draft-ietf-core-oscore-groupcomm-00.txt   draft-ietf-core-oscore-groupcomm-01.txt 
CoRE Working Group M. Tiloca CoRE Working Group M. Tiloca
Internet-Draft RISE SICS AB Internet-Draft RISE SICS AB
Intended status: Standards Track G. Selander Intended status: Standards Track G. Selander
Expires: August 16, 2018 F. Palombini Expires: September 6, 2018 F. Palombini
Ericsson AB Ericsson AB
J. Park J. Park
Universitaet Duisburg-Essen Universitaet Duisburg-Essen
February 12, 2018 March 05, 2018
Secure group communication for CoAP Secure group communication for CoAP
draft-ietf-core-oscore-groupcomm-00 draft-ietf-core-oscore-groupcomm-01
Abstract Abstract
This document describes a method for protecting group communication This document describes a mode for protecting group communication
over the Constrained Application Protocol (CoAP). The proposed over the Constrained Application Protocol (CoAP). The proposed mode
approach relies on Object Security for Constrained RESTful relies on Object Security for Constrained RESTful Environments
Environments (OSCORE) and the CBOR Object Signing and Encryption (OSCORE) and the CBOR Object Signing and Encryption (COSE) format.
(COSE) format. All security requirements fulfilled by OSCORE are In particular, it is defined how OSCORE should be used in a group
maintained for multicast OSCORE request messages and related OSCORE communication setting, while fulfilling the same security
response messages. Source authentication of all messages exchanged requirements for request messages and related response messages.
within the group is ensured, by means of digital signatures produced Source authentication of all messages exchanged within the group is
through private keys of sender devices and embedded in the protected ensured, by means of digital signatures produced through private keys
CoAP messages. of sender endpoints and embedded in the protected CoAP messages.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 16, 2018. This Internet-Draft will expire on September 6, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Assumptions and Security Objectives . . . . . . . . . . . . . 5 2. OSCORE Security Context . . . . . . . . . . . . . . . . . . . 5
2.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Management of Group Keying Material . . . . . . . . . . . 7
2.2. Security Objectives . . . . . . . . . . . . . . . . . . . 7 3. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 8
3. OSCORE Security Context . . . . . . . . . . . . . . . . . . . 7 3.1. Example: Request . . . . . . . . . . . . . . . . . . . . 10
3.1. Management of Group Keying Material . . . . . . . . . . . 9 3.2. Example: Response . . . . . . . . . . . . . . . . . . . . 10
4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 10 4. Message Processing . . . . . . . . . . . . . . . . . . . . . 11
5. Message Processing . . . . . . . . . . . . . . . . . . . . . 12 4.1. Protecting the Request . . . . . . . . . . . . . . . . . 11
5.1. Protecting the Request . . . . . . . . . . . . . . . . . 12 4.2. Verifying the Request . . . . . . . . . . . . . . . . . . 12
5.2. Verifying the Request . . . . . . . . . . . . . . . . . . 13 4.3. Protecting the Response . . . . . . . . . . . . . . . . . 12
5.3. Protecting the Response . . . . . . . . . . . . . . . . . 13 4.4. Verifying the Response . . . . . . . . . . . . . . . . . 12
5.4. Verifying the Response . . . . . . . . . . . . . . . . . 13 5. Synchronization of Sequence Numbers . . . . . . . . . . . . . 13
6. Synchronization of Sequence Numbers . . . . . . . . . . . . . 14 6. Responsibilities of the Group Manager . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7.1. Group-level Security . . . . . . . . . . . . . . . . . . 15 7.1. Group-level Security . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . 15 10.1. Normative References . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 16 10.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. List of Use Cases . . . . . . . . . . . . . . . . . 18 Appendix A. Assumptions and Security Objectives . . . . . . . . 19
Appendix B. Example of Group Identifier Format . . . . . . . . . 20 A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 19
Appendix C. Set-up of New Endpoints . . . . . . . . . . . . . . 21 A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 20
C.1. Join Process . . . . . . . . . . . . . . . . . . . . . . 21 Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 21
C.2. Provisioning and Retrieval of Public Keys . . . . . . . . 23 Appendix C. Example of Group Identifier Format . . . . . . . . . 23
C.3. Group Joining Based on the ACE Framework . . . . . . . . 24 Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 24
Appendix D. Examples of Synchronization Approaches . . . . . . . 25 D.1. Join Process . . . . . . . . . . . . . . . . . . . . . . 24
D.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 25 D.2. Provisioning and Retrieval of Public Keys . . . . . . . . 27
D.2. Baseline Synchronization . . . . . . . . . . . . . . . . 25 D.3. Group Joining Based on the ACE Framework . . . . . . . . 29
D.3. Challenge-Response Synchronization . . . . . . . . . . . 26 Appendix E. Examples of Synchronization Approaches . . . . . . . 29
Appendix E. No Verification of Signatures . . . . . . . . . . . 27 E.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 E.2. Baseline Synchronization . . . . . . . . . . . . . . . . 30
E.3. Challenge-Response Synchronization . . . . . . . . . . . 30
Appendix F. No Verification of Signatures . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
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]. networks [RFC7228].
Group communication for CoAP [RFC7390] addresses use cases where Group communication for CoAP [RFC7390] addresses use cases where
deployed devices benefit from a group communication model, for deployed devices benefit from a group communication model, for
example to reduce latencies and improve performance. Use cases example to reduce latencies and improve performance. Use cases
include lighting control, integrated building control, software and include lighting control, integrated building control, software and
firmware updates, parameter and configuration updates, commissioning firmware updates, parameter and configuration updates, commissioning
of constrained networks, and emergency multicast (see Appendix A). of constrained networks, and emergency multicast (see Appendix B).
Furthermore, [RFC7390] recognizes the importance to introduce a Furthermore, [RFC7390] recognizes the importance to introduce a
secure mode for CoAP group communication. This specification defines secure mode for CoAP group communication. This specification defines
such a mode. such a mode.
Object Security for Constrained RESTful Environments Object Security for Constrained RESTful Environments
(OSCORE)[I-D.ietf-core-object-security] describes a security protocol (OSCORE)[I-D.ietf-core-object-security] describes a security protocol
based on the exchange of protected CoAP messages. OSCORE builds on based on the exchange of protected CoAP messages. OSCORE builds on
CBOR Object Signing and Encryption (COSE) [RFC8152] and provides end- CBOR Object Signing and Encryption (COSE) [RFC8152] and provides end-
to-end encryption, integrity, and replay protection between a sending to-end encryption, integrity, and replay protection between a sending
endpoint and a receiving endpoint across intermediary nodes. To this endpoint and a receiving endpoint possibly involving intermediary
end, a CoAP message is protected by including payload (if any), endpoints. To this end, a CoAP message is protected by including its
certain options, and header fields in a COSE object, which finally payload (if any), certain options, and header fields in a COSE
replaces the authenticated and encrypted fields in the protected object, which finally replaces the authenticated and encrypted fields
message. in the protected message.
This document describes multicast OSCORE, providing end-to-end This document describes group OSCORE, providing end-to-end security
security of CoAP messages exchanged between members of a multicast of CoAP messages exchanged between members of a group. In
group. In particular, the described approach defines how OSCORE particular, the described approach defines how OSCORE should be used
should be used in a group communication context, while fulfilling the in a group communication setting, so that end-to-end security is
same security requirements. That is, end-to-end security is assured assured by using the same security method. That is, end-to-end
for multicast CoAP requests sent by multicaster nodes to the group security is assured for multicast CoAP requests sent by multicaster
and for related CoAP responses sent as reply by multiple listener endpoints to the group and for related CoAP responses sent as reply
nodes. Multicast OSCORE provides source authentication of all CoAP by multiple listener endpoints. Group OSCORE provides source
messages exchanged within the group, by means of digital signatures authentication of all CoAP messages exchanged within the group, by
produced through private keys of sender devices and embedded in the means of digital signatures produced through private keys of sender
protected CoAP messages. As in OSCORE, it is still possible to devices and embedded in the protected CoAP messages. As in OSCORE,
simultaneously rely on DTLS to protect hop-by-hop communication it is still possible to simultaneously rely on DTLS to protect hop-
between a multicaster node and a proxy (and vice versa), and between by-hop communication between a multicaster endpoint and a proxy (and
a proxy and a listener node (and vice versa). vice versa), and between a proxy and a listener endpoint (and vice
versa).
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]; group communication for CoAP [RFC7390]; described in CoAP [RFC7252] including "endpoint", "sender" and
COSE and counter signatures [RFC8152]. "recipient"; group communication for CoAP [RFC7390]; COSE and counter
signatures [RFC8152].
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", "Master Secret" and "Master Salt", defined in as "Security Context", "Master Secret" and "Master Salt", defined in
[I-D.ietf-core-object-security]. [I-D.ietf-core-object-security].
Terminology for constrained environments, such as "constrained Terminology for constrained environments, such as "constrained
device", "constrained-node network", is defined in [RFC7228]. device", "constrained-node network", is defined in [RFC7228].
This document refers also to the following terminology. This document refers also to the following terminology.
o Keying material: data that is necessary to establish and maintain o Keying material: data that is necessary to establish and maintain
secure communication among member of a multicast group. This secure communication among endpoints. This includes, for
includes, for instance, keys and IVs [RFC4949]. instance, keys and IVs [RFC4949].
o Group Manager (GM): entity responsible for creating a multicast o Group: a set of endpoints that share group keying material and
group, establishing and provisioning Security Contexts among parameters (Common Context of the group's Security Context, see
authorized group members, as well as managing the joining of new Section 2). That is, the term group used in this specification
group members and the leaving of current group members. A GM can refers to a "security group", not to be confused with network/
be responsible for multiple multicast groups. Besides, a GM is multicast groups or application groups.
not required to be an actual group member and to take part in the
group communication. The GM is also responsible for renewing/
updating Security Contexts and related keying material in the
multicast groups of its competence. Each endpoint in a multicast
group securely communicates with the respective GM.
o Multicaster: member of a multicast group that sends multicast CoAP o Group Manager (GM): entity responsible for a set of OSCORE groups.
