Group Object Security for Constrained RESTful Environments (Group OSCORE)
draft-ietf-core-oscore-groupcomm-20
| Document | Type | Active Internet-Draft (core WG) | |
|---|---|---|---|
| Authors | Marco Tiloca , Göran Selander , Francesca Palombini , John Preuß Mattsson , Jiye Park | ||
| Last updated | 2023-09-02 | ||
| Replaces | draft-tiloca-core-multicast-oscoap | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources |
Working Group Repo
Mailing list discussion |
||
| Stream | WG state | WG Consensus: Waiting for Write-Up | |
| Document shepherd | Christian Amsüss | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | christian@amsuess.com |
draft-ietf-core-oscore-groupcomm-20
CoRE Working Group M. Tiloca
Internet-Draft RISE AB
Intended status: Standards Track G. Selander
Expires: 5 March 2024 F. Palombini
J. Mattsson
Ericsson AB
J. Park
Universitaet Duisburg-Essen
2 September 2023
Group Object Security for Constrained RESTful Environments (Group
OSCORE)
draft-ietf-core-oscore-groupcomm-20
Abstract
This document defines the security protocol Group Object Security for
Constrained RESTful Environments (Group OSCORE), providing end-to-end
security of CoAP messages exchanged between members of a group, e.g.,
sent over IP multicast. In particular, the described protocol
defines how OSCORE is used in a group communication setting to
provide source authentication for CoAP group requests, sent by a
client to multiple servers, and for protection of the corresponding
CoAP responses. Group OSCORE also defines a pairwise mode where each
member of the group can efficiently derive a symmetric pairwise key
with any other member of the group for pairwise OSCORE communication.
Group OSCORE can be used between endpoints communicating with CoAP or
CoAP-mappable HTTP.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 March 2024.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2. Security Context . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Common Context . . . . . . . . . . . . . . . . . . . . . 11
2.1.1. AEAD Algorithm . . . . . . . . . . . . . . . . . . . 11
2.1.2. ID Context . . . . . . . . . . . . . . . . . . . . . 11
2.1.3. Common IV . . . . . . . . . . . . . . . . . . . . . . 12
2.1.4. Authentication Credential Format . . . . . . . . . . 12
2.1.5. Group Manager Authentication Credential . . . . . . . 12
2.1.6. Group Encryption Algorithm . . . . . . . . . . . . . 12
2.1.7. Signature Algorithm . . . . . . . . . . . . . . . . . 12
2.1.8. Signature Encryption Key . . . . . . . . . . . . . . 13
2.1.9. Pairwise Key Agreement Algorithm . . . . . . . . . . 13
2.2. Sender Context and Recipient Context . . . . . . . . . . 14
2.3. Establishment of Security Context Parameters . . . . . . 15
2.4. Authentication Credentials . . . . . . . . . . . . . . . 15
2.5. Pairwise Keys . . . . . . . . . . . . . . . . . . . . . . 17
2.5.1. Derivation of Pairwise Keys . . . . . . . . . . . . . 17
2.5.2. ECDH with Montgomery Coordinates . . . . . . . . . . 19
2.5.3. Usage of Sequence Numbers . . . . . . . . . . . . . . 20
2.5.4. Security Context for Pairwise Mode . . . . . . . . . 21
2.6. Update of Security Context . . . . . . . . . . . . . . . 21
2.6.1. Loss of Mutable Security Context . . . . . . . . . . 22
2.6.2. Exhaustion of Sender Sequence Number Space . . . . . 24
2.6.3. Retrieving New Security Context Parameters . . . . . 24
3. The Group Manager . . . . . . . . . . . . . . . . . . . . . . 27
3.1. Set-up of New Endpoints . . . . . . . . . . . . . . . . . 27
3.2. Management of Group Keying Material . . . . . . . . . . . 29
3.2.1. Recycling of Identifiers . . . . . . . . . . . . . . 32
3.3. Support for Signature Checkers . . . . . . . . . . . . . 35
3.4. Responsibilities of the Group Manager . . . . . . . . . . 35
4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 37
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4.1. Countersignature . . . . . . . . . . . . . . . . . . . . 37
4.1.1. Clarifications on Using a Countersignature . . . . . 37
4.2. The 'kid' and 'kid context' parameters . . . . . . . . . 38
4.3. AEAD Nonce . . . . . . . . . . . . . . . . . . . . . . . 38
4.4. external_aad . . . . . . . . . . . . . . . . . . . . . . 38
5. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 41
5.1. Keystream Derivation for Countersignature Encryption . . 42
5.2. Examples of Compressed COSE Objects . . . . . . . . . . . 43
5.2.1. Examples in Group Mode . . . . . . . . . . . . . . . 43
5.2.2. Examples in Pairwise Mode . . . . . . . . . . . . . . 44
6. Message Binding, Sequence Numbers, Freshness, and Replay
Protection . . . . . . . . . . . . . . . . . . . . . . . 45
6.1. Supporting Multiple Responses in Long Exchanges . . . . . 46
6.2. Synchronization and Freshness . . . . . . . . . . . . . . 46
6.3. Replay Protection . . . . . . . . . . . . . . . . . . . . 47
6.3.1. Replay Protection of Responses . . . . . . . . . . . 47
7. Message Reception . . . . . . . . . . . . . . . . . . . . . . 49
8. Message Processing in Group Mode . . . . . . . . . . . . . . 50
8.1. Protecting the Request . . . . . . . . . . . . . . . . . 51
8.2. Verifying the Request . . . . . . . . . . . . . . . . . . 52
8.3. Protecting the Response . . . . . . . . . . . . . . . . . 55
8.4. Verifying the Response . . . . . . . . . . . . . . . . . 57
8.5. External Signature Checkers . . . . . . . . . . . . . . . 60
9. Message Processing in Pairwise Mode . . . . . . . . . . . . . 61
9.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 62
9.2. Main Differences from OSCORE . . . . . . . . . . . . . . 63
9.3. Protecting the Request . . . . . . . . . . . . . . . . . 63
9.4. Verifying the Request . . . . . . . . . . . . . . . . . . 63
9.5. Protecting the Response . . . . . . . . . . . . . . . . . 64
9.6. Verifying the Response . . . . . . . . . . . . . . . . . 65
10. Challenge-Response Based Freshness and Synchronization . . . 66
11. Implementation Compliance . . . . . . . . . . . . . . . . . . 69
12. Web Linking . . . . . . . . . . . . . . . . . . . . . . . . . 70
13. Security Considerations . . . . . . . . . . . . . . . . . . . 71
13.1. Security of the Group Mode . . . . . . . . . . . . . . . 72
13.2. Security of the Pairwise Mode . . . . . . . . . . . . . 74
13.3. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 75
13.4. Management of Group Keying Material . . . . . . . . . . 76
13.5. Update of Security Context and Key Rotation . . . . . . 76
13.5.1. Late Update on the Sender . . . . . . . . . . . . . 77
13.5.2. Late Update on the Recipient . . . . . . . . . . . . 77
13.6. Collision of Group Identifiers . . . . . . . . . . . . . 78
13.7. Cross-group Message Injection . . . . . . . . . . . . . 78
13.7.1. Attack Description . . . . . . . . . . . . . . . . . 78
13.7.2. Attack Prevention in Group Mode . . . . . . . . . . 79
13.8. Prevention of Group Cloning Attack . . . . . . . . . . . 81
13.9. Group OSCORE for Unicast Requests . . . . . . . . . . . 81
13.10. End-to-end Protection . . . . . . . . . . . . . . . . . 82
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13.11. Master Secret . . . . . . . . . . . . . . . . . . . . . 83
13.12. Replay Protection . . . . . . . . . . . . . . . . . . . 83
13.13. Message Ordering . . . . . . . . . . . . . . . . . . . . 84
13.14. Message Freshness . . . . . . . . . . . . . . . . . . . 84
13.15. Client Aliveness . . . . . . . . . . . . . . . . . . . . 85
13.16. Cryptographic Considerations . . . . . . . . . . . . . . 85
13.17. Message Segmentation . . . . . . . . . . . . . . . . . . 87
13.18. Privacy Considerations . . . . . . . . . . . . . . . . . 87
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 88
14.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . 88
14.2. Target Attributes Registry . . . . . . . . . . . . . . . 89
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 89
15.1. Normative References . . . . . . . . . . . . . . . . . . 89
15.2. Informative References . . . . . . . . . . . . . . . . . 91
Appendix A. Assumptions and Security Objectives . . . . . . . . 94
A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 95
A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 96
Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 97
Appendix C. Example of Group Identifier Format . . . . . . . . . 99
Appendix D. Document Updates . . . . . . . . . . . . . . . . . . 100
D.1. Version -19 to -20 . . . . . . . . . . . . . . . . . . . 100
D.2. Version -18 to -19 . . . . . . . . . . . . . . . . . . . 101
D.3. Version -17 to -18 . . . . . . . . . . . . . . . . . . . 101
D.4. Version -16 to -17 . . . . . . . . . . . . . . . . . . . 102
D.5. Version -15 to -16 . . . . . . . . . . . . . . . . . . . 102
D.6. Version -14 to -15 . . . . . . . . . . . . . . . . . . . 102
D.7. Version -13 to -14 . . . . . . . . . . . . . . . . . . . 102
D.8. Version -12 to -13 . . . . . . . . . . . . . . . . . . . 103
D.9. Version -11 to -12 . . . . . . . . . . . . . . . . . . . 103
D.10. Version -10 to -11 . . . . . . . . . . . . . . . . . . . 104
D.11. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 105
D.12. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 105
D.13. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 106
D.14. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 108
D.15. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 108
D.16. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 109
D.17. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 109
D.18. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 110
D.19. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 111
D.20. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 112
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 112
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 113
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1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a web
transfer protocol specifically designed for constrained devices and
networks [RFC7228]. Group communication for CoAP
[I-D.ietf-core-groupcomm-bis] addresses use cases where deployed
devices benefit from a group communication model, for example to
reduce latencies, improve performance, and reduce bandwidth
utilization. Use cases include lighting control, integrated building
control, software and firmware updates, parameter and configuration
updates, commissioning of constrained networks, and emergency
multicast (see Appendix B). Group communication for CoAP
[I-D.ietf-core-groupcomm-bis] mainly uses UDP/IP multicast as the
underlying data transport.
Object Security for Constrained RESTful Environments (OSCORE)
[RFC8613] describes a security protocol based on the exchange of
protected CoAP messages. OSCORE builds on CBOR Object Signing and
Encryption (COSE) [RFC9052][RFC9053] and provides end-to-end
encryption, integrity, replay protection, and binding of response to
request between a sender and a recipient, independent of the
transport layer also in the presence of intermediaries. To this end,
a CoAP message is protected by including its payload (if any),
certain options, and header fields into a COSE object, which is
conveyed within the CoAP payload and the CoAP OSCORE option of the
protected message, thereby replacing those message fields with an
authenticated and encrypted object.
This document defines Group OSCORE, a security protocol for group
communication with CoAP [I-D.ietf-core-groupcomm-bis] and for CoAP-
mappable HTTP requests and responses, providing the same end-to-end
security properties as OSCORE also in the case where requests have
multiple recipients. In particular, the described protocol defines
how OSCORE is used in a group communication setting to provide source
authentication for group requests sent by a client to multiple
servers, and for protection of the corresponding responses. Group
OSCORE also defines a pairwise mode where each member of the group
can efficiently derive a symmetric pairwise key with any other member
of the group for pairwise-protected OSCORE communication. Just like
OSCORE, Group OSCORE is independent of the transport layer and works
wherever CoAP does.
As with OSCORE, it is possible to combine Group OSCORE with
communication security on other layers. One example is the use of
transport layer security, such as DTLS [RFC9147], between one client
and one proxy (and vice versa), or between one proxy and one server
(and vice versa). This prevents observers from accessing addressing
information conveyed in CoAP options that would not be protected by
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Group OSCORE, but would be protected by DTLS. These options include
Uri-Host, Uri-Port, and Proxy-Uri. Note that DTLS does not define how
to secure messages sent over IP multicast. Group OSCORE is also
intended to work with OSCORE-capable proxies
[I-D.tiloca-core-oscore-capable-proxies] thereby enabling, for
example, nested OSCORE operations with OSCORE-protected communication
between a CoAP client and a proxy, carrying messages that are
additionally protected with Group OSCORE between the CoAP client and
the target CoAP servers.
Group OSCORE defines two modes of operation, that can be used
independently or together:
* In the group mode, Group OSCORE requests and responses are
digitally signed with the private key of the sender and the
signature is embedded in the protected CoAP message. The group
mode supports all COSE signature algorithms as well as signature
verification by intermediaries. This mode is defined in
Section 8.
* In the pairwise mode, two group members exchange OSCORE requests
and responses (typically) over unicast, and the messages are
protected with symmetric keys. These symmetric keys are derived
from Diffie-Hellman shared secrets, calculated with the asymmetric
keys of the sender and recipient, allowing for shorter integrity
tags and therefore lower message overhead. This mode is defined
in Section 9.
Both modes provide source authentication of CoAP messages. The
application decides what mode to use, potentially on a per-message
basis. Such decisions can be based, for instance, on pre-configured
policies or dynamic assessing of the target recipient and/or
resource, among other things. One important case is when requests
are protected in group mode, and responses in pairwise mode. Since
such responses convey shorter integrity tags instead of bigger, full-
fledged signatures, this significantly reduces the message overhead
in case of many responses to one request.
A special deployment of Group OSCORE consists in using the pairwise
mode only. For example, consider the case of a constrained-node
network [RFC7228] with a large number of CoAP endpoints and the
objective to establish secure communication between any pair of
endpoints with a small provisioning effort and message overhead.
Since the total number of security associations that needs to be
established grows with the square of the number of endpoints, it is
desirable to restrict the amount of secret keying material provided
to each endpoint. Moreover, a key establishment protocol would need
to be executed for each security association. One solution to this
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issue is to deploy Group OSCORE, with the endpoints being part of a
group, and to use the pairwise mode. This solution has the benefit
of providing a single shared secret, while distributing only the
public keys of group members or a subset of those. After that, a
CoAP endpoint can locally derive the OSCORE Security Context for the
other endpoint in the group, and protect CoAP communications with
very low overhead [I-D.ietf-iotops-security-protocol-comparison].
In some circumstances, Group OSCORE messages may be transported in
HTTP, e.g., when they are protected with the pairwise mode and target
a single recipient, or when they are protected with the group mode
and target multiple CoAP recipients through cross-protocol
translators such as HTTP-to-CoAP proxies
[RFC8075][I-D.tiloca-core-groupcomm-proxy]. The use of Group OSCORE
with HTTP is as defined for OSCORE in Section 11 of [RFC8613].
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with the terms and concepts
described in CoAP [RFC7252], including "endpoint", "client",
"server", "sender", and "recipient"; group communication for CoAP
[I-D.ietf-core-groupcomm-bis]; Observe [RFC7641]; CBOR [RFC8949];
COSE [RFC9052][RFC9053] and related countersignatures [RFC9338].
Readers are also expected to be familiar with the terms and concepts
for protection and processing of CoAP messages through OSCORE, such
as "Security Context" and "Master Secret", defined in [RFC8613].
Terminology for constrained environments, such as "constrained
device" and "constrained-node network", is defined in [RFC7228].
This document refers also to the following terminology.
* Keying material: data that is necessary to establish and maintain
secure communication among endpoints. This includes, for
instance, keys and IVs [RFC4949].
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* Authentication credential: information associated with an entity,
including that entity's public key and parameters associated with
the public key. Examples of authentication credentials are CBOR
Web Tokens (CWTs) and CWT Claims Sets [RFC8392], X.509
certificates [RFC5280] and C509 certificates
[I-D.ietf-cose-cbor-encoded-cert]. Further details about
authentication credentials are provided in Section 2.4.
* Group: a set of endpoints that share group keying material and
security parameters (Common Context, see Section 2). That is,
unless otherwise specified, the term group used in this document
refers to a "security group" (see Section 2.1 of
[I-D.ietf-core-groupcomm-bis]), not to be confused with "CoAP
group" or "application group".
* Group Manager: an entity responsible for a group, neither required
to be an actual group member nor to take part in the group
communication. The responsibilities of the Group Manager are
listed in Section 3.4.
* Silent server: a member of a group that performs only group mode
processing on incoming requests and never sends responses
protected with Group OSCORE. For CoAP group communications,
requests are normally sent without necessarily expecting a
response. A silent server may send unprotected responses, as
error responses reporting a Group OSCORE error.
* Group Identifier (Gid): identifier assigned to the group, unique
within the set of groups of a given Group Manager.
* Birth Gid: with respect to a group member, the Gid obtained by
that group member upon (re-)joining the group.
* Key Generation Number: an integer value identifying the current
version of the keying material used in a group.
* Source authentication: evidence that a received message in the
group originated from a specific identified group member. This
also provides assurance that the message was not tampered with by
anyone, be it a different legitimate group member or an endpoint
which is not a group member.
* Group request: a CoAP request message sent by a client in the
group to all the servers in that group.
* Long exchange: an exchange of messages associated with a request
that is a group request and/or an Observe request [RFC7641].
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In either case, multiple responses can follow from the same server
to the request associated with the long exchange. The client
terminates a long exchange when freeing up the CoAP Token value
used for the associated request, for which no further responses
will be accepted afterwards.
2. Security Context
This document refers to a group as a set of endpoints sharing keying
material and security parameters for executing the Group OSCORE
protocol, see Section 1.1. All members of a group maintain a
Security Context as defined in this section.
How the Security Context is established by the group members is out
of scope for this document, but if there is more than one Security
Context applicable to a message, then the endpoints MUST be able to
tell which Security Context was latest established. How to manage
information about the group is described in terms of a Group Manager
(see Section 3).
An endpoint of the group may use the group mode (see Section 8), the
pairwise mode (see Section 9), or both, depending on the modes it
supports and on the parameters of the Security Context. The Security
Context of Group OSCORE extends the OSCORE Security Context defined
in Section 3 of [RFC8613] as follows (see Figure 1).
* One Common Context, shared by all the endpoints in the group and
extended as defined below.
- The new parameter Authentication Credential Format, specifying
the format of authentication credentials used in the group (see
Section 2.1.4).
- The new parameter Group Manager Authentication Credential,
specifying the authentication credential of the Group Manager
responsible for the group (see Section 2.1.5).
- For the group mode, the Common Context is extended with the
following new parameters.
o Group Encryption Algorithm, used for encrypting and
decrypting messages protected in group mode (see
Section 2.1.6).
o Signature Algorithm, used for computing and verifying the
countersignature of messages protected in group mode (see
Section 2.1.7).
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o Signature Encryption Key, used for encrypting and decrypting
the countersignature of messages protected in group mode
(see Section 2.1.8).
- For the pairwise mode, the Common Context is extended with a
Pairwise Key Agreement Algorithm (see Section 2.1.9) used for
the agreement on a static-static Diffie-Hellman shared secret,
from which pairwise keys are derived (see Section 2.5.1).
* One Sender Context, extended with the following new parameters.
- The endpoint's own private key used to sign messages protected
in group mode (see Section 8), or for deriving pairwise keys
used with the pairwise mode (see Section 2.5).
- The endpoint's own authentication credential containing its
public key (see Section 2.4).
- For the pairwise mode, the Sender Context is extended with the
Pairwise Sender Keys associated with the other endpoints (see
Section 2.5).
If the endpoint is configured exclusively as silent server (see
Section 1.1), then the Sender Context is omitted.
* One Recipient Context for each other endpoint from which messages
are received. It is not necessary to maintain Recipient Contexts
associated with endpoints from which messages are not (expected to
be) received.
- Each Recipient Context is extended with the authentication
credential of the other endpoint, used to verify the signature
of messages protected in group mode, or for deriving the
pairwise keys used with the pairwise mode (see Section 2.5).
- For the pairwise mode, each Recipient Context is extended with
the Pairwise Recipient Key associated with the other endpoint
(see Section 2.5).
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+-------------------+-------------------------------------------------+
| Context Component | New Information Elements |
+-------------------+-------------------------------------------------+
| Common Context | Authentication Credential Format |
| | Group Manager Authentication Credential |
| | * Group Encryption Algorithm |
| | * Signature Algorithm |
| | * Signature Encryption Key |
| | ^ Pairwise Key Agreement Algorithm |
+-------------------+-------------------------------------------------+
| Sender Context | Endpoint's own private key |
| | Endpoint's own authentication credential |
| | ^ Pairwise Sender Keys for the other endpoints |
+-------------------+-------------------------------------------------+
| Each | Other endpoint's authentication credential |
| Recipient Context | ^ Pairwise Recipient Key for the other endpoint |
+-------------------+-------------------------------------------------+
Figure 1: Additions to the OSCORE Security Context. The elements
labeled with * and with ^ are relevant only for the group mode
and only for the pairwise mode, respectively.
