draft-ietf-core-oscore-groupcomm-09.txt   draft-ietf-core-oscore-groupcomm-10.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: December 25, 2020 F. Palombini Expires: May 6, 2021 F. Palombini
Ericsson AB Ericsson AB
J. Park J. Park
Universitaet Duisburg-Essen Universitaet Duisburg-Essen
June 23, 2020 November 02, 2020
Group OSCORE - Secure Group Communication for CoAP Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-09 draft-ietf-core-oscore-groupcomm-10
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.
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Copyright Notice Copyright Notice
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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 . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Common Context . . . . . . . . . . . . . . . . . . . . . 8 2.1. Common Context . . . . . . . . . . . . . . . . . . . . . 9
2.1.1. ID Context . . . . . . . . . . . . . . . . . . . . . 8 2.1.1. ID Context . . . . . . . . . . . . . . . . . . . . . 9
2.1.2. Counter Signature Algorithm . . . . . . . . . . . . . 9 2.1.2. Counter Signature Algorithm . . . . . . . . . . . . . 9
2.1.3. Counter Signature Parameters . . . . . . . . . . . . 9 2.1.3. Counter Signature Parameters . . . . . . . . . . . . 9
2.1.4. Counter Signature Key Parameters . . . . . . . . . . 10 2.1.4. Secret Derivation Algorithm . . . . . . . . . . . . . 10
2.2. Sender Context and Recipient Context . . . . . . . . . . 10 2.1.5. Secret Derivation Parameters . . . . . . . . . . . . 10
2.3. Pairwise Keys . . . . . . . . . . . . . . . . . . . . . . 11 2.2. Sender Context and Recipient Context . . . . . . . . . . 11
2.3.1. Derivation of Pairwise Keys . . . . . . . . . . . . . 11 2.3. Pairwise Keys . . . . . . . . . . . . . . . . . . . . . . 12
2.3.2. Usage of Sequence Numbers . . . . . . . . . . . . . . 12 2.3.1. Derivation of Pairwise Keys . . . . . . . . . . . . . 12
2.3.3. Security Context for Pairwise Mode . . . . . . . . . 12 2.3.2. Usage of Sequence Numbers . . . . . . . . . . . . . . 13
2.4. Update of Security Context . . . . . . . . . . . . . . . 13 2.3.3. Security Context for Pairwise Mode . . . . . . . . . 13
2.4.1. Loss of Mutable Security Context . . . . . . . . . . 13 2.4. Update of Security Context . . . . . . . . . . . . . . . 14
2.4.2. Exhaustion of Sender Sequence Numbers . . . . . . . . 13 2.4.1. Loss of Mutable Security Context . . . . . . . . . . 14
2.4.3. Retrieving New Security Context Parameters . . . . . 14 2.4.2. Exhaustion of Sender Sequence Number . . . . . . . . 15
3. The Group Manager . . . . . . . . . . . . . . . . . . . . . . 15 2.4.3. Retrieving New Security Context Parameters . . . . . 16
3.1. Management of Group Keying Material . . . . . . . . . . . 16 3. The Group Manager . . . . . . . . . . . . . . . . . . . . . . 18
3.2. Responsibilities of the Group Manager . . . . . . . . . . 17 3.1. Management of Group Keying Material . . . . . . . . . . . 19
4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 18 3.2. Responsibilities of the Group Manager . . . . . . . . . . 20
4.1. Counter Signature . . . . . . . . . . . . . . . . . . . . 18 4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 21
4.2. The 'kid' and 'kid context' parameters . . . . . . . . . 19 4.1. Counter Signature . . . . . . . . . . . . . . . . . . . . 21
4.3. external_aad . . . . . . . . . . . . . . . . . . . . . . 19 4.2. The 'kid' and 'kid context' parameters . . . . . . . . . 21
4.3.1. external_aad for Encryption . . . . . . . . . . . . . 19 4.3. external_aad . . . . . . . . . . . . . . . . . . . . . . 22
4.3.2. external_aad for Signing . . . . . . . . . . . . . . 20 4.3.1. external_aad for Encryption . . . . . . . . . . . . . 22
5. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 21 4.3.2. external_aad for Signing . . . . . . . . . . . . . . 23
5.1. Examples of Compressed COSE Objects . . . . . . . . . . . 21 5. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 24
5.1.1. Examples in Group Mode . . . . . . . . . . . . . . . 22 5.1. Examples of Compressed COSE Objects . . . . . . . . . . . 25
5.1.2. Examples in Pairwise Mode . . . . . . . . . . . . . . 23 5.1.1. Examples in Group Mode . . . . . . . . . . . . . . . 25
5.1.2. Examples in Pairwise Mode . . . . . . . . . . . . . . 26
6. Message Binding, Sequence Numbers, Freshness and Replay 6. Message Binding, Sequence Numbers, Freshness and Replay
Protection . . . . . . . . . . . . . . . . . . . . . . . . . 24 Protection . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.1. Update of Replay Window . . . . . . . . . . . . . . . . . 24 6.1. Update of Replay Window . . . . . . . . . . . . . . . . . 27
7. Message Reception . . . . . . . . . . . . . . . . . . . . . . 24 7. Message Reception . . . . . . . . . . . . . . . . . . . . . . 28
8. Message Processing in Group Mode . . . . . . . . . . . . . . 25 8. Message Processing in Group Mode . . . . . . . . . . . . . . 29
8.1. Protecting the Request . . . . . . . . . . . . . . . . . 26 8.1. Protecting the Request . . . . . . . . . . . . . . . . . 29
8.1.1. Supporting Observe . . . . . . . . . . . . . . . . . 26 8.1.1. Supporting Observe . . . . . . . . . . . . . . . . . 30
8.2. Verifying the Request . . . . . . . . . . . . . . . . . . 26 8.2. Verifying the Request . . . . . . . . . . . . . . . . . . 31
8.2.1. Supporting Observe . . . . . . . . . . . . . . . . . 27 8.2.1. Supporting Observe . . . . . . . . . . . . . . . . . 32
8.3. Protecting the Response . . . . . . . . . . . . . . . . . 32
8.3. Protecting the Response . . . . . . . . . . . . . . . . . 28 8.3.1. Supporting Observe . . . . . . . . . . . . . . . . . 33
8.3.1. Supporting Observe . . . . . . . . . . . . . . . . . 28 8.4. Verifying the Response . . . . . . . . . . . . . . . . . 34
8.4. Verifying the Response . . . . . . . . . . . . . . . . . 29 8.4.1. Supporting Observe . . . . . . . . . . . . . . . . . 34
8.4.1. Supporting Observe . . . . . . . . . . . . . . . . . 30 9. Message Processing in Pairwise Mode . . . . . . . . . . . . . 35
9. Message Processing in Pairwise Mode . . . . . . . . . . . . . 30 9.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 36
9.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 31 9.2. Protecting the Request . . . . . . . . . . . . . . . . . 36
9.2. Protecting the Request . . . . . . . . . . . . . . . . . 31 9.3. Verifying the Request . . . . . . . . . . . . . . . . . . 37
9.3. Verifying the Request . . . . . . . . . . . . . . . . . . 32 9.4. Protecting the Response . . . . . . . . . . . . . . . . . 37
9.4. Protecting the Response . . . . . . . . . . . . . . . . . 32 9.5. Verifying the Response . . . . . . . . . . . . . . . . . 38
9.5. Verifying the Response . . . . . . . . . . . . . . . . . 33 10. Security Considerations . . . . . . . . . . . . . . . . . . . 38
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33 10.1. Group-level Security . . . . . . . . . . . . . . . . . . 39
10.1. Group-level Security . . . . . . . . . . . . . . . . . . 34 10.2. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 40
10.2. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 35 10.3. Management of Group Keying Material . . . . . . . . . . 40
10.3. Management of Group Keying Material . . . . . . . . . . 35 10.4. Update of Security Context and Key Rotation . . . . . . 41
10.4. Update of Security Context and Key Rotation . . . . . . 36 10.4.1. Late Update on the Sender . . . . . . . . . . . . . 41
10.4.1. Late Update on the Sender . . . . . . . . . . . . . 36 10.4.2. Late Update on the Recipient . . . . . . . . . . . . 42
10.4.2. Late Update on the Recipient . . . . . . . . . . . . 37 10.5. Collision of Group Identifiers . . . . . . . . . . . . . 42
10.5. Collision of Group Identifiers . . . . . . . . . . . . . 37 10.6. Cross-group Message Injection . . . . . . . . . . . . . 43
10.6. Cross-group Message Injection . . . . . . . . . . . . . 38 10.6.1. Attack Description . . . . . . . . . . . . . . . . . 43
10.6.1. Attack Description . . . . . . . . . . . . . . . . . 38 10.6.2. Attack Prevention in Group Mode . . . . . . . . . . 44
10.6.2. Attack Prevention in Group Mode . . . . . . . . . . 39 10.7. Group OSCORE for Unicast Requests . . . . . . . . . . . 45
10.7. Group OSCORE for Unicast Requests . . . . . . . . . . . 40 10.8. End-to-end Protection . . . . . . . . . . . . . . . . . 46
10.8. End-to-end Protection . . . . . . . . . . . . . . . . . 41 10.9. Master Secret . . . . . . . . . . . . . . . . . . . . . 46
10.9. Master Secret . . . . . . . . . . . . . . . . . . . . . 41 10.10. Replay Protection . . . . . . . . . . . . . . . . . . . 47
10.10. Replay Protection . . . . . . . . . . . . . . . . . . . 42 10.11. Client Aliveness . . . . . . . . . . . . . . . . . . . . 48
10.11. Client Aliveness . . . . . . . . . . . . . . . . . . . . 43 10.12. Cryptographic Considerations . . . . . . . . . . . . . . 48
10.12. Cryptographic Considerations . . . . . . . . . . . . . . 43 10.13. Message Segmentation . . . . . . . . . . . . . . . . . . 49
10.13. Message Segmentation . . . . . . . . . . . . . . . . . . 44 10.14. Privacy Considerations . . . . . . . . . . . . . . . . . 49
10.14. Privacy Considerations . . . . . . . . . . . . . . . . . 44 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45 11.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . 50
11.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . 45 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 45 12.1. Normative References . . . . . . . . . . . . . . . . . . 50
12.1. Normative References . . . . . . . . . . . . . . . . . . 45 12.2. Informative References . . . . . . . . . . . . . . . . . 52
12.2. Informative References . . . . . . . . . . . . . . . . . 47 Appendix A. Assumptions and Security Objectives . . . . . . . . 54
Appendix A. Assumptions and Security Objectives . . . . . . . . 49 A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 55
A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 49 A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 56
A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 51 Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 57
Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 52 Appendix C. Example of Group Identifier Format . . . . . . . . . 60
Appendix C. Example of Group Identifier Format . . . . . . . . . 54 Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 60
Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 55 Appendix E. Examples of Synchronization Approaches . . . . . . . 61
Appendix E. Examples of Synchronization Approaches . . . . . . . 56 E.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 61
E.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 56 E.2. Baseline Synchronization . . . . . . . . . . . . . . . . 62
E.2. Baseline Synchronization . . . . . . . . . . . . . . . . 56 E.3. Challenge-Response Synchronization . . . . . . . . . . . 62
E.3. Challenge-Response Synchronization . . . . . . . . . . . 57 Appendix F. No Verification of Signatures in Group Mode . . . . 65
Appendix F. No Verification of Signatures in Group Mode . . . . 60 Appendix G. Example Values with COSE Capabilities . . . . . . . 66
Appendix G. Optimized Request . . . . . . . . . . . . . . . . . 60 Appendix H. Document Updates . . . . . . . . . . . . . . . . . . 67
Appendix H. Example Values of Parameters for Countersignatures . 61 H.1. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 67
Appendix I. Document Updates . . . . . . . . . . . . . . . . . . 61 H.2. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 68
I.1. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 62 H.3. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 69
I.2. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 62 H.4. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 70
I.3. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 64 H.5. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 71
I.4. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 64 H.6. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 72
I.5. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 65 H.7. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 72
I.6. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 65 H.8. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 73
I.7. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 66 H.9. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 74
I.8. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 67 H.10. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 74
I.9. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 68 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 75
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 75
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 69
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
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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.
Just like OSCORE, Group OSCORE is independent of transport layer and Just like OSCORE, Group OSCORE is independent of transport layer and
works wherever CoAP does. Group communication for CoAP works wherever CoAP does. Group communication for CoAP
[I-D.ietf-core-groupcomm-bis] uses UDP/IP multicast as the underlying [I-D.ietf-core-groupcomm-bis] uses UDP/IP multicast as the underlying
data transport. 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 [RFC6347], between one client transport layer security, such as DTLS
and one proxy (and vice versa), or between one proxy and one server [RFC6347][I-D.ietf-tls-dtls13], between one client and one proxy (and
(and vice versa), in order to protect the routing information of vice versa), or between one proxy and one server (and vice versa), in
packets from observers. Note that DTLS [RFC6347] does not define how order to protect the routing information of packets from observers.
to secure messages sent over IP multicast. Note that DTLS does not define how to secure messages sent over IP
multicast.
Group OSCORE defines two modes of operation: Group OSCORE defines two modes of operation:
o In the group mode, Group OSCORE requests and responses are o 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 algorithms as well as signature
verification by intermediaries. This mode is defined in Section 8 verification by intermediaries. This mode is defined in Section 8
and MUST be supported. and MUST be supported.
o In the pairwise mode, two group members exchange Group OSCORE o In the pairwise mode, two group members exchange Group OSCORE
requests and responses over unicast, and the messages are requests and responses 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 OPTIONAL tags and therefore lower message overhead. This mode is defined
to support as defined in Section 9. in Section 9 and is OPTIONAL to support.
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 decision 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 this significantly reduces the overhead in case of many mode. Since such responses convey shorter integrity tags instead of
responses to one request. bigger, full-fledged signatures, this significantly reduces the
message overhead in case of many responses to one request.
A special deployment of Group OSCORE is to use pairwise mode only. A special deployment of Group OSCORE is to use pairwise mode only.
For example, consider the case of a constrained-node network For example, consider the case of a constrained-node network
[RFC7228] with a large number of CoAP endpoints and the objective to [RFC7228] with a large number of CoAP endpoints and the objective to
establish secure communication between any pair of endpoints with a establish secure communication between any pair of endpoints with a
small provisioning effort and message overhead. Since the total small provisioning effort and message overhead. Since the total
number of security associations that needs to be established grows number of security associations that needs to be established grows
with the square of the number of nodes, it is desirable to restrict with the square of the number of nodes, it is desirable to restrict
the provisioned keying material. Moreover, a key establishment the provisioned keying material. Moreover, a key establishment
protocol would need to be executed for each security association. protocol would need to be executed for each security association.