Each endpoint in a group securely communicates with the respective
GM, which is not required to be an actual group member and to take
part in the group communication. The full list of
responsibilities of the Group Manager is provided in Section 6.
o Multicaster: member of a group that sends multicast CoAP request
messages intended for all members of the group. In a 1-to-N messages intended for all members of the group. In a 1-to-N
multicast group, only a single multicaster transmits data to the communication model, only a single multicaster transmits data to
group; in an M-to-N multicast group (where M and N do not the group; in an M-to-N communication model (where M and N do not
necessarily have the same value), M group members are necessarily have the same value), M group members are
multicasters. According to [RFC7390], any possible proxy entity multicasters. According to [RFC7390], any possible proxy entity
is supposed to know about the multicasters in the group and to not is supposed to know about the multicasters in the group and to not
perform aggregation of response messages. Also, every multicaster perform aggregation of response messages. Also, every multicaster
expects and is able to handle multiple response messages expects and is able to handle multiple response messages
associated to a given multicast request message that it has associated to a given multicast request message that it has
previously sent to the group. previously sent to the group.
o Listener: member of a multicast group that receives multicast CoAP o Listener: member of a group that receives multicast CoAP request
messages when listening to the multicast IP address associated to messages when listening to the multicast IP address associated to
the multicast group. A listener may reply back, by sending a the group. A listener may reply back, by sending a response
response message to the multicaster which has sent the multicast message to the multicaster which has sent the request message.
message.
o Pure listener: member of a multicast group that is configured as o Pure listener: member of a group that is configured as listener
listener and never replies back to multicasters after receiving and never replies back to multicasters after receiving request
multicast messages. messages.
o Endpoint ID: identifier assigned by the Group Manager to an o Group ID: group identifier assigned to the group. Group IDs are
endpoint upon joining the group as a new member, unless configured unique within the set of groups of a same Group Manager.
exclusively as pure listener. The Group Manager generates and
manages Endpoint IDs in order to ensure their uniqueness within a o Endpoint ID: Sender ID of the endpoint, as defined in
same multicast group. That is, within a single multicast group, [I-D.ietf-core-object-security]. An Endpoint ID is provided to an
the same Endpoint ID cannot be associated to more endpoints at the endpoint upon joining a group, is valid only within that group,
same time. Endpoint IDs are not necessarily related to any and is unique within the same group. Endpoints which are
protocol-relevant identifiers, such as IP addresses. configured only as pure listeners do not have an Endpoint ID.
o Group request: multicast CoAP request message sent by a o Group request: multicast CoAP request message sent by a
multicaster in the group to all listeners in the group through multicaster in the group to all listeners in the group through
multicast IP, unless otherwise specified. multicast IP, unless otherwise specified.
o Source authentication: evidence that a received message in the o Source authentication: evidence that a received message in the
group originated from a specifically identified group member. group originated from a specifically identified group member.
This also provides assurances that the message was not tampered This also provides assurances that the message was not tampered
with either by a different group member or by a non-group member. with by a different group member or by a non-group member.
2. Assumptions and Security Objectives
This section presents a set of assumptions and security objectives
for the approach described in this document.
2.1. Assumptions
The following assumptions are assumed to be already addressed and are
out of the scope of this document.
o Multicast communication topology: this document considers both
1-to-N (one multicaster and multiple listeners) and M-to-N
(multiple multicasters and multiple listeners) communication
topologies. The 1-to-N communication topology is the simplest
group communication scenario that would serve the needs of a
typical low-power and lossy network (LLN). For instance, in a
typical lighting control use case, a single switch is the only
entity responsible for sending commands to a group of lighting
devices. In more advanced lighting control use cases, a M-to-N
communication topology would be required, for instance in case
multiple sensors (presence or day-light) are responsible to
trigger events to a group of lighting devices.
o Multicast group size: security solutions for group communication
should be able to adequately support different, possibly large,
group sizes. Group size is the combination of the number of
multicasters and listeners in a multicast group, with possible
overlap (i.e. a multicaster may also be a listener at the same
time). In the use cases mentioned in this document, the number of
multicasters (normally the controlling devices) is expected to be
much smaller than the number of listeners (i.e. the controlled
devices). A security solution for group communication that
supports 1 to 50 multicasters would be able to properly cover the
group sizes required for most use cases that are relevant for this
document. The total number of group members is expected to be in
the range of 2 to 100 devices. Groups larger than that should be
divided into smaller independent multicast groups, e.g. by
grouping lights in a building on a per floor basis.
o Establishment and management of Security Contexts: a Security
Context must be established among the group members by the Group
Manager which manages the multicast group. A secure mechanism
must be used to generate, revoke and (re-)distribute keying
material, multicast security policies and security parameters in
the multicast group. The actual establishment and management of
the Security Context is out of the scope of this document, and it
is anticipated that an activity in IETF dedicated to the design of
a generic key management scheme will include this feature,
preferably based on [RFC3740][RFC4046][RFC4535].
o Multicast data security ciphersuite: all group members MUST agree
on a ciphersuite to provide authenticity, integrity and
confidentiality of messages in the multicast group. The
ciphersuite is specified as part of the Security Context.
o Backward security: a new device joining the multicast group should
not have access to any old Security Contexts used before its
joining. This ensures that a new group member is not able to
decrypt confidential data sent before it has joined the group.
The adopted key management scheme should ensure that the Security
Context is updated to ensure backward confidentiality. The actual
mechanism to update the Security Context and renew the group
keying material upon a group member's joining has to be defined as
part of the group key management scheme.
o Forward security: entities that leave the multicast group should
not have access to any future Security Contexts or message
exchanged within the group after their leaving. This ensures that
a former group member is not able to decrypt confidential data
sent within the group anymore. Also, it ensures that a former
member is not able to send encrypted and/or integrity protected
messages to the group anymore. The actual mechanism to update the
Security Context and renew the group keying material upon a group
member's leaving has to be defined as part of the group key
management scheme.
2.2. Security Objectives
The approach described in this document aims at fulfilling the
following security objectives:
o Data replay protection: replayed group request messages or
response messages MUST be detected.
o Group-level data confidentiality: messages sent within the
multicast group SHALL be encrypted if privacy sensitive data is
exchanged within the group. In fact, some control commands and/or
associated responses could pose unforeseen security and privacy
risks to the system users, when sent as plaintext. This document
considers group-level data confidentiality since messages are
encrypted at a group level, i.e. in such a way that they can be
decrypted by any member of the multicast group, but not by an
external adversary or other external entities.
o Source authentication: messages sent within the multicast group
SHALL be authenticated. That is, it is essential to ensure that a
message is originated by a member of the group in the first place
(group authentication), and in particular by a specific member of
the group (source authentication).
o Message integrity: messages sent within the multicast group SHALL
be integrity protected. That is, it is essential to ensure that a
message has not been tampered with by an external adversary or
other external entities which are not group members.
o Message ordering: it MUST be possible to determine the ordering of
messages coming from a single sender endpoint. In accordance with
OSCORE [I-D.ietf-core-object-security], this results in providing
relative freshness of group requests and absolute freshness of
responses. It is not required to determine ordering of messages
from different sender endpoints.
3. OSCORE Security Context 2. OSCORE Security Context
To support multicast communication secured with OSCORE, each endpoint To support group communication secured with OSCORE, each endpoint
registered as member of a multicast group maintains a Security registered as member of a group maintains a Security Context as
Context as defined in Section 3 of [I-D.ietf-core-object-security]. defined in Section 3 of [I-D.ietf-core-object-security]. In
In particular, each endpoint in a group stores: particular, each endpoint in a group stores:
1. one Common Context, received from the Group Manager upon joining 1. one Common Context, shared by all the endpoints in the group.
the multicast group and shared by all the endpoints in the group.
All the endpoints in the group agree on the same COSE AEAD All the endpoints in the group agree on the same COSE AEAD
algorithm. In addition to what is defined in Section 3 of algorithm. In addition to what is defined in Section 3 of
[I-D.ietf-core-object-security], the Common Context includes the [I-D.ietf-core-object-security], the Common Context includes the
following information. following information.
* Group Identifier (Gid). Variable length byte string * Group Identifier (Gid). Variable length byte string
identifying the Security Context and used as Master Salt identifying the Security Context. A Gid MUST have a random
parameter in the derivation of keying material. The Gid is component and be long enough, in order to achieve a negligible
used together with the multicast IP address of the group to probability of collisions between Group Identifiers from
retrieve the Security Context, upon receiving a secure different Group Managers. A Group ID is used i) alone or
multicast request message (see Section 5.2). The Gid together with other parameters, such as the multicast IP
associated to a multicast group is determined by the address of the group, to retrieve the OSCORE Security Context
responsible Group Manager. The choice of the Gid for a given of the associated group (see Section 4); and ii) as OSCORE
group's Security Context is application specific. However, a Master Salt (see Section 3.1 of
Gid MUST be random as well as long enough, in order to achieve [I-D.ietf-core-object-security]). The choice of the Gid for a
a negligible probability of collisions between Group given group's Security Context is application specific. It is
Identifiers from different Group Managers. It is the role of the role of the application to specify how to handle possible
the application to specify how to handle possible collisions. collisions. An example of specific formatting of the Group
An example of specific formatting of the Group Identifier that Identifier that would follow this specification is given in
would follow this specification is given in Appendix B. Appendix C.
* Counter signature algorithm. Value identifying the algorithm * Counter Signature Algorithm. Value identifying the algorithm
used for source authenticating messages sent within the group, used for source authenticating messages sent within the group,
by means of a counter signature (see Section 4.5 of by means of a counter signature (see Section 4.5 of
[RFC8152]). Its value is immutable once the Security Context [RFC8152]). Its value is immutable once the Common Context is
is established. All the endpoints in the group agree on the established. All the endpoints in the group agree on the same
same counter signature algorithm. The Group Manager MUST counter signature algorithm. The list of supported signature
define a list of supported signature algorithms as part of the algorithms is part of the group communication policy and MUST
group communication policy. Such a list MUST include the include the EdDSA signature algorithm ed25519 [RFC8032].
EdDSA signature algorithm ed25519 [RFC8032].
2. one Sender Context, unless the endpoint is configured exclusively 2. one Sender Context, unless the endpoint is configured exclusively
as pure listener. The Sender Context is used to secure outgoing as pure listener. The Sender Context is used to secure outgoing
messages and is initialized according to Section 3 of group messages and is initialized according to Section 3 of
[I-D.ietf-core-object-security], once the endpoint has joined the [I-D.ietf-core-object-security], once the endpoint has joined the
multicast group. In practice, the sender endpoint shares the group. In practice, the symmetric keying material in the Sender
same symmetric keying material stored in the Sender Context with Context of the sender endpoint is shared with all the recipient
all the recipient endpoints receiving its outgoing OSCORE endpoints that have received group messages from that same sender
messages. The Sender ID in the Sender Context coincides with the endpoint. Besides, in addition to what is defined in
Endpoint ID received upon joining the group. It is
responsibility of the Group Manager to assign Endpoint IDs to new
joining endpoints in such a way that uniquess is ensured within
the multicast group. Besides, in addition to what is defined in
[I-D.ietf-core-object-security], the Sender Context stores also [I-D.ietf-core-object-security], the Sender Context stores also
the endpoint's public-private key pair. the endpoint's public-private key pair.
3. one Recipient Context for each distinct endpoint from which 3. one Recipient Context for each distinct endpoint from which group
messages are received, used to process such incoming secure messages are received, used to process such incoming messages.
messages. The endpoint creates a new Recipient Context upon The recipient endpoint creates a new Recipient Context upon
receiving an incoming message from another endpoint in the group receiving an incoming message from another endpoint in the group
for the first time. In practice, the recipient endpoint shares for the first time (see Section 4.2 and Section 4.4). In
the symmetric keying material stored in the Recipient Context practice, the symmetric keying material in a given Recipient
with the associated other endpoint from which secure messages are Context of the recipient endpoint is shared with the associated
received. Besides, in addition to what is defined in sender endpoint from which group messages are received. Besides,
in addition to what is defined in
[I-D.ietf-core-object-security], each Recipient Context stores [I-D.ietf-core-object-security], each Recipient Context stores
also the public key of the associated other endpoint from which also the public key of the associated other endpoint from which
secure messages are received. group messages are received.
The table in Figure 1 overviews the new information included in the
OSCORE Security Context, with respect to what defined in Section 3 of
[I-D.ietf-core-object-security].