2.1. Common Context
The following sections specify how the Common Context is used and
extended compared to [RFC8613]. All algorithms (AEAD, Group
Encryption, Signature, Pairwise Key Agreement) are immutable once the
Common Context is established. The Common Context may be acquired
from the Group Manager (see Section 3).
2.1.1. AEAD Algorithm
The AEAD Algorithm (see Section 3.1 of [RFC8613]) SHALL identify the
COSE AEAD algorithm to use for encryption and decryption when
messages are protected using the pairwise mode (see Section 9). This
algorithm MUST provide integrity protection. If this parameter is
not set, the pairwise mode is not used in the group.
2.1.2. ID Context
The ID Context parameter (see Sections 3.1 and 3.3 of [RFC8613])
SHALL contain the Group Identifier (Gid) of the group. The choice of
the Gid format is application specific. An example of specific
formatting of the Gid is given in Appendix C. The application needs
to specify how to handle potential collisions between Gids (see
Section 13.6).
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2.1.3. Common IV
The Common IV parameter (see Section 3.1 of [RFC8613]) SHALL identify
the Common IV used in the group. Differently from OSCORE, the length
of the Common IV is determined as follows.
* If only one among the AEAD Algorithm and the Group Encryption
Algorithm is set (see Section 2.1.1 and Section 2.1.6), the length
of the Common IV is the AEAD nonce length for the set algorithm.
* If both the AEAD Algorithm and the Group Encryption Algorithm are
set, the length of the Common IV is the greatest AEAD nonce length
among those of the two algorithms.
2.1.4. Authentication Credential Format
The new parameter Authentication Credential Format specifies the
format of authentication credentials used in the group. Further
details on authentication credentials are compiled in Section 2.4.
2.1.5. Group Manager Authentication Credential
The new parameter Group Manager Authentication Credential specifies
the authentication credential of the Group Manager, including the
Group Manager's public key. The endpoint MUST achieve proof-of-
possession of the corresponding private key. Further details on the
provisioning of the Group Manager's authentication credential to the
group members are out of the scope of this document.
2.1.6. Group Encryption Algorithm
The new parameter Group Encryption Algorithm identifies the algorithm
to use for encryption and decryption, when messages are protected in
group mode (see Section 8). This algorithm MAY provide integrity
protection. If this parameter is not set, the group mode is not used
in the group.
2.1.7. Signature Algorithm
The new parameter Signature Algorithm identifies the digital
signature algorithm used for computing and verifying the
countersignature on the COSE object (see Sections 3.2 and 3.3 of
[RFC9338]), when messages are protected in group mode (see
Section 8). If this parameter is not set, the group mode is not used
in the group.
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2.1.8. Signature Encryption Key
The new parameter Signature Encryption Key specifies the encryption
key for deriving a keystream to encrypt/decrypt a countersignature,
when a message is protected in group mode (see Section 8).
The Signature Encryption Key is derived as defined for Sender/
Recipient Keys in Section 3.2.1 of [RFC8613], with the following
differences.
* The 'id' element of the 'info' array is the empty byte string.
* The 'alg_aead' element of the 'info' array specifies the Group
Encryption Algorithm from the Common Context (see Section 2.1.6),
encoded as a CBOR integer or text string, consistently with the
"Value" field in the "COSE Algorithms" Registry for this
algorithm.
* The 'type' element of the 'info' array is "SEKey". The label is
an ASCII string and does not include a trailing NUL byte.
* L and the 'L' element of the 'info' array are the size of the key
for the Group Encryption Algorithm specified in the Common Context
(see Section 2.1.6), in bytes. While the obtained Signature
Encryption Key is never used with the Group Encryption Algorithm,
its length was chosen to obtain a matching level of security.
2.1.9. Pairwise Key Agreement Algorithm
The new parameter Pairwise Key Agreement Algorithm identifies the
elliptic curve Diffie-Hellman algorithm used to derive a static-
static Diffie-Hellman shared secret, from which pairwise keys are
derived (see Section 2.5.1) to protect messages with the pairwise
mode (see Section 9). If this parameter is not set, the pairwise
mode is not used in the group.
If the HKDF Algorithm specified in the Common Context is "HKDF SHA-
256" (identified as "HMAC 256/256", COSE algorithm encoding: 5), then
the Pairwise Key Agreement Algorithm is "ECDH-SS + HKDF-256" (COSE
algorithm encoding: -27).
If the HKDF Algorithm specified in the Common Context is "HKDF SHA-
512" (identified as "HMAC 512/512", COSE algorithm encoding: 7), then
the Pairwise Key Agreement Algorithm is "ECDH-SS + HKDF-512" (COSE
algorithm encoding: -28).
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Note that the HKDF Algorithm in the Common Context is denoted by the
corresponding COSE HMAC Algorithm. For example, the HKDF Algorithm
"HKDF SHA-256" is specified as the HMAC Algorithm "HMAC 256/256"
(COSE algorithm encoding: 5).
More generally, if Pairwise Key Agreement Algorithm is set, it MUST
identify a COSE algorithm such that: i) it performs a direct ECDH
Static-Static key agreement; and ii) it indicates the use of the same
HKDF Algorithm used in the group as specified in the Common Context.
2.2. Sender Context and Recipient Context
The Sender ID SHALL be unique for each endpoint in a group with a
certain triplet (Master Secret, Master Salt, Group Identifier), see
Section 3.3 of [RFC8613].
The maximum length of a Sender ID in bytes equals L minus 6, where L
is determined as follows.
* If only one among the AEAD Algorithm and the Group Encryption
Algorithm is set (see Section 2.1.1 and Section 2.1.6), then L is
the AEAD nonce length for the set algorithm.
* If both the AEAD Algorithm and the Group Encryption Algorithm are
set, then L is the smallest AEAD nonce length among those of the
two algorithms.
With the exception of the authentication credential of the sender
endpoint, a receiver endpoint can derive a complete Security Context
from a received Group OSCORE message and the Common Context (see
Section 2.3).
The authentication credentials in the Recipient Contexts can be
retrieved from the Group Manager (see Section 3) upon joining the
group. An authentication credential can alternatively be acquired
from the Group Manager at a later time, for example the first time a
message is received from a particular endpoint in the group (see
Section 8.2 and Section 8.4).
For severely constrained devices, it may be infeasible to
simultaneously handle the ongoing processing of a recently received
message in parallel with the retrieval of the sender endpoint's
authentication credential. Such devices can be configured to drop a
received message for which there is no (complete) Recipient Context,
and retrieve the sender endpoint's authentication credential in order
to have it available to verify subsequent messages from that
endpoint.
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An endpoint may admit a maximum number of Recipient Contexts for a
same Security Context, e.g., due to memory limitations. After
reaching that limit, the endpoint has to delete a current Recipient
Context to install a new one (see Section 2.6.1.2). It is up to the
application to define policies for Recipient Contexts to delete.
2.3. Establishment of Security Context Parameters
OSCORE defines the derivation of Sender Context and Recipient Context
(specifically, of Sender/Recipient Keys) and of the Common IV, from a
set of input parameters (see Section 3.2 of [RFC8613]).
The derivation of Sender/Recipient Keys and of the Common IV defined
in OSCORE applies also to Group OSCORE, with the following
modifications compared to Section 3.2.1 of [RFC8613].
* If Group Encryption Algorithm in the Common Context is set (see
Section 2.1.6), then the 'alg_aead' element of the 'info' array
MUST specify Group Encryption Algorithm from the Common Context as
a CBOR integer or text string, consistently with the "Value" field
in the "COSE Algorithms" Registry for this algorithm.
* If Group Encryption Algorithm in the Common Context is not set,
then the 'alg_aead' element of the 'info' array MUST specify AEAD
Algorithm from the Common Context (see Section 2.1.1), as per
Section 5.4 of [RFC8613].
* When deriving the Common IV, the 'L' element of the 'info' array
MUST specify the length of the Common IV in bytes, which is
determined as defined in Section 2.1.3.
2.4. Authentication Credentials
The authentication credentials of the endpoints in a group MUST be
encoded according to the format used in the group, as indicated by
the Authentication Credential Format parameter in the Common Context
(see Section 2.1.4). The authentication credential of the Group
Manager SHOULD be encoded according to that same format. The format
of authentication credentials MUST provide the public key and a
comprehensive set of information related to the public key algorithm,
including, e.g., the used elliptic curve (when applicable).
If Group Encryption Algorithm in the Common Context is not set (see
Section 2.1.6), then the public key algorithm is the Pairwise Key
Agreement Algorithm used in the group (see Section 2.1.9), else the
Signature Algorithm used in the group (see Section 2.1.7).
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If the authentication credentials are X.509 certificates [RFC5280] or
C509 certificates [I-D.ietf-cose-cbor-encoded-cert], the public key
algorithm is fully described by the "algorithm" field of the
"SubjectPublicKeyInfo" structure, and by the
"subjectPublicKeyAlgorithm" element, respectively.
If authentication credentials are CBOR Web Tokens (CWTs) or CWT
Claims Sets [RFC8392], the public key algorithm is fully described by
a COSE key type and its "kty" and "crv" parameters.
Authentication credentials are used to derive pairwise keys (see
Section 2.5.1) and are included in the external additional
authenticated data when processing messages (see Section 4.4). In
both these cases, an endpoint in a group MUST treat authentication
credentials as opaque data, i.e., by considering the same binary
representation made available to other endpoints in the group,
possibly through a designated trusted source (e.g., the Group
Manager).
For example, an X.509 certificate is provided as its direct binary
serialization. If C509 certificates or CWTs are used as
authentication credentials, each is provided as the binary
serialization of a (possibly tagged) CBOR array. If CWT Claims Sets
are used as authentication credentials, each is provided as the
binary serialization of a CBOR map.
If authentication credentials are CWTs, then the untagged CWT
associated with an entity is stored in the Security Context and used
as authentication credential for that entity.
If authentication credentials are X.509 / C509 certificates or CWTs,
and the authentication credential associated with an entity is
provided within a chain or a bag, then only the end-entity
certificate or end-entity untagged CWT is stored in the Security
Context and used as authentication credential for that entity.
Storing whole authentication credentials rather than only a subset of
those may result in a non-negligible storage overhead. On the other
hand, it also ensures that authentication credentials are correctly
used in a simple, flexible and non-error-prone way, also taking into
account future credential formats as entirely new or extending
existing ones. In particular, it is ensured that:
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* When used to derive pairwise keys and when included in the
external additional authenticated data, authentication credentials
can also specify possible metadata and parameters related to the
included public key. Besides the public key algorithm, these
comprise other relevant pieces of information such as key usage,
expiration time, issuer, and subject.
* All endpoints using another endpoint's authentication credential
use exactly the same binary serialization, as obtained and
distributed by the credential provider (e.g., the Group Manager),
and as originally crafted by the credential issuer. In turn, this
does not require to define and maintain canonical subsets of
authentication credentials and their corresponding encoding, and
spares endpoints from error-prone re-encoding operations.
Depending on the particular deployment and the intended group size,
limiting the storage overhead of endpoints in a group can be an
incentive for system/network administrators to prefer using a compact
format of authentication credentials in the first place.
2.5. Pairwise Keys
Certain signature schemes, such as EdDSA and ECDSA, support a secure
combined signature and encryption scheme. This section specifies the
derivation of "pairwise keys", for use in the pairwise mode defined
in Section 9.
Group OSCORE keys used for both signature and encryption MUST be used
only for purposes related to Group OSCORE. These include the
processing of messages with Group OSCORE, as well as performing
proof-of-possession of private keys, e.g., upon joining a group
through the Group Manager (see Section 3).
2.5.1. Derivation of Pairwise Keys
Using the Group OSCORE Security Context (see Section 2), a group
member can derive AEAD keys, to protect point-to-point communication
between itself and any other endpoint X in the group by means of the
AEAD Algorithm from the Common Context (see Section 2.1.1). The key
derivation of these so-called pairwise keys follows the same
construction as in Section 3.2.1 of [RFC8613]:
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Pairwise Sender Key = HKDF(Sender Key, IKM-Sender, info, L)
Pairwise Recipient Key = HKDF(Recipient Key, IKM-Recipient, info, L)
with
IKM-Sender = Sender Auth Cred | Recipient Auth Cred | Shared Secret
IKM-Recipient = Recipient Auth Cred | Sender Auth Cred | Shared Secret
where:
* The Pairwise Sender Key is the AEAD key for processing outgoing
messages addressed to endpoint X.
* The Pairwise Recipient Key is the AEAD key for processing incoming
messages from endpoint X.
* HKDF is the OSCORE HKDF algorithm [RFC8613] from the Common
Context.
* The Sender Key from the Sender Context is used as salt in the
HKDF, when deriving the Pairwise Sender Key.
* The Recipient Key from the Recipient Context associated with
endpoint X is used as salt in the HKDF, when deriving the Pairwise
Recipient Key.
* Sender Auth Cred is the endpoint's own authentication credential
from the Sender Context.
* Recipient Auth Cred is the endpoint X's authentication credential
from the Recipient Context associated with the endpoint X.
* The Shared Secret is computed as a cofactor Diffie-Hellman shared
secret, see Section 5.7.1.2 of [NIST-800-56A], using the Pairwise
Key Agreement Algorithm. The endpoint uses its private key from
the Sender Context and the other endpoint's public key included in
Recipient Auth Cred. Note the requirement of validation of public
keys in Section 13.16.
In case the other endpoint's public key has COSE Key Type "EC2"
(e.g., for the curves P-256, P-384, and P-512), then the public
key is used as is. In case the other endpoint's public key has
COSE Key Type "OKP", the procedure is described in Section 5 of
[RFC7748]. In particular, if the public key is for X25519 or
X448, it is used as is. Otherwise, if the public key is for the
curve Ed25519 or Ed448, it is first mapped to Montgomery
coordinates (see Section 2.5.2).
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* IKM-Sender is the Input Keying Material (IKM) used in the HKDF for
the derivation of the Pairwise Sender Key. IKM-Sender is the byte
string concatenation of Sender Auth Cred, Recipient Auth Cred, and
the Shared Secret. The authentication credentials Sender Auth
Cred and Recipient Auth Cred are binary encoded as defined in
Section 2.4.
* IKM-Recipient is the Input Keying Material (IKM) used in the HKDF
for the derivation of the Pairwise Recipient Key. IKM-Recipient is
the byte string concatenation of Recipient Auth Cred, Sender Auth
Cred, and the Shared Secret. The authentication credentials
Recipient Auth Cred and Sender Auth Cred are binary encoded as
defined in Section 2.4.
* info and L are as defined in Section 3.2.1 of [RFC8613]. That is:
- The 'alg_aead' element of the 'info' array takes the value of
AEAD Algorithm from the Common Context (see Section 2.1.1).
- L and the 'L' element of the 'info' array are the size of the
key for the AEAD Algorithm from the Common Context (see
Section 2.1.1), in bytes.
If EdDSA asymmetric keys are used, the Edward coordinates are mapped
to Montgomery coordinates using the maps defined in Sections 4.1 and
4.2 of [RFC7748], before using the X25519 or X448 function defined in
Section 5 of [RFC7748]. For further details, see Section 2.5.2. ECC
asymmetric keys in Montgomery or Weierstrass form are used directly
in the key agreement algorithm, without coordinate mapping.
After establishing a partially or completely new Security Context
(see Section 2.6 and Section 3.2), the old pairwise keys MUST be
deleted. Since new Sender/Recipient Keys are derived from the new
group keying material (see Section 2.2), every group member MUST use
the new Sender/Recipient Keys when deriving new pairwise keys.
As long as any two group members preserve the same asymmetric keys,
their Diffie-Hellman shared secret does not change across updates of
the group keying material.
2.5.2. ECDH with Montgomery Coordinates
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2.5.2.1. Curve25519
The y-coordinate of the other endpoint's Ed25519 public key is
decoded as specified in Section 5.1.3 of [RFC8032]. The Curve25519
u-coordinate is recovered as u = (1 + y) / (1 - y) (mod p) following
the map in Section 4.1 of [RFC7748]. Note that the mapping is not
defined for y = 1, and that y = -1 maps to u = 0 which corresponds to
the neutral group element and thus will result in a degenerate shared
secret. Therefore, implementations MUST abort if the y-coordinate of
the other endpoint's Ed25519 public key is 1 or -1 (mod p).
The private signing key byte strings (i.e., the lower 32 bytes used
for generating the public key, see step 1 of Section 5.1.5 of
[RFC8032]) are decoded the same way for signing in Ed25519 and scalar
multiplication in X25519. Hence, in order to compute the shared
secret, the endpoint applies the X25519 function to the Ed25519
private signing key byte string and the encoded u-coordinate byte
string as specified in Section 5 of [RFC7748].
2.5.2.2. Curve448
The y-coordinate of the other endpoint's Ed448 public key is decoded
as specified in Section 5.2.3. of [RFC8032]. The Curve448
u-coordinate is recovered as u = y^2 * (d * y^2 - 1) / (y^2 - 1) (mod
p) following the map from "edwards448" in Section 4.2 of [RFC7748],
and also using the relation x^2 = (y^2 - 1)/(d * y^2 - 1) from the
curve equation. Note that the mapping is not defined for y = 1 or
-1. Therefore, implementations MUST abort if the y-coordinate of the
peer endpoint's Ed448 public key is 1 or -1 (mod p).
The private signing key byte strings (i.e., the lower 57 bytes used
for generating the public key, see step 1 of Section 5.2.5 of
[RFC8032]) are decoded the same way for signing in Ed448 and scalar
multiplication in X448. Hence, in order to compute the shared
secret, the endpoint applies the X448 function to the Ed448 private
signing key byte string and the encoded u-coordinate byte string as
specified in Section 5 of [RFC7748].
2.5.3. Usage of Sequence Numbers
When using any of its Pairwise Sender Keys, a sender endpoint
including the 'Partial IV' parameter in the protected message MUST
use the current fresh value of the Sender Sequence Number from its
Sender Context (see Section 2.2). That is, the same Sender Sequence
Number space is used for all outgoing messages protected with Group
OSCORE, thus limiting both storage and complexity.
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On the other hand, when combining communications with the group mode
and the pairwise mode, this may result in the Partial IV values
moving forward more often. This can happen when a client engages in
frequent or long sequences of one-to-one exchanges with servers in
the group, by sending requests over unicast. In turn, this
contributes to a sooner exhaustion of the Sender Sequence Number
space of the client, which would then require to take actions for
deriving a new Sender Context before resuming communications in the
group (see Section 2.6.2).
2.5.4. Security Context for Pairwise Mode
If the pairwise mode is supported, the Security Context additionally
includes the Pairwise Key Agreement Algorithm and the pairwise keys,
as described at the beginning of Section 2.
The pairwise keys as well as the shared secrets used in their
derivation (see Section 2.5.1) may be stored in memory or recomputed
every time they are needed. The shared secret changes only when a
public/private key pair used for its derivation changes, which
results in the pairwise keys also changing. Additionally, the
pairwise keys change if the Sender ID changes or if a new Security
Context is established for the group (see Section 2.6.3). In order
to optimize protocol performance, an endpoint may store the derived
pairwise keys for easy retrieval.
In the pairwise mode, the Sender Context includes the Pairwise Sender
Keys to use with the other endpoints (see Figure 1). In order to
identify the right key to use, the Pairwise Sender Key for endpoint X
may be associated with the Recipient ID of endpoint X, as defined in
the Recipient Context (i.e., the Sender ID from the point of view of
endpoint X). In this way, the Recipient ID can be used to lookup for
the right Pairwise Sender Key. This association may be implemented in
different ways, e.g., by storing the pair (Recipient ID, Pairwise
Sender Key) or linking a Pairwise Sender Key to a Recipient Context.
2.6. Update of Security Context
It is RECOMMENDED that the immutable part of the Security Context is
stored in non-volatile memory, or that it can otherwise be reliably
accessed throughout the operation of the group, e.g., after device
reboots. However, also immutable parts of the Security Context may
need to be updated, for example due to scheduled key renewal, new or
re-joining members in the group, or the fact that the endpoint
changes Sender ID (see Section 2.6.3).