One solution to this is to deploy Group OSCORE, with the endpoints
One solution to this is to deploy Group OSCORE with the endpoints being part of a group, and use the pairwise mode. This solution
being part of a group and use the pairwise mode. This solution
assumes a trusted third party called Group Manager (see Section 3), assumes a trusted third party called Group Manager (see Section 3),
but has the benefit of restricting the symmetric keying material but has the benefit of restricting the symmetric keying material
while distributing only the public key of each group member. After while distributing only the public key of each group member. After
that, a CoAP endpoint can locally derive the OSCORE Security Context that, a CoAP endpoint can locally derive the OSCORE Security Context
for the other endpoint and protect the CoAP communication with very for the other endpoint in the group, and protect CoAP communications
low overhead [I-D.ietf-lwig-security-protocol-comparison]. with very low overhead [I-D.ietf-lwig-security-protocol-comparison].
1.1. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"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]; COSE and counter signatures [I-D.ietf-core-groupcomm-bis]; CBOR [I-D.ietf-cbor-7049bis]; COSE
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs]. [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
related counter signatures [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.
skipping to change at page 7, line 15 skipping to change at page 7, line 18
o Silent server: member of a group that never sends protected o 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 pairwise mode. not have a Sender ID, they cannot support the pairwise mode.
o Group Identifier (Gid): identifier assigned to the group, unique o 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 o 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 o 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
skipping to change at page 7, line 37 skipping to change at page 7, line 40
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 specification refers to a group as a set of endpoints sharing
keying material and security parameters for executing the Group keying material and security parameters for executing the Group
OSCORE protocol (see Section 1.1). Each endpoint which is member of OSCORE protocol (see Section 1.1). Each endpoint which is member of
a group maintains a Security Context as defined in Section 3 of a group maintains a Security Context as defined in Section 3 of
[RFC8613], extended as follows (see Figure 1): [RFC8613], extended as follows (see Figure 1):
o One Common Context, shared by all the endpoints in the group. o One Common Context, shared by all the endpoints in the group. Two
Three new parameters are included in the Common Context: Counter new parameters are included in the Common Context, namely Counter
Signature Algorithm, Counter Signature Parameters and Counter Signature Algorithm and Counter Signature Parameters. These
Signature Key Parameters, which all relate to the signature of the relate to the computation of counter signatures, when messages are
message included in group mode (see Section 8). protected using the group mode (see Section 8).
If the pairwise mode is supported, the Common Context is further
extended with two new parameters, namely Secret Derivation
Algorithm and Secret Derivation Parameters. These relate to the
derivation of a static-static Diffie-Hellman shared secret, from
which pairwise keys are derived (see Section 2.3.1) to protect
messages with the pairwise mode (see Section 9).
o One Sender Context, extended with the endpoint's private key. The o One Sender Context, extended with the endpoint's private key. The
private key is used to sign the message in group mode, and for private key is used to sign the message in group mode, and for
calculating the pairwise keys in pairwise mode (see Section 2.3). deriving the pairwise keys in pairwise mode (see Section 2.3). If
If the pairwise mode is supported, the Sender Context is also the pairwise mode is supported, the Sender Context is also
extended with the Pairwise Sender Keys associated to the other extended with the Pairwise Sender Keys associated to the other
endpoints (see Section 2.3). The Sender Context is omitted if the endpoints (see Section 2.3). The Sender Context is omitted if the
endpoint is configured exclusively as silent server. endpoint is configured exclusively as silent server.
o One Recipient Context for each endpoint from which messages are o 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 calculating 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 2.3). If the pairwise mode is supported, then the
Recipient Context is also extended with the Pairwise Recipient Key Recipient Context is also extended with the Pairwise Recipient Key
associated to the other endpoint (see Section 2.3). associated to the other endpoint (see Section 2.3).
+-------------------+-----------------------------------------------+ +-------------------+-----------------------------------------------+
| Context Component | New Information Elements | | Context Component | New Information Elements |
+-------------------+-----------------------------------------------+ +-------------------+-----------------------------------------------+
| | Counter Signature Algorithm | | Common Context | Counter Signature Algorithm |
| Common Context | Counter Signature Parameters | | | Counter Signature Parameters |
| | Counter Signature Key Parameters | | | *Secret Derivation Algorithm |
| | *Secret Derivation Parameters |
+-------------------+-----------------------------------------------+ +-------------------+-----------------------------------------------+
| Sender Context | Endpoint's own private key | | Sender Context | Endpoint's own private key |
| | *Pairwise Sender Keys for the other endpoints | | | *Pairwise Sender Keys for the other endpoints |
+-------------------+-----------------------------------------------+ +-------------------+-----------------------------------------------+
| Each | Public key of the other endpoint | | Each | Public key of the other endpoint |
| Recipient Context | *Pairwise Recipient Key of the other endpoint | | Recipient Context | *Pairwise Recipient Key of the other endpoint |
+-------------------+-----------------------------------------------+ +-------------------+-----------------------------------------------+
Figure 1: Additions to the OSCORE Security Context. Optional Figure 1: Additions to the OSCORE Security Context. Optional
additions are labeled with an asterisk. additions are labeled with an asterisk.
skipping to change at page 9, line 4 skipping to change at page 9, line 17
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. ID Context
The ID Context parameter (see Sections 3.3 and 5.1 of [RFC8613]) in The ID Context parameter (see Sections 3.3 and 5.1 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 10.5).
2.1.2. Counter Signature Algorithm 2.1.2. Counter Signature Algorithm
Counter Signature Algorithm identifies the digital signature Counter Signature Algorithm identifies the digital signature
algorithm used to compute a counter signature on the COSE object (see algorithm used to compute a counter signature on the COSE object (see
Section 5.2 of [I-D.ietf-cose-rfc8152bis-struct]). Sections 3.2 and 3.3 of [I-D.ietf-cose-countersign]), when messages
are protected using the group mode (see Section 8).
This parameter is immutable once the Common Context is established. This parameter is immutable once the Common Context is established.
Counter Signature Algorithm MUST take value from the "Value" column Counter Signature Algorithm MUST take value from the "Value" column
of the "COSE Algorithms" Registry [COSE.Algorithms]. The value is of the "COSE Algorithms" Registry [COSE.Algorithms]. The value is
associated to a COSE key type, specified in the "Capabilities" column associated to a COSE key type, as specified in the "Capabilities"
of the Registry. COSE capabilities for algorithms are defined in column of the "COSE Algorithms" Registry [COSE.Algorithms]. COSE
Section 8 of [I-D.ietf-cose-rfc8152bis-algs]. capabilities for algorithms are defined in Section 8 of
[I-D.ietf-cose-rfc8152bis-algs].
The EdDSA signature algorithm Ed25519 [RFC8032] is mandatory to The EdDSA signature algorithm and the elliptic curve Ed25519
implement. For endpoints that support the pairwise mode of Group [RFC8032] are mandatory to implement. If elliptic curve signatures
OSCORE, the X25519 function [RFC7748] is also mandatory to implement. are used, it is RECOMMENDED to implement deterministic signatures
If elliptic curve signatures are used, it is RECOMMENDED to implement with additional randomness as specified in
deterministic signatures with additional randomness as specified in
[I-D.mattsson-cfrg-det-sigs-with-noise]. [I-D.mattsson-cfrg-det-sigs-with-noise].
2.1.3. Counter Signature Parameters 2.1.3. Counter Signature Parameters
Counter Signature Parameters identifies the parameters associated to Counter Signature Parameters identifies the parameters associated to
the digital signature algorithm specified in Counter Signature the digital signature algorithm specified in Counter Signature
Algorithm. This parameter is immutable once the Common Context is Algorithm. This parameter is immutable once the Common Context is
established. established.
This parameter is a CBOR array including the following two elements, This parameter is a CBOR array including the following two elements,
whose exact structure and value depend on the value of Counter whose exact structure and value depend on the value of Counter
Signature Algorithm: Signature Algorithm:
o The first element is the array of COSE capabilities for Counter o The first element is the array of COSE capabilities for Counter
Signature Algorithm, as specified for that algorithm in the Signature Algorithm, as specified for that algorithm in the
"Capabilities" column of the "COSE Algorithms" Registry "Capabilities" column of the "COSE Algorithms" Registry
[COSE.Algorithms] (see Section 8.2 of [COSE.Algorithms] (see Section 8.1 of
[I-D.ietf-cose-rfc8152bis-algs]). [I-D.ietf-cose-rfc8152bis-algs]).
o The second element is the array of COSE capabilities for the COSE o The second element is the array of COSE capabilities for the COSE
key type associated to Counter Signature Algorithm, as specified key type associated to Counter Signature Algorithm, as specified
for that key type in the "Capabilities" column of the "COSE Key for that key type in the "Capabilities" column of the "COSE Key
Types" Registry [COSE.Key.Types] (see Section 8.1 of Types" Registry [COSE.Key.Types] (see Section 8.2 of
[I-D.ietf-cose-rfc8152bis-algs]). [I-D.ietf-cose-rfc8152bis-algs]).
Examples of Counter Signature Parameters are in Appendix H. Examples of Counter Signature Parameters are in Appendix G.
2.1.4. Counter Signature Key Parameters 2.1.4. Secret Derivation Algorithm
Counter Signature Key Parameters identifies the parameters associated Secret Derivation Algorithm identifies the elliptic curve Diffie-
to the keys used with the digital signature algorithm specified in Hellman algorithm used to derive a static-static Diffie-Hellman
Counter Signature Algorithm. This parameter is immutable once the shared secret, from which pairwise keys are derived (see
Common Context is established. Section 2.3.1) to protect messages with the pairwise mode (see
Section 9).
The exact structure and value of this parameter depends on the value This parameter is immutable once the Common Context is established.
of Counter Signature Algorithm. In particular, this parameter takes Secret Derivation Algorithm MUST take value from the "Value" column
the same structure and value of the array of COSE capabilities for of the "COSE Algorithms" Registry [COSE.Algorithms]. The value is
the COSE key type associated to Counter Signature Algorithm, as associated to a COSE key type, as specified in the "Capabilities"
specified for that key type in the "Capabilities" column of the "COSE column of the "COSE Algorithms" Registry [COSE.Algorithms]. COSE
Key Types" Registry [COSE.Key.Types] (see Section 8.1 of capabilities for algorithms are defined in Section 8 of
[I-D.ietf-cose-rfc8152bis-algs]). [I-D.ietf-cose-rfc8152bis-algs].
Examples of Counter Signature Key Parameters are in Appendix H. 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 curve [RFC7748] are
mandatory to implement.
2.1.5. Secret Derivation Parameters
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
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]).
Examples of Secret Derivation Parameters are in Appendix G.
2.2. Sender Context and Recipient Context 2.2. Sender Context and Recipient Context
OSCORE specifies the derivation of Sender Context and Recipient OSCORE specifies the derivation of Sender Context and Recipient
Context, specifically of Sender/Recipient Keys and Common IV, from a Context, specifically of Sender/Recipient Keys and Common IV, from a
set of input parameters (see Section 3.2 of [RFC8613]). This set of input parameters (see Section 3.2 of [RFC8613]). This
derivation applies also to Group OSCORE, and the mandatory-to- derivation applies also to Group OSCORE, and the mandatory-to-
implement HKDF and AEAD algorithms are the same as in [RFC8613]. The 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 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 Master Secret, Master Salt and Group Identifier (see Section 3.3 of
[RFC8613]). [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 sending endpoint, a With the exception of the public key of the sender endpoint, a
receiving endpoint can derive a complete security context from a receiver endpoint can derive a complete Security Context from a
received Group OSCORE message and the Common Context. The public received Group OSCORE message and the Common Context. The public
keys in the Recipient Contexts can be accessed from the Group Manager keys in the Recipient Contexts can be retrieved from the Group
(see Section 3) upon joining the group. A public key can Manager (see Section 3) upon joining the group. A public key can
alternatively be acquired from the Group Manager at a later time, for alternatively be acquired from the Group Manager at a later time, for
example the first time a message is received from a particular example the first time a message is received from a particular
endpoint in the group (see Section 8.2 and Section 8.4). endpoint in the group (see Section 8.2 and Section 8.4).
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 associated 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 public key in order to have it available to verify retrieve the sender endpoint's public key in order to have it
subsequent messages from that endpoint. available to verify subsequent messages from that endpoint.
Furthermore, sufficiently large replay windows should be considered,
to handle Partial IV values moving forward fast. This can happen
when a client engages in frequent or long sequences of one-to-one
exchanges with servers in the group, such as a large number of block-
wise transfers to a single server. When receiving following group
requests from that client, other servers in the group may believe to
have lost synchronization with the client's Sender Sequence Number.
If these servers use an Echo exchange to re-gain synchronization (see
Appendix E.3), this in itself may consume a considerable amount of
client's Sender Sequence Numbers, hence later resulting in the
servers possibly performing a new Echo exchange.
2.3. Pairwise Keys 2.3. 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 of Group derivation of "pairwise keys", for use in the pairwise mode of Group
OSCORE defined in Section 9. OSCORE defined in Section 9.
2.3.1. Derivation of Pairwise Keys 2.3.1. Derivation of Pairwise Keys
skipping to change at page 11, line 28 skipping to change at page 12, line 35
between itself and any other endpoint in the group. The same AEAD between itself and any other endpoint in the group. The same AEAD
algorithm as in the group mode is used. The key derivation of these algorithm as in the group mode is used. The key derivation of these
so-called pairwise keys follows the same construction as in so-called pairwise keys follows the same construction as in
Section 3.2.1 of [RFC8613]: Section 3.2.1 of [RFC8613]:
Pairwise Recipient Key = HKDF(Recipient Key, Shared Secret, info, L) Pairwise Recipient Key = HKDF(Recipient Key, Shared Secret, info, L)
Pairwise Sender Key = HKDF(Sender Key, Shared Secret, info, L) Pairwise Sender Key = HKDF(Sender Key, Shared Secret, info, L)
where: where:
o The Pairwise Recipient Key is the AEAD key for receiving from o The Pairwise Recipient Key is the AEAD key for processing incoming
endpoint X. messages from endpoint X.
o The Pairwise Sender Key is the AEAD key for sending to endpoint X. o The Pairwise Sender Key is the AEAD key for processing outgoing
messages addressed to endpoint X.
o HKDF is the HKDF algorithm specified by Secret Derivation
Algorithm from the Common Context (see Section 2.1.4).
o The Shared Secret is computed as a static-static Diffie-Hellman o The Shared Secret is computed as a static-static Diffie-Hellman
shared secret [NIST-800-56A], where the endpoint uses its private shared secret [NIST-800-56A], where the endpoint uses its private
key and the public key of the other endpoint X. key and the public key of the other endpoint X.
o The Recipient Key and the public key are from the Recipient o The Recipient Key and the public key are from the Recipient
Context associated to endpoint X. Context associated to endpoint X.
o The Sender Key and private key are from the Sender Context. o The Sender Key and private key are from the Sender Context.
skipping to change at page 12, line 26 skipping to change at page 13, line 37
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.