+---------------------------+-----------------------------+
| Context portion | New information |
+---------------------------+-----------------------------+
| | |
| Common Context | Group Identifier (Gid) |
| | |
| Common Context | Counter signature algorithm |
| | |
| Sender Context | Endpoint's private key |
| | |
| Sender Context | Endpoint's public key |
| | |
| Each Recipient Context | Public key of the |
| | associated other endpoint |
| | |
+---------------------------+-----------------------------+
Figure 1: Additions to the OSCORE Security Context
Upon receiving a secure CoAP message, a recipient endpoint relies on Upon receiving a secure CoAP message, a recipient endpoint relies on
the sender endpoint's public key, in order to verify the counter the sender endpoint's public key, in order to verify the counter
signature conveyed in the COSE Object. signature conveyed in the COSE Object.
If not already stored in the Recipient Context associated to the If not already stored in the Recipient Context associated to the
sender endpoint, the recipient endpoint retrieves the public key from sender endpoint, the recipient endpoint retrieves the public key from
a trusted key repository. In such a case, the correct binding a trusted key repository. In such a case, the correct binding
between the sender endpoint and the retrieved public key MUST be between the sender endpoint and the retrieved public key must be
assured, for instance by means of public key certificates. assured, for instance by means of public key certificates. Further
discussion about how public keys can be handled and retrieved in the
It is RECOMMENDED that the Group Manager acts as trusted key group is provided in Appendix D.2.
repository, and hence is configured to store public keys of group
members and provide them to other members of the same group upon
request. Possible approaches to provision public keys upon joining
the group and to retrieve public keys of group members are discussed
in Appendix C.2.
The Sender Key/IV stored in the Sender Context and the Recipient The Sender Key/IV stored in the Sender Context and the Recipient
Keys/IVs stored in the Recipient Contexts are derived according to Keys/IVs stored in the Recipient Contexts are derived according to
the same scheme defined in Section 3.2 of the same scheme defined in Section 3.2 of
[I-D.ietf-core-object-security]. [I-D.ietf-core-object-security].
3.1. Management of Group Keying Material 2.1. Management of Group Keying Material
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. Such a risk is reduced when the risk of compromise of group members. In particular, the adoption
multicast groups are deployed in physically secured locations, like of key management schemes for secure revocation and renewal of
lighting inside office buildings. Nevertheless, the adoption of key Security Contexts and group keying material should be considered.
management schemes for secure revocation and renewal of Security
Contexts and group keying material should be considered.
Consistently with the security assumptions in Section 2, it is Consistently with the security assumptions in Appendix A.1, it is
RECOMMENDED to adopt a group key management scheme, and securely RECOMMENDED to adopt a group key management scheme, and securely
distribute a new value for the Master Secret parameter of the group's distribute a new value 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 in order to preserve backward security and forward security necessary in order to preserve backward security and forward security
in the multicast group. The Group Manager responsible for the group in the group.
is entrusted with such a task.
In particular, the Group Manager MUST distribute also a new Group In particular, a new Group Identifier (Gid) for that group and a new
Identifier (Gid) for that group, together with a new value for the value for the Master Secret parameter must also be distributed. An
Master Secret parameter. An example of how this can be done is example of Group Identifier format supporting this operation is
provided in Appendix B. Then, each group member re-derives the provided in Appendix C. Then, each group member re-derives the
keying material stored in its own Sender Context and Recipient keying material stored in its own Sender Context and Recipient
Contexts as described in Section 3, using the updated Group Contexts as described in Section 2, using the updated Group
Identifier. Identifier.
Especially in dynamic, large-scale, multicast groups where endpoints Especially in dynamic, large-scale, groups where endpoints can join
can join and leave at any time, it is important that the considered and leave at any time, it is important that the considered group key
group key management scheme is efficient and highly scalable with the management scheme is efficient and highly scalable with the group
group size, in order to limit the impact on performance due to the size, in order to limit the impact on performance due to the Security
Security Context and keying material update. Context and keying material update.
4. The COSE Object 3. The COSE Object
When creating a protected CoAP message, an endpoint in the group When creating a protected CoAP message, an endpoint in the group
computes the COSE object using the untagged COSE_Encrypt0 structure computes the COSE object using the untagged COSE_Encrypt0 structure
[RFC8152] as defined in Section 5 of [I-D.ietf-core-object-security], [RFC8152] as defined in Section 5 of [I-D.ietf-core-object-security],
with the following modifications. with the following modifications.
o The value of the "kid" parameter in the "unprotected" field of o The value of the "kid" parameter in the "unprotected" field of
responses SHALL be set to the Sender ID of the endpoint response messagess SHALL be set to the Endpoint ID of the endpoint
transmitting the group message. transmitting the message, i.e. the Sender ID.
o The "unprotected" field of the "Headers" field SHALL additionally o The "unprotected" field of the "Headers" field SHALL additionally
include the following parameters: include the following parameter:
* gid : its value is set to the Group Identifier (Gid) of the
group's Security Context. This parameter MAY be omitted if the
message is a CoAP response.
* countersign : its value is set to the counter signature of the
COSE object (Appendix C.3.3 of [RFC8152]), computed by the
endpoint by means of its own private key as described in
Section 4.5 of [RFC8152].
In particular, "gid" is included as COSE header parameter as defined
in Figure 1.
+------+-------+------------+----------------+-------------------+
| name | label | value type | value registry | description |
+------+-------+------------+----------------+-------------------+
| gid | TBD | bstr | | Identifies the |
| | | | | OSCORE group |
| | | | | Security Context |
+------+-------+------------+----------------+-------------------+
Figure 1: Additional common header parameter for the COSE object * CounterSignature0 : its value is set to the counter signature
of the COSE object, computed by the endpoint by means of its
own private key as described in Section 4.5 of [RFC8152]. The
presence of this parameter is explicitly signaled, by using the
reserved sixth least significant bit of the first byte of flag
bits in the value of the Object-Security option (see
Section 6.1 of [I-D.ietf-core-object-security]).
o The Additional Authenticated Data (AAD) considered to compute the o The Additional Authenticated Data (AAD) considered to compute the
COSE object is extended, in order to include also the Group COSE object is extended, by adding the countersignature algorithm
Identifier (Gid) of the Security Context used to protect the used to protect group messages. In particular, the "external_aad"
request message. In particular, the "external_aad" in Section 5.3 defined in Section 5.4 of [I-D.ietf-core-object-security] SHALL
of [I-D.ietf-core-object-security] SHALL include also gid as also include "alg_countersign", which contains the Counter
follows: Signature Algorithm from the Common Context (see Section 2).
external_aad = [ external_aad = [
version : uint, oscore_version : uint,
alg : int, [alg_aead : int / tstr , alg_countersign : int / tstr],
request_kid : bstr, request_kid : bstr,
request_piv : bstr, request_piv : bstr,
gid : bstr,
options : bstr options : bstr
] ]
o The OSCORE compression defined in Section 8 of o The OSCORE compression defined in Section 6 of
[I-D.ietf-core-object-security] is used, with the following [I-D.ietf-core-object-security] is used, with the following
additions for the encoding of the object-security option. additions for the encoding of the Object-Security option.
* The fourth least significant bit of the first byte of the * The fourth least significant bit of the first byte of flag bits
object-security option value SHALL be set to 1, to indicate the SHALL be set to 1, to indicate the presence of the "kid"
presence of the "kid" parameter for both multicast requests and parameter for both group requests and responses.
responses.
* The fifth least significant bit of the first byte MUST be set * The fifth least significant bit of the first byte of flag bits
to 1 for multicast requests, to indicate the presence of the MUST be set to 1 for group requests, to indicate the presence
Context Hint in the OSCORE payload. The Context Hint flag MAY of the kid context in the OSCORE payload. The kid context flag
be set to 1 for responses. MAY be set to 1 for responses.
* The sixth least significant bit of the first byte is set to 1 * The sixth least significant bit of the first byte of flag bits
if the "countersign" parameter is present, or to 0 otherwise. is originally marked as reserved in
[I-D.ietf-core-object-security] and its usage is defined in
this specification. This bit is set to 1 if the
"CounterSignature0" parameter is present, or to 0 otherwise.
In order to ensure source authentication of group messages as In order to ensure source authentication of group messages as
described in this specification, this bit SHALL be set to 1. described in this specification, this bit SHALL be set to 1.
* The Context Hint value encodes the Group Identifier value (Gid) * The 'kid context' value encodes the Group Identifier value
of the group's Security Context. (Gid) of the group's Security Context.
* The following q bytes (q given by the counter signature * The following q bytes (q given by the Counter Signature
algorithm specified in the Security Context) encode the value Algorithm specified in the Security Context) encode the value
of the "countersign" parameter including the counter signature of the "CounterSignature0" parameter including the counter
of the COSE object. signature of the COSE object.
* The remaining bytes in the Object-Security value encode the * The remaining bytes in the Object-Security value encode the
value of the "kid" parameter, which is always present both in value of the "kid" parameter, which is always present both in
multicast requests and in responses. group requests and in responses.
0 1 2 3 4 5 6 7 <----------- n bytes -----------> <-- 1 byte --> 0 1 2 3 4 5 6 7 <----------- n bytes -----------> <-- 1 byte -->
+-+-+-+-+-+-+-+-+---------------------------------+--------------+ +-+-+-+-+-+-+-+-+---------------------------------+--------------+
|0 0|1|h|1| n | Partial IV (if any) | s (if any) | |0 0|1|h|1| n | Partial IV (if any) | s (if any) |
+-+-+-+-+-+-+-+-+---------------------------------+--------------+ +-+-+-+-+-+-+-+-+---------------------------------+--------------+
<------ s bytes ------> <--------- q bytes ---------> <------ s bytes ------> <--------- q bytes --------->
-----------------------+-----------------------------+-----------+ -----------------------+-----------------------------+-----------+
Gid (if any) | countersign | kid | kid context = Gid | CounterSignature0 | kid |
-----------------------+-----------------------------+-----------+ -----------------------+-----------------------------+-----------+
Figure 2: Object-Security Value Figure 2: Object-Security Value
5. Message Processing 3.1. Example: Request
Each multicast request message and response message is protected and Request with kid = 0x25, Partial IV = 5 and kid context = 0x44616c,
processed as specified in [I-D.ietf-core-object-security], with the assuming the label for the new kid context defined in
modifications described in the following sections. [I-D.ietf-core-object-security] has value 10. COUNTERSIGN is the
CounterSignature0 byte string as described in Section 3 and is 64
bytes long in this example. The ciphertext in this example is 14
bytes long.
Furthermore, endpoints in the multicast group locally perform error Before compression (96 bytes):
handling and processing of invalid messages according to the same
principles adopted in [I-D.ietf-core-object-security]. However, a
receiver endpoint MUST stop processing and silently reject any
message which is malformed and does not follow the format specified
in Section 4, without sending back any error message. This prevents
listener endpoints from sending multiple error messages to a
multicaster endpoint, so avoiding the risk of flooding the multicast
group.
