Version -07 to -08 F.5. Version -07 to -08
o Clarified relation between pairwise mode and group communication * Clarified relation between pairwise mode and group communication
(Section 1). (Section 1).
o Improved definition of "silent server" (Section 1.1). * Improved definition of "silent server" (Section 1.1).
o Clarified when a Recipient Context is needed (Section 2). * Clarified when a Recipient Context is needed (Section 2).
o Signature checkers as entities supported by the Group Manager * Signature checkers as entities supported by the Group Manager
(Section 2.3). (Section 2.3).
o Clarified that the Group Manager is under exclusive control of Gid * Clarified that the Group Manager is under exclusive control of Gid
and Sender ID values in a group, with Sender ID values under each and Sender ID values in a group, with Sender ID values under each
Gid value (Section 2.3). Gid value (Section 2.3).
o Mitigation policies in case of recycled 'kid' values * Mitigation policies in case of recycled 'kid' values
(Section 2.4). (Section 2.4).
o More generic exhaustion (not necessarily wrap-around) of sender * More generic exhaustion (not necessarily wrap-around) of sender
sequence numbers (Sections 2.5 and 10.11). sequence numbers (Sections 2.5 and 10.11).
o Pairwise key considerations, as to group rekeying and Sender * Pairwise key considerations, as to group rekeying and Sender
Sequence Numbers (Section 3). Sequence Numbers (Section 3).
o Added reference to static-static Diffie-Hellman shared secret * Added reference to static-static Diffie-Hellman shared secret
(Section 3). (Section 3).
o Note for implementation about the external_aad for signing * Note for implementation about the external_aad for signing
(Sectino 4.3.2). (Sectino 4.3.2).
o Retransmission by the application for group requests over * Retransmission by the application for group requests over
multicast as Non-Confirmable (Section 7). multicast as Non-Confirmable (Section 7).
o A server MUST use its own Partial IV in a response, if protecting * A server MUST use its own Partial IV in a response, if protecting
it with a different context than the one used for the request it with a different context than the one used for the request
(Section 7.3). (Section 7.3).
o Security considerations: encryption of pairwise mode as * Security considerations: encryption of pairwise mode as
alternative to group-level security (Section 10.1). alternative to group-level security (Section 10.1).
o Security considerations: added approach to reduce the chance of * Security considerations: added approach to reduce the chance of
global collisions of Gid values from different Group Managers global collisions of Gid values from different Group Managers
(Section 10.5). (Section 10.5).
o Security considerations: added implications for block-wise * Security considerations: added implications for block-wise
transfers when using the signature mode for requests over unicast transfers when using the signature mode for requests over unicast
(Section 10.7). (Section 10.7).
o Security considerations: (multiple) supported signature algorithms * Security considerations: (multiple) supported signature algorithms
(Section 10.13). (Section 10.13).
o Security considerations: added privacy considerations on the * Security considerations: added privacy considerations on the
approach for reducing global collisions of Gid values approach for reducing global collisions of Gid values
(Section 10.15). (Section 10.15).
o Updates to the methods for synchronizing with clients' sequence * Updates to the methods for synchronizing with clients' sequence
number (Appendix E). number (Appendix E).
o Simplified text on discovery services supporting the pairwise mode * Simplified text on discovery services supporting the pairwise mode
(Appendix G.1). (Appendix G.1).
o Editorial improvements. * Editorial improvements.
I.5. Version -06 to -07 F.6. Version -06 to -07
o Updated abstract and introduction. * Updated abstract and introduction.
o Clarifications of what pertains a group rekeying. * Clarifications of what pertains a group rekeying.
o Derivation of pairwise keying material. * Derivation of pairwise keying material.
o Content re-organization for COSE Object and OSCORE header * Content re-organization for COSE Object and OSCORE header
compression. compression.
o Defined the Pairwise Flag bit for the OSCORE option. * Defined the Pairwise Flag bit for the OSCORE option.
o Supporting CoAP Observe for group requests and responses. * Supporting CoAP Observe for group requests and responses.
o Considerations on message protection across switching to new * Considerations on message protection across switching to new
keying material. keying material.
o New optimized mode based on pairwise keying material. * New optimized mode based on pairwise keying material.
o More considerations on replay protection and Security Contexts * More considerations on replay protection and Security Contexts
upon key renewal. upon key renewal.
o Security considerations on Group OSCORE for unicast requests, also * Security considerations on Group OSCORE for unicast requests, also
as affecting the usage of the Echo option. as affecting the usage of the Echo option.
o Clarification on different types of groups considered * Clarification on different types of groups considered
(application/security/CoAP). (application/security/CoAP).
o New pairwise mode, using pairwise keying material for both * New pairwise mode, using pairwise keying material for both
requests and responses. requests and responses.