As a consequence, replay checks may be invoked more often on the
recipient side, where larger replay windows should be considered.
2.3.3. Security Context for Pairwise Mode 2.3.3. Security Context for Pairwise Mode
If the pairwise mode is supported, the pairwise keys are added to the If the pairwise mode is supported, the Security Context additionally
Security Context, as described at the beginning of Section 2. includes Secret Derivation Algorithm, Secret Derivation Parameters
and the pairwise keys, as 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 2.3.1) may be stored in memory or recomputed
each 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 2.4.3). 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 for the other endpoints (see Figure 1). In order to identify Keys to use with the other endpoints (see Figure 1). In order to
the right key to use, the Pairwise Sender Key for endpoint X may be identify the right key to use, the Pairwise Sender Key for endpoint X
associated to the Recipient ID of endpoint X, as defined in the may be associated to the Recipient ID of endpoint X, as defined in
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 2.4. Update of Security Context
The mutable parts of the Security Context are updated by the endpoint
when executing the security protocol, but may nevertheless become
outdated, e.g. due to loss of the mutable Security Context (see
Section 2.4.1) or exhaustion of Sender Sequence Numbers (see
Section 2.4.2). The endpoint MUST be able to detect a loss of
mutable security context (see Section 2.4.1). If an endpoint detects
a loss of mutable Sender Security Context, it MUST NOT protect
further messages using this Security Context to avoid reusing a nonce
with the same AEAD key.
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 device accessed throughout the operation of the group, e.g. after a device
reboot. 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 2.4.3).
On the other hand, the mutable parts of the Security Context are
updated by the endpoint when executing the security protocol, but may
nevertheless become outdated, e.g. due to loss of the mutable
Security Context (see Section 2.4.1) or exhaustion of Sender Sequence
Numbers (see Section 2.4.2).
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
limited number of write operations), the endpoint MUST be able to
detect a loss of the mutable Security Context.
When a loss of mutable Security Context is detected (e.g., following
a reboot), the endpoint MUST NOT protect further messages using this
Security Context to avoid reusing a nonce with the same AEAD key, and
SHOULD instead retrieve new security parameters from the Group
Manager (see Section 2.4.1).
2.4.1. Loss of Mutable Security Context 2.4.1. Loss of Mutable Security Context
An endpoint losing its mutable Security Context, e.g., due to reboot, An endpoint that has lost its mutable Security Context, e.g. due to a
needs to prevent the re-use of Sender Sequence Numbers, and to handle reboot, needs to prevent the re-use of a nonce with the same AEAD
incoming replayed messages. Appendix B.1 of [RFC8613] describes key, and to handle incoming replayed messages.
secure procedures for handling the loss of Sender Sequence Number and
the update of Replay Window. The procedure in Appendix B.1.1 of
[RFC8613] applies also to servers in Group OSCORE and is RECOMMENDED
to use. A variant of Appendix B.1.2 of [RFC8613] applicable to Group
OSCORE is specified in Appendix E.3 of this specification.
If an endpoint is not able to establish an updated Sender Security To this end, after a loss of mutable Security Context, the endpoint
Context, e.g. because of lack of connectivity with the Group Manager, SHOULD inform the Group Manager, retrieve new Security Context
it MUST NOT protect further messages using this Security Context. parameters from the Group Manager (see Section 2.4.3), and use them
The endpoint SHOULD inform the Group Manager and retrieve new to derive a new Sender Context (see Section 2.2). In particular,
Security Context parameters from the Group Manager (see regardless the exact actions taken by the Group Manager, the endpoint
Section 2.4.3). resets its Sender Sequence Number to 0, and derives a new Sender Key.
This is in turn used to possibly derive new Pairwise Sender Keys.
2.4.2. Exhaustion of Sender Sequence Numbers From then on, the endpoint MUST use its latest installed Sender
Context to protect outgoing messages.
An endpoint can eventually exhaust the Sender Sequence Numbers, which If an endpoint is not able to establish an updated Sender Context,
are incremented for each new outgoing message including a Partial IV. e.g. because of lack of connectivity with the Group Manager, the
endpoint MUST NOT protect further messages using the current Security
Context.
In order to handle the update of Replay Window in Recipient Contexts,
three approaches are discussed in Appendix E. In particular, the
approach specified in Appendix E.3 and based on the Echo Option
[I-D.ietf-core-echo-request-tag] is a variant of the approach defined
in Appendix B.1.2 of [RFC8613] as applicable to Group OSCORE.
2.4.2. Exhaustion of Sender Sequence Number
An endpoint can eventually exhaust the Sender Sequence Number, which
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.
If an implementation's integers support wrapping addition, when a Implementations MUST be able to detect an exhaustion of Sender
wrap-around is detected the implementation MUST treat Sender Sequence Sequence Number, after the endpoint has consumed the largest usable
Numbers as exhausted. value. If an implementation's integers support wrapping addition,
the implementation MUST treat Sender Sequence Number as exhausted
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
protect further messages using this Security Context. The endpoint use this Security Context to protect further messages including a
SHOULD inform the Group Manager and retrieve new Security Context Partial IV.
parameters from the Group Manager (see Section 2.4.3).
The endpoint SHOULD inform the Group Manager, retrieve new Security
Context parameters from the Group Manager (see Section 2.4.3), and
use them to derive a new Sender Context (see Section 2.2). In
particular, regardless the exact actions taken by the Group Manager,
the endpoint resets its Sender Sequence Number to 0, and derives a
new Sender Key. This is in turn used to possibly derive new Pairwise
Sender Keys.
From then on, the endpoint MUST use its latest installed Sender
Context to protect outgoing messages.
2.4.3. Retrieving New Security Context Parameters 2.4.3. 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
Security Context to retrieve missing data of the Security Context and Context to retrieve missing data of the Security Context and thereby
thereby become fully operational in the group again. The two main become fully operational in the group again. The two main options
options are described in this section: i) assignment of a new Sender for the Group Manager are described in this section: i) assignment of
ID (see Section 2.4.3.1); and ii) establishment of a new Security a new Sender ID to the endpoint (see Section 2.4.3.1); and ii)
Context for the group (see Section 2.4.3.2). Update of Replay Window establishment of a new Security Context for the group (see
in Recipient Contexts is discussed in Section 6.1. Section 2.4.3.2). Update of Replay Window in Recipient Contexts is
discussed in Section 6.1.
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 2.4.3.1) 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
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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 2.4.3.1. New Sender ID for the Endpoint
The Group Manager may assign the endpoint a new Sender ID, leaving The Group Manager may assign a new Sender ID to an endpoint, while
the Gid, Master Secret and Master Salt unchanged. In this case the leaving the Gid, Master Secret and Master Salt unchanged in the
Group Manager MUST assign an unused Sender ID. Having retrieved the group. In this case, the Group Manager MUST assign a Sender ID that
new Sender ID, and potentially other missing data of the immutable has never been assigned before in the group.
Security Context, the endpoint can derive a new Sender Context (see
Section 2.2). The Sender Sequence Number is initialized to 0. Having retrieved the new Sender ID, and potentially other missing
data of the immutable Security Context, the endpoint can derive a new
Sender Context (see Section 2.2). When doing so, the endpoint re-
initilizes the Sender Sequence Number in its Sender Context to 0.
From then on, the endpoint MUST use its latest installed Sender
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 of
exhaustion of Sender Sequence Numbers (see Section 2.4.2). An exhaustion of Sender Sequence Numbers (see Section 2.4.2). 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 its
mutable Security Context (see Section 2.4.1), and receive as response mutable Security Context (see Section 2.4.1), and receive as response
a new Sender ID together with the latest immutable Security Context. a new Sender ID together with the latest immutable Security Context.
The Recipient Context of the other group members corresponding to the For the other group members, the Recipient Context corresponding to
old Sender ID becomes stale (see Section 3.1). the old Sender ID becomes stale (see Section 3.1).
2.4.3.2. New Security Context for the Group 2.4.3.2. 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 3.1). 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 2.4.3.1), unless that was already
planned or required for other reasons. All endpoints in the group planned or required for other reasons.
need to acquire new Security Context parameters from the Group
Manager.
Having acquired new Security Context parameters, each member can re- All the group members need to acquire new Security Context parameters
derive the keying material stored in its Sender Context and Recipient from the Group Manager. Once having acquired new Security Context
Contexts (see Section 2.2). The Master Salt used for the re- parameters, each group member performs the following actions.
derivations is the updated Master Salt parameter if provided by the
Group Manager, or the empty byte string otherwise. Unless otherwise o From then on, it MUST NOT use the current Security Context to
specified by the application, a group member does not reset the start processing new messages for the considered group.
Sender Sequence Number in its Sender Context, and does not reset the
Replay Windows in its Recipient Contexts. From then on, each group o It completes any ongoing message processing for the considered
member MUST use its latest installed Sender Context to protect group.
outgoing messages.
o It derives and install a new Security Context. In particular:
* It re-derives the keying material stored in its Sender Context
and Recipient Contexts (see Section 2.2). The Master Salt used
for the re-derivations is the updated Master Salt parameter if
provided by the Group Manager, or the empty byte string
otherwise.
* It resets to 0 its Sender Sequence Number in its Sender
Context.
* It re-initializes the Replay Window of each Recipient Context.
* It resets to 0 the sequence number of each ongoing observation
where it is an observer client and that it wants to keep
active.
From then on, it can resume processing new messages for the
considered group. In particular:
o It MUST use its latest installed Sender Context to protect
outgoing messages.
o It SHOULD use its latest installed Recipient Contexts to process
incoming messages, unless application policies admit to
temporarily retain and use the old, recent, Security Context (see
Section 10.4.1).
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 10.4.
3. The Group Manager 3. The Group Manager
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interacts with the group members. The responsibilities of the Group interacts with the group members. The responsibilities of the Group
Manager are compiled in Section 3.2. Manager are compiled in Section 3.2.
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 unique Group Identifiers (Gids) to
different groups under its control, as well as unique Sender IDs (and different groups under its control, as well as unique Sender IDs (and
thereby Recipient IDs) to the members of those groups. According to thereby Recipient IDs) to the members of those groups. The Group
a hierarchical approach, the Gid value assigned to a group is Manager MUST NOT reassign a Sender ID within the same group, and MUST
NOT reassign a Gid value to the same group. 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 associated to a dedicated space for the values of Sender ID and
Recipient ID of the members of that group. In addition, the Group Recipient ID of the members of that group.
Manager maintains records of the public keys of endpoints in a group,
and provides information about the group and its members to other In addition, the Group Manager maintains records of the public keys
members and selected roles. Upon nodes' joining, the Group Manager of endpoints in a group, and provides information about the group and
collects such public keys and MUST verify proof-of-possession of the its members to other members and selected roles. Upon nodes'
respective private key. 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 A group member can retrieve from the Group Manager the public key and
other information associated to another group member, with which it other information associated to another group member, with which it
can generate the corresponding Recipient Context. An application can can generate the corresponding Recipient Context. An application can
configure a group member to asynchronously retrieve information about configure a group member to asynchronously retrieve information about
Recipient Contexts, e.g. by Observing [RFC7641] the Group Manager to Recipient Contexts, e.g. by Observing [RFC7641] a resource at the
get updates on the group membership. Group Manager to get updates on the group membership.
The Group Manager MAY serve additional entities acting as signature The Group Manager MAY serve additional entities acting as signature
checkers, e.g. intermediary gateways. These entities do not join a checkers, e.g. intermediary gateways. These entities do not join a
group as members, but can retrieve public keys of group members from group as members, but can retrieve public keys of group members from
the Group Manager, in order to verify counter signatures of group the Group Manager, in order to verify counter signatures of group
messages. A signature checker MUST be authorized for retrieving messages. A signature checker MUST be authorized for retrieving
public keys of members in a specific group from the Group Manager. public keys of members in a specific group from the Group Manager.
To this end, the same method mentioned above based on the ACE To this end, the same method mentioned above based on the ACE
framework [I-D.ietf-ace-oauth-authz] can be used. framework [I-D.ietf-ace-oauth-authz] can be used.
3.1. Management of Group Keying Material 3.1. Management of Group Keying Material
In order to establish a new Security Context for a group, a new Group In order to establish a new Security Context for a group, a new Group
Identifier (Gid) for that group and a new value for the Master Secret Identifier (Gid) for that group and a new value for the Master Secret
parameter MUST be generated. An example of Gid format supporting parameter MUST be generated. When distributing the new Gid and
this operation is provided in Appendix C. When distributing the new Master Secret, the Group Manager MAY distribute also a new value for
Gid and Master Secret, the Group Manager MAY distribute also a new the Master Salt parameter, and SHOULD preserve the current value of
value for the Master Salt parameter, and SHOULD preserve the current the Sender ID of each group member.
value of the Sender ID of each group member.
The Group Manager MUST NOT reassign a Gid value to the same group.
That is, each 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 Group Manager MUST NOT reassign a previously used Sender ID
('kid') with the same Gid, Master Secret and Master Salt. Even if ('kid') with the same Gid, Master Secret and Master Salt. Even if
Gid and Master Secret are renewed as described in this section, the Gid and Master Secret are renewed as described in this section, the
Group Manager SHOULD NOT reassign an endpoint's Sender ID ('kid') Group Manager MUST NOT reassign an endpoint's Sender ID ('kid')
within a same group, especially in the short term. within a same group (see Section 2.4.3.1).
If required by the application (see Appendix A.1), it is RECOMMENDED If required by the application (see Appendix A.1), it is RECOMMENDED
to adopt a group key management scheme, and securely distribute a new 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 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 Security Context, before a new joining endpoint is added to the group
or after a currently present endpoint leaves the group. This is or after a currently present endpoint leaves the group. This is
necessary to preserve backward security and forward security in the necessary to preserve backward security and forward security in the
group, if the application requires it. group, if the application requires it.
The specific approach used to distribute new group data is out of the The specific approach used to distribute new group data is out of the
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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. Generating and managing Sender IDs within its OSCORE groups, as 5. 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. This includes ensuring uniqueness of Sender IDs join process, or to current group members upon request of
within each of its OSCORE groups. renewal. This includes ensuring that each Sender ID is unique
within each of the OSCORE groups, and that it is not reassigned
within the same group.
6. Defining communication policies for each of its OSCORE groups, 6. Defining communication policies for each of its OSCORE groups,
and signalling them to new endpoints during the join process. and signalling them to new endpoints during the join process.