5.1. Protecting the Request [
h'',
{ 4:h'25', 6:h'05', 10:h'44616c', 9:COUNTERSIGN },
h'aea0155667924dff8a24e4cb35b9'
]
A multicaster endpoint transmits a secure multicast request message After compression (85 bytes):
as described in Section 7.1 of [I-D.ietf-core-object-security], with
the following modifications. Flag byte: 0b00111001 = 0x39
Option Value: 39 05 03 44 61 6c COUNTERSIGN 25 (7 bytes + size of
COUNTERSIGN)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)
3.2. Example: Response
Response with kid = 0x52. COUNTERSIGN is the CounterSignature0 byte
string as described in Section 3 and is 64 bytes long in this
example. The ciphertext in this example is 14 bytes long.
Before compression (88 bytes):
[
h'',
{ 4:h'52', 9:COUNTERSIGN },
h'60b035059d9ef5667c5a0710823b'
]
After compression (80 bytes):
Flag byte: 0b00101000 = 0x28
Option Value: 28 COUNTERSIGN 52 (2 bytes + size of COUNTERSIGN)
Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b (14 bytes)
4. Message Processing
Each request message and response message is protected and processed
as specified in [I-D.ietf-core-object-security], with the
modifications described in the following sections. The following
security objectives are fulfilled, as further discussed in
Appendix A.2: data replay protection, group-level data
confidentiality, source authentication, message integrity, and
message ordering.
Furthermore, endpoints in the group locally perform error handling
and processing of invalid messages according to the same principles
adopted in [I-D.ietf-core-object-security]. However, a receiver
endpoint MUST stop processing and silently reject any message which
is malformed and does not follow the format specified in Section 3,
without sending back any error message. This prevents listener
endpoints from sending multiple error messages to a multicaster
endpoint, so avoiding the risk of flooding and possibly congesting
the group.
4.1. Protecting the Request
A multicaster endpoint transmits a secure group request as described
in Section 8.1 of [I-D.ietf-core-object-security], with the following
modifications.
1. The multicaster endpoint stores the association Token - Group 1. The multicaster endpoint stores the association Token - Group
Identifier. That is, it SHALL be able to find the correct Identifier. That is, it SHALL be able to find the correct
Security Context used to protect the multicast request and verify Security Context used to protect the group request and verify the
the response(s) by using the CoAP Token used in the message response(s) by using the CoAP Token used in the message exchange.
exchange.
2. The multicaster computes the COSE object as defined in Section 4 2. The multicaster computes the COSE object as defined in Section 3
of this specification. of this specification.
5.2. Verifying the Request 4.2. Verifying the Request
Upon receiving a secure multicast request message, a listener Upon receiving a secure group request, a listener endpoint proceeds
endpoint proceeds as described in Section 7.2 of as described in Section 8.2 of [I-D.ietf-core-object-security], with
[I-D.ietf-core-object-security], with the following modifications. the following modifications.
1. The listener endpoint retrieves the Group Identifier from the 1. The listener endpoint retrieves the Group Identifier from the
"gid" parameter of the received COSE object. Then, it uses the 'kid context' parameter of the received COSE object. Then, it
Group Identifier together with the destination IP address of the uses the Group Identifier together with the destination IP
multicast request message to identify the correct group's address of the group request to identify the correct group's
Security Context. Security Context.
2. The listener endpoint retrieves the Sender ID from the "kid" 2. The listener endpoint retrieves the Sender ID from the "kid"
parameter of the received COSE object. Then, the Sender ID is parameter of the received COSE object. Then, the Sender ID is
used to retrieve the correct Recipient Context associated to the used to retrieve the correct Recipient Context associated to the
multicaster endpoint and used to process the request message. multicaster endpoint and used to process the group request. When
When receiving a secure multicast CoAP request message from that receiving a secure group request message from that multicaster
multicaster endpoint for the first time, the listener endpoint endpoint for the first time, the listener endpoint creates a new
creates a new Recipient Context, initializes it according to Recipient Context, initializes it according to Section 3 of
Section 3 of [I-D.ietf-core-object-security], and includes the [I-D.ietf-core-object-security], and includes the multicaster
multicaster endpoint's public key. endpoint's public key.
3. The listener endpoint retrieves the corresponding public key of 3. The listener endpoint retrieves the corresponding public key of
the multicaster endpoint from the associated Recipient Context. the multicaster endpoint from the associated Recipient Context.
Then, it verifies the counter signature and decrypts the request Then, it verifies the counter signature and decrypts the group
message. request.
5.3. Protecting the Response 4.3. Protecting the Response
A listener endpoint that has received a multicast request message may A listener endpoint that has received a secure group request may
reply with a secure response message, which is protected as described reply with a secure response, which is protected as described in
in Section 7.3 of [I-D.ietf-core-object-security], with the following Section 8.3 of [I-D.ietf-core-object-security], with the following
modifications. modifications.
1. The listener endpoint computes the COSE object as defined in 1. The listener endpoint computes the COSE object as defined in
Section 4 of this specification. Section 3 of this specification.
5.4. Verifying the Response 4.4. Verifying the Response
Upon receiving a secure response message, a multicaster endpoint Upon receiving a secure response message, a multicaster endpoint
proceeds as described in Section 7.4 of proceeds as described in Section 8.4 of
[I-D.ietf-core-object-security], with the following modifications. [I-D.ietf-core-object-security], with the following modifications.
1. The multicaster endpoint retrieves the Security Context by using 1. The multicaster endpoint retrieves the Security Context by using
the Token of the received response message. the Token of the received response message.
2. The multicaster endpoint retrieves the Sender ID from the "kid" 2. The multicaster endpoint retrieves the Sender ID from the "kid"
parameter of the received COSE object. Then, the Sender ID is parameter of the received COSE object. Then, the Sender ID is
used to retrieve the correct Recipient Context associated to the used to retrieve the correct Recipient Context associated to the
listener endpoint and used to process the response message. When listener endpoint and used to process the response message. When
receiving a secure CoAP response message from that listener receiving a secure response message from that listener endpoint
endpoint for the first time, the multicaster endpoint creates a for the first time, the multicaster endpoint creates a new
new Recipient Context, initializes it according to Section 3 of Recipient Context, initializes it according to Section 3 of
[I-D.ietf-core-object-security], and includes the listener [I-D.ietf-core-object-security], and includes the listener
endpoint's public key. endpoint's public key.
3. The multicaster endpoint retrieves the corresponding public key 3. The multicaster endpoint retrieves the corresponding public key
of the listener endpoint from the associated Recipient Context. of the listener endpoint from the associated Recipient Context.
Then, it verifies the counter signature and decrypts the response Then, it verifies the counter signature and decrypts the response
message. message.
The mapping between response messages from listener endpoints and the The mapping between response messages from listener endpoints and the
associated multicast request message from a multicaster endpoint associated group request from a multicaster endpoint relies on the
relies on the 3-tuple (Group ID, Sender ID, Partial IV) associated to pair (Sender ID, Partial IV) associated to the secure group request.
the secure multicast request message. This is used by listener This is used by listener endpoints as part of the Additional
endpoints as part of the Additional Authenticated Data when Authenticated Data when protecting their own response message, as
protecting their own response message, as described in Section 4. described in Section 3.
6. Synchronization of Sequence Numbers 5. Synchronization of Sequence Numbers
Upon joining the multicast group, new listeners are not aware of the Upon joining the group, new listeners are not aware of the sequence
sequence number values currently used by different multicasters to number values currently used by different multicasters to transmit
transmit multicast request messages. This means that, when such group requests. This means that, when such listeners receive a
listeners receive a secure multicast request from a given multicaster secure group request from a given multicaster for the first time,
for the first time, they are not able to verify if that request is they are not able to verify if that request is fresh and has not been
fresh and has not been replayed. The same applies when a listener replayed. The same holds when a listener endpoint loses
endpoint loses synchronization with sequence numbers of multicasters, synchronization with sequence numbers of multicasters, for instance
for instance after a device reboot. after a device reboot.
The exact way to address this issue depends on the specific use case The exact way to address this issue depends on the specific use case
and its synchronization requirements. The Group Manager should and its synchronization requirements. The list of methods to handle
define also how to handle synchronization of sequence numbers, as synchronization of sequence numbers is part of the group
part of the policies enforced in the multicast group. In particular, communication policy, and different listener endpoints can use
the Group Manager can suggest to single specific listener endpoints different methods. Appendix E describes three possible approaches
how they can exceptionally behave in order to synchronize with that can be considered.
sequence numbers of multicasters. Appendix D describes three
possible approaches that can be considered. 6. Responsibilities of the Group Manager
The Group Manager is responsible for performing the following tasks:
o Creating and managing OSCORE groups. This includes the assignment
of a Group ID to every newly created group, as well as ensuring
uniqueness of Group IDs within the set of its OSCORE groups.
o Defining policies for authorizing the joining of its OSCORE
groups. Such policies can be enforced by a third party, which is
in a trust relation with the Group Manager and enforces join
policies on behalf of the Group Manager.
o Driving the join process to add new endpoints as group members.
o Establishing Security Common Contexts and providing them to
authorized group members during the join process, together with a
corresponding Security Sender Context.
o Generating and managing Endpoint IDs within its OSCORE groups, as
well as assigning and providing them to new endpoints during the
join process. This includes ensuring uniqueness of Endpoints IDs
within each of its OSCORE groups.
o Defining a set of supported signature algorithms as part of the
communication policy of each of its OSCORE groups, and signalling
it to new endpoints during the join process.
o Defining the methods to handle loss of synchronization with
sequence numbers as part of the communication policy of each of
its OSCORE groups, and signaling the one(s) to use to new
endpoints during the join process.
o Renewing the Security Context of an OSCORE group upon membership
change, by revoking and renewing common security parameters and
keying material (rekeying).
o Providing the management keying material that a new endpoint
requires to participate in the rekeying process, consistently with
the key management scheme used in the group joined by the new
endpoint.
o Updating the Group ID of its OSCORE groups, upon renewing the
respective Security Context.
The Group Manager may additionally be responsible for the following
tasks:
o Acting as trusted key repository, in order to store the public
keys of the members of its OSCORE groups, and provide such public
keys to other members of the same group upon request. This
specification recommends that the Group Manager is entrusted to
perform this task.
o Acting as network router device where endpoints register to
correctly receive group messages sent to the multicast IP address
of that group.
o Autonomously and locally enforcing access policies to authorize
new endpoints to join its OSCORE groups.
7. Security Considerations 7. Security Considerations
The same security considerations from OSCORE (Section 11 of The same security considerations from OSCORE (Section 11 of
[I-D.ietf-core-object-security]) apply to this specification. [I-D.ietf-core-object-security]) apply to this specification.
Additional security aspects to be taken into account are discussed Additional security aspects to be taken into account are discussed
below. below.
7.1. Group-level Security 7.1. Group-level Security
The approach described in this document relies on commonly shared The approach described in this document relies on commonly shared
group keying material to protect communication within a multicast group keying material to protect communication within a group. This
group. This means that messages are encrypted at a group level means that messages are encrypted at a group level (group-level data
(group-level data confidentiality), i.e. they can be decrypted by any confidentiality), i.e. they can be decrypted by any member of the
member of the multicast group, but not by an external adversary or group, but not by an external adversary or other external entities.
other external entities.
In addition, it is required that all group members are trusted, i.e. In addition, it is required that all group members are trusted, i.e.
they do not forward the content of group messages to unauthorized they do not forward the content of group messages to unauthorized
entities. However, in many use cases, the devices in the multicast entities. However, in many use cases, the devices in the group
group belong to a common authority and are configured by a belong to a common authority and are configured by a commissioner
commissioner. For instance, in a professional lighting scenario, the (see Appendix B).
roles of multicaster and listener are configured by the lighting
commissioner, and devices strictly follow those roles.