I.6. Version -05 to -06 F.7. Version -05 to -06
o Group IDs mandated to be unique under the same Group Manager. * Group IDs mandated to be unique under the same Group Manager.
o Clarifications on parameter update upon group rekeying. * Clarifications on parameter update upon group rekeying.
o Updated external_aad structures. * Updated external_aad structures.
o Dynamic derivation of Recipient Contexts made optional and * Dynamic derivation of Recipient Contexts made optional and
application specific. application specific.
o Optional 4.00 response for failed signature verification on the * Optional 4.00 response for failed signature verification on the
server. server.
o Removed client handling of duplicated responses to multicast * Removed client handling of duplicated responses to multicast
requests. requests.
o Additional considerations on public key retrieval and group * Additional considerations on public key retrieval and group
rekeying. rekeying.
o Added Group Manager responsibility on validating public keys. * Added Group Manager responsibility on validating public keys.
o Updates IANA registries. * Updates IANA registries.
o Reference to RFC 8613. * Reference to RFC 8613.
o Editorial improvements. * Editorial improvements.
I.7. Version -04 to -05 F.8. Version -04 to -05
o Added references to draft-dijk-core-groupcomm-bis. * Added references to draft-dijk-core-groupcomm-bis.
o New parameter Counter Signature Key Parameters (Section 2). * New parameter Counter Signature Key Parameters (Section 2).
o Clarification about Recipient Contexts (Section 2). * Clarification about Recipient Contexts (Section 2).
o Two different external_aad for encrypting and signing * Two different external_aad for encrypting and signing
(Section 3.1). (Section 3.1).
o Updated response verification to handle Observe notifications * Updated response verification to handle Observe notifications
(Section 6.4). (Section 6.4).
o Extended Security Considerations (Section 8). * Extended Security Considerations (Section 8).
o New "Counter Signature Key Parameters" IANA Registry * New "Counter Signature Key Parameters" IANA Registry
(Section 9.2). (Section 9.2).
I.8. Version -03 to -04 F.9. Version -03 to -04
o Added the new "Counter Signature Parameters" in the Common Context * Added the new "Counter Signature Parameters" in the Common Context
(see Section 2). (see Section 2).
o Added recommendation on using "deterministic ECDSA" if ECDSA is * Added recommendation on using "deterministic ECDSA" if ECDSA is
used as counter signature algorithm (see Section 2). used as countersignature algorithm (see Section 2).
o Clarified possible asynchronous retrieval of keying material from * Clarified possible asynchronous retrieval of keying material from
the Group Manager, in order to process incoming messages (see the Group Manager, in order to process incoming messages (see
Section 2). Section 2).
o Structured Section 3 into subsections. * Structured Section 3 into subsections.
o Added the new 'par_countersign' to the aad_array of the * Added the new 'par_countersign' to the aad_array of the
external_aad (see Section 3.1). external_aad (see Section 3.1).
o Clarified non reliability of 'kid' as identity indicator for a * Clarified non reliability of 'kid' as identity indicator for a
group member (see Section 2.1). group member (see Section 2.1).
o Described possible provisioning of new Sender ID in case of * Described possible provisioning of new Sender ID in case of
Partial IV wrap-around (see Section 2.2). Partial IV wrap-around (see Section 2.2).
o The former signature bit in the Flag Byte of the OSCORE option * The former signature bit in the Flag Byte of the OSCORE option
value is reverted to reserved (see Section 4.1). value is reverted to reserved (see Section 4.1).
o Updated examples of compressed COSE object, now with the sixth * Updated examples of compressed COSE object, now with the sixth
less significant bit in the Flag Byte of the OSCORE option value less significant bit in the Flag Byte of the OSCORE option value
set to 0 (see Section 4.3). set to 0 (see Section 4.3).
o Relaxed statements on sending error messages (see Section 6). * Relaxed statements on sending error messages (see Section 6).
o Added explicit step on computing the counter signature for * Added explicit step on computing the countersignature for outgoing
outgoing messages (see Sections 6.1 and 6.3). messages (see Sections 6.1 and 6.3).
o Handling of just created Recipient Contexts in case of * Handling of just created Recipient Contexts in case of
unsuccessful message verification (see Sections 6.2 and 6.4). unsuccessful message verification (see Sections 6.2 and 6.4).
o Handling of replied/repeated responses on the client (see * Handling of replied/repeated responses on the client (see
Section 6.4). Section 6.4).
o New IANA Registry "Counter Signature Parameters" (see * New IANA Registry "Counter Signature Parameters" (see
Section 9.1). Section 9.1).
I.9. Version -02 to -03 F.10. Version -02 to -03
o Revised structure and phrasing for improved readability and better * Revised structure and phrasing for improved readability and better
alignment with draft-ietf-core-object-security. alignment with draft-ietf-core-object-security.
o Added discussion on wrap-Around of Partial IVs (see Section 2.2). * Added discussion on wrap-Around of Partial IVs (see Section 2.2).
o Separate sections for the COSE Object (Section 3) and the OSCORE * Separate sections for the COSE Object (Section 3) and the OSCORE
Header Compression (Section 4). Header Compression (Section 4).
o The countersignature is now appended to the encrypted payload of * The countersignature is now appended to the encrypted payload of
the OSCORE message, rather than included in the OSCORE Option (see the OSCORE message, rather than included in the OSCORE Option (see
Section 4). Section 4).
o Extended scope of Section 5, now titled " Message Binding, * Extended scope of Section 5, now titled " Message Binding,
Sequence Numbers, Freshness and Replay Protection".