7. Renewing the Security Context of an OSCORE group upon membership 7. 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).
8. Providing the management keying material that a new endpoint 8. Providing the management keying material that a new endpoint
requires to participate in the rekeying process, consistent with requires to participate in the rekeying process, consistently
the key management scheme used in the group joined by the new with the key management scheme used in the group joined by the
endpoint. new endpoint.
9. Updating the Gid of its OSCORE groups, upon renewing the 9. Updating the Gid of its OSCORE groups, upon renewing the
respective Security Context. respective Security Context. This includes ensuring that the
same Gid value is not reassigned to the same group.
10. Acting as key repository, in order to handle the public keys of 10. 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. storage device or service.
11. Validating that the format and parameters of public keys of 11. Validating that the format and parameters of public keys of
group members are consistent with the countersignature algorithm group members are consistent with the countersignature algorithm
and related parameters used in the respective OSCORE group. and 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
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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. Counter Signature
For the group mode only, the 'unprotected' field MUST additionally For the group mode only, the 'unprotected' field MUST additionally
include the following parameter: include the following parameter:
o CounterSignature0: its value is set to the counter signature of o COSE_CounterSignature0: its value is set to the counter signature
the COSE object, computed by the sender as described in of the COSE object, computed by the sender as described in
Section 5.2 of [I-D.ietf-cose-rfc8152bis-struct], by using the Sections 3.2 and 3.3 of [I-D.ietf-cose-countersign], by using its
private key and according to the Counter Signature Algorithm and private key and according to the Counter Signature Algorithm and
Counter Signature Parameters in the Security Context. In Counter Signature Parameters in the Security Context.
particular, the Sig_structure contains the external_aad as defined
in Section 4.3.2 and the ciphertext of the COSE_Encrypt0 object as In particular, the Countersign_structure contains the context text
payload. string "CounterSignature0", the external_aad as defined in
Section 4.3.2 of this specification, and the ciphertext of the
COSE object as payload.
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. That is, unlike in [RFC8613], the 'kid' transmitting the message. That is, unlike in [RFC8613], the 'kid'
parameter is always present in all messages, both requests and parameter is always present in all messages, both requests and
responses. responses.
The value of the 'kid context' parameter in the 'unprotected' field The value of the 'kid context' parameter in the 'unprotected' field
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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. In particular, there is one different compared to OSCORE. In particular, there is one
external_aad used for encryption (both in group mode and pairwise external_aad used for encryption (both in group mode and pairwise
mode), and another external_aad used for signing (only in group mode), and another external_aad used for signing (only in group
mode). mode).
4.3.1. external_aad for Encryption 4.3.1. external_aad for Encryption
The external_aad for encryption (see Section 6.3 of The external_aad for encryption (see Section 4.3 of
[I-D.ietf-cose-rfc8152bis-struct]), used both in group mode and [I-D.ietf-cose-rfc8152bis-struct]), used both in group mode and
pairwise mode, includes also the counter signature algorithm and pairwise mode, includes also the counter signature algorithm and
related signature parameters, see Figure 2. related signature parameters, as well as the value of the 'kid
context' in the COSE object of the request (see Figure 2).
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,
alg_countersign : int / tstr, alg_countersign : int / tstr,
par_countersign : [countersign_alg_capab, par_countersign : [countersign_alg_capab,
countersign_key_type_capab], countersign_key_type_capab],
par_countersign_key : countersign_key_type_capab], par_countersign_key : countersign_key_type_capab],
request_kid : bstr, request_kid : bstr,
request_piv : bstr, request_piv : bstr,
options : bstr options : bstr,
request_kid_context : bstr
] ]
Figure 2: external_aad for Encryption Figure 2: external_aad for Encryption
Compared with Section 5.4 of [RFC8613], the 'algorithms' array in the Compared with Section 5.4 of [RFC8613], the aad_array has the
aad_array additionally includes: following differences.
o 'alg_countersign', which specifies Counter Signature Algorithm o The 'algorithms' array in the aad_array additionally includes:
from the Common Context (see Section 2.1.2). This parameter MUST
encode the value of Counter Signature Algorithm as a CBOR integer
or text string, consistently with the "Value" field in the "COSE
Algorithms" Registry for this counter signature algorithm.
o 'par_countersign', which specifies the CBOR array Counter * 'alg_countersign', which specifies Counter Signature Algorithm
Signature Parameters from the Common Context (see Section 2.1.3). from the Common Context (see Section 2.1.2). This parameter
In particular: MUST encode the value of Counter Signature Algorithm as a CBOR
integer or text string, consistently with the "Value" field in
the "COSE Algorithms" Registry for this counter signature
algorithm.
* 'countersign_alg_capab' is the array of COSE capabilities for * 'par_countersign', which specifies the CBOR array Counter
the countersignature algorithm indicated in 'alg_countersign'. Signature Parameters from the Common Context (see
Section 2.1.3). In particular:
* 'countersign_key_type_capab' is the array of COSE capabilities + 'countersign_alg_capab' is the array of COSE capabilities
for the COSE key type used by the countersignature algorithm for the countersignature algorithm indicated in
indicated in 'alg_countersign'. 'alg_countersign'. This is the first element of the CBOR
array Counter Signature Parameters from the Common Context.
o 'par_countersign_key', which specifies Counter Signature Key + 'countersign_key_type_capab' is the array of COSE
Parameters from the Common Context (see Section 2.1.4). In capabilities for the COSE key type used by the
particular, 'countersign_key_type_capab' is the array of COSE countersignature algorithm indicated in 'alg_countersign'.
capabilities for the COSE key type used by the countersignature This is the second element of the CBOR array Counter
algorithm indicated in 'alg_countersign'. Signature Parameters from the Common Context.
* 'par_countersign_key', which specifies the parameters
associated to the keys used with the countersignature algorithm
indicated in 'alg_countersign'. These parameters are encoded
as a CBOR array 'countersign_key_type_capab', whose exact
structure and value depend on the value of 'alg_countersign'.
In particular, 'countersign_key_type_capab' is the array of
COSE capabilities for the COSE key type of the keys used with
the countersignature algorithm. This is the second element of
the CBOR array Counter Signature Parameters from the Common
Context.
Examples of 'par_countersign_key' are in Appendix G.
o The new element 'request_kid_context' contains the value of the
'kid context' in the COSE object of the request (see Section 4.2).
4.3.2. external_aad for Signing 4.3.2. external_aad for Signing
The external_aad for signing (see Section 4.4 of The external_aad for signing (see Section 4.3 of
[I-D.ietf-cose-rfc8152bis-struct]) used in group mode is identical to [I-D.ietf-cose-rfc8152bis-struct]) used in group mode is identical to
the external_aad for encryption (see Section 4.3.1) with the addition the external_aad for encryption (see Section 4.3.1) with the addition
of the OSCORE option, see Figure 3. of the OSCORE option (see Figure 3).
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,
alg_countersign : int / tstr, alg_countersign : int / tstr,
par_countersign : [countersign_alg_capab, par_countersign : [countersign_alg_capab,
countersign_key_type_capab], countersign_key_type_capab],
par_countersign_key : countersign_key_type_capab], par_countersign_key : countersign_key_type_capab],
request_kid : bstr, request_kid : bstr,
request_piv : bstr, request_piv : bstr,
options : bstr, options : bstr,
request_kid_context : bstr,
OSCORE_option: bstr OSCORE_option: bstr
] ]
Figure 3: external_aad for Signing Figure 3: external_aad for Signing
Compared with Section 5.4 of [RFC8613] the aad_array additionally Compared with Section 5.4 of [RFC8613] the aad_array additionally
includes: includes:
o the 'algorithms' array as defined in the external_aad for o the 'algorithms' array, as defined in the external_aad for
encryption, see Section 4.3.1; encryption (see Section 4.3.1);
o the value of the OSCORE Option encoded as a binary string. o the 'request_kid_context' element, as defined in the external_aad
for encryption (see Section 4.3.1);
o the value of the OSCORE Option present in the protected message,
encoded as a binary string.
Note for implementation: this construction requires the OSCORE option Note for implementation: this construction requires the OSCORE option
of the message to be generated before calculating the signature. of the message to be generated before calculating the signature.
Also, the aad_array needs to be large enough to contain the largest Also, the aad_array needs to be large enough to contain the largest
possible OSCORE option. possible OSCORE option.
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 o The payload of the OSCORE message SHALL encode the ciphertext of
the COSE object. In the group mode, the ciphertext above is the COSE_Encrypt0 object. In the group mode, the ciphertext above
concatenated with the value of the CounterSignature0 of the COSE is concatenated with the value of the COSE_CounterSignature0 of
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 o This specification defines the usage of the sixth least
significant bit, called the "Group Flag", in the first byte of the significant bit, called the "Group Flag", in the first byte of the
OSCORE option containing the OSCORE flag bits. This flag bit is OSCORE option containing the OSCORE flag bits. This flag bit is
specified in Section 11.1. specified in Section 11.1.
o The Group Flag MUST be set to 1 if the OSCORE message is protected o 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 o The Group Flag MUST be set to 0 if the OSCORE message is protected
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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, COUNTERSIGN denotes the CounterSignature0 byte string as group mode, COUNTERSIGN denotes the COSE_CounterSignature0 byte
described in Section 4, and is 64 bytes long. string as described in Section 4, and is 64 bytes long.
5.1.1. Examples in Group Mode 5.1.1. Examples in Group Mode
o Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid = o 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', 9:COUNTERSIGN }, { 4:h'25', 6:h'05', 10:h'44616c', 11:COUNTERSIGN },
h'aea0155667924dff8a24e4cb35b9' h'aea0155667924dff8a24e4cb35b9'
] ]
After compression (85 bytes): After compression (85 bytes):
Flag byte: 0b00111001 = 0x39 Flag byte: 0b00111001 = 0x39
Option Value: 39 05 03 44 61 6c 25 (7 bytes) Option Value: 39 05 03 44 61 6c 25 (7 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 COUNTERSIGN Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 COUNTERSIGN
(14 bytes + size of COUNTERSIGN) (14 bytes + size of COUNTERSIGN)
o Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid = o Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid =
skipping to change at page 22, line 42 skipping to change at page 26, line 20
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 COUNTERSIGN Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 COUNTERSIGN
(14 bytes + size of COUNTERSIGN) (14 bytes + size of COUNTERSIGN)
o Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid = o 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', 9:COUNTERSIGN }, { 4:h'52', 11:COUNTERSIGN },
h'60b035059d9ef5667c5a0710823b' h'60b035059d9ef5667c5a0710823b'
] ]
After compression (80 bytes): After compression (80 bytes):
Flag byte: 0b00101000 = 0x28 Flag byte: 0b00101000 = 0x28
Option Value: 28 52 (2 bytes) Option Value: 28 52 (2 bytes)
Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b COUNTERSIGN Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b COUNTERSIGN
(14 bytes + size of COUNTERSIGN) (14 bytes + size of COUNTERSIGN)
5.1.2. Examples in Pairwise Mode 5.1.2. Examples in Pairwise Mode
skipping to change at page 24, line 24 skipping to change at page 27, line 43
The requirements and properties described in Section 7 of [RFC8613] The requirements and properties described in Section 7 of [RFC8613]
also apply to OSCORE used in group communication. In particular, also apply to OSCORE used in group communication. In particular,
Group OSCORE provides message binding of responses to requests, which Group OSCORE provides message binding of responses to requests, which
enables absolute freshness of responses that are not notifications, enables absolute freshness of responses that are not notifications,
relative freshness of requests and notification responses, and replay relative freshness of requests and notification responses, and replay
protection of requests. protection of requests.
6.1. Update of Replay Window 6.1. Update of Replay Window
A new server joining a group may not be aware of the current Partial A new server joining a group may not be aware of the current Partial
IVs (Sender Sequence Numbers of the clients). The first time the new IVs (Sender Sequence Numbers of the clients). Hence, when receiving
server receives a request from a particular client, it is not able to a request from a particular client for the first time, the new server
verify if that request is a replay. The same holds when a server is not able to verify if that request is a replay. The same holds
loses its mutable Security Context (see Section 2.4.1), for instance when a server loses its mutable Security Context (see Section 2.4.1),
after a device reboot. for instance after a device reboot.
The exact way to address this issue is application specific, and The exact way to address this issue is application specific, and
depends on the particular use case and its replay requirements. The depends on the particular use case and its replay requirements. The
list of methods to handle the update of a Replay Window is part of list of methods to handle the update of a Replay Window is part of
the group communication policy, and different servers can use the group communication policy, and different servers can use
different methods. different methods. Appendix E describes three possible approaches
that can be considered to address the issue discussed above.
Appendix E describes three possible approaches that can be considered Furthermore, when the Group Manager establishes a new Security
to update a Replay Window. Context for the group (see Section 2.4.3.2), every server re-
initializes the Replay Window in each of its Recipient Contexts.
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 Security Context to process Group OSCORE messages in [RFC8613] and a Security Context to process Group OSCORE messages
as defined in this specification. as defined in this specification.
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 Counter Signature Algorithm in the Common
Context. Alternatively implementations can use an additional Context. Alternatively implementations can use an additional
parameter in the Security Context, to explicitly signal that it is parameter in the Security Context, to explicitly signal that it is
intended for processing Group OSCORE messages. intended for processing Group OSCORE messages.
If any of the following two conditions holds, a recipient endpoint If either of the following two conditions holds, a recipient endpoint
MUST discard the incoming protected message: MUST discard the incoming protected message:
o The Group Flag is set to 1, and the recipient endpoint can not o 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.
o The Group Flag is set to 0, and the recipient endpoint retrieves a o 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.
skipping to change at page 25, line 37 skipping to change at page 29, line 12
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 mode MUST be supported.
During all the steps of the message processing, an endpoint MUST use
the same Security Context for the considered group. That is, an
endpoint MUST NOT install a new Security Context for that group (see
Section 2.4.3.2) 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].
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 cannot be 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 pre-defined, sufficient should store such outgoing messages for a pre-defined, 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].
skipping to change at page 26, line 25 skipping to change at page 30, line 6
prevents servers from replying with multiple error messages to a prevents servers from replying with multiple error messages to a
client sending a group request, so avoiding the risk of flooding and client sending a group request, so avoiding the risk of flooding and
possibly congesting the network. 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 o In step 2, the Additional Authenticated Data is modified as
described in Section 4. described in Section 4 of this document.
o In step 4, the encryption of the COSE object is modified as o In step 4, the encryption of the COSE object is modified as
described in Section 4. The encoding of the compressed COSE described in Section 4 of this document. The encoding of the
object is modified as described in Section 5. In particular, the compressed COSE object is modified as described in Section 5 of
Group Flag MUST be set to 1. this document. In particular, the Group Flag MUST be set to 1.
o In step 5, the counter signature is computed and the format of the o In step 5, the counter signature is computed and the format of the
OSCORE message is modified as described in Section 4 and OSCORE message is modified as described in Section 4 and Section 5
Section 5. In particular, the payload of the OSCORE message of this document. In particular, the payload of the OSCORE
includes also the counter signature. message includes also the counter signature.