8. IANA Considerations 8. IANA Considerations
TBD. Header parameter 'gid'. This document has no actions for IANA.
9. Acknowledgments 9. Acknowledgments
The authors sincerely thank Stefan Beck, Rolf Blom, Carsten Bormann, The authors sincerely thank Stefan Beck, Rolf Blom, Carsten Bormann,
Klaus Hartke, Richard Kelsey, John Mattsson, Jim Schaad and Ludwig Esko Dijk, Klaus Hartke, Richard Kelsey, John Mattsson, Jim Schaad,
Seitz for their feedback and comments. Ludwig Seitz and Peter van der Stok for their feedback and comments.
The work on this document has been partly supported by the EIT-
Digital High Impact Initiative ACTIVE.
10. References 10. References
10.1. Normative References 10.1. Normative References
[I-D.ietf-core-object-security] [I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz, Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments "Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-08 (work in (OSCORE)", draft-ietf-core-object-security-08 (work in
progress), January 2018. progress), January 2018.
skipping to change at page 16, line 20 skipping to change at page 16, line 30
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017, RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>. <https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References 10.2. Informative References
[I-D.amsuess-core-repeat-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "Repeat And
Request-Tag", draft-amsuess-core-repeat-request-tag-00
(work in progress), July 2017.
[I-D.aragon-ace-ipsec-profile]
Aragon, S., Tiloca, M., and S. Raza, "IPsec profile of
ACE", draft-aragon-ace-ipsec-profile-01 (work in
progress), October 2017.
[I-D.ietf-ace-dtls-authorize] [I-D.ietf-ace-dtls-authorize]
Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
L. Seitz, "Datagram Transport Layer Security (DTLS) L. Seitz, "Datagram Transport Layer Security (DTLS)
Profiles for Authentication and Authorization for Profiles for Authentication and Authorization for
Constrained Environments (ACE)", draft-ietf-ace-dtls- Constrained Environments (ACE)", draft-ietf-ace-dtls-
authorize-02 (work in progress), October 2017. authorize-02 (work in progress), October 2017.
[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)", draft-ietf-ace-oauth- Constrained Environments (ACE)", draft-ietf-ace-oauth-
authz-09 (work in progress), November 2017. authz-10 (work in progress), February 2018.
[I-D.ietf-ace-oscore-profile] [I-D.ietf-ace-oscore-profile]
Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
profile of the Authentication and Authorization for profile of the Authentication and Authorization for
Constrained Environments Framework", draft-ietf-ace- Constrained Environments Framework", draft-ietf-ace-
oscore-profile-00 (work in progress), December 2017. oscore-profile-00 (work in progress), December 2017.
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "Echo and
Request-Tag", draft-ietf-core-echo-request-tag-00 (work in
progress), October 2017.
[I-D.palombini-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", draft-palombini-ace-key-
groupcomm-00 (work in progress), March 2018.
[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-ace-oscoap-joining] [I-D.tiloca-ace-oscoap-joining]
Tiloca, M. and J. Park, "Joining of OSCORE multicast Tiloca, M. and J. Park, "Joining of OSCORE multicast
groups in ACE", draft-tiloca-ace-oscoap-joining-02 (work groups in ACE", draft-tiloca-ace-oscoap-joining-02 (work
in progress), October 2017. in progress), October 2017.
skipping to change at page 18, line 42 skipping to change at page 19, line 5
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014, DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>. <https://www.rfc-editor.org/info/rfc7228>.
[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for [RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390, the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014, DOI 10.17487/RFC7390, October 2014,
<https://www.rfc-editor.org/info/rfc7390>. <https://www.rfc-editor.org/info/rfc7390>.
Appendix A. List of Use Cases Appendix A. Assumptions and Security Objectives
This section presents a set of assumptions and security objectives
for the approach described in this document.
A.1. Assumptions
The following assumptions are assumed to be already addressed and are
out of the scope of this document.
o Multicast communication topology: this document considers both
1-to-N (one multicaster and multiple listeners) and M-to-N
(multiple multicasters and multiple listeners) communication
topologies. The 1-to-N communication topology is the simplest
group communication scenario that would serve the needs of a
typical low-power and lossy network (LLN). Examples of use cases
that benefit from secure group communication are provided in
Appendix B.
o Group size: security solutions for group communication should be
able to adequately support different and possibly large groups.
The group size is the current number of members in a group. In
the use cases mentioned in this document, the number of
multicasters (normally the controlling devices) is expected to be
much smaller than the number of listeners (i.e. the controlled
devices). A security solution for group communication that
supports 1 to 50 multicasters would be able to properly cover the
group sizes required for most use cases that are relevant for this
document. The maximum group size is expected to be in the range
of 2 to 100 devices. Groups larger than that should be divided
into smaller independent groups, e.g. by grouping lights in a
building on a per floor basis.
o Communication with the Group Manager: an endpoint must use a
secure dedicated channel when communicating with the Group
Manager, even when not registered as group member. In particular,
communications with the Group Manager occuring during the join
process to become a group member must also be secured.
o Establishment and management of Security Contexts: an OSCORE
Security Context must be established among the group members. In
particular, a Common Context must be provided to a new joining
endpoint together with a corresponding Sender Context. On the
other hand, Recipient Contexts are locally and individually
derived by each group member. A secure mechanism must be used to
generate, revoke and (re-)distribute keying material, multicast
security policies and security parameters in the group. The
actual establishment and management of the Security Context is out
of the scope of this document, and it is anticipated that an
activity in IETF dedicated to the design of a generic key
management scheme will include this feature, preferably based on
[RFC3740][RFC4046][RFC4535].
o Multicast data security ciphersuite: all group members must agree
on a ciphersuite to provide authenticity, integrity and
confidentiality of messages in the group. The ciphersuite is
specified as part of the Security Context.
o Backward security: a new device joining the group should not have
access to any old Security Contexts used before its joining. This
ensures that a new group member is not able to decrypt
confidential data sent before it has joined the group. The
adopted key management scheme should ensure that the Security
Context is updated to ensure backward confidentiality. The actual
mechanism to update the Security Context and renew the group
keying material upon a group member's joining has to be defined as
part of the group key management scheme.
o Forward security: entities that leave the group should not have
access to any future Security Contexts or message exchanged within
the group after their leaving. This ensures that a former group
member is not able to decrypt confidential data sent within the
group anymore. Also, it ensures that a former member is not able
to send encrypted and/or integrity protected messages to the group
anymore. The actual mechanism to update the Security Context and
renew the group keying material upon a group member's leaving has
to be defined as part of the group key management scheme.
A.2. Security Objectives
The approach described in this document aims at fulfilling the
following security objectives:
o Data replay protection: replayed group request messages or
response messages must be detected.
o Group-level data confidentiality: messages sent within the group
shall be encrypted if privacy sensitive data is exchanged within
the group. This document considers group-level data
confidentiality since messages are encrypted at a group level,
i.e. in such a way that they can be decrypted by any member of the
group, but not by an external adversary or other external
entities.
o Source authentication: messages sent within the group shall be
authenticated. That is, it is essential to ensure that a message
is originated by a member of the group in the first place, and in
particular by a specific member of the group.
o Message integrity: messages sent within the group shall be
integrity protected. That is, it is essential to ensure that a
message has not been tampered with by an external adversary or
other external entities which are not group members.
o Message ordering: it must be possible to determine the ordering of
messages coming from a single sender endpoint. In accordance with
OSCORE [I-D.ietf-core-object-security], this results in providing
relative freshness of group requests and absolute freshness of
responses. It is not required to determine ordering of messages
from different sender endpoints.
Appendix B. List of Use Cases
Group Communication for CoAP [RFC7390] provides the necessary Group Communication for CoAP [RFC7390] provides the necessary
background for multicast-based CoAP communication, with particular background for multicast-based CoAP communication, with particular
reference to low-power and lossy networks (LLNs) and resource reference to low-power and lossy networks (LLNs) and resource
constrained environments. The interested reader is encouraged to constrained environments. The interested reader is encouraged to
first read [RFC7390] to understand the non-security related details. first read [RFC7390] to understand the non-security related details.
This section discusses a number of use cases that benefit from secure This section discusses a number of use cases that benefit from secure
group communication. Specific security requirements for these use group communication. Specific security requirements for these use
cases are discussed in Section 2. cases are discussed in Appendix A.
o Lighting control: consider a building equipped with IP-connected o Lighting control: consider a building equipped with IP-connected
lighting devices, switches, and border routers. The devices are lighting devices, switches, and border routers. The devices are
organized into groups according to their physical location in the organized into groups according to their physical location in the
building. For instance, lighting devices and switches in a room building. For instance, lighting devices and switches in a room
or corridor can be configured as members of a single multicast or corridor can be configured as members of a single group.
group. Switches are then used to control the lighting devices by Switches are then used to control the lighting devices by sending
sending on/off/dimming commands to all lighting devices in a on/off/dimming commands to all lighting devices in a group, while
group, while border routers connected to an IP network backbone border routers connected to an IP network backbone (which is also
(which is also multicast-enabled) can be used to interconnect multicast-enabled) can be used to interconnect routers in the
routers in the building. Consequently, this would also enable building. Consequently, this would also enable logical groups to
logical multicast groups to be formed even if devices in the be formed even if devices in the lighting group may be physically
lighting group may be physically in different subnets (e.g. on in different subnets (e.g. on wired and wireless networks).
wired and wireless networks). Connectivity between ligthing Connectivity between lighting devices may be realized, for
devices may be realized, for instance, by means of IPv6 and instance, by means of IPv6 and (border) routers supporting 6LoWPAN
(border) routers supporting 6LoWPAN [RFC4944][RFC6282]. Group [RFC4944][RFC6282]. Group communication enables synchronous
communication enables synchronous operation of a group of operation of a group of connected lights, ensuring that the light
connected lights, ensuring that the light preset (e.g. dimming preset (e.g. dimming level or color) of a large group of
level or color) of a large group of luminaires are changed at the luminaires are changed at the same perceived time. This is
same perceived time. This is especially useful for providing a especially useful for providing a visual synchronicity of light
visual synchronicity of light effects to the user. Devices may effects to the user. As a practical guideline, events within a
reply back to the switches that issue on/off/dimming commands, in 200 ms interval are perceived as simultaneous by humans, which is
order to report about the execution of the requested operation necessary to ensure in many setups. Devices may reply back to the
(e.g. OK, failure, error) and their current operational status. switches that issue on/off/dimming commands, in order to report
about the execution of the requested operation (e.g. OK, failure,
error) and their current operational status. In a typical
lighting control scenario, a single switch is the only entity
responsible for sending commands to a group of lighting devices.