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The mutable parts of the Security Context are updated by the endpoint
when executing the security protocol, but may be lost (see
Section 2.6.1) or become outdated by exhaustion of Sender Sequence
Numbers (see Section 2.6.2).
2.6.1. Loss of Mutable Security Context
An endpoint may lose its mutable Security Context, e.g., due to a
reboot occurred in an unprepared way (see Section 2.6.1.1) or due to
a deleted Recipient Context (see Section 2.6.1.2).
If it is not feasible or practically possible to store and maintain
up-to-date the mutable part in non-volatile memory (e.g., due to
limited number of write operations), the endpoint MUST be able to
detect a loss of the mutable Security Context, to prevent the re-use
of a nonce with the same AEAD key, and to handle incoming replayed
messages.
2.6.1.1. Total Loss
In case a loss of the Sender Context and/or of the Recipient Contexts
is detected (e.g., following a reboot occurred in an unprepared way),
the endpoint MUST NOT protect further messages using this Security
Context, to avoid reusing an AEAD nonce with the same AEAD key.
Before resuming its operations in the group, the endpoint MUST
retrieve new Security Context parameters from the Group Manager (see
Section 2.6.3) and use them to derive a new Sender Context and
Recipient Contexts (see Section 2.2). Since the new Sender Context
includes newly derived encryption keys, an endpoint will not reuse
the same pair (key, nonce), even when it is a server using the
Partial IV of (old re-injected) requests to build the AEAD nonce for
protecting the responses.
From then on, the endpoint MUST use the latest installed Sender
Context to protect outgoing messages. Newly derived Recipient
Contexts will have a Replay Window which is initialized as valid.
If an endpoint is not configured as silent server and is not able to
establish an updated Sender Context, e.g., because of lack of
connectivity with the Group Manager, then the endpoint MUST NOT
protect further messages using the current Security Context and MUST
NOT accept incoming messages from other group members, as currently
unable to detect possible replays.
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If an endpoint is configured as silent server and is not able to
establish an updated Security Context, e.g., because of lack of
connectivity with the Group Manager, then the endpoint MUST NOT
accept incoming messages from other group members, as currently
unable to detect possible replays.
An adversary may leverage the above to perform a Denial of Service
attack and prevent some group members from communicating altogether.
That is, the adversary can first block the communication path between
the Group Manager and some individual group members. This can be
achieved, for instance, by injecting fake responses to DNS queries
for the Group Manager hostname, or by removing a network link used
for routing traffic towards the Group Manager. Then, the adversary
can induce an unprepared reboot for some endpoints in the group,
e.g., by triggering a short power outage. After that, such endpoints
that have lost their Sender Context and/or Recipient Contexts
following the reboot would not be able to obtain new Security Context
parameters from the Group Manager, as specified above. Thus, they
would not be able to further communicate in the group until
connectivity with the Group Manager is restored.
2.6.1.2. Deleted Recipient Contexts
The Security Context may contain a large and variable number of
Recipient Contexts. A Recipient Context may need to be deleted,
because the maximum number of Recipient Contexts has been reached
(see Section 2.2), or due to some other reason.
When a Recipient Context is deleted, information about previously
received messages from the corresponding other endpoint is lost.
Therefore, if the Recipient Context is derived again from the same
Security Context, there is a risk that a replayed message is not
detected. If one Recipient Context has been deleted from the current
Security Context, then the Replay Window of any new Recipient Context
in this Security Context MUST be initialized as invalid. Messages
associated with a Recipient Context that has an invalid Replay Window
MUST NOT be delivered to the application.
If the endpoint receives a request to process with the new Recipient
Context and the endpoint supports the CoAP Echo Option [RFC9175],
then it is RECOMMENDED to follow the procedure specified in
Section 10 which establishes a valid Replay Window. In particular,
the endpoint MUST use its Partial IV when generating the AEAD nonce
and MUST include the Partial IV in the response message conveying the
Echo Option.
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Alternatively, the endpoint MAY retrieve or wait for new Security
Context parameters from the Group Manager and derive new Sender and
Recipient Contexts, as defined in Section 2.6.1.1. In this case the
Replay Windows of all Recipient Contexts become valid if they are not
already.
2.6.2. Exhaustion of Sender Sequence Number Space
Since an endpoint increments its Sender Sequence Number for each new
outgoing message including a Partial IV, an endpoint can eventually
exhaust the Sender Sequence Number space.
Implementations MUST be able to detect an exhaustion of Sender
Sequence Number space, after the endpoint has consumed the largest
usable value. This may be influenced by additional limitations
besides the mere 40-bit size limit of the Partial IV. If an
implementation's integers support wrapping addition, the
implementation MUST treat the Sender Sequence Number as exhausted
when a wrap-around is detected.
Upon exhausting the Sender Sequence Number space, the endpoint MUST
NOT use this Security Context to protect further messages including a
Partial IV.
The endpoint SHOULD inform the Group Manager, retrieve new Security
Context parameters from the Group Manager (see Section 2.6.3), and
use them to derive a new Sender Context (see Section 2.2).
From then on, the endpoint MUST use its latest installed Sender
Context to protect outgoing messages.
2.6.3. Retrieving New Security Context Parameters
The Group Manager can assist an endpoint with an incomplete Sender
Context to retrieve missing data of the Security Context and thereby
become fully operational in the group again. The two main options
for the Group Manager are described in this section: i) assignment of
a new Sender ID to the endpoint (see Section 2.6.3.1); and ii)
establishment of a new Security Context for the group (see
Section 2.6.3.2). The update of the Replay Window in each of the
Recipient Contexts is discussed in Section 2.6.1.
As group membership changes, or as group members get new Sender IDs
(see Section 2.6.3.1), so do the relevant Recipient IDs that the
other endpoints need to keep track of. As a consequence, group
members may end up retaining stale Recipient Contexts, that are no
longer useful to verify incoming secure messages.
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The Recipient ID ('kid') SHOULD NOT be considered as a persistent and
reliable identifier of a group member. Such an indication can be
achieved only by using that member's public key, when verifying
countersignatures of received messages (in group mode), or when
verifying messages integrity-protected with pairwise keying material
derived from authentication credentials and associated asymmetric
keys (in pairwise mode).
Furthermore, applications MAY define policies to: i) delete
(long-)unused Recipient Contexts and reduce the impact on storage
space; as well as ii) check with the Group Manager that an
authentication credential with the public key included therein is
currently the one associated with a 'kid' value, after a number of
consecutive failed verifications.
2.6.3.1. New Sender ID for the Endpoint
The Group Manager may assign a new Sender ID to an endpoint, while
leaving the Gid, Master Secret and Master Salt unchanged in the
group. In this case, the Group Manager MUST assign a Sender ID that
has not been used in the group since the latest time when the current
Gid value was assigned to the group (see Section 3.2).
Having retrieved the new Sender ID, and potentially other missing
data of the immutable Security Context, the endpoint can derive a new
Sender Context (see Section 2.2). When doing so, the endpoint resets
the Sender Sequence Number in its Sender Context to 0, and derives a
new Sender Key. This is in turn used to possibly derive new Pairwise
Sender Keys.
From then on, the endpoint MUST use its latest installed Sender
Context to protect outgoing messages.
The assignment of a new Sender ID may be the result of different
processes. The endpoint may request a new Sender ID, e.g., because
of the exhaustion of the Sender Sequence Number space (see
Section 2.6.2). An endpoint may request to re-join the group, e.g.,
because of losing its mutable Security Context (see Section 2.6.1),
and is provided with a new Sender ID together with the latest
immutable Security Context.
For the other group members, the Recipient Context corresponding to
the old Sender ID becomes stale (see Section 3.2).
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2.6.3.2. New Security Context for the Group
The Group Manager may establish a new Security Context for the group
(see Section 3.2). The Group Manager does not necessarily establish
a new Security Context for the group if one member has an outdated
Security Context (see Section 2.6.3.1), unless that was already
planned or required for other reasons.
All the group members need to acquire new Security Context parameters
from the Group Manager. Once having acquired new Security Context
parameters, each group member performs the following actions.
* From then on, it MUST NOT use the current Security Context to
start processing new messages for the considered group.
* It completes any ongoing message processing for the considered
group.
* It derives and installs a new Security Context. In particular:
- It re-derives the keying material stored in its Sender Context
and Recipient Contexts (see Section 2.2). The Master Salt used
for the re-derivations is the updated Master Salt parameter if
provided by the Group Manager, or the empty byte string
otherwise.
- It resets its Sender Sequence Number in its Sender Context to
0.
- It re-initializes the Replay Window of each Recipient Context
as valid and with 0 as its current lower limit.
- For each long exchange where it is a client and that it wants
to keep active, it sets the Response Number of each associated
server as not initialized (see Section 6.1).
From then on, it can resume processing new messages for the
considered group. In particular:
* It MUST use its latest installed Sender Context to protect
outgoing messages.
* It SHOULD use only its latest installed Recipient Contexts to
process incoming messages, unless application policies admit to
temporarily retain and use the old, recent, Security Context (see
Section 13.5.1).
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The distribution of a new Gid and Master Secret may result in
temporarily misaligned Security Contexts among group members. In
particular, this may result in a group member not being able to
process messages received right after a new Gid and Master Secret
have been distributed. A discussion on practical consequences and
possible ways to address them, as well as on how to handle the old
Security Context, is provided in Section 13.5.
3. The Group Manager
As with OSCORE, endpoints communicating with Group OSCORE need to
establish the relevant Security Context. Group OSCORE endpoints need
to acquire OSCORE input parameters, information about the group(s)
and about other endpoints in the group(s). This document is based on
the existence of an entity called Group Manager that is responsible
for the group, but it does not mandate how the Group Manager
interacts with the group members. The list of responsibilities of
the Group Manager is compiled in Section 3.4.
The Group Manager assigns unique Group Identifiers (Gids) to the
groups under its control. Within each of such groups, the Group
Manager assigns unique Sender IDs (and thus Recipient IDs) to the
respective group members. The maximum length of Sender IDs depends
on the length of the AEAD nonce for the algorithms used in the group
(see Section 2.2).
According to a hierarchical approach, the Gid value assigned to a
group is associated with a dedicated space for the values of Sender
ID and Recipient ID of the members of that group. When an endpoint
(re-)joins a group, it is provided with the current Gid to use in the
group. The Group Manager also assigns an integer Key Generation
Number counter to each of its groups, identifying the current version
of the keying material used in that group. Further details about
identifiers and keys are provided in Section 3.2.
The Group Manager maintains records of the authentication credentials
of endpoints in a group, and provides information about the group and
its members to other group members (see Section 3.1), and to external
entities with a specific role (see Section 3.3).
3.1. Set-up of New Endpoints
From the Group Manager, an endpoint acquires group data such as the
Gid and OSCORE input parameters including its own Sender ID, with
which it can derive the Sender Context.
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When joining the group or later on as a group member, an endpoint can
also retrieve from the Group Manager the authentication credential of
the Group Manager as well as the authentication credential and other
information associated with other members of the group, with which it
can derive the corresponding Recipient Context. An application can
configure a group member to asynchronously retrieve information about
Recipient Contexts, e.g., by Observing [RFC7641] a resource at the
Group Manager to get updates on the group membership.
Upon endpoints' joining, the Group Manager collects their
authentication credentials and MUST verify proof-of-possession of the
respective private key. Together with the requested authentication
credentials of other group members, the Group Manager MUST provide
the joining endpoints with the Sender ID of the associated group
members and the current Key Generation Number in the group (see
Section 3.2).
An endpoint may join a group, for example, by explicitly interacting
with the responsible Group Manager, or by being configured with some
tool performing the tasks of the Group Manager. When becoming
members of a group, endpoints are not required to know how many and
what endpoints are in the same group.
Communications that the Group Manager has with joining endpoints and
group members MUST be secured. Specific details on how to secure
such communications are out of the scope of this document.
The Group Manager MUST verify that the joining endpoint is authorized
to join the group. To this end, the Group Manager can directly
authorize the joining endpoint, or expect it to provide authorization
evidence previously obtained from a trusted entity. Further details
about the authorization of joining endpoints are out of the scope of
this document.
In case of successful authorization check, the Group Manager provides
the joining endpoint with the keying material to initialize the
Security Context. The actual provisioning of keying material and
parameters to the joining endpoint is out of the scope of this
document.
One realization of a Group Manager is specified in
[I-D.ietf-ace-key-groupcomm-oscore], where the join process is based
on the ACE framework for authentication and authorization in
constrained environments [RFC9200].
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3.2. Management of Group Keying Material
In order to establish a new Security Context for a group, the Group
Manager MUST generate and assign to the group a new Group Identifier
(Gid) and a new value for the Master Secret parameter. When doing
so, a new value for the Master Salt parameter MAY also be generated
and assigned to the group. When establishing the new Security
Context, the Group Manager should preserve the current value of the
Sender ID of each group member.
The specific group key management scheme used to distribute new
keying material is out of the scope of this document. A simple group
key management scheme is defined in
[I-D.ietf-ace-key-groupcomm-oscore]. When possible, the delivery of
rekeying messages should use a reliable transport, in order to reduce
chances of group members missing a rekeying instance.
The set of group members should not be assumed as fixed, i.e., the
group membership is subject to changes, possibly on a frequent basis.
The Group Manager MUST rekey the group without undue delay in case
one or more endpoints leave the group. An endpoint may leave the
group at own initiative, or may be evicted from the group by the
Group Manager, e.g., in case an endpoint is compromised, or is
suspected to be compromised. In either case, rekeying the group
excludes such endpoints from future communications in the group, and
thus preserves forward security. If a network node is compromised or
suspected to be compromised, the Group Manager MUST evict from the
group all the endpoints hosted by that node that are member of the
group and rekey the group accordingly.
If required by the application, the Group Manager MUST rekey the
group also before one or more new joining endpoints are added to the
group, thus preserving backward security.
Separately for each group, the value of the Key Generation Number
increases by one each time the Group Manager distributes new keying
material to that group (see Section 3.2). The first Key Generation
Number assigned to every group MUST be 0.
The establishment of the new Security Context for the group takes the
following steps.
1. The Group Manager MUST increment the Key Generation Number for
the group by 1.
2. The Group Manager MUST build a set of stale Sender IDs including:
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* The Sender IDs that, during the current Gid, were both
assigned to an endpoint and subsequently relinquished (see
Section 2.6.3.1).
* The current Sender IDs of the group members that the upcoming
group rekeying aims to exclude from future group
communications, if any.
3. The Group Manager rekeys the group, by distributing:
* The new keying material, i.e., the new Master Secret, the new
Gid and (optionally) the new Master Salt.
* The new Key Generation Number from step 1.
* The set of stale Sender IDs from step 2.
Further information may be distributed, depending on the specific
group key management scheme used in the group.
When receiving the new group keying material, a group member
considers the received stale Sender IDs and performs the following
actions.
* The group member MUST remove every authentication credential
associated with a stale Sender ID from its list of group members'
authentication credentials used in the group.
* The group member MUST delete each of its Recipient Contexts used
in the group whose corresponding Recipient ID is a stale Sender
ID.
After that, the group member installs the new keying material and
derives the corresponding new Security Context.
A group member might miss one or more consecutive instances of group
rekeying. As a result, the group member will retain old group keying
material with Key Generation Number GEN_OLD. Eventually, the group
member can notice the discrepancy, e.g., by repeatedly failing to
verify incoming messages, or by explicitly querying the Group Manager
for the current Key Generation Number. Once the group member gains
knowledge of having missed a group rekeying, it MUST delete the old
keying material it stores.
Then, the group member proceeds according to the following steps.
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1. The group member retrieves from the Group Manager the current
group keying material, together with the current Key Generation
Number GEN_NEW. The group member MUST NOT install the obtained
group keying material yet.
2. The group member asks the Group Manager for the set of stale
Sender IDs.
3. If no exact indication can be obtained from the Group Manager,
the group member MUST remove all the authentication credentials
from its list of group members' authentication credentials used
in the group and MUST delete all its Recipient Contexts used in
the group.
Otherwise, the group member MUST remove every authentication
credential associated with a stale Sender ID from its list of
group members' authentication credentials used in the group, and
MUST delete each of its Recipient Contexts used in the group
whose corresponding Recipient ID is a stale Sender ID.
4. The group member installs the current group keying material, and
derives the corresponding new Security Context.
Alternatively, the group member can re-join the group. In such a
case, the group member MUST take one of the following two actions.
* First, the group member performs steps 2 and 3 above. Then, the
group member re-joins the group.
* The group member re-joins the group with the same roles it
currently has in the group, and, during the re-joining process, it
asks the Group Manager for the authentication credentials of all
the current group members.
Then, given Z, the set of authentication credentials received from
the Group Manager, the group member removes every authentication
credential which is not in Z from its list of group members'
authentication credentials used in the group, and deletes each of
its Recipient Contexts used in the group that does not include any
of the authentication credentials in Z.
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By removing authentication credentials and deleting Recipient
Contexts associated with stale Sender IDs, it is ensured that a
recipient endpoint storing the latest group keying material does not
store the authentication credentials of sender endpoints that are not
current group members. This in turn allows group members to rely on
stored authentication credentials to confidently assert the group
membership of sender endpoints, when receiving incoming messages
protected in group mode (see Section 8).
Such a strictness in managing the authentication credentials and
Recipient Contexts associated with other group members is required
for two reasons. First, as further discussed in Section 13.1, it
ensures that the group mode can be used securely, even in a group
where the Group Encryption Algorithm does not provide integrity
protection (see Section 2.1.6) and external signature checkers are
used (see Section 8.5). Second, it ensures that the wrong (old)
authentication credential associated with a group member A is never
used with a sender ID that used to be associated with A and has been
later issued to a different group member B (see Section 3.2.1.2),
thus preventing the need to recover from an identity mix-up.
3.2.1. Recycling of Identifiers
This section specifies how the Group Manager handles and possibly
reassigns Gid values and Sender ID values in a group.
3.2.1.1. Recycling of Group Identifiers
Since the Gid value changes every time a group is rekeyed, it can
happen that, after several rekeying instances, the whole space of Gid
values has been used for the group in question. When this happens,
the Group Manager has no available Gid values to use that have never
been assigned to the group during the group's lifetime.
The occurrence of such an event and how long it would take to occur
depend on the format and encoding of Gid values used in the group
(see, e.g., Appendix C), as well as on the frequency of rekeying
instances yielding a change of Gid value. Independently for each
group under its control, the Group Manager can take one of the two
following approaches.
* The Group Manager does not reassign Gid values. That is, once the
whole space of Gid values has been used for a group, the Group
Manager terminates the group and may re-establish a new group.
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* While the Gid value changes every time a group is rekeyed, the
Group Manager can reassign Gid values previously used during a
group's lifetime. By doing so, the group can continue to exist
even once the whole space of Gid values has been used.
The Group Manager MAY support and use this approach. In such a
case, the Group Manager MUST take additional actions when handling
Gid values and rekeying the group, as specified below.
When a node (re-)joins the group and it is provided with the
current Gid to use in the group, the Group Manager considers such
a Gid as the Birth Gid of that endpoint for that group. For each
group member, the Group Manager MUST store the latest
corresponding Birth Gid until that member leaves the group. In
case the endpoint has in fact re-joined the group, the newly
determined Birth Gid overwrites the one currently stored.
When establishing a new Security Context for the group, the Group
Manager takes the additional following step between steps 1 and 2
of Section 3.2.
A. The Group Manager MUST check if the new Gid to be distributed
is equal to the Birth Gid of any of the current group members. If
any of such "elder members" is found in the group, then:
- The Group Manager MUST evict the elder members from the group.
That is, the Group Manager MUST terminate their membership and,
in the following steps, it MUST rekey the group in such a way
that the new keying material is not provided to those evicted
elder members.
This ensures that any response from the same server to the
request of a long exchange can never successfully match against
the request of two different long exchanges.
In fact, the excluded elder members could eventually re-join
the group, thus terminating any of their ongoing long exchanges
(see Section 6.1).
Therefore, it is ensured by construction that no client can
have with the same server two ongoing long exchanges, such that
the two respective requests were protected using the same
Partial IV, Gid, and Sender ID.