8.1.1. Supporting Observe 8.1.1. Supporting Observe
If Observe [RFC7641] is supported, for each newly started If Observe [RFC7641] is supported, the following holds for each newly
observation, the client MUST store the value of the 'kid' parameter started observation.
from the original Observe request.
The client MUST NOT update the stored value, even in case it is o If the client intends to keep the observation active beyond a
individually rekeyed and receives a new Sender ID from the Group possible change of Sender ID, the client MUST store the value of
Manager (see Section 2.4.3.1). the 'kid' parameter from the original Observe request, and retain
it for the whole duration of the observation. Even in case the
client is individually rekeyed and receives a new Sender ID from
the Group Manager (see Section 2.4.3.1), the client MUST NOT
update the stored value associated to a particular Observe
request.
o If the client intends to keep the observation active beyond a
possible change of ID Context following a group rekeying (see
Section 3.1), then the following applies.
* The client MUST store the value of the 'kid context' parameter
from the original Observe request, and retain it for the whole
duration of the observation. Upon establishing a new Security
Context with a new ID Context as Gid (see Section 2.4.3.2), the
client MUST NOT update the stored value associated to a
particular Observe request.
* The client MUST store an invariant identifier of the group,
which is immutable even in case the Security Context of the
group is re-established. For example, this invariant
identifier can be the "group name" in
[I-D.ietf-ace-key-groupcomm-oscore], where it is used for
joining the group and retrieving the current group keying
material from the Group Manager.
After a group rekeying, such an invariant information makes it
simpler for the observer client to retrieve the current group
keying material from the Group Manager, in case the client has
missed both the rekeying messages and the first observe
notification protected with the new Security Context (see
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 o In step 2, the decoding of the compressed COSE object follows
Section 5. 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 4.02 (Bad Option) error. When doing so, the respond with a 4.02 (Bad Option) error. When doing so, the
server MAY set an Outer Max-Age option with value zero, and MAY server MAY set an Outer Max-Age option with value zero, and MAY
include a descriptive string as diagnostic payload. 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 o In step 4, the Additional Authenticated Data is modified as
described in Section 4. described in Section 4 of this document.
o In step 6, the server also verifies the counter signature using o In step 6, the server also verifies the counter signature using
the public key of the client from the associated Recipient the public key of the client from the associated Recipient
Context. If the signature verification fails, the server SHALL Context. In particular:
stop processing the request and MAY respond with a 4.00 (Bad
Request) response. If the verification fails, the same steps are * If the server does not have the public key of the client yet,
taken as if the decryption had failed. In particular, the Replay the server MUST stop processing the request and MAY respond
Window is only updated if both the signature verification and the with a 5.03 (Service Unavailable) response. The response MAY
decryption succeed. include a Max-Age Option, indicating to the client the number
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
client, as default value defined in Section 5.10.5 of
[RFC7252].
* If the signature verification fails, the server SHALL stop
processing the request and MAY respond with a 4.00 (Bad
Request) response. If the verification fails, the same steps
are taken as if the decryption had failed. In particular, the
Replay Window is only updated if both the signature
verification and the decryption succeed.
o Additionally, if the used Recipient Context was created upon o 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 5.2.1 in an example where this is not fulfilled, see Section 6.2.1 in
[I-D.tiloca-core-observe-multicast-notifications]. [I-D.tiloca-core-observe-multicast-notifications].
8.2.1. Supporting Observe 8.2.1. Supporting Observe
If Observe [RFC7641] is supported, for each newly started If Observe [RFC7641] is supported, the following holds for each newly
observation, the server MUST store the value of the 'kid' parameter started observation.
from the original Observe request.
The server MUST NOT update the stored value of a 'kid' parameter o The server MUST store the value of the 'kid' parameter from the
associated to a particular Observe request, even in case the observer original Observe request, and retain it for the whole duration of
client is individually rekeyed and starts using a new Sender ID the observation. The server MUST NOT update the stored value of a
received from the Group Manager (see Section 2.4.3.1). 'kid' parameter associated to a particular Observe request, even
in case the observer client is individually rekeyed and starts
using a new Sender ID received from the Group Manager (see
Section 2.4.3.1).
o The server MUST store the value of the 'kid context' parameter
from the original Observe request, and retain it for the whole
duration of the observation, beyond a possible change of ID
Context following a group rekeying (see Section 3.1). That is,
upon establishing a new Security Context with a new ID Context as
Gid (see Section 2.4.3.2), the server MUST NOT update the 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 a new Security Context from its latest Security Context, and that establishing a new
Context does not reset the Sender Sequence Number unless otherwise Security Context resets the Sender Sequence Number to 0 (see
specified by the application (see Section 3.1). Section 3.1).
o In step 2, the Additional Authenticated Data is modified as o In step 2, the Additional Authenticated Data is modified as
described in Section 4. described in Section 4 of this document.
o In step 3, if the server is using a different Security Context for o 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 AEAD nonce from the request MUST NOT be Section 3.1), then the server MUST include its Sender Sequence
used. 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
the request from being reused.
o In step 4, the encryption of the COSE object is modified as o In step 4, the encryption of the COSE object is modified as
described in Section 4. The encoding of the compressed COSE described in Section 4 of this document. The encoding of the
object is modified as described in Section 5. In particular, the compressed COSE object is modified as described in Section 5 of
Group Flag MUST be set to 1. If the server is using a different this document. In particular, the Group Flag MUST be set to 1.
ID Context (Gid) for the response compared to what was used to If the server is using a different ID Context (Gid) for the
verify the request (see Section 3.1), then the new ID Context MUST response compared to what was used to verify the request (see
be included in the 'kid context' parameter of the response. Section 3.1), then the new ID Context MUST be included in the 'kid
context' parameter of the response.
o In step 5, the counter signature is computed and the format of the o In step 5, the counter signature is computed and the format of the
OSCORE message is modified as described in Section 5. In OSCORE message is modified as described in Section 5 of this
particular, the payload of the OSCORE message includes also the document. In particular, the payload of the OSCORE message
counter signature. includes also the counter signature.
8.3.1. Supporting Observe 8.3.1. Supporting Observe
If Observe [RFC7641] is supported, the server may have ongoing If Observe [RFC7641] is supported, the following holds when
observations, started by Observe requests protected with an old protecting notifications for an ongoing observation.
Security Context.
After completing the establishment of a new Security Context, the o The server MUST use the stored value of the 'kid' parameter from
server MUST protect the following notifications with the Sender the original Observe request (see Section 8.2.1), as value for the
Context of the new Security Context. 'request_kid' parameter in the two external_aad structures (see
Section 4.3.1 and Section 4.3.2).
o The server MUST use the stored value of the 'kid context'
parameter from the original Observe request (see Section 8.2.1),
as value for the 'request_kid_context' parameter in the two
external_aad structures (see Section 4.3.1 and Section 4.3.2).
Furthermore, the server may have ongoing observations started by
Observe requests protected with an old Security Context. After
completing the establishment of a new Security Context, the server
MUST protect the following notifications with the Sender Context of
the new Security Context.
For each ongoing observation, the server MUST include in the first For each ongoing observation, the server MUST include in the first
notification protected with the new Security Context also the 'kid notification protected with the new Security Context also the 'kid
context' parameter, which is set to the ID Context (Gid) of the new context' parameter, which is set to the ID Context (Gid) of the new
Security Context. It is OPTIONAL for the server to include the ID Security Context. It is OPTIONAL for the server to include the ID
Context (Gid) in the 'kid context' parameter also in further Context (Gid) in the 'kid context' parameter also in further
following notifications for those observations. following notifications for those observations.
Furthermore, for each ongoing observation, the server MUST use the
stored value of the 'kid' parameter from the original Observe
request, as value for the 'request_kid' parameter in the two
external_aad structures (see Section 4.3.1 and Section 4.3.2), when
protecting notifications for that observation.
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 group request, and that, upon the establishment of a new Security
Context, the client does not reset its own replay windows in its Context, the client re-initializes its replay windows in its
Recipient Contexts, unless otherwise specified by the application Recipient Contexts (see Section 3.1).
(see Section 3.1).
o In step 2, the decoding of the compressed COSE object is modified o In step 2, the decoding of the compressed COSE object is modified
as described in Section 5. If the received 'kid context' matches as described in Section 5 of this document. If the received 'kid
an existing ID Context (Gid) but the received 'kid' does not match context' matches an existing ID Context (Gid) but the received
any Recipient ID in this Security Context, then the client MAY 'kid' does not match any Recipient ID in this Security Context,
create a new Recipient Context for this Recipient ID and then the client MAY create a new Recipient Context for this
initialize it according to Section 3 of [RFC8613], and also Recipient ID and initialize it according to Section 3 of
retrieve the associated public key. If the application does not [RFC8613], and also retrieve the associated public key. If the
specify dynamic derivation of new Recipient Contexts, then the application does not specify dynamic derivation of new Recipient
client SHALL stop processing the response. Contexts, then the client SHALL stop processing the response.
o In step 3, the Additional Authenticated Data is modified as o In step 3, the Additional Authenticated Data is modified as
described in Section 4. described in Section 4 of this document.
o In step 5, the client also verifies the counter signature using o In step 5, the client also verifies the counter signature using
the public key of the server from the associated Recipient the public key of the server from the associated Recipient
Context. If the verification fails, the same steps are taken as Context. If the verification fails, the same steps are taken as
if the decryption had failed. if the decryption had failed.
o Additionally, if the used Recipient Context was created upon o 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, for each ongoing observation, the If Observe [RFC7641] is supported, the following holds when verifying
client MUST use the stored value of the 'kid' parameter from the notifications for an ongoing observation.
original Observe request, as value for the 'request_kid' parameter in
the two external_aad structures (see Section 4.3.1 and o The client MUST use the stored value of the 'kid' parameter from
Section 4.3.2), when verifying notifications for that observation. the original Observe request (see Section 8.1.1), as value for the
'request_kid' parameter in the two external_aad structures (see
Section 4.3.1 and Section 4.3.2).
o The client MUST use the stored value of the 'kid context'
parameter from the original Observe request (see Section 8.1.1),
as value for the 'request_kid_context' parameter in the two
external_aad structures (see Section 4.3.1 and Section 4.3.2).
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). received from the Group Manager (see Section 2.4.3.1), as well as
when it establishes a new Security Context with a new ID Context
(Gid) following a group rekeying (see Section 3.1).
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 Section 8, with the modifications described in and processed as in Section 8, 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, the
signature scheme of the group mode MUST support a combined signature signature scheme of the group mode MUST support a combined signature
and encryption scheme. This can be, for example, signature using and encryption scheme. This can be, for example, signature using
ECDSA, and encryption using AES-CCM with a key derived with ECDH. ECDSA, and encryption using AES-CCM with a key derived with ECDH.
The pairwise mode does not support intermediary verification of
source authentication or integrity.
The pairwise mode MAY be supported. The pairwise mode MUST be The pairwise mode does not support the use of additional entities
supported by endpoints that use the CoAP Echo Option acting as verifiers of source authentication and integrity of group
[I-D.ietf-core-echo-request-tag] and/or block-wise transfers messages, such as intermediary gateways (see Section 3).
[RFC7959], for instance for responses after the first block-wise
request, possibly targeting all servers in the group and including The pairwise mode MAY be supported. An endpoint implementing only a
the CoAP Block2 option (see Section 2.3.6 of
[I-D.ietf-core-groupcomm-bis]). An endpoint implementing only a
silent server does not support the pairwise mode. silent server does not support the pairwise mode.
If the signature algorithm used in the group supports ECDH (e.g.,
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
block-wise transfers [RFC7959], for instance for responses after the
first block-wise request, which possibly targets all servers in the
group and includes the CoAP Block2 option (see Section 2.3.6 of
[I-D.ietf-core-groupcomm-bis]). This prevents the attack described
in Section 10.7, which leverages requests sent over unicast to a
single group member and protected with the group mode.
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 o The 'kid' and 'kid context' parameters of the COSE object are used
as defined in Section 4.2. as defined in Section 4.2 of this document.
o The external_aad defined in Section 4.3.1 is used for the o The external_aad defined in Section 4.3.1 of this document is used
encryption process. for the encryption process.
o The Sender/Recipient Keys used in the pairwise mode are derived as o The Pairwise Sender/Recipient Keys used as Sender/Recipient keys
defined in Section 2.3. are derived as defined in Section 2.3 of this document.
Senders MUST NOT use the pairwise mode to protect a message intended Senders MUST NOT use the pairwise mode to protect a message intended
for multiple recipients. The pairwise mode is defined only between for multiple recipients. The pairwise mode is defined only between
two endpoints and the keying material is thus only available to one two endpoints and the keying material is thus only available to one
recipient. recipient.
The Group Manager MAY indicate that the group uses also the pairwise The Group Manager MAY indicate that the group uses also the pairwise
mode, as part of the group communication policies signalled to mode, as part of the group data provided to candidate group members
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 Pairwise Sender Context of Section 2.3.3). endpoint (see Pairwise Sender Context of Section 2.3.3).
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
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identifying all the servers in the group. Further details of such an identifying all the servers in the group. Further details of such an
interface are out of scope for this document. interface are out of scope for this document.
9.2. Protecting the Request 9.2. 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, with the following differences. Section 8.1, with the following differences.
o The Group Flag MUST be set to 0. o The Group Flag MUST be set to 0.
o The Sender Key used is the Pairwise Sender Key (see Section 2.3). o The used Sender Key is the Pairwise Sender Key (see Section 2.3).
o The counter signature is not computed and therefore not included o The counter signature is not computed and therefore not included
in the message. The payload of the protected request thus in the message. The payload of the protected request thus
terminates with the encoded ciphertext of the COSE object, just as terminates with the encoded ciphertext of the COSE object, just
in [RFC8613]. like in [RFC8613].
Note that, just as in the group mode, the external_aad for encryption Note that, like in the group mode, the external_aad for encryption is
is generated as in Section 4.3.1, and the Partial IV is the current generated as in Section 4.3.1, and the Partial IV is the current
fresh value of the Sender Sequence Number (see Section 2.3.2). fresh value of the client's Sender Sequence Number (see
Section 2.3.2).