In more advanced lighting control use cases, a M-to-N
communication topology would be required, for instance in case
multiple sensors (presence or day-light) are responsible to
trigger events to a group of lighting devices. Especially in
professional lighting scenarios, the roles of multicaster and
listener are configured by the lighting commissioner, and devices
strictly follow those roles.
o Integrated building control: enabling Building Automation and o Integrated building control: enabling Building Automation and
Control Systems (BACSs) to control multiple heating, ventilation Control Systems (BACSs) to control multiple heating, ventilation
and air-conditioning units to pre-defined presets. Controlled and air-conditioning units to pre-defined presets. Controlled
units can be organized into multicast groups in order to reflect units can be organized into groups in order to reflect their
their physical position in the building, e.g. devices in the same physical position in the building, e.g. devices in the same room
room can be configured as members of a single multicast group. can be configured as members of a single group. As a practical
Furthermore, controlled units are expected to possibly reply back guideline, events within intervals of seconds are typically
acceptable. Controlled units are expected to possibly reply back
to the BACS issuing control commands, in order to report about the to the BACS issuing control commands, in order to report about the
execution of the requested operation (e.g. OK, failure, error) execution of the requested operation (e.g. OK, failure, error)
and their current operational status. and their current operational status.
o Software and firmware updates: software and firmware updates often o Software and firmware updates: software and firmware updates often
comprise quite a large amount of data. This can overload a LLN comprise quite a large amount of data. This can overload a LLN
that is otherwise typically used to deal with only small amounts that is otherwise typically used to deal with only small amounts
of data, on an infrequent base. Rather than sending software and of data, on an infrequent base. Rather than sending software and
firmware updates as unicast messages to each individual device, firmware updates as unicast messages to each individual device,
multicasting such updated data to a larger group of devices at multicasting such updated data to a larger group of devices at
skipping to change at page 20, line 26 skipping to change at page 23, line 16
configuration updates are expected to possibly reply back, to configuration updates are expected to possibly reply back, to
provide a feedback about the execution of the update operation provide a feedback about the execution of the update operation
(e.g. OK, failure, error) and their current operational status. (e.g. OK, failure, error) and their current operational status.
o Commissioning of LLNs systems: a commissioning device is o Commissioning of LLNs systems: a commissioning device is
responsible for querying all devices in the local network or a responsible for querying all devices in the local network or a
selected subset of them, in order to discover their presence, and selected subset of them, in order to discover their presence, and
be aware of their capabilities, default configuration, and be aware of their capabilities, default configuration, and
operating conditions. Queried devices displaying similarities in operating conditions. Queried devices displaying similarities in
their capabilities and features, or sharing a common physical their capabilities and features, or sharing a common physical
location can be configured as members of a single multicast group. location can be configured as members of a single group. Queried
Queried devices are expected to reply back to the commissioning devices are expected to reply back to the commissioning device, in
device, in order to notify their presence, and provide the order to notify their presence, and provide the requested
requested information and their current operational status. information and their current operational status.
o Emergency multicast: a particular emergency related information o Emergency multicast: a particular emergency related information
(e.g. natural disaster) is generated and multicast by an emergency (e.g. natural disaster) is generated and multicast by an emergency
notifier, and relayed to multiple devices. The latters may reply notifier, and relayed to multiple devices. The latters may reply
back to the emergency notifier, in order to provide their feedback back to the emergency notifier, in order to provide their feedback
and local information related to the ongoing emergency. and local information related to the ongoing emergency. This kind
of setups should additionally rely on a fault tolerance multicast
algorithm, such as MPL.
Appendix B. Example of Group Identifier Format Appendix C. Example of Group Identifier Format
This section provides an example of how the Group Identifier (Gid) This section provides an example of how the Group Identifier (Gid)
can be specifically formatted. That is, the Gid can be composed of can be specifically formatted. That is, the Gid can be composed of
two parts, namely a Group Prefix and a Group Epoch. two parts, namely a Group Prefix and a Group Epoch.
The Group Prefix is uniquely defined in the set of all the multicast The Group Prefix is uniquely defined in the set of all the groups
groups associated to the same Group Manager. The choice of the Group associated to the same Group Manager. The choice of the Group Prefix
Prefix for a given group's Security Context is application specific. for a given group's Security Context is application specific. A
Group Prefixes are random as well as long enough, in order to achieve Group Prefix is random, constant over time, and long enough to
a negligible probability of collisions between Group Identifiers from achieve a negligible probability of collisions between Group
different Group Managers. Identifiers from different Group Managers. The size of the Group
Prefix directly impact on the maximum number of distinct groups under
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 upon completing each renewal of the Security Context incremented by 1 upon completing each renewal of the Security Context
and keying material in the group (see Section 3.1). In particular, and keying material in the group (see Section 2.1). In particular,
once a new Master Secret has been distributed to the group, all the once a new Master Secret has been distributed to the group, all the
group members increment by 1 the Group Epoch in the Group Identifier group members increment by 1 the Group Epoch in the Group Identifier
of that group (see Section 3). of that group.
Appendix C. Set-up of New Endpoints As an example, a 3-byte Group Identifier can be composed of: i) a
1-byte Group Prefix '0xb1' interpreted as a raw byte string; and ii)
a 2-byte Group Epoch interpreted as an unsigned integer ranging from
0 to 65535. Then, after having established the Security Common
Context 61532 times in the group, its Group Identifier will assume
value '0xb1f05c'.
An endpoint joins a multicast group by explicitly interacting with Appendix D. Set-up of New Endpoints
the responsible Group Manager. All communications between a joining
endpoint and the Group Manager rely on the CoAP protocol and MUST be An endpoint joins a group by explicitly interacting with the
secured. Specific details on how to secure communications between responsible Group Manager. Communications between a joining endpoint
joining endpoints and a Group Manager are out of the scope of this and the Group Manager rely on the CoAP protocol and must be secured.
specification. Specific details on how to secure communications between joining
endpoints and a Group Manager are out of scope.
In order to receive multicast messages sent to the group, a joining In order to receive multicast messages sent to the group, a joining
endpoint has to register with a network router device endpoint has to register with a network router device
[RFC3376][RFC3810], signaling its intent to receive packets sent to [RFC3376][RFC3810], signaling its intent to receive packets sent to
the multicast IP address of that group. As a particular case, the the multicast IP address of that group. As a particular case, the
Group Manager can also act as such a network router device. Upon Group Manager can also act as such a network router device. Upon
joining the group, endpoints are not required to know how many and joining the group, endpoints are not required to know how many and
what endpoints are active in the same group. what endpoints are active in the same group.
Furthermore, in order to participate in the secure group Furthermore, in order to participate in the secure group
communication, an endpoint needs to maintain a number of information communication, an endpoint needs to be properly initialized upon
elements stored in its own Security Context (see Section 3). The joining the group. In particular, the Group Manager provides keying
following Appendix C.1 describes which of this information is material and parameters to a joining endpoint, which can then
provided to an endpoint upon joining a multicast group through the initialize its own Security Context (see Section 2).
responsible Group Manager.
C.1. Join Process The following Appendix D.1 provides an example describing how such
information can be provided to an endpoint upon joining a group
through the responsible Group Manager. Then, Appendix D.2 discusses
how public keys of group members can be handled and made available to
group members. Finally, Appendix D.3 overviews how the ACE framework
for Authentication and Authorization in constrained environments
[I-D.ietf-ace-oauth-authz] can be possibly used to support such a
join process.
An endpoint requests to join a multicast group by sending a D.1. Join Process
confirmable CoAP POST request to the Group Manager responsible for
that group. The join request is addressed to a CoAP resource
associated to that group and carries the following information.
o Role: the exact role of the joining endpoint in the multicast An endpoint requests to join a group by sending a confirmable CoAP
group. Possible values are: "multicaster", "listener", "pure POST request to the Group Manager responsible for that group. This
listener", "multicaster and listener", or "multicaster and pure join request can reflect the format of the Key Distribution Request
listener". message defined in Section 4.1 of [I-D.palombini-ace-key-groupcomm].
Besides, it can be addressed to a CoAP resource associated to that
group and carries the following information.
o Group identifier: the Group Identifier (Gid) of the group, as
known to the joining endpoint at this point in time. This may not
fully coincide with the Gid currently associated to the group,
e.g. if it includes a dynamic component. This information can be
mapped to the first element of the "scope" parameter of the Key
Distribution Request message defined in Section 4.1 of
[I-D.palombini-ace-key-groupcomm].
o Role: the exact role of the joining endpoint in the group.
Possible values are: "multicaster", "listener", "pure listener",
"multicaster and listener", or "multicaster and pure listener".
This information can be mapped to the second element of the
"scope" parameter of the Key Distribution Request message defined
in Section 4.1 of [I-D.palombini-ace-key-groupcomm].
o Retrieval flag: indication of interest to receive the public keys
of the endpoints currently in the group, as included in the
following join response. This flag must not be present if the
Group Manager is not configured to store the public keys of group
members, or if the joining endpoint is configured exclusively as
pure listener for the group to join. This information can be
mapped to the "get_pub_keys" parameter of the Key Distribution
Request message defined in Section 4.1 of
[I-D.palombini-ace-key-groupcomm].
o Identity credentials: information elements to enforce source o Identity credentials: information elements to enforce source
authentication of group messages from the joining endpoint, such authentication of group messages from the joining endpoint, such
as its public key. The exact content depends on whether the Group as its public key. The exact content depends on whether the Group
Manager is configured to store the public keys of group members. Manager is configured to store the public keys of group members.
If this is the case, this information is omitted if it has been If this is the case, this information is omitted if it has been
provided to the same Group Manager upon previously joining the provided to the same Group Manager upon previously joining the
same or a different multicast group under its control. This same or a different group under its control. This information is
information is also omitted if the joining endpoint is configured also omitted if the joining endpoint is configured exclusively as
exclusively as pure listener for the joined group. Appendix C.2 pure listener for the joined group. Appendix D.2 discusses
discusses additional details on provisioning of public keys and additional details on provisioning of public keys and other
other information to enforce source authentication of joining information to enforce source authentication of joining
node's messages. endpoints's messages. This information can be mapped to the
"client_cred" parameter of the Key Distribution Request message
o Retrieval flag: indication of interest to receive the public keys defined in Section 4.1 of [I-D.palombini-ace-key-groupcomm].
of the endpoints currently in the multicast group, as included in
the following join response. This flag MUST be set to false if
the Group Manager is not configured to store the public keys of
group members, or if the joining endpoint is configured
exclusively as pure listener for the joined group.
The Group Manager MUST be able to verify that the joining enpoint is The Group Manager must be able to verify that the joining endpoint is
authorized to become a member of the multicast group. To this end, authorized to become a member of the group. To this end, the Group
the Group Manager can directly authorize the joining endpoint, or Manager can directly authorize the joining endpoint, or expect it to
expect it to provide authorization evidence previously obtained from provide authorization evidence previously obtained from a trusted
a trusted entity. Appendix C.3 describes how this can be achieved by entity. Appendix D.3 describes how this can be achieved by
leveraging the ACE framework for Authentication and Authorization in leveraging the ACE framework for Authentication and Authorization in
constrained environments [I-D.ietf-ace-oauth-authz]. constrained environments [I-D.ietf-ace-oauth-authz].
In case of successful authorization check, the Group Manager In case of successful authorization check, the Group Manager
generates an Endpoint ID assigned to the joining node, before generates an Endpoint ID assigned to the joining endpoint, before
proceeding with the rest of the join process. Instead, in case the proceeding with the rest of the join process. Instead, in case the
authorization check fails, the Group Manager MUST abort the join authorization check fails, the Group Manager aborts the join process.
process. Further details about the authorization of joining endpoint Further details about the authorization of joining endpoint are out
are out of the scope of this specification. of scope.