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3.2.1.2. Recycling of Sender IDs
From the moment when a Gid is assigned to a group until the moment a
new Gid is assigned to that same group, the Group Manager MUST NOT
reassign a Sender ID within the group. This prevents to reuse a
Sender ID ('kid') with the same triple (Gid, Master Secret, Master
Salt). Within this restriction, the Group Manager can assign a
Sender ID used under an old Gid value (including under a same,
recycled Gid value), thus avoiding Sender ID values to irrecoverably
grow in size.
Even when an endpoint joining a group is recognized as a current
member of that group, e.g., through the ongoing secure communication
association, the Group Manager MUST assign a new Sender ID different
than the one currently used by the endpoint in the group, unless the
group is rekeyed first and a new Gid value is established.
3.2.1.3. Relation between Identifiers and Keying Material
Figure 2 overviews the different identifiers and keying material
components, considering their relation and possible reuse across
group rekeying.
Components changed in lockstep
upon a group rekeying
+----------------------------+ * Changing a kid does not
| | need changing the Group ID
| Master Group |<--> kid1
| Secret <---> o <---> ID | * A kid is not reassigned
| ^ |<--> kid2 under the ongoing usage of
| | | the current Group ID
| | |<--> kid3
| v | * Upon changing the Group ID,
| Master Salt | ... ... every current kid should
| (optional) | be preserved for efficient
| | key rollover
| The Key Generation Number |
| is incremented by 1 | * After changing Group ID, an
| | unused kid can be assigned,
+----------------------------+ even if it was used before
the Group ID change
Figure 2: Relations among keying material components.
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3.3. Support for Signature Checkers
The Group Manager may serve signature checkers, e.g., intermediary
gateways, which verify countersignatures of messages protected in
group mode (see Section 8.5). These entities do not join a group as
members, but can retrieve authentication credentials of group members
and other selected group data from the Group Manager.
In order to verify countersignatures of messages in a group, a
signature checker needs to retrieve the following information about
the group:
* The current ID Context (Gid) used in the group.
* The authentication credentials of the group members and of the
Group Manager.
If the signature checker is provided with a CWT for a given
entity, then the authentication credential associated with that
entity is the untagged CWT.
If the signature checker is provided with a chain or a bag of
X.509 / C509 certificates or of CWTs for a given entity, then the
authentication credential associated with that entity is the end-
entity certificate or end-entity untagged CWT.
* The current Signature Encryption Key (see Section 2.1.8).
* The identifiers of the algorithms used in the group (see
Section 2), i.e.: i) Group Encryption Algorithm and Signature
Algorithm; and ii) AEAD Algorithm and Pairwise Key Agreement
Algorithm, if such parameters are set in the Common Context (see
Section 2.1.1 and Section 2.1.9).
A signature checker MUST be authorized before it can retrieve such
information, for example with the use of
[I-D.ietf-ace-key-groupcomm-oscore].
3.4. Responsibilities of the Group Manager
The Group Manager is responsible for performing the following tasks:
1. Creating and managing OSCORE groups. This includes the
assignment of a Gid to every newly created group, ensuring
uniqueness of Gids within the set of its OSCORE groups and,
optionally, the secure recycling of Gids.
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2. Defining policies for authorizing the joining of its OSCORE
groups.
3. Handling the join process to add new endpoints as group members.
4. Establishing the Common Context part of the Security Context,
and providing it to authorized group members during the join
process, together with the corresponding Sender Context.
5. Updating the Key Generation Number and the Gid of its OSCORE
groups, upon renewing the respective Security Context.
6. Generating and managing Sender IDs within its OSCORE groups, as
well as assigning and providing them to new endpoints during the
join process, or to current group members upon request of
renewal or re-joining. This includes ensuring that:
* Each Sender ID is unique within each of the OSCORE groups;
* Each Sender ID is not reassigned within the same group since
the latest time when the current Gid value was assigned to
the group. That is, the Sender ID is not reassigned even to
a current group member re-joining the same group, without a
rekeying happening first.
7. Defining communication policies for each of its OSCORE groups,
and signaling them to new endpoints during the join process.
8. Renewing the Security Context of an OSCORE group upon membership
change, by revoking and renewing common security parameters and
keying material (rekeying).
9. 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.
10. Assisting a group member that has missed a group rekeying
instance to understand which authentication credentials and
Recipient Contexts to delete, as associated with former group
members.
11. Acting as key repository, in order to handle the authentication
credentials of the members of its OSCORE groups, and providing
such authentication credentials to other members of the same
group upon request. The actual storage of authentication
credentials may be entrusted to a separate secure storage device
or service.
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12. Validating that the format and parameters of authentication
credentials of group members are consistent with the public key
algorithm and related parameters used in the respective OSCORE
group.
The Group Manager specified in [I-D.ietf-ace-key-groupcomm-oscore]
provides this functionality.
4. The COSE Object
Building on Section 5 of [RFC8613], this section defines how to use
COSE [RFC9052] to wrap and protect data in the original message.
Like OSCORE, Group OSCORE uses the untagged COSE_Encrypt0 structure
with an Authenticated Encryption with Associated Data (AEAD)
algorithm. Unless otherwise specified, the following modifications
to what is defined for OSCORE apply, for both the group mode and the
pairwise mode of Group OSCORE.
4.1. Countersignature
When protecting a message in group mode, the 'unprotected' field MUST
additionally include the following parameter:
* Countersignature0 version 2: its value is set to the
countersignature of the COSE object.
The countersignature is computed by the sender as described in
Sections 3.2 and 3.3 of [RFC9338], by using its private key and
according to the Signature Algorithm in the Security Context.
In particular, the Countersign_structure contains the context text
string "CounterSignature0", the external_aad as defined in
Section 4.4 of this document, and the ciphertext of the COSE
object as payload.
4.1.1. Clarifications on Using a Countersignature
The literature commonly refers to a countersignature as a signature
computed by an entity A over a document already protected by a
different entity B.
However, the COSE_Countersignature0 structure belongs to the set of
abbreviated countersignatures defined in Sections 3.2 and 3.3 of
[RFC9338], which were designed primarily to deal with the problem of
encrypted group messaging, but where it is required to know who
originated the message.
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Since the parameters for computing or verifying the abbreviated
countersignature generated by A are provided by the same context used
to describe the security processing performed by B and to be
countersigned, these structures are applicable also when the two
entities A and B are actually the same one, like the sender of a
Group OSCORE message protected in group mode.
4.2. The 'kid' and 'kid context' parameters
The value of the 'kid' parameter in the 'unprotected' field of
response messages MUST be set to the Sender ID of the endpoint
transmitting the message, if the request was protected in group mode.
That is, unlike in [RFC8613], the 'kid' parameter is always present
in responses to a request that was protected in group mode.
The value of the 'kid context' parameter in the 'unprotected' field
of request messages MUST be set to the ID Context, i.e., the Group
Identifier value (Gid) of the group. That is, unlike in [RFC8613],
the 'kid context' parameter is always present in requests.
4.3. AEAD Nonce
The AEAD nonce is constructed like in OSCORE, with the difference
that step 4 in Section 5.2 of [RFC8613] is replaced with:
A. and then XOR with the X least significant bits of the Common IV,
where X is the length in bits of the AEAD nonce.
4.4. external_aad
The external_aad of the Additional Authenticated Data (AAD) is
different compared to OSCORE [RFC8613], and is defined in this
section.
The same external_aad structure is used in group mode and pairwise
mode for encryption/decryption (see Section 5.3 of [RFC9052]), as
well as in group mode for computing and verifying the
countersignature (see Sections 3.2 and 3.3 of [RFC9338]).
In particular, the external_aad includes also the Signature
Algorithm, the Group Encryption Algorithm, the Pairwise Key Agreement
Algorithm, the value of the 'kid context' in the COSE object of the
request, the OSCORE option of the protected message, the sender's
authentication credential, and the Group Manager's authentication
credential.
The external_aad SHALL be a CBOR array wrapped in a bstr object as
defined below, following the notation of [RFC8610]:
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external_aad = bstr .cbor aad_array
aad_array = [
oscore_version : uint,
algorithms : [alg_aead : int / tstr / null,
alg_group_enc : int / tstr / null,
alg_signature : int / tstr / null,
alg_pairwise_key_agreement : int / tstr / null],
request_kid : bstr,
request_piv : bstr,
options : bstr,
request_kid_context : bstr,
OSCORE_option : bstr,
sender_cred : bstr,
gm_cred : bstr
]
Figure 3: external_aad
Compared with Section 5.4 of [RFC8613], the aad_array has the
following differences.
* The 'algorithms' array is extended as follows.
The parameter 'alg_aead' MUST be set to the CBOR simple value
"null" (0xf6) if the parameter AEAD Algorithm in the Security
Context is not set (see Section 2). Otherwise, regardless of
whether the endpoint supports the pairwise mode or not, this
parameter MUST specify AEAD Algorithm from the Common Context (see
Section 2.1.1) as per Section 5.4 of [RFC8613].
Furthermore, the 'algorithms' array additionally includes:
- 'alg_group_enc', which specifies Group Encryption Algorithm
from the Common Context (see Section 2.1.6). This parameter
MUST be set to the CBOR simple value "null" (0xf6) if the
parameter Group Encryption Algorithm in the Common Context is
not set. Otherwise, regardless of whether the endpoint
supports the group mode or not, this parameter MUST specify
Group Encryption Algorithm as a CBOR integer or text string,
consistently with the "Value" field in the "COSE Algorithms"
Registry for this algorithm.
- 'alg_signature', which specifies Signature Algorithm from the
Common Context (see Section 2.1.7). This parameter MUST be set
to the CBOR simple value "null" (0xf6) if the parameter
Signature Algorithm in the Common Context is not set.
Otherwise, regardless of whether the endpoint supports the
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group mode or not, this parameter MUST specify Signature
Algorithm as a CBOR integer or text string, consistently with
the "Value" field in the "COSE Algorithms" Registry for this
algorithm.
- 'alg_pairwise_key_agreement', which specifies Pairwise Key
Agreement Algorithm from the Common Context (see
Section 2.1.9). This parameter MUST be set to the CBOR simple
value "null" (0xf6) if Pairwise Key Agreement Algorithm in the
Common Context is not set. Otherwise, regardless of whether
the endpoint supports the pairwise mode or not, this parameter
MUST specify Pairwise Key Agreement Algorithm as a CBOR integer
or text string, consistently with the "Value" field in the
"COSE Algorithms" Registry for this algorithm.
* The new element 'request_kid_context' contains the value of the
'kid context' in the COSE object of the request (see Section 4.2).
This enables endpoints to safely keep a long exchange active
beyond a possible change of Gid (i.e., of ID Context), following a
group rekeying (see Section 3.2). In fact, it ensures that every
response within a long exchange cryptographically matches with
only one request (i.e., the request associated with that long
exchange), rather than with multiple requests that were protected
with different keying material but share the same 'request_kid'
and 'request_piv' values.
* The new element 'OSCORE_option', containing the value of the
OSCORE Option present in the protected message, encoded as a
binary string. This prevents the attack described in Section 13.7
when using the group mode, as further explained in Section 13.7.2.
Note for implementation: this construction requires the OSCORE
option of the message to be generated and finalized before
computing the ciphertext of the COSE_Encrypt0 object (when using
the group mode or the pairwise mode) and before calculating the
countersignature (when using the group mode). Also, the aad_array
needs to be large enough to contain the largest possible OSCORE
option.
* The new element 'sender_cred', containing the sender's
authentication credential. This parameter MUST be set to a CBOR
byte string, which encodes the sender's authentication credential
in its original binary representation made available to other
endpoints in the group (see Section 2.4).
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* The new element 'gm_cred', containing the Group Manager's
authentication credential. This parameter MUST be set to a CBOR
byte string, which encodes the Group Manager's authentication
credential in its original binary representation made available to
other endpoints in the group (see Section 2.4). This prevents the
attack described in Section 13.8.
5. OSCORE Header Compression
The OSCORE header compression defined in Section 6 of [RFC8613] is
used for compactly encoding the COSE_Encrypt0 object specified in
Section 4 of this document, with the following differences.
* When the Group OSCORE message is protected in group mode, the
message payload SHALL encode the ciphertext of the COSE object,
concatenated with the encrypted countersignature of the COSE
object. That is:
- The plain, original countersignature of the COSE object, namely
SIGNATURE, is specified in the "Countersignature0 version 2"
parameter within the 'unprotected' field of the COSE object
(see Section 4.1).
- The encrypted countersignature, namely ENC_SIGNATURE, is
computed as
ENC_SIGNATURE = SIGNATURE XOR KEYSTREAM
where KEYSTREAM is derived as per Section 5.1.
* When the Group OSCORE message is protected in pairwise mode, the
message payload SHALL encode the ciphertext of the COSE object.
* This document defines the usage of the sixth least significant
bit, called "Group Flag", in the first byte of the OSCORE option
containing the OSCORE flag bits. This flag bit is specified in
Section 14.1.
* The Group Flag MUST be set to 1 if the Group OSCORE message is
protected using the group mode (see Section 8).
* The Group Flag MUST be set to 0 if the Group OSCORE message is
protected using the pairwise mode (see Section 9). The Group Flag
MUST also be set to 0 for ordinary OSCORE messages processed
according to [RFC8613].
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5.1. Keystream Derivation for Countersignature Encryption
The following defines how an endpoint derives the keystream
KEYSTREAM, used to encrypt/decrypt the countersignature of an
outgoing/incoming message M protected in group mode.
The keystream SHALL be derived as follows, by using the HKDF
Algorithm from the Common Context (see Section 3.2 of [RFC8613]),
which consists of composing the HKDF-Extract and HKDF-Expand steps
[RFC5869].
KEYSTREAM = HKDF(salt, IKM, info, L)
The input parameters of HKDF are as follows.
* salt takes as value the Partial IV (PIV) used to protect M. Note
that, if M is a response, salt takes as value either: i) the fresh
Partial IV generated by the server and included in the response;
or ii) the same Partial IV of the request generated by the client
and not included in the response.
* IKM is the Signature Encryption Key from the Common Context (see
Section 2.1.8).
* info is the serialization of a CBOR array consisting of (the
notation follows [RFC8610]):
info = [
id : bstr,
id_context : bstr,
type : bool,
L : uint
]
where:
* id is the Sender ID of the endpoint that generated PIV.
* id_context is the ID Context (Gid) used when protecting M.
Note that, in case of group rekeying, a server might use a
different Gid when protecting a response, compared to the Gid that
it used to verify (that the client used to protect) the request,
see Section 8.3.
* type is the CBOR simple value "true" (0xf5) if M is a request, or
the CBOR simple value "false" (0xf4) otherwise.
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* L is the size of the countersignature, as per Signature Algorithm
from the Common Context (see Section 2.1.7), in bytes.
5.2. Examples of Compressed COSE Objects
This section covers a list of OSCORE Header Compression examples of
Group OSCORE used in group mode (see Section 5.2.1) or in pairwise
mode (see Section 5.2.2).
The examples assume that the COSE_Encrypt0 object is set (which means
the CoAP message and cryptographic material is known). Note that the
examples do not include the full CoAP unprotected message or the full
Security Context, but only the input necessary to the compression
mechanism, i.e., the COSE_Encrypt0 object. The output is the
compressed COSE object as defined in Section 5 and divided into two
parts, since the object is transported in two CoAP fields: OSCORE
option and payload.
The examples assume that the plaintext (see Section 5.3 of [RFC8613])
is 6 bytes long, and that the AEAD tag is 8 bytes long, hence
resulting in a ciphertext which is 14 bytes long. When using the
group mode, the COSE_Countersignature0 byte string as described in
Section 4 is assumed to be 64 bytes long.
5.2.1. Examples in Group Mode
Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid = 0x25,
Partial IV = 5 and kid context = 0x44616c.
* Before compression (96 bytes):
[
/ protected / h'',
/ unprotected / {
/ kid / 4 : h'25',
/ Partial IV / 6 : h'05',
/ kid context / 10 : h'44616c',
/ Countersignature0 version 2 / 12 : h'66e6d9b0
db009f3e105a673f8855611726caed57f530f8cae9d0b168
513ab949fedc3e80a96ebe94ba08d3f8d3bf83487458e2ab
4c2f936ff78b50e33c885e35'
},
/ ciphertext / h'aea0155667924dff8a24e4cb35b9'
]
* After compression (85 bytes):
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Flag byte: 0b00111001 = 0x39 (1 byte)
Option Value: 0x39 05 03 44 61 6c 25 (7 bytes)
Payload: 0xaea0155667924dff8a24e4cb35b9 de9e ... f1
(14 bytes + size of the encrypted countersignature)
Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid = 0x52
and no Partial IV.
* Before compression (88 bytes):
[
/ protected / h'',
/ unprotected / {
/ kid / 4 : h'52',
/ Countersignature0 version 2 / 12 : h'f5b659b8
24487eb349c5c5c8a3fe401784cade2892725438e8be0fab
daa2867ee6d29f68edb0818e50ebf98c28b923d0205f5162
e73662e27c1a3ec562a49b80'
},
/ ciphertext / h'60b035059d9ef5667c5a0710823b'
]
* After compression (80 bytes):
Flag byte: 0b00101000 = 0x28 (1 byte)
Option Value: 0x28 52 (2 bytes)
Payload: 0x60b035059d9ef5667c5a0710823b ca1e ... b3
(14 bytes + size of the encrypted countersignature)
5.2.2. Examples in Pairwise Mode
Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid = 0x25,
Partial IV = 5 and kid context = 0x44616c.
* Before compression (29 bytes):
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[
/ protected / h'',
/ unprotected / {
/ kid / 4 : h'25',
/ Partial IV / 6 : h'05',
/ kid context / 10 : h'44616c'
},
/ ciphertext / h'aea0155667924dff8a24e4cb35b9'
]
* After compression (21 bytes):
Flag byte: 0b00011001 = 0x19 (1 byte)
Option Value: 0x19 05 03 44 61 6c 25 (7 bytes)
Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)
Response with ciphertext = 0x60b035059d9ef5667c5a0710823b and no
Partial IV.
* Before compression (18 bytes):
[
/ protected / h'',
/ unprotected / {},
/ ciphertext / h'60b035059d9ef5667c5a0710823b'
]
* After compression (14 bytes):
Flag byte: 0b00000000 = 0x00 (1 byte)
Option Value: 0x (0 bytes)
Payload: 0x60b035059d9ef5667c5a0710823b (14 bytes)
6. Message Binding, Sequence Numbers, Freshness, and Replay Protection
Like OSCORE, Group OSCORE provides message binding of responses to
requests, as well as uniqueness of AEAD (key, nonce) pair (see
Sections 7.1 and 7.2 of [RFC8613], respectively).
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6.1. Supporting Multiple Responses in Long Exchanges
For each of its ongoing long exchange, a client maintains one
Response Number for each different server. Then, separately for each
server, the client uses the associated Response Number to perform
ordering and replay protection of responses received from that server
within that long exchange (see Section 6.3.1).
That is, the Response Number has the same purpose that the
Notification Number has in OSCORE (see Section 4.1.3.5.2 of
[RFC8613]), but a client uses it for handling any response from the
associated server within a long exchange.
Group OSCORE allows to preserve a long exchange active indefinitely,
even in case the group is rekeyed, with consequent change of ID
Context, or in case the client obtains a new Sender ID.
As defined in Section 8, this is achieved by the client and server(s)
storing the 'kid' and 'kid context' used in the original request,
throughout the whole duration of the long exchange.
Upon leaving the group or before re-joining the group, a group member
MUST terminate all the ongoing long exchanges that it has started in
the group as a client, and hence frees up the CoAP Token associated
with the corresponding request.
6.2. Synchronization and Freshness
If the application requires freshness, e.g., according to time- or
event-based policies (see Section 2.5.1 of [RFC9175]), a server can
use the approach in Section 10 as a variant of the procedure in
Appendix B.1.2 of [RFC8613], before delivering request messages from
a client to the application. This also makes the server
(re-)synchronize with the client's Sender Sequence Number.
Like in OSCORE [RFC8613], assuming an honest server, the message
binding guarantees that a response is not older than the request it
replies to. Therefore, building on Section 7.3 of [RFC8613], the
following properties hold for Group OSCORE.
* The freshness of a response can be assessed if it is received soon
after the request.
For responses within a long exchange, this assessment gets weaker
with time. If such responses are Observe notifications [RFC7641],
it is RECOMMENDED that the client regularly re-register the
observation.
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If the request was neither a group request nor an Observe request,
there is at most a single valid response and only from one,
individually targeted server in the group. Thus, freshness can be
assessed depending on when the request was sent.