9.3. Verifying the Request 9.3. 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, with the following differences. Section 8.2, with the following differences.
o If the server discards the request due to not retrieving a o 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 4.02 supporting the pairwise mode, the server MAY respond with a 4.02
(Bad Option) error. When doing so, the server MAY set an Outer (Bad Option) error. When doing so, the server MAY set an Outer
Max-Age option with value zero, and MAY include a descriptive Max-Age option with value zero, and MAY include a descriptive
string as diagnostic payload. string as diagnostic payload.
o If a new Recipient Context is created for this Recipient ID, new o 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 2.3.1). 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 The Recipient Key used is the Pairwise Recipient Key (see o The used Recipient Key is the Pairwise Recipient Key (see
Section 2.3). Section 2.3).
o No verification of counter signature occurs, as there is none o No verification of counter signature occurs, as there is none
included in the message. included in the message.
9.4. Protecting the Response 9.4. 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, with the following differences. Section 8.3, with the following differences.
o The Group Flag MUST be set to 0. o The Group Flag MUST be set to 0.
o The Sender Key used is the Pairwise Sender Key (see Section 2.3). o The used Sender Key is the Pairwise Sender Key (see Section 2.3).
o The counter signature is not computed and therefore not included o The counter signature is not computed and therefore not included
in the message. The payload of the protected response thus in the message. The payload of the protected response thus
terminates with the encoded ciphertext of the COSE object, just as terminates with the encoded ciphertext of the COSE object, just
in [RFC8613]. like in [RFC8613].
9.5. Verifying the Response 9.5. 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, with the following differences. Section 8.4, with the following differences.
o If a new Recipient Context is created for this Recipient ID, new o 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 2.3.1). 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 The Recipient Key used is the Pairwise Recipient Key (see o The used Recipient Key is the Pairwise Recipient Key (see
Section 2.3). Section 2.3).
o No verification of counter signature occurs, as there is none o No verification of counter signature occurs, as there is none
included in the message. included in the message.
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 counter signatures, as discussed in Section 10.1.
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. This is further discussed in Section 10.2 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 difference. The counter signature included in a Group the following difference. The counter signature included in a Group
OSCORE message protected in group mode is computed also over the OSCORE message protected in group mode is computed also over the
value of the OSCORE option, which is part of the Additional value of the OSCORE option, which is part of the Additional
Authenticated Data used in the signing process. This is further Authenticated Data used in the signing process. This is further
discussed in Section 10.6. discussed in Section 10.6 of this 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 (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. Group-level Security
The group mode described in Section 8 relies on commonly shared group The group mode described 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. This has
the following implications. the following implications.
o Messages are encrypted at a group level (group-level data o 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 o The AEAD algorithm provides only group authentication, i.e. it
ensures that a message sent to a group has been sent by a member ensures that a message sent to a group has been sent by a member
of that group, but not by the alleged sender. This is why source of that group, but not necessarily by the alleged sender. This is
authentication of messages sent to a group is ensured through a why source authentication of messages sent to a group is ensured
counter signature, which is computed by the sender using its own through a counter signature, which is computed by the sender using
private key and then appended to the message payload. its own private key and then appended to the message payload.
Instead, the pairwise mode described in Section 9 protects messages Instead, the pairwise mode described in Section 9 protects messages
by using pairwise symmetric keys, derived from the static-static by using pairwise symmetric keys, derived from the static-static
Diffie-Hellman shared secret computed from the asymmetric keys of the Diffie-Hellman shared secret computed from the asymmetric keys of the
sender and recipient endpoint (see Section 2.3). Therefore, in the sender and recipient endpoint (see Section 2.3). Therefore, in the
parwise mode, the AEAD algorithm provides both pairwise data- parwise mode, the AEAD algorithm provides both pairwise data-
confidentiality and source authentication of messages, without using confidentiality and source authentication of messages, without using
counter signatures. counter signatures.
The long-term storing of the Diffie-Hellman shared secret is a The long-term storing of the Diffie-Hellman shared secret is a
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practically limited risk and enables a prompt detection/reaction in practically limited risk and enables a prompt detection/reaction in
case of misbehaving. case of misbehaving.
10.2. Uniqueness of (key, nonce) 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 o Uniqueness of Sender IDs within the group is enforced by the Group
Manager. Manager, which never reassigns the same Sender ID within the same
group.
o The case A in Appendix D.4 of [RFC8613] concerns all group o 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 o 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.
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The approach described in this specification should take into account The approach described in this specification should take into account
the risk of compromise of group members. In particular, this the risk of compromise of group members. In particular, this
document specifies that a key management scheme for secure revocation document specifies that a key management scheme for secure revocation
and renewal of Security Contexts and group keying material should be and renewal of Security Contexts and group keying material should 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 size may be needed in dynamic, large-scale groups where endpoints can
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 10.4. 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 group request, and a server using a different new Security Context a group request, and a server using a different new Security Context
to protect a corresponding response. As a consequence, clients may to 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 group request. the one used to protect the corresponding group request.
In particular, a server may first get a group request protected with In particular, a server may first get a group request protected with
the old Security Context, then install the new Security Context, and the old Security Context, then install the new Security Context, and
only after that produce a response to send back to the client. In only after that produce a response to send back to the client. In
such a case, as specified in Section 8.3, the server MUST protect the such a case, as specified in Section 8.3, the server MUST protect the
potential response using the new Security Context. Specifically, the potential response using the new Security Context. Specifically, the
server MUST use its own Sender Sequence Number as Partial IV to server MUST include its Sender Sequence Number as Partial IV in the
protect that response, and not the Partial IV from the request, in response and use it to build the AEAD nonce to protect the response.
order to prevent reuse of AEAD nonces in the new Security Context. This prevents the AEAD nonce from the request from being reused with
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 reception. 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 7.2 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 10.4.1. 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, hence not being able to correctly process the new Security Context, and is thus unable to correctly process it.
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 second received message using the old retained Security Context as 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
group request. Instead, a recipient server would better and more group request. Instead, a recipient server would better and more
simply discard an incoming group request which is not successfully simply discard an incoming group request which is not successfully
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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 10.4.2. 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 uses CoAP retransmissions, 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 10.5. Collision of Group Identifiers
In case endpoints are deployed in multiple groups managed by In case endpoints are deployed in multiple groups managed by
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can be used to forge a response M2 for a given request would be can be used to forge a response M2 for a given request would be
equal to 2^-24, since there are more MAC values (8 bytes in size) equal to 2^-24, since there are more MAC values (8 bytes in size)
than Partial IV values (5 bytes in size). than Partial IV values (5 bytes in size).
Note that, by changing the Partial IV as discussed above, any Note that, by changing the Partial IV as discussed above, any
member of G1 would also be able to forge a valid signed response member of G1 would also be able to forge a valid signed response
message M2 to be injected in G1. message M2 to be injected in G1.
10.7. Group OSCORE for Unicast Requests 10.7. Group OSCORE for Unicast Requests
With reference to the processing defined in Section 8.1 for the group If a request is intended to be sent over unicast as addressed to a
mode and in Appendix G for the optimized request, it is NOT single group member, it is NOT RECOMMENDED for the client to protect
RECOMMENDED for a client to use the group mode for securing a request the request by using the group mode as defined in Section 8.1.
intended for a single group member and sent over unicast.
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
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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 defined in Section 9.2, in A client can instead use the pairwise mode as defined in Section 9.2,
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 2.3). 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 5.2.1 in see Section 6.2.1 in
[I-D.tiloca-core-observe-multicast-notifications]. [I-D.tiloca-core-observe-multicast-notifications].
With particular reference to block-wise transfers [RFC7959], With particular reference to block-wise transfers [RFC7959],
Section 2.3.6 of [I-D.ietf-core-groupcomm-bis] points out that, while Section 2.3.6 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.3, with respect Additional considerations are discussed in Appendix E.3, 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
client Sender Sequence Numbers. clients' Sender Sequence Number.
10.8. End-to-end Protection 10.8. 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 DTLS 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 10.9. 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.
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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 2.4.3, 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 10.2) is preserved also when a new Security
Context is established. Context is established.
As discussed in Section 6.1, an endpoint that has just joined a group As discussed in Section 6.1, an endpoint that has just joined a group
is exposed to replay attack, as it is not aware of the sender is exposed to replay attack, as it is not aware of the Sender
sequence numbers currently used by other group members. Appendix E Sequence Numbers currently used by other group members. Appendix E
describes how endpoints can synchronize with senders' sequence describes how endpoints can synchronize with others' Sender Sequence
numbers. Number.
Unless exchanges in a group rely only on unicast messages, Group Unless exchanges in a group rely only on unicast messages, Group
OSCORE cannot be used with reliable transport. Thus, unless only OSCORE cannot be used with reliable transport. Thus, unless only
unicast messages are sent in the group, it cannot be defined that unicast messages are sent in the group, it cannot be defined that
only messages with sequence numbers that are equal to the previous only messages with sequence numbers that are equal to the previous
sequence number + 1 are accepted. sequence number + 1 are accepted.
The processing of response messages described in Section 2.3.1 of The processing of response messages described in Section 2.3.1 of
[I-D.ietf-core-groupcomm-bis] also ensures that a client accepts a [I-D.ietf-core-groupcomm-bis] also ensures that a client accepts a
single valid response to a given request from each replying server, single valid response to a given request from each replying server,
unless CoAP observation is used. unless CoAP observation is used.
10.11. Client Aliveness 10.11. Client Aliveness
As discussed in Section 12.5 of [RFC8613], a server may use the CoAP As discussed in Section 12.5 of [RFC8613], a server may use the CoAP
Echo Option [I-D.ietf-core-echo-request-tag] to verify the aliveness Echo Option [I-D.ietf-core-echo-request-tag] to verify the aliveness
of the client that originated a received request. This would also of the client that originated a received request. This would also
allow the server to (re-)synchronize with the client's sequence allow the server to (re-)synchronize with the client's Sender
number, as well as to ensure that the request is fresh and has not Sequence Number, as well as to ensure that the request is fresh and
been replayed or (purposely) delayed, if it is the first one received has not been replayed or (purposely) delayed, if it is the first one
from that client after having joined the group or rebooted (see received from that client after having joined the group or rebooted
Appendix E.3). (see Appendix E.3).
10.12. Cryptographic Considerations 10.12. 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 2.4.2, an endpoint that experiences an
exhaustion of its own Sender Sequence Number MUST NOT transmit 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 Ed25519 [RFC8032] is mandatory to The EdDSA signature algorithm and the elliptic curve Ed25519
implement. For endpoints that support the pairwise mode of Group [RFC8032] are mandatory to implement. For endpoints that support the
OSCORE, the X25519 function [RFC7748] is also mandatory to implement. pairwise mode, the ECDH-SS + HKDF-256 algorithm specified in
Montgomery curves and (twisted) Edwards curves [RFC7748] can be Section 6.3.1 of [I-D.ietf-cose-rfc8152bis-algs] and the X25519 curve
alternatively represented in short-Weierstrass form as described in [RFC7748] are also mandatory to implement.
[I-D.ietf-lwig-curve-representations].
Constrained IoT devices may alternatively represent Montgomery curves
and (twisted) Edwards curves [RFC7748] in the short-Weierstrass form
Wei25519, with which the algorithms ECDSA25519 and ECDH25519 can be
used for signature operations and Diffie-Hellman secret calculation,
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 2.3.1 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. The security of using the same
skipping to change at page 44, line 34 skipping to change at page 49, line 40
o The 'kid' parameter, which is intended to help a recipient o 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. Since both requests and information about the Sender Endpoint. Since both requests and
responses always include the 'kid' parameter, this may reveal responses always include the 'kid' parameter, this may reveal
information about both a client sending a group request and all information about both a client sending a group 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 o The 'kid context' parameter, which is intended to help a recipient
endpoint to find the right Recipient 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.
skipping to change at page 45, line 31 skipping to change at page 50, line 37
paragraph. 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.
+--------------+------------+-------------------------------+-----------+ +--------------+------------+----------------------------+-----------+
| Bit Position | Name | Description | Reference | | Bit Position | Name | Description | Reference |
+--------------+------------+-------------------------------+-----------+ +--------------+------------+----------------------------+-----------+
| 2 | Group Flag | Set to 1 if the message is | [This | | 2 | Group Flag | Set to 1 if the message is | [This |
| | | protected with the group mode | Document] | | | | protected with the group | Document] |
| | | of Group OSCORE | | | | | mode of Group OSCORE | |
+--------------+------------+-------------------------------+-----------+ +--------------+------------+----------------------------+-----------+
12. References 12. References
12.1. Normative References 12.1. Normative References
[COSE.Algorithms] [COSE.Algorithms]
IANA, "COSE Algorithms", IANA, "COSE Algorithms",
<https://www.iana.org/assignments/cose/ <https://www.iana.org/assignments/cose/
cose.xhtml#algorithms>. cose.xhtml#algorithms>.
[COSE.Key.Types] [COSE.Key.Types]
IANA, "COSE Key Types", IANA, "COSE Key Types",
<https://www.iana.org/assignments/cose/ <https://www.iana.org/assignments/cose/cose.xhtml#key-
cose.xhtml#key-type>. type>.
[I-D.ietf-cbor-7049bis]
Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", draft-ietf-cbor-7049bis-16 (work
in progress), September 2020.
[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)", draft-
ietf-core-groupcomm-bis-00 (work in progress), March 2020. ietf-core-groupcomm-bis-02 (work in progress), November
2020.
[I-D.ietf-cose-countersign]
Schaad, J. and R. Housley, "CBOR Object Signing and
Encryption (COSE): Countersignatures", draft-ietf-cose-
countersign-01 (work in progress), October 2020.
[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-09 Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-12
(work in progress), June 2020. (work in progress), September 2020.
[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", draft-ietf-cose-rfc8152bis-
struct-10 (work in progress), June 2020. struct-14 (work in progress), September 2020.
[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 47, line 24 skipping to change at page 52, line 38
12.2. Informative References 12.2. Informative References
[Degabriele] [Degabriele]
Degabriele, J., Lehmann, A., Paterson, K., Smart, N., and Degabriele, J., Lehmann, A., Paterson, K., Smart, N., and
M. Strefler, "On the Joint Security of Encryption and M. Strefler, "On the Joint Security of Encryption and
Signature in EMV", December 2011, Signature in EMV", December 2011,
<https://eprint.iacr.org/2011/615>. <https://eprint.iacr.org/2011/615>.
[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-07 Communication using ACE", draft-ietf-ace-key-groupcomm-10
(work in progress), June 2020. (work in progress), November 2020.
[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", draft-ietf-ace-key-groupcomm-
oscore-07 (work in progress), June 2020. oscore-09 (work in progress), November 2020.
[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-34 Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-35
(work in progress), June 2020. (work in progress), June 2020.