As discussed in Section 3.1, it is then RECOMMENDED that the Security As discussed in Section 2.1, it is recommended that the Security
Context is renewed before the joining endpoint becomes a new active Context is renewed before the joining endpoint receives the group
member of the multicast group. This is achieved by securely keying material and becomes a new active member of the group. This
distributing a new Master Secret and a new Group Identifier to the is achieved by securely distributing a new Master Secret and a new
endpoints currently present in the same group. Group Identifier to the endpoints currently present in the same
group.
Once renewed the Security Context in the multicast group, the Group Once renewed the Security Context in the group, the Group Manager
Manager replies to the joining endpoint with a CoAP response carrying replies to the joining endpoint with a CoAP response carrying the
the following information. following information. This join response can reflect the format of
the Key Distribution Response message defined in Section 4.2 of
[I-D.palombini-ace-key-groupcomm].
o Security Common Context: the OSCORE Security Common Context o Security Common Context: the OSCORE Security Common Context
associated to the joined multicast group (see Section 3). associated to the joined group (see Section 2). This information
can be mapped to the "key" parameter of the Key Distribution
Response message defined in Section 4.2 of
[I-D.palombini-ace-key-groupcomm].
o Endpoint ID: the Endpoint ID associated to the joining node. This o Endpoint ID: the Endpoint ID associated to the joining endpoint.
information is not included in case "Role" in the join request is This information is not included in case "Role" in the join
equal to "pure listener". request is equal to "pure listener". This information can be
mapped to the "clientID" parameter within the "key" parameter of
the Key Distribution Response message defined in Section 4.2 of
[I-D.palombini-ace-key-groupcomm].
o Member public keys: the public keys of the endpoints currently
present in the group. This includes: the public keys of the non-
pure listeners currently in the group, if the joining endpoint is
configured (also) as multicaster; and the public keys of the
multicasters currently in the group, if the joining endpoint is
configured (also) as listener or pure listener. This information
is omitted in case the Group Manager is not configured to store
the public keys of group members or if the "Retrieval flag" was
not present in the join request. Appendix D.2 discusses
additional details on provisioning public keys upon joining the
group and on retrieving public keys of group members. This
information can be mapped to the "pub_keys" parameter of the Key
Distribution Response message defined in Section 4.2 of
[I-D.palombini-ace-key-groupcomm].
o Group policies: a list of key words indicating the particular
policies enforced in the group. This includes, for instance, the
list of supported signature algorithms and the method to achieve
synchronization of sequence numbers among group members (see
Appendix E). This information can be mapped to the
"group_policies" parameter of the Key Distribution Response
message defined in Section 4.2 of
[I-D.palombini-ace-key-groupcomm].
o Management keying material: the set of administrative keying o Management keying material: the set of administrative keying
material used to participate in the group rekeying process run by material used to participate in the group rekeying process run by
the Group Manager (see Section 3.1). The specific elements of the Group Manager (see Section 2.1). The specific elements of
this management keying material depend on the group rekeying this management keying material depend on the group rekeying
protocol used in the group. For instance, this can simply consist protocol used in the group. For instance, this can simply consist
in a group key encryption key and a pairwise symmetric key shared in a group key encryption key and a pairwise symmetric key shared
between the joining node and the Group Manager, in case GKMP between the joining endpoint and the Group Manager, in case GKMP
[RFC2093][RFC2094] is used. Instead, if key-tree based rekeying [RFC2093][RFC2094] is used. Instead, if key-tree based rekeying
protocols like LKH [RFC2627] are used, it can consist in the set protocols like LKH [RFC2627] are used, it can consist in the set
of symmetric keys associated to the key-tree leaf representing the of symmetric keys associated to the key-tree leaf representing the
group member up to the key-tree root representing the group key group member up to the key-tree root representing the group key
encryption key. encryption key. This information can be mapped to the
"mgt_key_material" parameter of the Key Distribution Response
o Member public keys: the public keys of the endpoints currently message defined in Section 4.2 of
present in the multicast group. This includes: the public keys of [I-D.palombini-ace-key-groupcomm].
the non-pure listeners currently in the group, if the joining
endpoint is configured (also) as multicaster; and the public keys
of the multicasters currently in the group, if the joining
endpoint is configured (also) as listener or pure listener. This
information is omitted in case the Group Manager is not configured
to store the public keys of group members or if the "Retrieval
flag" was set to false in the join request. Appendix C.2
discusses additional details on provisioning public keys upon
joining the group and on retrieving public keys of group members.
C.2. Provisioning and Retrieval of Public Keys D.2. Provisioning and Retrieval of Public Keys
As mentioned in Section 3, it is RECOMMENDED that the Group Manager As mentioned in Section 6, it is recommended that the Group Manager
acts as trusted key repository, stores public keys of group members acts as trusted key repository, so storing public keys of group
and provide them to other members of the same group upon request. In members and providing them to other members of the same group upon
such a case, a joining endpoint provides its own public key to the request. In such a case, a joining endpoint provides its own public
Group Manager, as "Identity credentials" of the join request, when key to the Group Manager, as "Identity credentials" of the join
joining the multicast group (see Appendix C.1). request, when joining the group (see Appendix D.1).
After that, the Group Manager MUST verify that the joining endpoint After that, the Group Manager should verify that the joining endpoint
actually owns the associated private key, for instance by performing actually owns the associated private key, for instance by performing
a proof-of-possession challenge-response. In case of success, the a proof-of-possession challenge-response, whose details are out of
Group Manager stores the received public key as associated to the scope. In case of failure, the Group Manager performs up to a pre-
joining endpoint and its Endpoint ID, before sending the join defined maximum number of retries, after which it aborts the join
response and continuing with the rest of the join process. From then process.
on, that public key will be available for secure and trusted delivery
to other endpoints in the multicast group.
The joining node does not have to provide its own public key if that In case of successful challenge-response, the Group Manager stores
already occurred upon previously joining the same or a different the received public key as associated to the joining endpoint and its
multicast group under the same Group Manager. However, separately Endpoint ID. From then on, that public key will be available for
for each multicast group under its control, the Group Manager secure and trusted delivery to other endpoints in the group.
maintains an updated list of active Endpoint IDs associated to a same Finally, the Group Manager sends the join response to the joining
endpoint's public key. endpoint, as described in Appendix D.1.
The joining endpoint does not have to provide its own public key if
that already occurred upon previously joining the same or a different
group under the same Group Manager. However, separately for each
group under its control, the Group Manager maintains an updated list
of active Endpoint IDs associated to the respective endpoint's public
key.
Instead, in case the Group Manager does not act as trusted key Instead, in case the Group Manager does not act as trusted key
repository, the following information is exchanged with the Group repository, the following exchange with the Group Manager can occur
Manager during the join process. during the join process.
1. The joining endpoint signs its own certificate by using its own 1. The joining endpoint signs its own certificate by using its own
private key. There is no restriction on the Certificate Subject private key. The certificate includes also the identifier of the
included in the joining node's certificate. issuer Certification Authority (CA). There is no restriction on
the Certificate Subject included in the joining endpoint's
certificate.
2. The joining endpoint includes the following information as 2. The joining endpoint specifies the signed certificate as
"Identity credentials" in the join request (Appendix C.1): the "Identity credentials" in the join request (Appendix D.1). The
signed certificate; and the identifier of the Certification joining endpoint can optionally specify also a list of public key
Authority that issued the certificate. The joining endpoint can repositories storing its own certificate. In such a case, this
optionally specify also a list of public key repositories storing information can be mapped to the "pub_keys_repos" parameter of
its own certificate. the Key Distribution Request message defined in Section 4.1 of
[I-D.palombini-ace-key-groupcomm].
3. When processing the join request, the Group Manager first 3. When processing the join request, the Group Manager first
validates the certificate by verifying the signature of the validates the certificate by verifying the signature of the
issuer CA, and then verifies the signature of the joining node. issuer CA, and then verifies the signature of the joining
endpoint.
4. The Group Manager stores the association between the Certificate 4. The Group Manager stores the association between the Certificate
Subject of the joining node's certificate and the pair {Group ID, Subject of the joining endpoint's certificate and the pair {Group
Endpoint ID of the joining node}. If received from the joining ID, Endpoint ID of the joining endpoint}. If received from the
endpoint, the Group Manager also stores the list of public key joining endpoint, the Group Manager also stores the list of
repositories storing the certificate of the joining endpoint. public key repositories storing the certificate of the joining
endpoint.
When a group member X wants to retrieve the public key of another When a group member X wants to retrieve the public key of another
group member Y in the same multicast group, the endpoint X proceeds group member Y in the same group, the endpoint X proceeds as follows.
as follows.
1. The endpoint X contacts the Group Manager, specifying the pair 1. The endpoint X contacts the Group Manager, specifying the pair
{Group ID, Endpoint ID of the endpoint Y}. {Group ID, Endpoint ID of the endpoint Y}.
2. The Group Manager provides the endpoint X with the Certificate 2. The Group Manager provides the endpoint X with the Certificate
Subject CS from the certificate of endpoint Y. If available, the Subject CS from the certificate of endpoint Y. If available, the
Group Manager provides the endpoint X also with the list of Group Manager provides the endpoint X also with the list of
public key repositories storing the certificate of the endpoint public key repositories storing the certificate of the endpoint
Y. Y.
3. The endpoint X retrieves the certificate of the endpoint X from a 3. The endpoint X retrieves the certificate of the endpoint X from a
key repository storing it, by using the Certificate Subject CS. key repository storing it, by using the Certificate Subject CS.
C.3. Group Joining Based on the ACE Framework D.3. Group Joining Based on the ACE Framework
The join process to register an endpoint as a new member of a The join process to register an endpoint as a new member of a group
multicast group can be based on the ACE framework for Authentication can be based on the ACE framework for Authentication and
and Authorization in constrained environments Authorization in constrained environments [I-D.ietf-ace-oauth-authz],
[I-D.ietf-ace-oauth-authz], built on re-use of OAuth 2.0 [RFC6749]. built on re-use of OAuth 2.0 [RFC6749].
In particular, the approach described in In particular, the approach described in
[I-D.tiloca-ace-oscoap-joining] uses the ACE framework to delegate [I-D.tiloca-ace-oscoap-joining] uses the ACE framework to delegate
the authentication and authorization of joining endpoints to an the authentication and authorization of joining endpoints to an
Authorization Server in a trust relation with the Group Manager. At Authorization Server in a trust relation with the Group Manager. At
the same time, it allows a joining endpoint to establish a secure the same time, it allows a joining endpoint to establish a secure
channel with the Group Manager, by leveraging protocol-specific channel with the Group Manager, by leveraging protocol-specific
profiles of ACE [I-D.ietf-ace-oscore-profile] profiles of ACE, such as [I-D.ietf-ace-oscore-profile] and
[I-D.ietf-ace-dtls-authorize] [I-D.aragon-ace-ipsec-profile] to [I-D.ietf-ace-dtls-authorize], to achieve communication security,
achieve communication security, proof-of-possession and server proof-of-possession and server authentication.
authentication.