* It is not guaranteed that a misbehaving server did not create the
response before receiving the request, i.e., Group OSCORE does not
verify server aliveness.
* For requests and responses, the received Partial IV allows a
recipient to determine the relative order of requests or
responses.
6.3. Replay Protection
Like in OSCORE [RFC8613], the replay protection relies on the Partial
IV of incoming messages. A server updates the Replay Window of its
Recipient Contexts based on the Partial IV values in received request
messages, which correspond to the Sender Sequence Numbers of the
clients. Note that there can be large jumps in these Sender Sequence
Number values, for example when a client exchanges unicast messages
with other servers. The operation of validating the Partial IV and
performing replay protection MUST be atomic. The update of Replay
Windows is described in Section 2.6.1.
The protection from replay of requests is performed as per
Section 7.4 of [RFC8613], separately for each client and by
leveraging the Replay Window in the corresponding Recipient Context.
The protection from replay of responses in a long exchange is
performed as defined in Section 6.3.1.
6.3.1. Replay Protection of Responses
A client uses the method defined in this section in order to check
whether a received response is a replay.
This especially applies to responses received within a long exchange,
during which multiple such responses can be received from the same
server to the corresponding request. These include Observe
notifications [RFC7641]; and non-notification responses as a reply to
a group request, which the client can receive until the CoAP Token
value associated with the group request is freed up (see
Section 3.1.6 of [I-D.ietf-core-groupcomm-bis]).
When sending a response (both successful and error), a server MUST
include its Sender Sequence Number as Partial IV in the response,
except when sending the first response to the corresponding request,
in which case the Partial IV in the response MAY be omitted.
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In order to protect against replay, the client SHALL maintain for
each ongoing long exchange one Response Number for each different
server. The Response Number is a non-negative integer containing the
largest Partial IV of the responses received from that server during
the long exchange, while using the same Security Context.
Then, separately for each server, the client uses the associated
Response Number to perform ordering and replay protection of the
responses from that server during the long exchange, by comparing
their Partial IVs with one another and against the Response Number.
For every long exchange, the Response Number associated with a server
is initialized to the Partial IV of the response from that server
such that, within the long exchange, it is the first response from
that server to include a Partial IV and to be successfully verified
with the used Security Context. Note that, when a new Security
Context is established in the group, the client sets the Response
Number of each associated server as not initialized (see
Section 2.6.3.2), hence later responses within the same long exchange
and protected with the new Security Context will result in a new
initialization of Response Numbers. Furthermore, for every long
exchange, a client MUST only accept at most one response without
Partial IV from each server, and treat it as the oldest response from
that server within that long exchange.
During a long exchange, a client receiving a response containing a
Partial IV SHALL compare the Partial IV with the Response Number
associated with the replying server within that long exchange. The
client MUST stop processing a response from a server, if that
response has a Partial IV that has been previously received from that
server during that long exchange, while using the same Security
Context. Applications MAY decide that a client only processes
responses within a long exchange if those have a greater Partial IV
than the Response Number associated with the replying server within
that long exchange.
If the verification of the response succeeds, and the received
Partial IV (when included) was greater than the Response Number
associated with the replying server, then the client SHALL overwrite
that Response Number with the received Partial IV.
As long as a server uses the same Security Context to protect its
responses to the same request, the client MUST consider the response
with the highest Partial IV as the freshest response from that server
among those protected with that Security Context, regardless of the
order of arrival. Within a long exchange, implementations need to
make sure that a response without Partial IV is considered the oldest
response from the replying server within that long exchange.
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The method defined in this section is not relevant for responses to
requests that are neither group requests nor Observe requests. In
fact, for each of such requests, there is at most one response and
only from one, individually targeted server in the group.
7. Message Reception
Upon receiving a protected message, a recipient endpoint retrieves a
Security Context as in [RFC8613]. An endpoint MUST be able to
distinguish between a Security Context to process OSCORE messages as
in [RFC8613] and a Group OSCORE Security Context to process Group
OSCORE messages as defined in this document.
To this end, an endpoint can take into account the different
structure of the Security Context defined in Section 2, for example
based on the presence of Signature Algorithm and Pairwise Key
Agreement Algorithm in the Common Context. Alternatively,
implementations can use an additional parameter in the Security
Context, to explicitly mark that it is intended for processing Group
OSCORE messages.
If either of the following conditions holds, a recipient endpoint
MUST discard the incoming protected message:
* The Group Flag is set to 0 and the retrieved Security Context is
associated with an OSCORE group, but the endpoint does not support
the pairwise mode or any of the following algorithms is not set in
the Security Context: the AEAD Algorithm and the Pairwise Key
Agreement Algorithm.
* The Group Flag is set to 1 and the retrieved Security Context is
associated with an OSCORE group, but the endpoint does not support
the group mode or any of the following algorithms is not set in
the Security Context: the Group Encryption Algorithm and the
Signature Algorithm.
* The Group Flag is set to 1 but there is no Security Context
associated with an OSCORE group.
Future specifications may define how to process incoming messages
protected with Security Contexts as in [RFC8613], when the Group
Flag bit is set to 1.
Otherwise, if a Security Context associated with an OSCORE group is
retrieved, the recipient endpoint processes the message with Group
OSCORE, using the group mode (see Section 8) if the Group Flag is set
to 1, or the pairwise mode (see Section 9) if the Group Flag is set
to 0.
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Note that, if the Group Flag is set to 0, and the recipient endpoint
retrieves a Security Context which is valid to process the message
but is not associated with an OSCORE group, then the message is
processed according to [RFC8613].
8. Message Processing in Group Mode
When using the group mode, messages are protected and processed as
specified in [RFC8613], with the modifications described in this
section. The security objectives of the group mode are discussed in
Appendix A.2.
The possible use of the group mode is indicated by the Group Manager
as part of the group data provided to candidate group members when
joining the group, according to which the parameters Group Encryption
Algorithm and Signature Algorithm in the Security Context are set
(see Section 2).
During all the steps of the message processing, an endpoint MUST use
the same Security Context for the considered group. That is, an
endpoint MUST NOT install a new Security Context for that group (see
Section 2.6.3.2) until the message processing is completed.
The group mode SHOULD be used to protect group requests intended for
multiple recipients or for the whole group. For an example where
this is not fulfilled, see [I-D.amsuess-core-cachable-oscore]. This
applies to both requests directly addressed to multiple recipients,
e.g., sent by the client over multicast, as well as requests sent by
the client over unicast to a proxy, that forwards them to the
intended recipients over multicast [I-D.ietf-core-groupcomm-bis].
As per [RFC7252][I-D.ietf-core-groupcomm-bis], group requests sent
over multicast MUST be Non-confirmable, and thus are not
retransmitted by the CoAP messaging layer. Instead, applications
should store such outgoing messages for a predefined, sufficient
amount of time, in order to correctly perform potential
retransmissions at the application layer. If performed, these
retransmissions are repetitions of previous protected messages, which
the sender endpoint does not protect again with Group OSCORE.
According to Section 5.2.3 of [RFC7252], responses to Non-confirmable
group requests SHOULD also be Non-confirmable, but endpoints MUST be
prepared to receive Confirmable responses in reply to a Non-
confirmable group request. Confirmable group requests are
acknowledged when sent over non-multicast transports, as specified in
[RFC7252].
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Furthermore, endpoints in the group locally perform error handling
and processing of invalid messages according to the same principles
adopted in [RFC8613]. In addition, a recipient MUST stop processing
and reject any message that is malformed and that does not follow the
format specified in Section 4 of this document, or that is not
cryptographically validated in a successful way as per the processing
defined in Section 8.2 and Section 8.4 of this document.
In either case, it is RECOMMENDED that a server does not send back
any error message in reply to a received request, if any of the two
following conditions holds:
* The server is not able to identify the received request as a group
request, i.e., as sent to all servers in the group.
* The server identifies the received request as a group request.
This prevents servers from replying with multiple error messages to a
client sending a group request, so avoiding the risk of flooding and
possibly congesting the network.
8.1. Protecting the Request
When using the group mode to protect a request, a client SHALL
proceed as described in Section 8.1 of [RFC8613], with the following
modifications.
* In step 2, the Additional Authenticated Data is modified as
described in Section 4 of this document.
* In step 4, the encryption of the COSE object is modified as
described in Section 4 of this document. The encoding of the
compressed COSE object is modified as described in Section 5 of
this document. In particular, the Group Flag MUST be set to 1.
The Group Encryption Algorithm from the Common Context MUST be
used.
* In step 5, the countersignature is computed and the format of the
OSCORE message is modified as described in Section 4 and Section 5
of this document. In particular, the payload of the Group OSCORE
message includes also the encrypted countersignature.
In addition, the following applies when sending a request that
establishes a long exchange.
* If the client intends to keep the long exchange active beyond a
possible change of Sender ID, the client MUST store the value of
the 'kid' parameter from the request, and retain it until the long
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exchange is terminated. Even in case the client is individually
rekeyed and receives a new Sender ID from the Group Manager (see
Section 2.6.3.1), the client MUST NOT update the stored 'kid'
parameter value associated with the long exchange and the
corresponding request.
* If the client intends to keep the long exchange active beyond a
possible change of ID Context following a group rekeying (see
Section 3.2), then the following applies.
- The client MUST store the value of the 'kid context' parameter
from the request, and retain it until the long exchange is
terminated. Upon establishing a new Security Context with a
new Gid as ID Context (see Section 2.6.3.2), the client MUST
NOT update the stored 'kid context' parameter value associated
with the long exchange and the corresponding request.
- The client MUST store an invariant identifier of the group,
which is immutable even in case the Security Context of the
group is re-established. For example, this invariant
identifier can be the "group name" in
[I-D.ietf-ace-key-groupcomm-oscore], where it is used for
joining the group and retrieving the current group keying
material from the Group Manager.
After a group rekeying, such an invariant information makes it
simpler for the client to retrieve the current group keying
material from the Group Manager, in case the client has missed
both the rekeying messages and the first response protected
with the new Security Context (see Section 8.3).
8.2. Verifying the Request
Upon receiving a protected request with the Group Flag set to 1,
following the procedure in Section 7, a server SHALL proceed as
described in Section 8.2 of [RFC8613], with the following
modifications.
* In step 2, the decoding of the compressed COSE object follows
Section 5 of this document. In particular:
- If the server discards the request due to not retrieving a
Security Context associated with the OSCORE group, the server
MAY respond with a 4.01 (Unauthorized) error message. When
doing so, the server MAY set an Outer Max-Age option with value
zero, and MAY include a descriptive string as diagnostic
payload.
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- If the received 'kid context' matches an existing ID Context
(Gid) but the received 'kid' does not match any Recipient ID in
this Security Context, then the server MAY create a new
Recipient Context for this Recipient ID and initialize it
according to Section 3 of [RFC8613], and also retrieve the
authentication credential associated with the Recipient ID to
be stored in the new Recipient Context. Such a configuration
is application specific. If the application does not specify
dynamic derivation of new Recipient Contexts, then the server
SHALL stop processing the request.
* In step 4, the Additional Authenticated Data is modified as
described in Section 4 of this document.
* In step 6, the server also verifies the countersignature, by using
the public key from the client's authentication credential stored
in the associated Recipient Context. In particular:
- If the server does not have the public key of the client yet,
the server MUST stop processing the request and MAY respond
with a 5.03 (Service Unavailable) response. The response MAY
include a Max-Age Option, indicating to the client the number
of seconds after which to retry. If the Max-Age Option is not
present, a retry time of 60 seconds will be assumed by the
client, as default value defined in Section 5.10.5 of
[RFC7252].
- The server MUST perform signature verification before
decrypting the COSE object, as defined below. Implementations
that cannot perform the two steps in this order MUST ensure
that no access to the plaintext is possible before a successful
signature verification and MUST prevent any possible leak of
time-related information that can yield side-channel attacks.
- The server retrieves the encrypted countersignature
ENC_SIGNATURE from the message payload, and computes the
original countersignature SIGNATURE as
SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM
where KEYSTREAM is derived as per Section 5.1.
- The server verifies the original countersignature SIGNATURE as
described in Sections 3.2 and 3.3 of [RFC9338], by using the
client's public key and according to the Signature Algorithm in
the Security Context.
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In particular, the Countersign_structure contains the context
text string "CounterSignature0", the external_aad as defined in
Section 4.4 of this document, and the ciphertext of the COSE
object as payload.
- If the signature verification fails, the server SHALL stop
processing the request, SHALL NOT update the Replay Window, and
MAY respond with a 4.00 (Bad Request) response. Such a
response MAY include an Outer Max-Age option with value zero,
and its diagnostic payload MAY contain a string, which, if
present, MUST be "Decryption failed" as if the decryption of
the COSE object had failed.
- When decrypting the COSE object using the Recipient Key, the
Group Encryption Algorithm from the Common Context MUST be
used.
* Additionally, if the used Recipient Context was created upon
receiving this request and the message is not verified
successfully, the server MAY delete that Recipient Context. Such
a configuration, which is specified by the application, mitigates
attacks that aim at overloading the server's storage.
A server SHOULD NOT process a request if the received Recipient ID
('kid') is equal to its own Sender ID in its own Sender Context. For
an example where this is not fulfilled, see Section 9.2.1 of
[I-D.ietf-core-observe-multicast-notifications].
In addition, the following applies if the request establishes a long
exchange and the server intends to reply with multiple responses.
* The server MUST store the value of the 'kid' parameter from the
request, and retain it until the last response has been sent. The
server MUST NOT update the stored value of the 'kid' parameter
associated with the request, even in case the client is
individually rekeyed and starts using a new Sender ID received
from the Group Manager (see Section 2.6.3.1).
* The server MUST store the value of the 'kid context' parameter
from the request, and retain it until the last response has been
sent, i.e., beyond a possible change of ID Context following a
group rekeying (see Section 3.2). That is, upon establishing a
new Security Context with a new Gid as ID Context (see
Section 2.6.3.2), the server MUST NOT update the stored value of a
'kid context' parameter associated with the request.
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8.3. Protecting the Response
When using the group mode to protect a response, a server SHALL
proceed as described in Section 8.3 of [RFC8613], with the following
modifications.
Note that the server always protects a response with the Sender
Context from its latest Security Context, and that establishing a new
Security Context resets the Sender Sequence Number to 0 (see
Section 3.2).
* In step 2, the Additional Authenticated Data is modified as
described in Section 4 of this document.
In addition, the following applies if the server intends to reply
with multiple responses, within the long exchange established by
the corresponding request.
- The server MUST use the stored value of the 'kid' parameter
from the request (see Section 8.2), as value for the
'request_kid' parameter in the external_aad structure (see
Section 4.4).
- The server MUST use the stored value of the 'kid context'
parameter from the request (see Section 8.2), as value for the
'request_kid_context' parameter in the external_aad structure
(see Section 4.4).
* In step 3, if any of the following two conditions holds, the
server MUST include its Sender Sequence Number as Partial IV in
the response and use it to build the AEAD nonce to protect the
response. This prevents the server from reusing the AEAD nonce
from the request together with the same encryption key.
- The response is not the first response that the server sends to
the request.
- The server is using a different Security Context for the
response compared to what was used to verify the request (see
Section 3.2).
* In step 4, the encryption of the COSE object is modified as
described in Section 4 of this document. The encoding of the
compressed COSE object is modified as described in Section 5 of
this document. In particular, the Group Flag MUST be set to 1.
The Group Encryption Algorithm from the Common Context MUST be
used.
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In addition, the following applies.
- If the server is using a different ID Context (Gid) for the
response compared to what was used to verify the request (see
Section 3.2) and this is the first response from the server to
that request, then the new ID Context MUST be included in the
'kid context' parameter of the response.
- The server may be replying to a request that was protected with
an old Security Context. After completing the establishment of
a new Security Context, the server MUST protect all the
responses to that request with the Sender Context of the new
Security Context.
For each ongoing long exchange, the server can help the client
to synchronize, by including also the 'kid context' parameter
in responses following a group rekeying, with value set to the
ID Context (Gid) of the new Security Context.
If there is a known upper limit to the duration of a group
rekeying, the server SHOULD include the 'kid context' parameter
during that time. Otherwise, the server SHOULD include it
until the Max-Age has expired for the last response sent before
the installation of the new Security Context.
- The server can obtain a new Sender ID from the Group Manager,
when individually rekeyed (see Section 2.6.3.1) or when re-
joining the group. In such a case, the server can help the
client to synchronize, by including the 'kid' parameter in a
response protected in group mode, even when the request was
protected in pairwise mode (see Section 9.3).
That is, when responding to a request protected in pairwise
mode, the server SHOULD include the 'kid' parameter in a
response protected in group mode, if it is replying to that
client for the first time since the assignment of its new
Sender ID.
* In step 5, the countersignature is computed and the format of the
OSCORE message is modified as described in Section 4 and Section 5
of this document. In particular the payload of the Group OSCORE
message includes also the encrypted countersignature (see
Section 5).
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8.4. Verifying the Response
Upon receiving a protected response with the Group Flag set to 1,
following the procedure in Section 7, a client SHALL proceed as
described in Section 8.4 of [RFC8613], with the modifications
described in this section.
Note that a client may receive a response protected with a Security
Context different from the one used to protect the corresponding
request, and that, upon the establishment of a new Security Context,
the client re-initializes its Replay Windows in its Recipient
Contexts (see Section 3.2).
* In step 2, the decoding of the compressed COSE object is modified
as described in Section 5 of this document. In particular, a
'kid' may not be present, if the response is a reply to a request
protected in pairwise mode. In such a case, the client assumes
the response 'kid' to be the Recipient ID for the server to which
the request protected in pairwise mode was intended for.
If the response 'kid context' matches an existing ID Context (Gid)
but the received/assumed 'kid' does not match any Recipient ID in
this Security Context, then the client MAY create a new Recipient
Context for this Recipient ID and initialize it according to
Section 3 of [RFC8613], and also retrieve the authentication
credential associated with the Recipient ID to be stored in the
new Recipient Context. If the application does not specify
dynamic derivation of new Recipient Contexts, then the client
SHALL stop processing the response.
* In step 3, the Additional Authenticated Data is modified as
described in Section 4 of this document.
In addition, the following applies if the client processes a
response to a request within a long exchange.
- The client MUST use the stored value of the 'kid' parameter
from the request (see Section 8.1), as value for the
'request_kid' parameter in the external_aad structure (see
Section 4.4).
- The client MUST use the stored value of the 'kid context'
parameter from the request (see Section 8.1), as value for the
'request_kid_context' parameter in the external_aad structure
(see Section 4.4).
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This ensures that, throughout a long exchange, the client can
correctly verify the received responses, even in case the client
is individually rekeyed and starts using a new Sender ID received
from the Group Manager (see Section 2.6.3.1), as well as when it
installs a new Security Context with a new ID Context (Gid)
following a group rekeying (see Section 3.2).
* In step 5, the client also verifies the countersignature, by using
the public key from the server's authentication credential stored
in the associated Recipient Context. In particular:
- The client MUST perform signature verification as defined
below, before decrypting the COSE object. Implementations that
cannot perform the two steps in this order MUST ensure that no
access to the plaintext is possible before a successful
signature verification and MUST prevent any possible leak of
time-related information that can yield side-channel attacks.
- The client retrieves the encrypted countersignature
ENC_SIGNATURE from the message payload, and computes the
original countersignature SIGNATURE as
SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM
where KEYSTREAM is derived as per Section 5.1.
The client verifies the original countersignature SIGNATURE.
- If the verification of the countersignature fails, the client:
i) SHALL stop processing the response; and ii) SHALL NOT update
the Response Number associated with the server.
- After a successful verification of the countersignature, the
client performs also the following actions in case the request
was protected in pairwise mode (see Section 9.3).
o If the 'kid' parameter is present in the response, the
client checks whether this received 'kid' is equal to the
expected 'kid', i.e., the known Recipient ID for the server
to which the request was intended for.
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o If the 'kid' parameter is not present in the response, the
client checks whether the server that has sent the response
is the same one to which the request was intended for. This
can be done by checking that the public key used to verify
the countersignature of the response is equal to the public
key included in the authentication credential Recipient Auth
Cred, which was taken as input to derive the Pairwise Sender
Key used for protecting the request (see Section 2.5.1).
In either case, if the client determines that the response has
come from a different server than the expected one, then the
client: i) SHALL discard the response and SHALL NOT deliver it
to the application; ii) SHALL NOT update the Response Number
associated with the server.