[I-D.ietf-core-echo-request-tag] [I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo, Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
Request-Tag, and Token Processing", draft-ietf-core-echo- Request-Tag, and Token Processing", draft-ietf-core-echo-
request-tag-09 (work in progress), March 2020. request-tag-10 (work in progress), July 2020.
[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-10 (work in draft-ietf-lwig-curve-representations-12 (work in
progress), April 2020. progress), August 2020.
[I-D.ietf-lwig-security-protocol-comparison] [I-D.ietf-lwig-security-protocol-comparison]
Mattsson, J., Palombini, F., and M. Vucinic, "Comparison Mattsson, J., Palombini, F., and M. Vucinic, "Comparison
of CoAP Security Protocols", draft-ietf-lwig-security- of CoAP Security Protocols", draft-ietf-lwig-security-
protocol-comparison-04 (work in progress), March 2020. protocol-comparison-04 (work in progress), March 2020.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-38 (work in progress), May
2020.
[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., 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", draft-mattsson-cfrg-det-sigs-with-noise-02
(work in progress), March 2020. (work in progress), March 2020.
[I-D.somaraju-ace-multicast] [I-D.somaraju-ace-multicast]
Somaraju, A., Kumar, S., Tschofenig, H., and W. Werner, Somaraju, A., Kumar, S., Tschofenig, H., and W. Werner,
"Security for Low-Latency Group Communication", draft- "Security for Low-Latency Group Communication", draft-
somaraju-ace-multicast-02 (work in progress), October somaraju-ace-multicast-02 (work in progress), October
2016. 2016.
[I-D.tiloca-core-observe-multicast-notifications] [I-D.tiloca-core-observe-multicast-notifications]
Tiloca, M., Hoeglund, R., Amsuess, C., and F. Palombini, Tiloca, M., Hoeglund, R., Amsuess, C., and F. Palombini,
"Observe Notifications as CoAP Multicast Responses", "Observe Notifications as CoAP Multicast Responses",
draft-tiloca-core-observe-multicast-notifications-02 (work draft-tiloca-core-observe-multicast-notifications-04 (work
in progress), March 2020. in progress), November 2020.
[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>.
skipping to change at page 49, line 21 skipping to change at page 54, line 40
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 o 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. o Security group, as defined in Section 1.1 of this specification.
There can be a one-to-one or a one-to-many relation between There can be a one-to-one or a one-to-many relation between
security groups and application groups, and vice versa. Any two security groups and application groups, and vice versa.
application groups associated to the same security group do not
share any same resource.
o CoAP group, as defined in [I-D.ietf-core-groupcomm-bis] i.e. a set o CoAP group, as defined in [I-D.ietf-core-groupcomm-bis] i.e. a set
of CoAP endpoints, where each endpoint is configured to receive of CoAP endpoints, where each endpoint is configured to receive
CoAP multicast requests that are sent to the group's associated IP CoAP multicast requests that are sent to the group's associated IP
multicast address and UDP port. An endpoint may be a member of multicast address and UDP port. 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,
skipping to change at page 54, line 45 skipping to change at page 60, line 19
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, including a new Gid, incremented by 1 each time new keying material, together with a new
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 3.1).
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 between two consecutive usages of the same Group Epoch value elapses before all possible Group Epoch values are used, since the
in that group. This ensures that the Gid value is temporally unique Group Manager does not reassign the same Gid to the same group.
during the lifetime of a given message. Thus, the expected highest Thus, the expected highest rate for addition/removal of group members
rate for addition/removal of group members and consequent group and consequent group rekeying should be taken into account for a
rekeying should be taken into account for a proper dimensioning of proper dimensioning of the Group Epoch size.
the Group Epoch size.
As discussed in Section 10.5, if endpoints are deployed in multiple As discussed in Section 10.5, 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 favourable that message is correctly verified. Therefore, it is favourable that
Group Identifiers from different Group Managers have a size that Group Identifiers from different Group Managers have a size that
result in a small probability of collision. How small this result in a small probability of collision. How small this
skipping to change at page 56, line 20 skipping to change at page 61, line 39
Appendix E. Examples of Synchronization Approaches Appendix E. Examples of Synchronization Approaches
This section describes three possible approaches that can be This section describes three possible approaches that can be
considered by server endpoints to synchronize with Sender Sequence considered by server endpoints to synchronize with Sender Sequence
Numbers of client endpoints sending group requests. Numbers of client endpoints sending group requests.
The Group Manager MAY indicate which of such approaches are used in The Group Manager MAY indicate which of such approaches are used in
the group, as part of the group communication policies signalled to the group, as part of the group communication policies signalled to
candidate group members upon their group joining. candidate group members upon their group joining.
If a server has recently lost the mutable Security Context, e.g. due
to a reboot, the server has also to establish an updated Security
Context before resuming to send protected messages to the group (see
Section 2.4.1). Since this results in deriving a new Sender Key for
its Sender Context, the server does not reuse the same pair (key,
nonce), even when using the Partial IV of (old re-injected) requests
to build the AEAD nonce for protecting the corresponding responses.
E.1. Best-Effort Synchronization E.1. Best-Effort Synchronization
Upon receiving a group request from a client, a server does not take Upon receiving a group request from a client, a server does not take
any action to synchronize with the sender sequence number of that any action to synchronize with the Sender Sequence Number of that
client. This provides no assurance at all as to message freshness, client. This provides no assurance at all as to message freshness,
which can be acceptable in non-critical use cases. which can be acceptable in non-critical use cases.
With the notable exception of Observe notifications and responses With the notable exception of Observe notifications and responses
following a group rekeying, it is optional for the server to use the following a group rekeying, it is optional for the server to use its
sender sequence number as Partial IV. Instead, for efficiency Sender Sequence Number as Partial IV when protecting a response.
reasons, the server may rather use the request's Partial IV when Instead, for efficiency reasons, the server may rather use the
protecting a response. request's Partial IV when protecting a response to that request.
E.2. Baseline Synchronization E.2. Baseline Synchronization
Upon receiving a group request from a given client for the first Upon receiving a group request from a given client for the first
time, a server initializes its last-seen Sender Sequence Number in time, a server initializes the last-seen Sender Sequence Number
its Recipient Context associated to that client. The server may also associated to that client in its corresponding Recipient Context.
drop the group request without delivering it to the application. The server may also drop the group request without delivering it to
This method provides a reference point to identify if future group the application. This method provides a reference point to identify
requests from the same client are fresher than the last one received. if future group requests from the same client are fresher than the
last one received.
A replay time interval exists, between when a possibly replayed or A replay time interval exists, between when a possibly replayed or
delayed message is originally transmitted by a given client and the delayed message is originally transmitted by a given client and the
first authentic fresh message from that same client is received. first authentic fresh message from that same client is received.
This can be acceptable for use cases where servers admit such a This can be acceptable for use cases where servers admit such a
trade-off between performance and assurance of message freshness. trade-off between performance and assurance of message freshness.
With the notable exception of Observe notifications and responses With the notable exception of Observe notifications and responses
following a group rekeying, it is optional for the server to use its following a group rekeying, it is optional for the server to use its
own Sender Sequence Number as Partial IV. Instead, for efficiency Sender Sequence Number as Partial IV when protecting a response.
reasons, the server may rather use the request's Partial IV when Instead, for efficiency reasons, the server may rather use the
protecting a response. request's Partial IV when protecting a response to that request.
E.3. Challenge-Response Synchronization E.3. Challenge-Response Synchronization
A server performs a challenge-response exchange with a client, by A server performs a challenge-response exchange with a client, by
using the Echo Option for CoAP described in Section 2 of using the Echo Option for CoAP described in Section 2 of
[I-D.ietf-core-echo-request-tag] and according to Appendix B.1.2 of [I-D.ietf-core-echo-request-tag] and according to Appendix B.1.2 of
[RFC8613]. [RFC8613].
That is, upon receiving a group request from a particular client for That is, upon receiving a group request from a particular client for
the first time, the server processes the message as described in this the first time, the server processes the message as described in this
specification, but, even if valid, does not deliver it to the specification, but, even if valid, does not deliver it to the
application. Instead, the server replies to the client with an application. Instead, the server replies to the client with an
OSCORE protected 4.01 (Unauthorized) response message, including only OSCORE protected 4.01 (Unauthorized) response message, including only
the Echo Option and no diagnostic payload. Since this response is the Echo Option and no diagnostic payload. The server MUST NOT set
protected with the Security Context used in the group, the client the Echo Option to a value which is both predictable and reusable.
will consider the response valid upon successfully decrypting and Since this response is protected with the Security Context used in
verifying it. the group, the client will consider the response valid upon
successfully decrypting and 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 group request, respectively. After a group rekeying Option of the group request, respectively. After a group rekeying
has been completed and a new Security Context has been established in has been completed and a new Security Context has been established in
the group, which results also in a new Group Identifier (see the group, which results also in a new Group Identifier (see
Section 3.1), the server MUST delete all the stored Echo values Section 3.1), the server MUST delete all the stored Echo values
associated to members of that group. associated to members of that group.
skipping to change at page 57, line 43 skipping to change at page 63, line 23
Option and originates from a verified group member, a client sends a Option and originates from a verified group member, a client sends a
request as a unicast message addressed to the same server, echoing 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.
In particular, the client does not necessarily resend the same group In particular, the client does not necessarily resend the same group
request, but can instead send a more recent one, if the application request, but can instead send a more recent one, if the application
permits it. This makes it possible for the client to not retain permits it. This makes it possible for the client to not retain
previously sent group requests for full retransmission, unless the previously sent group requests for full retransmission, unless the
application explicitly requires otherwise. In either case, the application explicitly requires otherwise. In either case, the
client uses the Sender Sequence Number value currently stored in its client uses a fresh Sender Sequence Number value from its own Sender
own Sender Context. If the client stores group requests for possible Context. If the client stores group requests for possible
retransmission with the Echo Option, it should not store a given retransmission with the Echo Option, it should not store a given
request for longer than a pre-configured time interval. Note that request for longer than a pre-configured time interval. Note that
the unicast request echoing the Echo Option is correctly treated and the unicast request echoing the Echo Option is correctly treated and
processed as a message, since the 'kid context' field including the processed as a message, since the 'kid context' field including the
Group Identifier of the OSCORE group is still present in the OSCORE Group Identifier of the OSCORE group is still present in the OSCORE
Option as part of the COSE object (see Section 4). Option as part of the 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.
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(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) o 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 by 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.
In case of positive verification, the request is further processed In case of positive verification, the request is further processed
and verified. Finally, the server updates the Recipient Context and verified. Finally, the server updates the Recipient Context
associated to that client, by setting the Replay Window according to associated to that client, by setting the Replay Window according to
the Sequence Number from the unicast request conveying the Echo the Sender Sequence Number from the unicast request conveying the
Option. The server either delivers the request to the application if Echo Option. The server either delivers the request to the
it is an actual retransmission of the original one, or discards it application if it is an actual retransmission of the original one, or
otherwise. Mechanisms to signal whether the resent request is a full discards it otherwise. Mechanisms to signal whether the resent
retransmission of the original one are out of the scope of this request is a full retransmission of the original one are out of the
specification. scope of this specification.
A server should not deliver group requests from a given client to the A server should not deliver group requests from a given client to the
application until one valid request from that same client has been application until one valid request from that same client has been
verified as fresh, as conveying an echoed Echo Option verified as fresh, as conveying an echoed Echo Option
[I-D.ietf-core-echo-request-tag]. Also, a server may perform the [I-D.ietf-core-echo-request-tag]. Also, a server may perform the
challenge-response described above at any time, if synchronization challenge-response described above at any time, if synchronization
with Sender Sequence Numbers of clients is (believed to be) lost, for with Sender Sequence Numbers of clients is (believed to be) lost, for
instance after a device reboot. A client has to be always ready to instance after a device reboot. A client has to be always ready to
perform the challenge-response based on the Echo Option in case a perform the challenge-response based on the Echo Option in case a
server starts it. server starts it.
It is the role of the server application to define under what It is the role of the server application to define under what
circumstances Sender Sequence Numbers lose synchronization. This can circumstances Sender Sequence Numbers lose synchronization. This can
include experiencing a "large enough" gap D = (SN2 - SN1), between include experiencing a "large enough" gap D = (SN2 - SN1), between
the Sender Sequence Number SN1 of the latest accepted group request the Sender Sequence Number SN1 of the latest accepted group request
from a client and the Sender Sequence Number SN2 of a group request from a client and the Sender Sequence Number SN2 of a group request
just received from that client. However, a client may send several just received from that client. However, a client may send several
unicast requests to different group members as protected with the unicast requests to different group members as protected with the
pairwise mode (see Section 9.2), which may consume the gap D at the pairwise mode (see Section 9.2), which may result in the server
server relatively fast. This would induce the server to perform more experiencing the gap D in a relatively short time. This would induce
challenge-response exchanges than actually needed. the server to perform more challenge-response exchanges than actually
needed.
To ameliorate this, the server may rather rely on a trade-off between To ameliorate this, the server may rather rely on a trade-off between
the Sender Sequence Number gap D and a time gap T = (t2 - t1), where the Sender Sequence Number gap D and a time gap T = (t2 - t1), where
t1 is the time when the latest group request from a client was t1 is the time when the latest group request from a client was
accepted and t2 is the time when the latest group request from that accepted and t2 is the time when the latest group request from that
client has been received, respectively. Then, the server can start a client has been received, respectively. Then, the server can start a
challenge-response when experiencing a time gap T larger than a challenge-response when experiencing a time gap T larger than a
given, pre-configured threshold. Also, the server can start a given, pre-configured threshold. Also, the server can start a
challenge-response when experiencing a Sender Sequence Number gap D challenge-response when experiencing a Sender Sequence Number gap D
greater than a different threshold, computed as a monotonically greater than a different threshold, computed as a monotonically
skipping to change at page 60, line 41 skipping to change at page 66, line 20
originated by a group member, although not specifically identifiable originated by a group member, although not specifically identifiable
in a secure way. This can violate a number of security requirements, 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 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 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 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, be used not only for light bulbs but for locks as well. Therefore,
extreme care must be taken in situations where the security extreme care must be taken in situations where the security
requirements are relaxed, so that deployment of the system will requirements are relaxed, so that deployment of the system will
always be done safely. always be done safely.
Appendix G. Optimized Request Appendix G. Example Values with COSE Capabilities
An optimized request is processed as a request in group mode
(Section 8.1) and uses the OSCORE header compression defined in
Section 5 for the group mode, with the following difference: the
payload of the OSCORE message SHALL encode the ciphertext without the
tag, concatenated with the value of the CounterSignature0 of the COSE
object computed as described in Section 4.1.
The optimized request is compatible with all AEAD algorithms defined
in [I-D.ietf-cose-rfc8152bis-algs], but would not be compatible with
AEAD algorithms that do not have a well-defined tag.