More specifically and with reference to the terminology defined in More specifically and with reference to the terminology defined in
OAuth 2.0: OAuth 2.0:
o The joining endpoint acts as Client; o The joining endpoint acts as Client;
o The Group Manager acts as Resource Server, with different CoAP o The Group Manager acts as Resource Server, with different CoAP
resources for different multicast groups it is responsible for; resources for different groups it is responsible for;
o An Authorization Server enables and enforces authorized access of o An Authorization Server enables and enforces authorized access of
the joining endpoint to the Group Manager and its CoAP resources the joining endpoint to the Group Manager and its CoAP resources
paired with multicast groups to join. paired with groups to join.
Both the joining endpoint and the Group Manager MUST adopt secure Messages exchanged among the participants follow the formats defined
communication also for any message exchange with the Authorization in [I-D.palombini-ace-key-groupcomm]. Both the joining endpoint and
Server. To this end, different alternatives are possible, such as the Group Manager have to adopt secure communication also for any
OSCORE, DTLS [RFC6347] or IPsec [RFC4301]. message exchange with the Authorization Server. To this end,
different alternatives are possible, such as OSCORE, DTLS [RFC6347]
or IPsec [RFC4301].
Appendix D. 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 listener endpoints to synchronize with sequence numbers considered by listener endpoints to synchronize with sequence numbers
of multicasters. of multicasters.
D.1. Best-Effort Synchronization E.1. Best-Effort Synchronization
Upon receiving a multicast request from a multicaster, a listener Upon receiving a multicast request from a multicaster, a listener
endpoint does not take any action to synchonize with the sequence endpoint does not take any action to synchonize with the sequence
number of that multicaster. This provides no assurance at all as to number of that multicaster. This provides no assurance at all as to
message freshness, which can be acceptable in non-critical use cases. message freshness, which can be acceptable in non-critical use cases.
D.2. Baseline Synchronization E.2. Baseline Synchronization
Upon receiving a multicast request from a given multicaster for the Upon receiving a multicast request from a given multicaster for the
first time, a listener endpoint initializes its last-seen sequence first time, a listener endpoint initializes its last-seen sequence
number in its Recipient Context associated to that multicaster. number in its Recipient Context associated to that multicaster.
However, the listener drops the multicast request without delivering However, the listener drops the multicast request without delivering
it to the application layer. This provides a reference point to it to the application layer. This provides a reference point to
identify if future multicast requests from the same multicaster are identify if future group requests from the same multicaster are
fresher than the last one received. fresher than the last one received.
A replay time interval exists, between when a possibly replayed A replay time interval exists, between when a possibly replayed
message is originally transmitted by a given multicaster and the message is originally transmitted by a given multicaster and the
first authentic fresh message from that same multicaster is received. first authentic fresh message from that same multicaster is received.
This can be acceptable for use cases where listener endpoints admit This can be acceptable for use cases where listener endpoints admit
such a trade-off between performance and assurance of message such a trade-off between performance and assurance of message
freshness. freshness.
D.3. Challenge-Response Synchronization E.3. Challenge-Response Synchronization
A listener endpoint performs a challenge-response exchange with a A listener endpoint performs a challenge-response exchange with a
multicaster, by using the Repeat Option for CoAP described in multicaster, by using the Repeat Option for CoAP described in
Section 2 of [I-D.amsuess-core-repeat-request-tag]. Section 2 of [I-D.ietf-core-echo-request-tag].
That is, upon receiving a multicast request from a particular That is, upon receiving a group request from a particular multicaster
multicaster for the first time, the listener processes the message as for the first time, the listener processes the message as described
described in Section 5.2 of this specification, but, even if valid, in Section 4.2 of this specification, but, even if valid, does not
does not deliver it to the application. Instead, the listener deliver it to the application. Instead, the listener replies to the
replies to the multicaster with a 4.03 Forbidden response message multicaster with a 4.03 Forbidden response message including a Repeat
including a Repeat Option, and stores the option value included Option, and stores the option value included therein.
therein.
Upon receiving a 4.03 Forbidden response that includes a Repeat Upon receiving a 4.03 Forbidden response that includes a Repeat
Option and originates from a verified group member, a multicaster Option and originates from a verified group member, a multicaster
MUST send a group request as a unicast message addressed to the same sends a request as a unicast message addressed to the same listener,
listener, echoing the Repeat Option value. In particular, the echoing the Repeat Option value. In particular, the multicaster does
multicaster does not necessarily resend the same group request, but not necessarily resend the same group request, but can instead send a
can instead send a more recent one, if the application permits it. more recent one, if the application permits it. This makes it
This makes it possible for the multicaster to not retain previously possible for the multicaster to not retain previously sent group
sent group requests for full retransmission, unless the application requests for full retransmission, unless the application explicitly
explicitly requires otherwise. In either case, the multicaster uses requires otherwise. In either case, the multicaster uses the
the sequence number value currently stored in its own Sender Context. sequence number value currently stored in its own Sender Context. If
If the multicaster stores group requests for possible retransmission the multicaster stores group requests for possible retransmission
with the Repeat Option, it should not store a given request for with the Repeat Option, it should not store a given request for
longer than a pre-configured time interval. Note that the unicast longer than a pre-configured time interval. Note that the unicast
request echoing the Repeat Option is correctly treated and processed request echoing the Repeat Option is correctly treated and processed
as a group message, since the "gid" field including the Group as a group message, since the 'kid context' field including the Group
Identifier of the OSCORE group is still present in the Object- Identifier of the OSCORE group is still present in the Object-
Security Option as part of the COSE object (see Section 4). Security Option as part of the COSE object (see Section 3).
Upon receiving the unicast group request including the Repeat Option, Upon receiving the unicast request including the Repeat Option, the
the listener verifies that the option value equals the stored and listener verifies that the option value equals the stored and
previously sent value; otherwise, the request is silently discarded. previously sent value; otherwise, the request is silently discarded.
Then, the listener verifies that the unicast group request has been Then, the listener verifies that the unicast request has been
received within a pre-configured time interval, as described in received within a pre-configured time interval, as described in
[I-D.amsuess-core-repeat-request-tag]. In such a case, the request [I-D.ietf-core-echo-request-tag]. In such a case, the request is
is further processed and verified; otherwise, it is silently further processed and verified; otherwise, it is silently discarded.
discarded. Finally, the listener updates the Recipient Context Finally, the listener updates the Recipient Context associated to
associated to that multicaster, by setting the Replay Window that multicaster, by setting the Replay Window according to the
according to the Sequence Number from the unicast group request Sequence Number from the unicast request conveying the Repeat Option.
conveying the Repeat Option. The listener either delivers the The listener either delivers the request to the application if it is
request to the application if it is an actual retransmission of the an actual retransmission of the original one, or discards it
original one, or discard it otherwise. Mechanisms to signal whether otherwise. Mechanisms to signal whether the resent request is a full
the resent request is a full retransmission of the original one are retransmission of the original one are out of the scope of this
out of the scope of this specification. specification.
In case it does not receive a valid group request including the In case it does not receive a valid unicast request including the
Repeat Option within the configured time interval, the listener node Repeat Option within the configured time interval, the listener
SHOULD perform the same challenge-response upon receiving the next endpoint should perform the same challenge-response upon receiving
multicast request from that same multicaster. the next multicast request from that same multicaster.
A listener SHOULD NOT deliver group request messages from a given A listener should not deliver group requests from a given multicaster
multicaster to the application until one valid group request from to the application until one valid request from that same multicaster
that same multicaster has been verified as fresh, as conveying an has been verified as fresh, as conveying an echoed Repeat Option
echoed Repeat Option [I-D.amsuess-core-repeat-request-tag]. Also, a [I-D.ietf-core-echo-request-tag]. Also, a listener may perform the
listener MAY perform the challenge-response described above at any challenge-response described above at any time, if synchronization
time, if synchronization with sequence numbers of multicasters is with sequence numbers of multicasters is (believed to be) lost, for
(believed to be) lost, for instance after a device reboot. It is the instance after a device reboot. It is the role of the application to
role of the application to define under what circumstances sequence define under what circumstances sequence numbers lose
numbers lose synchronization. This can include a minimum gap between synchronization. This can include a minimum gap between the sequence
the sequence number of the latest accepted group request from a number of the latest accepted group request from a multicaster and
multicaster and the sequence number of a group request just received the sequence number of a group request just received from the same
from the same multicaster. A multicaster MUST always be ready to multicaster. A multicaster has to be always ready to perform the
perform the challenge-response based on the Repeat Option in case a challenge-response based on the Repeat Option in case a listener
listener starts it. starts it.
Note that endpoints configured as pure listeners are not able to Note that endpoints configured as pure listeners are not able to
perform the challenge-response described above, as they do not store perform the challenge-response described above, as they do not store
a Sender Context to secure the 4.03 Forbidden response to the a Sender Context to secure the 4.03 Forbidden response to the
multicaster. Therefore, pure listeners should adopt alternative multicaster. Therefore, pure listeners should adopt alternative
approaches to achieve and maintain synchronization with sequence approaches to achieve and maintain synchronization with sequence
numbers of multicasters. numbers of multicasters.
This approach provides an assurance of absolute message freshness. This approach provides an assurance of absolute message freshness.
However, it can result in an impact on performance which is However, it can result in an impact on performance which is
undesirable or unbearable, especially in large multicast groups where undesirable or unbearable, especially in large groups where many
many nodes at the same time might join as new members or lose endpoints at the same time might join as new members or lose
synchronization. synchronization.
Appendix E. No Verification of Signatures Appendix F. No Verification of Signatures
There are some application scenarios using group communications that There are some application scenarios using group communication that
have particularly strict requirements. One example of this is the have particularly strict requirements. One example of this is the
requirement of low message latency in non-emergency lighting requirement of low message latency in non-emergency lighting
applications [I-D.somaraju-ace-multicast]. For those applications applications [I-D.somaraju-ace-multicast]. For those applications
which have tight performance constraints and relaxed security which have tight performance constraints and relaxed security
requirements, it can be inconvenient for some endpoints to verify requirements, it can be inconvenient for some endpoints to verify
digital signatures in order to assert source authenticity of received digital signatures in order to assert source authenticity of received
group messages. In other cases, the signature verification can be group messages. In other cases, the signature verification can be
deferred or only checked for specific actions. For instance, a deferred or only checked for specific actions. For instance, a
command to turn a bulb on where the bulb is already on does not need command to turn a bulb on where the bulb is already on does not need
the signature to be checked. In such situations, the counter the signature to be checked. In such situations, the counter
signature needs to be included anyway as part of the group message, signature needs to be included anyway as part of the group message,
so that an endpoint that needs to validate the signature for any so that an endpoint that needs to validate the signature for any
reason has the ability to do so. reason has the ability to do so.
In this specification, it is NOT RECOMMENDED that endpoints do not In this specification, it is NOT RECOMMENDED that endpoints do not
verify the counter signature of received group messages. However, it verify the counter signature of received group messages. However, it
is recognized that there may be situations where it is not always is recognized that there may be situations where it is not always
required. The consequence of not doing the signature validation is required. The consequence of not doing the signature validation is
that security in the group is based only on the group-authenticity of that security in the group is based only on the group-authenticity of
the shared keying material used for encryption. That is, endpoints the shared keying material used for encryption. That is, endpoints
in the multicast group have evidence that a received message has been in the group have evidence that a received message has been
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.
 End of changes. 148 change blocks. 
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