Otherwise, the client hereafter considers the received 'kid' as
the current Recipient ID for the server.
* In step 5, when decrypting the COSE object using the Recipient
Key, the Group Encryption Algorithm from the Common Context MUST
be used.
In addition, the client performs the following actions if the
response is received within a long exchange.
- The ordering and the replay protection of responses received
from the server during the long exchange are performed as per
Section 6.3.1 of this document, by using the Response Number
associated with that server within that long exchange. In case
of unsuccessful decryption and verification of a response, the
client SHALL NOT update the Response Number associated with the
server.
- When receiving the first valid response from the server within
the long exchange, the client MUST store the current kid "kid1"
of that server for that long exchange. If the 'kid' field is
included in the OSCORE option of the response, its value
specifies "kid1". If the request was protected in pairwise
mode (see Section 9.3), the 'kid' field may not be present in
the OSCORE option of the response (see Section 4.2). In this
case, the client assumes "kid1" to be the Recipient ID for the
server to which the request was intended for.
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- When receiving another valid response to the same request from
the same server - which can be identified and recognized
through the same public key used to verify the countersignature
and included in the server's authentication credential - the
client determines the current kid "kid2" of the server as above
for "kid1", and MUST check whether "kid2" is equal to the
stored "kid1".
If "kid1" and "kid2" are different, the client SHOULD NOT
accept the response as valid to be delivered to the
application, and SHOULD NOT update the Response Number
associated with the server. Exceptions can apply if the client
has a means of preserving the order of responses in a more
elaborate way out of the scope of this document, or the client
application does not require response ordering altogether.
Note that, if "kid2" is different from "kid1" and the 'kid' field
is omitted from the response - which is possible if the request
was protected in pairwise mode - then the client will compute a
wrong keystream to decrypt the countersignature (i.e., by using
"kid1" rather than "kid2" in the 'id' field of the 'info' array in
Section 5.1), thus subsequently failing to verify the
countersignature and discarding the response.
This ensures that the client remains able to correctly perform the
ordering and replay protection of responses within a long
exchange, even in case the server legitimately starts using a new
Sender ID, as received from the Group Manager when individually
rekeyed (see Section 2.6.3.1) or when re-joining the group.
* In step 8, if the used Recipient Context was created upon
receiving this response and the message is not verified
successfully, the client MAY delete that Recipient Context. Such
a configuration, which is specified by the application, mitigates
attacks that aim at overloading the client's storage.
8.5. External Signature Checkers
When receiving a message protected in group mode, a signature checker
(see Section 3.3) proceeds as follows.
* The signature checker retrieves the encrypted countersignature
ENC_SIGNATURE from the message payload, and computes the original
countersignature SIGNATURE as
SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM
where KEYSTREAM is derived as per Section 5.1.
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* The signature checker verifies the original countersignature
SIGNATURE, by using the public key of the sender endpoint as
included in that endpoint's authentication credential. The
signature checker determines the right authentication credential
based on the ID Context (Gid) and the Sender ID of the sender
endpoint.
Note that the following applies when attempting to verify the
countersignature of a response message.
* The response may not include a Partial IV and/or an ID Context.
In such a case, the signature checker considers the same values
from the corresponding request, i.e., the request matching with
the response by CoAP Token value.
* The response may not include a Sender ID. This can happen when
the response protected in group mode matches a request protected
in pairwise mode (see Section 9.1), with a case in point provided
by [I-D.amsuess-core-cachable-oscore]. In such a case, the
signature checker needs to use other means (e.g., source
addressing information of the server endpoint) to identify the
correct authentication credential including the public key to use
for verifying the countersignature of the response.
The particular actions following a successful or unsuccessful
verification of the countersignature are application specific and out
of the scope of this document.
9. Message Processing in Pairwise Mode
When using the pairwise mode of Group OSCORE, messages are protected
and processed as in [RFC8613], with the modifications described in
this section. The security objectives of the pairwise mode are
discussed in Appendix A.2.
The possible use of the pairwise mode is indicated by the Group
Manager as part of the group data provided to candidate group members
when joining the group, according to which the parameters AEAD
Algorithm and Pairwise Key Agreement Algorithm in the Security
Context are set (see Section 2).
The pairwise mode takes advantage of an existing Security Context to
establish keying material shared exclusively with any other member.
For encryption and decryption operations in pairwise mode, the AEAD
Algorithm from the Common Context is used (see Section 2.1.1).
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In order to use the pairwise mode in a group where the group mode is
also used (i.e., Group Encryption Algorithm and Signature Algorithm
in the Security Context are set), the signature scheme of the group
mode MUST support a combined signature and encryption scheme. For
example, this can rely on signing operations using ECDSA, and
encryption operations using AES-CCM with keying material derived
through ECDH.
The pairwise mode does not support external verifiers of source
authentication and message integrity like the group mode does (see
Section 8.5).
An endpoint implementing only a silent server does not support the
pairwise mode.
Endpoints using the CoAP Echo Option [RFC9175] and/or block-wise
transfers [RFC7959] in a group where the AEAD Algorithm and Pairwise
Key Agreement Algorithm are set MUST support the pairwise mode. This
applies, for example, to block-wise exchanges after a first block-
wise request which targets all servers in the group and includes the
CoAP Block2 option (see Section 3.8 of
[I-D.ietf-core-groupcomm-bis]). This prevents the attack described
in Section 13.9, which leverages requests sent over unicast to a
single group member and protected in group mode.
The pairwise mode cannot be used to protect messages intended for
multiple recipients. In fact, the keying material used for the
pairwise mode is shared only between two endpoints.
However, a sender can use the pairwise mode to protect a message sent
to (but not intended for) multiple recipients, if interested in a
response from only one of them. For instance, this is useful to
support the address discovery service defined in Section 9.1, when a
single 'kid' value is indicated in the payload of a request sent to
multiple recipients, e.g., over multicast.
9.1. Pre-Conditions
In order to protect an outgoing message in pairwise mode, the sender
needs to know the authentication credential and the Recipient ID for
the recipient endpoint, as stored in the Recipient Context associated
with that endpoint (see Section 2.5.4).
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Furthermore, the sender needs to know the individual address of the
recipient endpoint. This information may not be known at any given
point in time. For instance, right after having joined the group, a
client may know the authentication credential and Recipient ID for a
given server, but not the addressing information required to reach it
with an individual, one-to-one request.
In order to make addressing information of individual endpoints
available, servers in the group MAY expose a resource to which a
client can send a request targeting a set of servers, identified by
their 'kid' values specified in the request payload, or implicitly if
the request is sent in pairwise mode. Further details of such an
interface are out of scope for this document.
9.2. Main Differences from OSCORE
The pairwise mode protects messages between two members of a group,
essentially following [RFC8613], but with the following notable
differences.
* The 'kid' and 'kid context' parameters of the COSE object are used
as defined in Section 4.2 of this document.
* The external_aad defined in Section 4.4 of this document is used
for the encryption and decryption process.
* The Pairwise Sender/Recipient Keys used as Sender/Recipient keys
are derived as defined in Section 2.5 of this document.
9.3. Protecting the Request
When using the pairwise mode to protect a request, a client SHALL
proceed as described in Section 8.1 of [RFC8613], with the
differences summarized in Section 9.2 of this document.
Furthermore, when sending a request that establishes a long exchange,
what is specified in Section 8.1 of this document holds, with respect
to storing the value of the 'kid' and 'kid context' parameters, and
to storing an invariant identifier of the group.
9.4. Verifying the Request
Upon receiving a protected request with the Group Flag set to 0,
following the procedure in Section 7, a server SHALL proceed as
described in Section 8.2 of [RFC8613], with the differences
summarized in Section 9.2 of this document. The following
differences also apply.
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* If the server discards the request due to not retrieving a
Security Context associated with the OSCORE group or to not
supporting the pairwise mode, the server MAY respond with a 4.01
(Unauthorized) error message or a 4.02 (Bad Option) error message,
respectively. When doing so, the server MAY set an Outer Max-Age
option with value zero, and MAY include a descriptive string as
diagnostic payload.
* If a new Recipient Context is created for this Recipient ID, new
Pairwise Sender/Recipient Keys are also derived (see
Section 2.5.1). The new Pairwise Sender/Recipient Keys are
deleted if the Recipient Context is deleted as a result of the
message not being successfully verified.
* What is specified in Section 8.2 of this document holds with
respect to the following points.
- The possible, dynamic creation and configuration of a Recipient
Context upon receiving the request.
- The possible deletion of a Recipient Context created upon
receiving the request, in case the request is not verified
successfully.
- The rule about processing the request where the received
Recipient ID ('kid') is equal to the server's Sender ID.
- The storing of the value of the 'kid' and 'kid context'
parameters from the request, if the server intends to reply
with multiple responses within the long exchange established by
the request.
9.5. Protecting the Response
When using the pairwise mode to protect a response, a server SHALL
proceed as described in Section 8.3 of [RFC8613], with the
differences summarized in Section 9.2 of this document. The
following differences also apply.
* What is specified in Section 8.3 of this document holds with
respect to the following points.
- The protection of a response when using a different Security
Context than the one used to protect the corresponding request.
That is, the server always protects a response with the Sender
Context from its latest Security Context, and establishing a
new Security Context resets the Sender Sequence Number to 0
(see Section 3.2).
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- The use of the stored value of the 'kid' and 'kid context'
parameters, if the server intends to reply with multiple
responses within the long exchange established by the request.
- The rules for the inclusion of the server's Sender Sequence
Number as Partial IV in a response, as used to build the AEAD
nonce to protect the response.
- The rules for the inclusion of the ID Context (Gid) in the 'kid
context' parameter of a response, if the ID Context used for
the response differs from the one used to verify the request
(see Section 3.2), also for helping the client to synchronize.
- The rules for the inclusion of the Sender ID in the 'kid'
parameter of a response to a request that was protected in
pairwise mode, if the server has obtained a new Sender ID from
the Group Manager when individually rekeyed (see
Section 2.6.3.1), thus helping the client to synchronize.
9.6. Verifying the Response
Upon receiving a protected response with the Group Flag set to 0,
following the procedure in Section 7, a client SHALL proceed as
described in Section 8.4 of [RFC8613], with the differences
summarized in Section 9.2 of this document. The following
differences also apply.
* The client may receive a response protected with a Security
Context different from the one used to protect the corresponding
request. Also, upon the establishment of a new Security Context,
the client re-initializes its Replay Windows in its Recipient
Contexts (see Section 2.2).
* The same as described in Section 8.4 holds with respect to
handling the 'kid' parameter of the response, when received as a
reply to a request protected in pairwise mode. The client can
also in this case check whether the replying server is the
expected one, by relying on the server's public key. However,
since the response is protected in pairwise mode, the public key
is not used for verifying a countersignature as in Section 8.4.
Instead, the expected server's authentication credential - namely
Recipient Auth Cred and including the server's public key - was
taken as input to derive the Pairwise Recipient Key used to
decrypt and verify the response (see Section 2.5.1).
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* If a new Recipient Context is created for this Recipient ID, new
Pairwise Sender/Recipient Keys are also derived (see
Section 2.5.1). The new Pairwise Sender/Recipient Keys are
deleted if the Recipient Context is deleted as a result of the
message not being successfully verified.
* What is specified in Section 8.4 of this document holds with
respect to the following points.
- The possible, dynamic creation and configuration of a Recipient
Context upon receiving the response.
- The use of the stored value of the 'kid' and 'kid context'
parameters, when processing a response received within a long
exchange.
- The performing of ordering and replay protection for responses
received within a long exchange.
- The possible deletion of a Recipient Context created upon
receiving the response, in case the response is not verified
successfully.
10. Challenge-Response Based Freshness and Synchronization
This section describes how a server endpoint can verify freshness of
a request and synchronize with Sender Sequence Numbers of client
endpoints in the group. Similarly to what is defined in
Appendix B.1.2 of [RFC8613], the server performs a challenge-response
exchange with a client, by using the Echo Option for CoAP specified
in Section 2 of [RFC9175].
Upon receiving a request from a particular client for the first time,
the server processes the message as described in this document, but,
even if valid, does not deliver it to the application. Instead, the
server replies to the client with a Group OSCORE protected 4.01
(Unauthorized) response message, including only the Echo Option and
no diagnostic payload. The server MUST use its Partial IV when
generating the AEAD nonce for protecting the response conveying the
Echo Option, and MUST include the Partial IV in the response.
The Echo option value SHOULD NOT be reused; if it is reused, it MUST
be highly unlikely to have been recently used with this client.
Since this response is protected with the Security Context used in
the group, the client will consider the response valid upon
successfully decrypting and verifying it.
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The server stores the Echo Option value included in the response
together with the pair (gid,kid), where 'gid' is the Group Identifier
of the OSCORE group and 'kid' is the Sender ID of the client in the
group. These are specified in the 'kid context' and 'kid' fields of
the OSCORE Option of the request, respectively. After a group
rekeying has been completed and a new Security Context has been
established in the group, which results also in a new Group
Identifier (see Section 3.2), the server MUST delete all the stored
Echo values associated with members of the group.
Upon receiving a 4.01 (Unauthorized) response that includes an Echo
Option and originates from a verified group member, the subsequent
client request echoing the Echo Option value MUST be sent as a
unicast message to the same server.
If in the group the AEAD Algorithm and Pairwise Key Agreement
Algorithm are set in the Security Context, the client MUST use the
pairwise mode to protect the request, as per Section 9.3. Note that,
as defined in Section 9, endpoints that are members of such a group
and that use the Echo Option support the pairwise mode. In a group
where the AEAD Algorithm and Pairwise Key Agreement Algorithm are not
set, only the group mode can be used. Hence, requests including the
Echo option can be protected only with the Group Mode, with the
caveat due to the risk for those requests to be redirected to a
different server than the intended one, as discussed in Section 13.9.
The client does not necessarily resend the same request, but can
instead send a more recent one, if the application permits it. This
allows the client to not retain previously sent requests for full
retransmission, unless the application explicitly requires otherwise.
In either case, the client uses a fresh Sender Sequence Number value
from its own Sender Context. If the client stores requests for
possible retransmission with the Echo Option, it should not store a
given request for longer than a preconfigured time interval. Note
that the unicast request echoing the Echo Option is correctly treated
and processed, since the 'kid context' field including the Group
Identifier of the OSCORE group is still present in the OSCORE Option
as part of the COSE object (see Section 4).
Upon receiving the unicast request including the Echo Option, the
server performs the following verifications.
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* If the server does not store an Echo Option value for the pair
(gid,kid), it considers: i) the time t1 when it has established
the Security Context used to protect the received request; and ii)
the time t2 when the request has been received. Since a valid
request cannot be older than the Security Context used to protect
it, the server verifies that (t2 - t1) is less than the largest
amount of time acceptable to consider the request fresh.
* If the server stores an Echo Option value for the pair (gid,kid)
associated with that same client in the same group, the server
verifies that the option value equals that same stored value
previously sent to that client.
If the verifications above fail, the server MUST NOT process the
request further and MAY send a 4.01 (Unauthorized) response including
an Echo Option, hence performing a new challenge-response exchange.
If the verifications above are successful, the server considers the
Recipient Context associated with the sender client and proceeds as
follows.
* In case the Replay Window is invalid, the steps below occur.
1. The server updates the Replay Window, by marking as received
the Sender Sequence Number from the latest received request.
This becomes the lower limit of the Replay Window, while all
the other values are marked as not received.
2. The server makes the Replay Window valid, and accepts the
request as fresh.
* In case the Replay Window is already valid, the server discards
the verification result and accepts the request as fresh or treats
it as a replay, according to the existing Replay Window.
A server should not deliver requests from a given client to the
application until one valid request from that same client has been
verified as fresh, as conveying an echoed Echo Option. A server may
perform the challenge-response described above at any time, if
synchronization with Sender Sequence Numbers of clients is lost,
e.g., after a device reboot occurred in an unprepared way. A client
has to be ready to perform the challenge-response based on the Echo
Option if a server starts it.
Message freshness is further discussed in Section 13.14.
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11. Implementation Compliance
Like in [RFC8613], HKDF SHA-256 is the mandatory to implement HKDF.
An endpoint may support only the group mode, or only the pairwise
mode, or both.
For endpoints that support the group mode, the following applies.
* For endpoints that use authenticated encryption, the AEAD
algorithm AES-CCM-16-64-128 defined in Section 4.2 of [RFC9053] is
mandatory to implement as Group Encryption Algorithm (see
Section 2.1.6).
* For many constrained IoT devices it is problematic to support more
than one signature algorithm. Existing devices can be expected to
support either EdDSA or ECDSA. In order to enable as much
interoperability as we can reasonably achieve, the following
applies with respect to the Signature Algorithm (see
Section 2.1.7).
Less constrained endpoints SHOULD implement both: the EdDSA
signature algorithm together with the elliptic curve Ed25519
[RFC8032]; and the ECDSA signature algorithm together with the
elliptic curve P-256.
Constrained endpoints SHOULD implement: the EdDSA signature
algorithm together with the elliptic curve Ed25519 [RFC8032]; or
the ECDSA signature algorithm together with the elliptic curve
P-256.
* Endpoints that implement the ECDSA signature algorithm MAY use
"deterministic ECDSA" as specified in [RFC6979]. Pure
deterministic elliptic-curve signature algorithms such as
deterministic ECDSA and EdDSA have the advantage of not requiring
access to a source of high-quality randomness. However, these
signature algorithms have been shown vulnerable to some side-
channel and fault injection attacks due to their determinism,
which can result in extracting a device's private key. As
suggested in Section 2.1.1 of [RFC9053], this can be addressed by
combining both randomness and determinism
[I-D.irtf-cfrg-det-sigs-with-noise].
For endpoints that support the pairwise mode, the following applies.
* The AEAD algorithm AES-CCM-16-64-128 defined in Section 4.2 of
[RFC9053] is mandatory to implement as AEAD Algorithm (see
Section 2.1.1).
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* The ECDH-SS + HKDF-256 algorithm specified in Section 6.3.1 of
[RFC9053] is mandatory to implement as Pairwise Key Agreement
Algorithm (see Section 2.1.9).
* In order to enable as much interoperability as we can reasonably
achieve in the presence of constrained devices (see above), the
following applies.
Less constrained endpoints SHOULD implement both the X25519 curve
[RFC7748] and the P-256 curve as ECDH curves.
Constrained endpoints SHOULD implement the X25519 curve [RFC7748]
or the P-256 curve as ECDH curve.
Constrained IoT devices may alternatively represent Montgomery curves
and (twisted) Edwards curves [RFC7748] in the short-Weierstrass form
Wei25519, with which the algorithms ECDSA25519 and ECDH25519 can be
used for signature operations and Diffie-Hellman secret calculation,
respectively [I-D.ietf-lwig-curve-representations].
12. Web Linking
The use of Group OSCORE or OSCORE [RFC8613] MAY be indicated by a
target "gosc" attribute in a web link [RFC8288] to a resource, e.g.,
using a link-format document [RFC6690] if the resource is accessible
over CoAP.
The "gosc" attribute is a hint indicating that the destination of
that link is only accessible using Group OSCORE or OSCORE, and
unprotected access to it is not supported. Note that this is simply
a hint, it does not include any security context material or any
other information required to run Group OSCORE or OSCORE.
A value MUST NOT be given for the "gosc" attribute; any present value
MUST be ignored by parsers. The "gosc" attribute MUST NOT appear
more than once in a given link-value; occurrences after the first
MUST be ignored by parsers.
When a link-value includes the "gosc" attribute, the link-value MUST
also include the "osc" attribute defined in Section 9 of [RFC8613].
If the endpoint parsing the link-value supports Group OSCORE and
understands the "gosc" attribute, then the parser MUST ignore the
"osc" attribute, which is overshadowed by the "gosc" attribute.
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The example in Figure 4 shows a use of the "gosc" attribute: the
client does resource discovery on a server and gets back a list of
resources, one of which includes the "gosc" attribute indicating that
the resource is protected with Group OSCORE or OSCORE. The link-
format notation (see Section 5 of [RFC6690]) is used.
REQ: GET /.well-known/core
RES: 2.05 Content
</sensors/temp>;gosc;osc,
</sensors/light>;if="sensor"
Figure 4: Example of using the "gosc" attribute in a web link.
13. Security Considerations
The same threat model discussed for OSCORE in Appendix D.1 of
[RFC8613] holds for Group OSCORE.