Appendix H. Example Values of Parameters for Countersignatures
The table below provides examples of values for Counter Signature The table below provides examples of values for Counter Signature
Parameters in the Common Context (see Section 2.1.3), for different Parameters in the Common Context (see Section 2.1.3), for different
values of Counter Signature Algorithm. values of Counter Signature Algorithm.
+-------------------+---------------------------------------------+ +-------------------+---------------------------------------------+
| Counter Signature | Example Values for Counter | | Counter Signature | Example Values for Counter |
| Algorithm | Signature Parameters | | Algorithm | Signature Parameters |
+-------------------+---------------------------------------------+ +-------------------+---------------------------------------------+
| (-8) // EdDSA | [1], [1, 6] // 1: OKP ; 1: OKP, 6: Ed25519 | | (-8) // EdDSA | [1], [1, 6] // 1: OKP ; 1: OKP, 6: Ed25519 |
| (-8) // EdDSA | [1], [1, 6] // 1: OKP ; 1: OKP, 7: Ed448 |
| (-7) // ES256 | [2], [2, 1] // 2: EC2 ; 2: EC2, 1: P-256 | | (-7) // ES256 | [2], [2, 1] // 2: EC2 ; 2: EC2, 1: P-256 |
| (-35) // ES384 | [2], [2, 2] // 2: EC2 ; 2: EC2, 2: P-384 | | (-35) // ES384 | [2], [2, 2] // 2: EC2 ; 2: EC2, 2: P-384 |
| (-36) // ES512 | [2], [2, 3] // 2: EC2 ; 2: EC2, 3: P-512 | | (-36) // ES512 | [2], [2, 3] // 2: EC2 ; 2: EC2, 3: P-512 |
| (-37) // PS256 | [], [3] // empty ; 3: RSA | | (-37) // PS256 | [], [3] // empty ; 3: RSA |
| (-38) // PS384 | [], [3] // empty ; 3: RSA | | (-38) // PS384 | [], [3] // empty ; 3: RSA |
| (-39) // PS512 | [], [3] // empty ; 3: RSA | | (-39) // PS512 | [], [3] // empty ; 3: RSA |
+-------------------+---------------------------------------------+ +-------------------+---------------------------------------------+
Figure 4: Examples of Counter Signature Parameters Figure 4: Examples of Counter Signature Parameters
The table below provides examples of values for Counter Signature Key The table below provides examples of values for Secret Derivation
Parameters in the Common Context (see Section 2.1.4), for different Parameters in the Common Context (see Section 2.1.5), for different
values of Counter Signature Algorithm. values of Secret Derivation Algorithm.
+-----------------------+--------------------------------------------+
| Secret Derivation | Example Values for Secret |
| Algorithm | Derivation Parameters |
+-----------------------+--------------------------------------------+
| (-27) // ECDH-SS | [1], [1, 6] // 1: OKP ; 1: OKP, 4: X25519 |
| // + HKDF-256 | |
| (-27) // ECDH-SS | [1], [1, 6] // 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
The table below provides examples of values for the
'par_countersign_key' element of the 'algorithms' array used in the
two external_aad structures (see Section 4.3.1 and Section 4.3.2),
for different values of Counter Signature Algorithm.
+-------------------+---------------------------------+ +-------------------+---------------------------------+
| Counter Signature | Example Values for Counter | | Counter Signature | Example Values for |
| Algorithm | Signature Key Parameters | | Algorithm | 'par_countersign_key' |
+-------------------+---------------------------------+ +-------------------+---------------------------------+
| (-8) // EdDSA | [1, 6] // 1: OKP , 6: Ed25519 | | (-8) // EdDSA | [1, 6] // 1: OKP , 6: Ed25519 |
| (-8) // EdDSA | [1, 6] // 1: OKP , 7: Ed448 |
| (-7) // ES256 | [2, 1] // 2: EC2 , 1: P-256 | | (-7) // ES256 | [2, 1] // 2: EC2 , 1: P-256 |
| (-35) // ES384 | [2, 2] // 2: EC2 , 2: P-384 | | (-35) // ES384 | [2, 2] // 2: EC2 , 2: P-384 |
| (-36) // ES512 | [2, 3] // 2: EC2 , 3: P-512 | | (-36) // ES512 | [2, 3] // 2: EC2 , 3: P-512 |
| (-37) // PS256 | [3] // 3: RSA | | (-37) // PS256 | [3] // 3: RSA |
| (-38) // PS384 | [3] // 3: RSA | | (-38) // PS384 | [3] // 3: RSA |
| (-39) // PS512 | [3] // 3: RSA | | (-39) // PS512 | [3] // 3: RSA |
+-------------------+---------------------------------+ +-------------------+---------------------------------+
Figure 5: Examples of Counter Signature Key Parameters Figure 6: Examples of 'par_countersign_key'
Appendix I. Document Updates Appendix H. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION. RFC EDITOR: PLEASE REMOVE THIS SECTION.
I.1. Version -08 to -09 H.1. Version -09 to -10
o Removed 'Counter Signature Key Parameters' from the Common
Context.
o New parameters in the Common Context covering the DH secret
derivation.
o New counter signature header parameter from draft-ietf-cose-
countersign.
o Stronger policies non non-recycling of Sender IDs and Gid.
o The Sender Sequence Number is reset when establishing a new
Security Context.
o Added 'request_kid_context' in the aad_array.
o The server can respond with 5.03 if the client's public key is not
available.
o The observer client stores an invariant identifier of the group.
o Relaxed storing of original 'kid' for observer clients.
o Both client and server store the 'kid_context' of the original
observation request.
o The server uses a fresh PIV if protecting the response with a
Security Context different from the one used to protect the
request.
o Clarifications on MTI algorithms and curves.
o Removed optimized requests.
o Overall clarifications and editorial revision.
H.2. Version -08 to -09
o Pairwise keys are discarded after group rekeying. o Pairwise keys are discarded after group rekeying.
o Signature mode renamed to group mode. o Signature mode renamed to group mode.
o The parameters for countersignatures use the updated COSE o 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 o 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.
skipping to change at page 62, line 43 skipping to change at page 69, line 29
o Normative support for the signature and pairwise mode. o Normative support for the signature and pairwise mode.
o Revised methods for synchronization with clients' sender sequence o Revised methods for synchronization with clients' sender sequence
number. number.
o Appendix with example values of parameters for countersignatures. o Appendix with example values of parameters for countersignatures.
o Clarifications and editorial improvements. o Clarifications and editorial improvements.
I.2. Version -07 to -08 H.3. Version -07 to -08
o Clarified relation between pairwise mode and group communication o Clarified relation between pairwise mode and group communication
(Section 1). (Section 1).
o Improved definition of "silent server" (Section 1.1). o Improved definition of "silent server" (Section 1.1).
o Clarified when a Recipient Context is needed (Section 2). o Clarified when a Recipient Context is needed (Section 2).
o Signature checkers as entities supported by the Group Manager o Signature checkers as entities supported by the Group Manager
(Section 2.3). (Section 2.3).
skipping to change at page 64, line 13 skipping to change at page 70, line 44
(Section 10.15). (Section 10.15).
o Updates to the methods for synchronizing with clients' sequence o 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 o Simplified text on discovery services supporting the pairwise mode
(Appendix G.1). (Appendix G.1).
o Editorial improvements. o Editorial improvements.
I.3. Version -06 to -07 H.4. Version -06 to -07
o Updated abstract and introduction. o Updated abstract and introduction.
o Clarifications of what pertains a group rekeying. o Clarifications of what pertains a group rekeying.
o Derivation of pairwise keying material. o Derivation of pairwise keying material.
o Content re-organization for COSE Object and OSCORE header o Content re-organization for COSE Object and OSCORE header
compression. compression.
skipping to change at page 64, line 45 skipping to change at page 71, line 29
o Security considerations on Group OSCORE for unicast requests, also o 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 o Clarification on different types of groups considered
(application/security/CoAP). (application/security/CoAP).
o New pairwise mode, using pairwise keying material for both o New pairwise mode, using pairwise keying material for both
requests and responses. requests and responses.
I.4. Version -05 to -06 H.5. Version -05 to -06
o Group IDs mandated to be unique under the same Group Manager. o Group IDs mandated to be unique under the same Group Manager.
o Clarifications on parameter update upon group rekeying. o Clarifications on parameter update upon group rekeying.
o Updated external_aad structures. o Updated external_aad structures.
o Dynamic derivation of Recipient Contexts made optional and o Dynamic derivation of Recipient Contexts made optional and
application specific. application specific.
skipping to change at page 65, line 25 skipping to change at page 72, line 9
rekeying. rekeying.
o Added Group Manager responsibility on validating public keys. o Added Group Manager responsibility on validating public keys.
o Updates IANA registries. o Updates IANA registries.
o Reference to RFC 8613. o Reference to RFC 8613.
o Editorial improvements. o Editorial improvements.
I.5. Version -04 to -05 H.6. Version -04 to -05
o Added references to draft-dijk-core-groupcomm-bis. o Added references to draft-dijk-core-groupcomm-bis.
o New parameter Counter Signature Key Parameters (Section 2). o New parameter Counter Signature Key Parameters (Section 2).
o Clarification about Recipient Contexts (Section 2). o Clarification about Recipient Contexts (Section 2).
o Two different external_aad for encrypting and signing o Two different external_aad for encrypting and signing
(Section 3.1). (Section 3.1).
o Updated response verification to handle Observe notifications o Updated response verification to handle Observe notifications
(Section 6.4). (Section 6.4).
o Extended Security Considerations (Section 8). o Extended Security Considerations (Section 8).
o New "Counter Signature Key Parameters" IANA Registry o New "Counter Signature Key Parameters" IANA Registry
(Section 9.2). (Section 9.2).
I.6. Version -03 to -04 H.7. Version -03 to -04
o Added the new "Counter Signature Parameters" in the Common Context o 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 o Added recommendation on using "deterministic ECDSA" if ECDSA is
used as counter signature algorithm (see Section 2). used as counter signature algorithm (see Section 2).
o Clarified possible asynchronous retrieval of keying material from o 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).
skipping to change at page 66, line 41 skipping to change at page 73, line 23
o Handling of just created Recipient Contexts in case of o 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 o Handling of replied/repeated responses on the client (see
Section 6.4). Section 6.4).
o New IANA Registry "Counter Signature Parameters" (see o New IANA Registry "Counter Signature Parameters" (see
Section 9.1). Section 9.1).
I.7. Version -02 to -03 H.8. Version -02 to -03
o Revised structure and phrasing for improved readability and better o 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). o Added discussion on wrap-Around of Partial IVs (see Section 2.2).
o Separate sections for the COSE Object (Section 3) and the OSCORE o 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 o The countersignature is now appended to the encrypted payload of
skipping to change at page 67, line 29 skipping to change at page 74, line 8
Section 7. Section 7.
o Revised and extended security considerations in Section 8. o Revised and extended security considerations in Section 8.
o Added IANA considerations for the OSCORE Flag Bits Registry in o Added IANA considerations for the OSCORE Flag Bits Registry in
Section 9. Section 9.
o Revised Appendix D, now giving a short high-level description of a o Revised Appendix D, now giving a short high-level description of a
new endpoint set-up. new endpoint set-up.
I.8. Version -01 to -02 H.9. Version -01 to -02
o Terminology has been made more aligned with RFC7252 and draft- o Terminology has been made more aligned with RFC7252 and draft-
ietf-core-object-security: i) "client" and "server" replace the ietf-core-object-security: i) "client" and "server" replace the
old "multicaster" and "listener", respectively; ii) "silent old "multicaster" and "listener", respectively; ii) "silent
server" replaces the old "pure listener". server" replaces the old "pure listener".
o Section 2 has been updated to have the Group Identifier stored in o Section 2 has been updated to have the Group Identifier stored in
the 'ID Context' parameter defined in draft-ietf-core-object- the 'ID Context' parameter defined in draft-ietf-core-object-
security. security.
skipping to change at page 68, line 16 skipping to change at page 74, line 43
implications of possible collisions of group identifiers. implications of possible collisions of group identifiers.
o Updated Appendix D.2, adding a pointer to draft-palombini-ace-key- o Updated Appendix D.2, adding a pointer to draft-palombini-ace-key-
groupcomm about retrieval of nodes' public keys through the Group groupcomm about retrieval of nodes' public keys through the Group
Manager. Manager.
o Minor updates to Appendix E.3 about Challenge-Response o Minor updates to Appendix E.3 about Challenge-Response
synchronization of sequence numbers based on the Echo option from synchronization of sequence numbers based on the Echo option from
draft-ietf-core-echo-request-tag. draft-ietf-core-echo-request-tag.
I.9. Version -00 to -01 H.10. Version -00 to -01
o Section 1.1 has been updated with the definition of group as o Section 1.1 has been updated with the definition of group as
"security group". "security group".
o Section 2 has been updated with: o Section 2 has been updated with:
* Clarifications on etablishment/derivation of Security Contexts. * Clarifications on etablishment/derivation of Security Contexts.
* A table summarizing the the additional context elements * A table summarizing the the additional context elements
compared to OSCORE. compared to OSCORE.
skipping to change at page 69, line 14 skipping to change at page 75, line 39
Acknowledgments Acknowledgments
The authors sincerely thank Christian Amsuess, Stefan Beck, Rolf The authors sincerely thank Christian Amsuess, Stefan Beck, Rolf
Blom, Carsten Bormann, Esko Dijk, Klaus Hartke, Rikard Hoeglund, Blom, Carsten Bormann, Esko Dijk, Klaus Hartke, Rikard Hoeglund,
Richard Kelsey, John Mattsson, Dave Robin, Jim Schaad, Ludwig Seitz, Richard Kelsey, John Mattsson, Dave Robin, Jim Schaad, Ludwig Seitz,
Peter van der Stok and Erik Thormarker for their feedback and Peter van der Stok and Erik Thormarker for their feedback and
comments. comments.
The work on this document has been partly supported by VINNOVA and The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; the SSF project SEC4Factory under the Celtic-Next project CRITISEC; the H2020 project SIFIS-Home (Grant
the grant RIT17-0032; and the EIT-Digital High Impact Initiative agreement 952652); the SSF project SEC4Factory under the grant
ACTIVE. RIT17-0032; and the EIT-Digital High Impact Initiative ACTIVE.
Authors' Addresses Authors' Addresses
Marco Tiloca Marco Tiloca
RISE AB RISE AB
Isafjordsgatan 22 Isafjordsgatan 22
Kista SE-16440 Stockholm Kista SE-16440 Stockholm
Sweden Sweden
Email: marco.tiloca@ri.se Email: marco.tiloca@ri.se
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