For Group OSCORE, the Sender Context and Recipient Context
additionally contain asymmetric keys, which are used to provide
source authentication: in group mode, by means of countersignatures
(see Section 13.1); in pairwise mode, by using Diffie-Hellman (see
Section 13.2). The key pair can, for example, be generated by the
endpoint or provisioned during manufacturing.
Note that, even if an endpoint is authorized to be a group member and
to take part in group communications, there is a risk that it behaves
inappropriately. For instance, it can forward the content of
messages in the group to unauthorized entities. However, in many use
cases, the devices in the group belong to a common authority and are
configured by a commissioner (see Appendix B), which results in a
practically limited risk and enables a prompt detection/reaction in
case of misbehaving.
The same considerations on supporting Proxy operations discussed for
OSCORE in Appendix D.2 of [RFC8613] hold for Group OSCORE.
The same considerations on protected message fields for OSCORE
discussed in Appendix D.3 of [RFC8613] hold for Group OSCORE.
The same considerations on uniqueness of (key, nonce) pairs for
OSCORE discussed in Appendix D.4 of [RFC8613] hold for Group OSCORE.
This is further discussed in Section 13.3 of this document.
The same considerations on unprotected message fields for OSCORE
discussed in Appendix D.5 of [RFC8613] hold for Group OSCORE, with
the following differences. First, the 'kid context' of request
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messages is part of the Additional Authenticated Data, thus safely
enabling to keep long exchanges active beyond a possible change of ID
Context (Gid), following a group rekeying (see Section 4.4). Second,
the countersignature included in a Group OSCORE message protected in
group mode is computed also over the value of the OSCORE option,
which is also part of the Additional Authenticated Data used in the
signing process. This is further discussed in Section 13.7 of this
document.
As discussed in Section 6.2.3 of [I-D.ietf-core-groupcomm-bis], Group
OSCORE addresses security attacks against CoAP listed in Sections
11.2-11.6 of [RFC7252], especially when run over IP multicast.
The rest of this section first discusses security aspects to be taken
into account when using Group OSCORE. Then it goes through aspects
covered in the security considerations of OSCORE (see Section 12 of
[RFC8613]), and discusses how they hold when Group OSCORE is used.
13.1. Security of the Group Mode
The group mode defined in Section 8 relies on commonly shared group
keying material to protect communication within a group. Using the
group mode has the implications discussed below. The following
refers to group members as the endpoints in the group storing the
latest version of the group keying material.
* Source authentication of messages sent to a group is ensured
through a countersignature, which is computed by the sender using
its own private key and then appended to the message payload.
Also, the countersignature is encrypted by using a keystream
derived from the group keying material (see Section 5). This
ensures group privacy, i.e., an attacker cannot track an endpoint
over two groups by linking messages between the two groups, unless
being also a member of those groups.
* Messages are encrypted at a group level (group-level data
confidentiality), i.e., they can be decrypted by any member of the
group, but not by an external adversary or other external
entities.
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* If the used Group Encryption Algorithm provides integrity
protection, then it also ensures group authentication and proof of
group membership, but not source authentication. That is, it
ensures that a message sent to a group has been sent by a member
of that group, but not necessarily by the alleged sender. In
fact, any group member is able to derive the Sender Key used by
the actual sender endpoint, and thus can compute a valid
authentication tag. Therefore, the message content could
originate from any of the current group members.
Furthermore, if the used Group Encryption Algorithm does not
provide integrity protection, then it does not ensure any level of
message authentication or proof of group membership.
On the other hand, proof of group membership is always ensured by
construction through the strict management of the group keying
material (see Section 3.2). That is, the group is rekeyed in case
of members' leaving, and the current group members are informed of
former group members. Thus, a current group member storing the
latest group keying material does not store the authentication
credential of any former group member.
This allows a recipient endpoint to rely on the stored
authentication credentials and public keys included therein, in
order to always confidently assert the group membership of a
sender endpoint when processing an incoming message, i.e., to
assert that the sender endpoint was a group member when it signed
the message. In turn, this prevents a former group member to
possibly re-sign and inject in the group a stored message that was
protected with old keying material.
A case in point is a group where the Group Encryption Algorithm
does not provide integrity protection; a group member leaves the
group; and, after the group rekeying, associates with the group as
external signature checker (see Section 8.5). When doing so, it
obtains from the Group Manager the new Signature Encryption Key,
from which it can derive keystreams for encrypting and decrypting
the countersignatures of messages protected in group mode.
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If, when participating in the group rekeying, the current group
members had not deleted the Recipient Context and authentication
credential of the former group member, then the signature checker
would be able to successfully inject messages protected in group
mode, as encrypted with the old group keying material, signed with
its own private key, and with the countersignature encrypted by
means of the latest Signature Encryption Key. Then, the group
members, as still retaining the authentication credential of the
signature checker, will verify and accept the message, even though
the sender was not a group member when signing the message.
The security properties of the group mode are summarized below.
1. Asymmetric source authentication, by means of a countersignature.
2. Symmetric group authentication, by means of an authentication tag
(only for Group Encryption Algorithms providing integrity
protection).
3. Symmetric group confidentiality, by means of symmetric
encryption.
4. Proof of group membership, by strictly managing the group keying
material, as well as by means of integrity tags when using a
Group Encryption Algorithm that provides also integrity
protection.
5. Group privacy, by encrypting the countersignature.
The group mode fulfills the security properties above while also
displaying the following benefits. First, the use of a Group
Encryption Algorithm that does not provide integrity protection
results in a minimal communication overhead, by limiting the message
payload to the ciphertext without integrity tag together with the
encrypted countersignature. Second, it is possible to deploy semi-
trusted entities such as signature checkers (see Section 3.3), which
can break property 5, but cannot break properties 1, 2, 3, and 4.
13.2. Security of the Pairwise Mode
The pairwise mode defined in Section 9 protects messages by using
pairwise symmetric keys, derived from the static-static Diffie-
Hellman shared secret computed from the asymmetric keys of the sender
and recipient endpoint (see Section 2.5).
The used AEAD Algorithm MUST provide integrity protection.
Therefore, the pairwise mode ensures both pairwise data-
confidentiality and source authentication of messages, without using
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countersignatures. Furthermore, the recipient endpoint achieves
proof of group membership for the sender endpoint, since only current
group members have the required keying material to derive a valid
Pairwise Sender/Recipient Key.
The long-term storing of the Diffie-Hellman shared secret is a
potential security issue. In fact, if the shared secret of two group
members is leaked, a third group member can exploit it to impersonate
any of those two group members, by deriving and using their pairwise
key. The possibility of such leakage should be contemplated, as more
likely to happen than the leakage of a private key, which could be
rather protected at a significantly higher level than generic memory,
e.g., by using a Trusted Platform Module. Therefore, there is a
trade-off between the maximum amount of time a same shared secret is
stored and the frequency of its re-computing.
13.3. Uniqueness of (key, nonce)
The proof for uniqueness of (key, nonce) pairs in Appendix D.4 of
[RFC8613] is also valid in group communication scenarios. That is,
given an OSCORE group:
* Uniqueness of Sender IDs within the group is enforced by the Group
Manager. In fact, from the moment when a Gid is assigned to a
group until the moment when a new Gid is assigned to that same
group, the Group Manager does not reassign a Sender ID within the
group (see Section 3.2).
* The case A in Appendix D.4 of [RFC8613] concerns all requests as
well as all responses including a Partial IV (e.g., Observe
notifications [RFC7641] or any other subsequent responses after
the first one). In this case, same considerations from [RFC8613]
apply here as well.
* The case B in Appendix D.4 of [RFC8613] concerns responses not
including a Partial IV (e.g., a single response to a request). In
this case, same considerations from [RFC8613] apply here as well.
As a consequence, each message encrypted/decrypted with the same
Sender Key is processed by using a different (ID_PIV, PIV) pair.
This means that nonces used by any fixed encrypting endpoint are
unique. Thus, each message is processed with a different (key,
nonce) pair.
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13.4. Management of Group Keying Material
The protocol described in this document should take into account the
risk of compromise of group members. In particular, this document
specifies that a key management scheme for secure revocation and
renewal of Security Contexts and group keying material MUST be
adopted.
[I-D.ietf-ace-key-groupcomm-oscore] specifies a simple rekeying
scheme for renewing the Security Context in a group.
Alternative rekeying schemes that are more scalable with the group
size may be needed in dynamic, large groups where endpoints can join
and leave at any time, in order to limit the impact on performance
due to the Security Context and keying material update.
13.5. Update of Security Context and Key Rotation
A group member can receive a message shortly after the group has been
rekeyed, and new security parameters and keying material have been
distributed by the Group Manager.
This may result in a client using an old Security Context to protect
a request, and a server using a different new Security Context to
protect a corresponding response. As a consequence, clients may
receive a response protected with a Security Context different from
the one used to protect the corresponding request.
In particular, a server may first get a request protected with the
old Security Context, then install the new Security Context, and only
after that produce a response to send back to the client. In such a
case, as specified in Section 8.3, the server MUST protect the
potential response using the new Security Context. Specifically, the
server MUST include its Sender Sequence Number as Partial IV in the
response and use it to build the AEAD nonce to protect the response.
This prevents the AEAD nonce from the request from being reused with
the new Security Context.
The client will process that response using the new Security Context,
provided that it has installed the new security parameters and keying
material before the message processing.
In case block-wise transfer [RFC7959] is used, the same
considerations from Section 10.3 of [I-D.ietf-ace-key-groupcomm]
hold.
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Furthermore, as described below, a group rekeying may temporarily
result in misaligned Security Contexts between the sender and
recipient of a same message.
13.5.1. Late Update on the Sender
In this case, the sender protects a message using the old Security
Context, i.e., before having installed the new Security Context.
However, the recipient receives the message after having installed
the new Security Context, and is thus unable to correctly process it.
A possible way to ameliorate this issue is to preserve the old
retained Security Context for a maximum amount of time defined by the
application. By doing so, the recipient can still try to process the
received message using the old retained Security Context.
This makes particular sense when the recipient is a client, that
would hence be able to process incoming responses protected with the
old retained Security Context used to protect the associated request.
If, as typically expected, the old Gid is not included in the
response, then the client will first fail to process the response
using the latest Security Context, and then use the old retained
Security Context as a second attempt.
Instead, a recipient server can immediately process an incoming
request with the old retained Security Context, as signaled by the
old Gid that is always included in requests. However, the server
would better and more simply discard such an incoming request.
This tolerance preserves the processing of secure messages throughout
a long-lasting key rotation, as group rekeying processes may likely
take a long time to complete, especially in large groups. On the
other hand, a former (compromised) group member can abusively take
advantage of this, and send messages protected with the old retained
Security Context. Therefore, a conservative application policy
should not admit the retention of old Security Contexts.
13.5.2. Late Update on the Recipient
In this case, the sender protects a message using the new Security
Context, but the recipient receives that message before having
installed the new Security Context. Therefore, the recipient would
not be able to correctly process the message and hence discards it.
If the recipient installs the new Security Context shortly after that
and the sender endpoint retransmits the message, the former will
still be able to receive and correctly process the message.
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In any case, the recipient should actively ask the Group Manager for
an updated Security Context according to an application-defined
policy, for instance after a given number of unsuccessfully decrypted
incoming messages.
13.6. Collision of Group Identifiers
In case endpoints are deployed in multiple groups managed by
different non-synchronized Group Managers, it is possible for Group
Identifiers of different groups to coincide.
This does not impair the security of the AEAD algorithm. In fact, as
long as the Master Secret is different for different groups and this
condition holds over time, AEAD keys are different among different
groups.
In case multiple groups use the same IP multicast address, the entity
assigning that address may help limiting the chances to experience
such collisions of Group Identifiers. In particular, it may allow
the Group Managers of those groups using the same IP multicast
address to share their respective list of assigned Group Identifiers
currently in use.
13.7. Cross-group Message Injection
A same endpoint is allowed to and would likely use the same pair
(private key, authentication credential) in multiple OSCORE groups,
possibly administered by different Group Managers.
When a sender endpoint sends a message protected in pairwise mode to
a recipient endpoint in an OSCORE group, a malicious group member may
attempt to inject the message to a different OSCORE group also
including the same endpoints (see Section 13.7.1).
This practically relies on altering the content of the OSCORE option,
and having the same MAC in the ciphertext still correctly validating,
which has a success probability depending on the size of the MAC.
As discussed in Section 13.7.2, the attack is practically infeasible
if the message is protected in group mode, thanks to the
countersignature also bound to the OSCORE option through the
Additional Authenticated Data used in the signing process (see
Section 4.4).
13.7.1. Attack Description
Let us consider:
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* Two OSCORE groups G1 and G2, with ID Context (Group ID) Gid1 and
Gid2, respectively. Both G1 and G2 use the AEAD cipher AES-CCM-
16-64-128, i.e., the MAC of the ciphertext is 8 bytes in size.
* A sender endpoint X which is member of both G1 and G2, and uses
the same pair (private key, authentication credential) in both
groups. The endpoint X has Sender ID Sid1 in G1 and Sender ID
Sid2 in G2. The pairs (Sid1, Gid1) and (Sid2, Gid2) identify the
same authentication credential of X in G1 and G2, respectively.
* A recipient endpoint Y which is member of both G1 and G2, and uses
the same pair (private key, authentication credential) in both
groups. The endpoint Y has Sender ID Sid3 in G1 and Sender ID
Sid4 in G2. The pairs (Sid3, Gid1) and (Sid4, Gid2) identify the
same authentication credential of Y in G1 and G2, respectively.
* A malicious endpoint Z is also member of both G1 and G2. Hence, Z
is able to derive the Sender Keys used by X in G1 and G2.
When X sends a message M1 addressed to Y in G1 and protected in
pairwise mode, Z can intercept M1, and attempt to forge a valid
message M2 to be injected in G2, making it appear as still sent by X
to Y and valid to be accepted.
More in detail, Z intercepts and stops message M1, and forges a
message M2 by changing the value of the OSCORE option from M1 as
follows: the 'kid context' is set to G2 (rather than G1); and the
'kid' is set to Sid2 (rather than Sid1). Then, Z injects message M2
as addressed to Y in G2.
Upon receiving M2, there is a probability equal to 2^-64 that Y
successfully verifies the same unchanged MAC by using the Pairwise
Recipient Key associated with X in G2.
Note that Z does not know the pairwise keys of X and Y, since it does
not know and is not able to compute their shared Diffie-Hellman
secret. Therefore, Z is not able to check offline if a performed
forgery is actually valid, before sending the forged message to G2.
13.7.2. Attack Prevention in Group Mode
When a Group OSCORE message is protected in group mode, the
countersignature is computed also over the value of the OSCORE
option, which is part of the Additional Authenticated Data used in
the signing process (see Section 4.4).
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That is, other than over the ciphertext, the countersignature is
computed over: the ID Context (Gid) and the Partial IV, which are
always present in requests; as well as the Sender ID of the message
originator, which is always present in requests as well as in
responses to requests protected in group mode.
Since the signing process takes as input also the ciphertext of the
COSE_Encrypt0 object, the countersignature is bound not only to the
intended OSCORE group, hence to the triplet (Master Secret, Master
Salt, ID Context), but also to a specific Sender ID in that group and
to its specific symmetric key used for AEAD encryption, hence to the
quartet (Master Secret, Master Salt, ID Context, Sender ID).
This makes it practically infeasible to perform the attack described
in Section 13.7.1, since it would require the adversary to
additionally forge a valid countersignature that replaces the
original one in the forged message M2.
If, hypothetically, the countersignature did not cover the OSCORE
option:
* The attack described in Section 13.7.1 would still be possible
against response messages protected in group mode, since the same
unchanged countersignature from message M1 would be also valid in
message M2.
* A simplification would also be possible in performing the attack,
since Z is able to derive the Sender/Recipient Keys of X and Y in
G1 and G2. That is, Z can also set a convenient Partial IV in the
response, until the same unchanged MAC is successfully verified by
using G2 as 'request_kid_context', Sid2 as 'request_kid', and the
symmetric key associated with X in G2.
Since the Partial IV is 5 bytes in size, this requires 2^40
operations to test all the Partial IVs, which can be done in real-
time. The probability that a single given message M1 can be used
to forge a response M2 for a given request would be equal to 2^-
24, since there are more MAC values (8 bytes in size) than Partial
IV values (5 bytes in size).
Note that, by changing the Partial IV as discussed above, any
member of G1 would also be able to forge a valid signed response
message M2 to be injected in the same group G1.
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13.8. Prevention of Group Cloning Attack
Both when using the group mode and the pairwise mode, the message
protection covers also the Group Manager's authentication credential.
This is included in the Additional Authenticated Data used in the
signing process and/or in the integrity-protected encryption process
(see Section 4.4).
By doing so, an endpoint X member of a group G1 cannot perform the
following attack.
1. X sets up a group G2 where it acts as Group Manager.
2. X makes G2 a "clone" of G1, i.e., G1 and G2 use the same
algorithms and have the same Master Secret, Master Salt and ID
Context.
3. X collects a message M sent to G1 and injects it in G2.
4. Members of G2 accept M and believe it to be originated in G2.
The attack above is effectively prevented, since message M is
protected by including the authentication credential of G1's Group
Manager in the Additional Authenticated Data. Therefore, members of
G2 do not successfully verify and decrypt M, since they correctly use
the authentication credential of X as Group Manager of G2 when
attempting to.
13.9. Group OSCORE for Unicast Requests
If a request is intended to be sent over unicast as addressed to a
single group member, it is NOT RECOMMENDED for the client to protect
the request by using the group mode as defined in Section 8.1.
This does not include the case where the client sends a request over
unicast to a proxy, to be forwarded to multiple intended recipients
over multicast [I-D.ietf-core-groupcomm-bis]. In this case, the
client typically protects the request with the group mode, even
though it is sent to the proxy over unicast (see Section 8).
If the client uses the group mode with its own Sender Key to protect
a unicast request to a group member, an on-path adversary can, right
then or later on, redirect that request to one/many different group
member(s) over unicast, or to the whole OSCORE group over multicast.
By doing so, the adversary can induce the target group member(s) to
perform actions intended for one group member only. Note that the
adversary can be external, i.e., they do not need to also be a member
of the OSCORE group.
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This is due to the fact that the client is not able to indicate the
single intended recipient in a way which is secure and possible to
process for Group OSCORE on the server side. In particular, Group
OSCORE does not protect network addressing information such as the IP
address of the intended recipient server. It follows that the
server(s) receiving the redirected request cannot assert whether that
was the original intention of the client, and would thus simply
assume so.
The impact of such an attack depends especially on the REST method of
the request, i.e., the Inner CoAP Code of the OSCORE request message.
In particular, safe methods such as GET and FETCH would trigger
(several) unintended responses from the targeted server(s), while not
resulting in destructive behavior. On the other hand, non safe
methods such as PUT, POST, and PATCH/iPATCH would result in the
target server(s) taking active actions on their resources and
possible cyber-physical environment, with the risk of destructive
consequences and possible implications for safety.
A client can instead use the pairwise mode as defined in Section 9.3,
in order to protect a request sent to a single group member by using
pairwise keying material (see Section 2.5). This prevents the attack
discussed above by construction, as only the intended server is able
to derive the pairwise keying material used by the client to protect
the request. In a group where the AEAD Algorithm and Pairwise Key
Agreement Algorithm are set in the Security Context, a client
supporting the pairwise mode SHOULD use it to protect requests sent
to a single group member over unicast. For an example where this is
not fulfilled, see Section 9.2.1 of
[I-D.ietf-core-observe-multicast-notifications].
The use of block-wise transfers [RFC7959] with group communication
for CoAP is as discussed in Section 3.8 of
[I-D.ietf-core-groupcomm-bis].
Additional considerations are discussed in Section 10, with respect
to requests including a CoAP Echo Option [RFC9175] that have to be
sent over unicast, as a challenge-response method for servers to
achieve synchronization of clients' Sender Sequence Number.
13.10. End-to-end Protection
The same considerations from Section 12.1 of [RFC8613] hold for Group
OSCORE.
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Additionally, (D)TLS and Group OSCORE can be combined for protecting
message exchanges occurring over unicast. However, it is not
possible to combine (D)TLS and Group OSCORE for protecting message
exchanges where messages are sent over multicast.
13.11. Master Secret
Group OSCORE derives the Security Context using the same construction
used by OSCORE, and by using the Group Identifier of a group as the
related ID Context. Hence, the same required properties of the
Security Context parameters discussed in Section 3.3 of [RFC8613]
hold for this document.
With particular reference to the OSCORE Master Secret, it has to be
kept secret among the members of the respective OSCORE group and the
Group Manager responsible for that group. Also, the Master Secret
must have a good amount of randomness, and the Group Manager can
generate it offline using a good random number generator. This
includes the case where the Group Manager rekeys the group by
generating and distributing a new Master Secret. Randomness
requirements for security are described in [RFC4086].
13.12. Replay Protection
As in OSCORE [RFC8613], also Group OSCORE relies on Sender Sequence
Numbers included in the COSE message field 'Partial IV' and used to
build AEAD nonces.
Note that the Partial IV of an endpoint does not necessarily grow
monotonically. For instance, upon exhaustion of the endpoint's
Sender Sequence Number space, the endpoint's Partial IV space also
gets exhausted. As discussed in Section 2.6.3, this results either
in the endpoint being individually rekeyed and getting a new Sender
ID, or in the establishment of a new Security Context in the group.
Therefore, uniqueness of (key, nonce) pairs (see Section 13.3) is
preserved also when a new Security Context is established.
Since one-to-many communication such as multicast usually involves
unreliable transports, the simplification of the Replay Window to a
size of 1 suggested in Section 7.4 of [RFC8613] is not viable with
Group OSCORE, unless exchanges in the group rely only on unicast
messages.
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A server may not be synchronized with a client's Sender Sequence
Number, or a Replay Window may be initialized as invalid (see
Section 2.6.1). The server can either retrieve a new Group OSCORE
Security Context, or synchronize with the clients' Sender Sequence
Numbers before resuming to accept incoming messages from other group
members.
13.13. Message Ordering
Assuming that the other endpoint is honest, Group OSCORE provides
relative ordering of received messages. For a given Group OSCORE
Security Context, the received Partial IV (when included) allows the
recipient endpoint to determine the order in which requests or
responses were sent by the other endpoint.
In case the Partial IV was omitted in a response, this indicates that
it was the oldest response from the sender endpoint to the
corresponding request (like notification responses in OSCORE, see
Section 7.4.1 of [RFC8613]). A received response is not older than
the corresponding request.
13.14. Message Freshness
As in OSCORE, Group OSCORE provides only the guarantee that the
request is not older than the Group OSCORE Security Context used to
protect it. Other aspects of freshness are discussed in Section 6.2.
The challenge-response approach described in Section 10 provides an
assurance of freshness of the request without depending on the
honesty of the client. However, it can result in an impact on
performance which is undesirable or unbearable, especially in large
groups where many endpoints at the same time might join as new
members or lose synchronization.
Endpoints configured as silent servers are not able to perform the
challenge-response described above, as they do not store a Sender
Context to secure the 4.01 (Unauthorized) response to the client.
Thus, silent servers should adopt alternative approaches to achieve
and maintain synchronization with Sender Sequence Numbers of clients.
Since requests including the Echo Option are sent over unicast, a
server can be victim of the attack discussed in Section 13.9, in case
such requests are protected in group mode. Instead, protecting those
requests with the pairwise mode prevents the attack above. In fact,
only the very server involved in the challenge-response exchange is
able to derive the pairwise key used by the client to protect the
request including the Echo Option.
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In either case, an internal on-path adversary would not be able to
mix up the Echo Option value of two different unicast requests, sent
by a same client to any two different servers in the group. In fact,
even if the group mode was used, this would require the adversary to
forge the countersignature of both requests. As a consequence, each
of the two servers remains able to selectively accept a request with
the Echo Option only if it is waiting for that exact integrity-
protected Echo Option value, and is thus the intended recipient.
13.15. Client Aliveness
Like in OSCORE (see Section 12.5 of [RFC8613]), a server may verify
the aliveness of the client by using the CoAP Echo Option [RFC9175]
as described in Section 10.
In the interest of avoiding otherwise unnecessary uses of such an
approach, the server can exploit the fact that the received request
cannot be older than the Security Context used to protect it. This
effectively allows the server to verify the client aliveness relative
to the installation of the latest group keying material.
13.16. Cryptographic Considerations
The same considerations from Section 12.6 of [RFC8613] about the
maximum Sender Sequence Number hold for Group OSCORE.
As discussed in Section 2.6.2, an endpoint that experiences an
exhaustion of its own Sender Sequence Number space MUST NOT protect
further messages including a Partial IV, until it has derived a new
Sender Context. This prevents the endpoint to reuse the same AEAD
nonce with the same Sender Key.
In order to renew its own Sender Context, the endpoint SHOULD inform
the Group Manager, which can either renew the whole Security Context
by means of group rekeying, or provide only that endpoint with a new
Sender ID value. In either case, the endpoint derives a new Sender
Context, and in particular a new Sender Key.
Additionally, the same considerations from Section 12.6 of [RFC8613]
hold for Group OSCORE, about building the AEAD nonce and the secrecy
of the Security Context parameters.
The group mode uses the "encrypt-then-sign" construction, i.e., the
countersignature is computed over the COSE_Encrypt0 object (see
Section 4.1). This is motivated by enabling signature checkers (see
Section 3.3), which do not join a group as members but are allowed to
verify countersignatures of messages protected in group mode without
being able to decrypt them (see Section 8.5).
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If the Group Encryption Algorithm used in group mode provides
integrity protection, countersignatures of COSE_Encrypt0 with short
authentication tags do not provide the security properties associated
with the same algorithm used in COSE_Sign (see Section 6 of
[RFC9338]). To provide 128-bit security against collision attacks,
the tag length MUST be at least 256-bits. A countersignature of a
COSE_Encrypt0 with AES-CCM-16-64-128 provides at most 32 bits of
integrity protection.
The derivation of pairwise keys defined in Section 2.5.1 is
compatible with ECDSA and EdDSA asymmetric keys, but is not
compatible with RSA asymmetric keys.
For the public key translation from Ed25519 (Ed448) to X25519 (X448)
specified in Section 2.5.1, variable time methods can be used since
the translation operates on public information. Any byte string of
appropriate length is accepted as a public key for X25519 (X448) in
[RFC7748]. It is therefore not necessary for security to validate
the translated public key (assuming the translation was successful).
The security of using the same key pair for Diffie-Hellman and for
signing (by considering the ECDH procedure in Section 2.5 as a Key
Encapsulation Mechanism (KEM)) is demonstrated in [Degabriele] and
[Thormarker].
Applications using ECDH (except X25519 and X448) based KEM in
Section 2.5 are assumed to verify that a peer endpoint's public key
is on the expected curve and that the shared secret is not the point
at infinity. The KEM in [Degabriele] checks that the shared secret
is different from the point at infinity, as does the procedure in
Section 5.7.1.2 of [NIST-800-56A] which is referenced in Section 2.5.
By extending Theorem 2 of [Degabriele], [Thormarker] shows that the
same key pair can be used with X25519 and Ed25519 (X448 and Ed448)
for the KEM specified in Section 2.5. By symmetry in the KEM used in
this document, both endpoints can consider themselves to have the
recipient role in the KEM - as discussed in Section 7 of [Thormarker]
- and rely on the mentioned proofs for the security of their key
pairs.
Theorem 3 in [Degabriele] shows that the same key pair can be used
for an ECDH based KEM and ECDSA. The KEM uses a different KDF than
in Section 2.5, but the proof only depends on that the KDF has
certain required properties, which are the typical assumptions about
HKDF, e.g., that output keys are pseudorandom. In order to comply
with the assumptions of Theorem 3, received public keys MUST be
successfully validated, see Section 5.6.2.3.4 of [NIST-800-56A]. The
validation MAY be performed by a trusted Group Manager. For
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[Degabriele] to apply as it is written, public keys need to be in the
expected subgroup. For this, we rely on cofactor Diffie-Hellman as
per Section 5.7.1.2 of [NIST-800-56A], which is referenced in
Section 2.5.1.
HashEdDSA variants of Ed25519 and Ed448 are not used by COSE (see
Section 2.2 of [RFC9053]), and are not covered by the analysis in
[Thormarker]. Hence, they MUST NOT be used with the public keys used
to derive pairwise keys as specified in this document.
13.17. Message Segmentation
The same considerations from Section 12.7 of [RFC8613] hold for Group
OSCORE.
13.18. Privacy Considerations
Group OSCORE ensures end-to-end integrity protection and encryption
of the message payload and of all the options that are not used for
proxy operations. In particular, options are processed according to
the same class U/I/E that they have for OSCORE. Therefore, the same
privacy considerations from Section 12.8 of [RFC8613] hold for Group
OSCORE, with the following addition.
* When protecting a message in group mode, the countersignature is
encrypted by using a keystream derived from the group keying
material (see Section 5 and Section 5.1). This ensures group
privacy. That is, an attacker cannot track an endpoint over two
groups by linking messages between the two groups, unless being
also a member of those groups.
Furthermore, the following privacy considerations hold about the
OSCORE option, which may reveal information on the communicating
endpoints.
* The 'kid' parameter, which is intended to help a recipient
endpoint to find the right Recipient Context, may reveal
information about the Sender Endpoint. When both a request and
the corresponding responses include the 'kid' parameter, this may
reveal information about both a client sending a request and all
the possibly replying servers sending their own individual
response.
* The 'kid context' parameter, which is intended to help a recipient
endpoint to find the right Security Context, reveals information
about the sender endpoint. In particular, it reveals that the
sender endpoint is a member of a particular OSCORE group, whose
current Group ID is indicated in the 'kid context' parameter.
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When receiving a group request, each of the recipient endpoints can
reply with a response that includes its Sender ID as 'kid' parameter.
All these responses will be matchable with the request through the
CoAP Token. Thus, even if these responses do not include a 'kid
context' parameter, it becomes possible to understand that the
responder endpoints are in the same group of the requester endpoint.
Furthermore, using the approach described in Section 10 to achieve
synchronization with a client's Sender Sequence Number may reveal
when a server device goes through a reboot. This can be mitigated by
the server device storing the precise state of the Replay Window of
each known client on a clean shutdown.
Finally, the approach described in Section 13.6 to prevent collisions
of Group Identifiers from different Group Managers may reveal
information about events in the respective OSCORE groups. In
particular, a Group Identifier changes when the corresponding group
is rekeyed. Thus, Group Managers might use the shared list of Group
Identifiers to infer the rate and patterns of group membership
changes triggering a group rekeying, e.g., due to newly joined
members or evicted (compromised) members. In order to alleviate this
privacy concern, it should be hidden from the Group Managers which
exact Group Manager has currently assigned which Group Identifiers in
its OSCORE groups.
14. IANA Considerations
Note to RFC Editor: Please replace "[RFC-XXXX]" with the RFC number
of this document and delete this paragraph.
This document has the following actions for IANA.
14.1. OSCORE Flag Bits Registry
IANA is asked to add the following entry to the "OSCORE Flag Bits"
registry within the "Constrained RESTful Environments (CoRE)
Parameters" registry group.
+--------------+-------+-----------------------------+------------+
| Bit Position | Name | Description | Reference |
+--------------+-------+-----------------------------+------------+
| 2 | Group | For using a Group OSCORE | [RFC-XXXX] |
| | Flag | Security Context, set to 1 | |
| | | if the message is protected | |
| | | with the group mode | |
+--------------+-------+-----------------------------+------------+
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14.2. Target Attributes Registry
IANA is asked to add the following entry to the "Target Attributes"
registry within the "CoRE Parameters" registry group.
Attribute Name: gosc
Brief Description: Hint: resource only accessible
using Group OSCORE or OSCORE
Change Controller: IETF
Reference: [RFC-XXXX]
15. References
15.1. Normative References
[I-D.ietf-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", Work in
Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
09, 10 July 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-groupcomm-bis-09>.
[NIST-800-56A]
Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
Davis, "Recommendation for Pair-Wise Key-Establishment
Schemes Using Discrete Logarithm Cryptography - NIST
Special Publication 800-56A, Revision 3", April 2018,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar3.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/rfc/rfc6979>.
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[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/rfc/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/rfc/rfc7641>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/rfc/rfc7748>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/rfc/rfc8288>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/rfc/rfc8613>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
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[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.
[RFC9175] Amsüss, C., Preuß Mattsson, J., and G. Selander,
"Constrained Application Protocol (CoAP): Echo, Request-
Tag, and Token Processing", RFC 9175,
DOI 10.17487/RFC9175, February 2022,
<https://www.rfc-editor.org/rfc/rfc9175>.
[RFC9338] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Countersignatures", STD 96, RFC 9338,
DOI 10.17487/RFC9338, December 2022,
<https://www.rfc-editor.org/rfc/rfc9338>.
15.2. Informative References
[Degabriele]
Degabriele, J. P., Lehmann, A., Paterson, K. G., Smart, N.
P., and M. Strefler, "On the Joint Security of Encryption
and Signature in EMV", December 2011,
<https://eprint.iacr.org/2011/615>.
[I-D.amsuess-core-cachable-oscore]
Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
Progress, Internet-Draft, draft-amsuess-core-cachable-
oscore-07, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-amsuess-core-
cachable-oscore-07>.
[I-D.ietf-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", Work in Progress, Internet-
Draft, draft-ietf-ace-key-groupcomm-16, 5 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-key-
groupcomm-16>.
[I-D.ietf-ace-key-groupcomm-oscore]
Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", Work in Progress, Internet-
Draft, draft-ietf-ace-key-groupcomm-oscore-16, 6 March
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
ace-key-groupcomm-oscore-16>.
[I-D.ietf-core-observe-multicast-notifications]
Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
"Observe Notifications as CoAP Multicast Responses", Work
in Progress, Internet-Draft, draft-ietf-core-observe-
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multicast-notifications-06, 26 April 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
observe-multicast-notifications-06>.
[I-D.ietf-cose-cbor-encoded-cert]
Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
ietf-cose-cbor-encoded-cert-06, 7 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cbor-encoded-cert-06>.
[I-D.ietf-iotops-security-protocol-comparison]
Mattsson, J. P., Palombini, F., and M. Vučinić,
"Comparison of CoAP Security Protocols", Work in Progress,
Internet-Draft, draft-ietf-iotops-security-protocol-
comparison-02, 11 April 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-iotops-
security-protocol-comparison-02>.
[I-D.ietf-lwig-curve-representations]
Struik, R., "Alternative Elliptic Curve Representations",
Work in Progress, Internet-Draft, draft-ietf-lwig-curve-
representations-23, 21 January 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-lwig-
curve-representations-23>.
[I-D.irtf-cfrg-det-sigs-with-noise]
Mattsson, J. P., Thormarker, E., and S. Ruohomaa,
"Deterministic ECDSA and EdDSA Signatures with Additional
Randomness", Work in Progress, Internet-Draft, draft-irtf-
cfrg-det-sigs-with-noise-00, 8 August 2022,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
det-sigs-with-noise-00>.
[I-D.tiloca-core-groupcomm-proxy]
Tiloca, M. and E. Dijk, "Proxy Operations for CoAP Group
Communication", Work in Progress, Internet-Draft, draft-
tiloca-core-groupcomm-proxy-09, 31 August 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
groupcomm-proxy-09>.
[I-D.tiloca-core-oscore-capable-proxies]
Tiloca, M. and R. Höglund, "OSCORE-capable Proxies", Work
in Progress, Internet-Draft, draft-tiloca-core-oscore-
capable-proxies-07, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-capable-proxies-07>.
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[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/rfc/rfc4944>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/rfc/rfc4949>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/rfc/rfc5869>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/rfc/rfc6282>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/rfc/rfc6690>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/rfc/rfc7228>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/rfc/rfc7959>.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/rfc/rfc8075>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/rfc/rfc8392>.
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[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/rfc/rfc9200>.
[Thormarker]
Thormarker, E., "On using the same key pair for Ed25519
and an X25519 based KEM", April 2021,
<https://eprint.iacr.org/2021/509>.
Appendix A. Assumptions and Security Objectives
This section presents a set of assumptions and security objectives
for the protocol described in this document. The rest of this
section refers to three types of groups:
* Application group, i.e., a set of CoAP endpoints that share a
common pool of resources.
* Security group, as defined in Section 1.1 of this document. There
can be a one-to-one or a one-to-many relation between security
groups and application groups, and vice versa.
* CoAP group, i.e., a set of CoAP endpoints where each endpoint is
configured to receive one-to-many CoAP requests, e.g., sent to the
group's associated IP multicast address and UDP port as defined in
[I-D.ietf-core-groupcomm-bis]. An endpoint may be a member of
multiple CoAP groups. There can be a one-to-one or a one-to-many
relation between application groups and CoAP groups. Note that a
device sending a CoAP request to a CoAP group is not necessarily
itself a member of that group: it is a member only if it also has
a CoAP server endpoint listening to requests for this CoAP group,
sent to the associated IP multicast address and port. In order to
provide secure group communication, all members of a CoAP group as
well as all further endpoints configured only as clients sending
CoAP (multicast) requests to the CoAP group have to be member of a
security group. There can be a one-to-one or a one-to-many
relation between security groups and CoAP groups, and vice versa.
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A.1. Assumptions
The following points are assumed to be already addressed and are out
of the scope of this document.
* Multicast communication topology: this document considers both
1-to-N (one sender and multiple recipients) and M-to-N (multiple
senders and multiple recipients) 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.
In a 1-to-N communication model, only a single client transmits
data to the CoAP group, in the form of request messages; in an
M-to-N communication model (where M and N do not necessarily have
the same value), M clients transmit data to the CoAP group.
According to [I-D.ietf-core-groupcomm-bis], any possible proxy
entity is supposed to know about the clients. Also, every client
expects and is able to handle multiple response messages
associated with a same request sent to the CoAP group.
* Group size: security solutions for group communication should be
able to adequately support different and possibly large security
groups. The group size is the current number of members in a
security group. In the use cases mentioned in this document, the
number of clients (normally the controlling devices) is expected
to be much smaller than the number of servers (i.e., the
controlled devices). A security solution for group communication
that supports 1 to 50 clients 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 hundreds to thousands of devices, with large groups easier to
manage if including several silent servers. Security groups
larger than that should be divided into smaller independent
groups. One should not assume that the set of members of a
security group remains fixed. That is, the group membership is
subject to changes, possibly on a frequent basis.
* Communication with the Group Manager: an endpoint must use a
secure dedicated channel when communicating with the Group
Manager, also when not registered as a member of the security
group.
* Provisioning and management of Security Contexts: a Security
Context must be established among the members of the security
group. A secure mechanism must be used to generate, revoke and
(re-)distribute keying material, communication policies and
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security parameters in the security group. The actual
provisioning and management of the Security Context is out of the
scope of this document.
* Multicast data security cipher suite: all members of a security
group must use the same cipher suite to provide authenticity,
integrity and confidentiality of messages in the group. The
cipher suite is specified as part of the Security Context.
* Ensuring backward security: a new device joining the security
group should not have access to any old Security Contexts used
before its joining. This ensures that a new member of the
security group is not able to decrypt confidential data sent
before it has joined the security 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
in the security group upon a new member's joining has to be
defined as part of the group key management scheme.
* Ensuring forward security: entities that leave the security group
should not have access to any future Security Contexts or message
exchanged within the security group after their leaving. This
ensures that a former member of the security group is not able to
decrypt confidential data sent within the security group anymore.
Also, it ensures that a former member is not able to send
protected messages to the security group anymore. The actual
mechanism to update the Security Context and renew the group
keying material in the security group upon a member's leaving has
to be defined as part of the group key management scheme.
A.2. Security Objectives
The protocol described in this document aims at fulfilling the
following security objectives:
* Data replay protection: request messages or response messages
replayed within the security group must be detected.
* Data confidentiality: messages sent within the security group
shall be encrypted.
* Group-level data confidentiality: the group mode provides 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 security group, but not by an external adversary or
other external entities.
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* Pairwise data confidentiality: the pairwise mode especially
provides pairwise data confidentiality, since messages are
encrypted using pairwise keying material shared between any two
group members, hence they can be decrypted only by the intended
single recipient.
* Source message authentication: messages sent within the security
group shall be authenticated. That is, it is essential to ensure
that a message is originated by a member of the security group in
the first place, and in particular by a specific, identifiable
member of the security group.
* Message integrity: messages sent within the security group shall
be integrity protected. That is, it is essential to ensure that a
message has not been tampered with, either by a group member, or
by an external adversary or other external entities which are not
members of the security group.
* Message ordering: it must be possible to determine the ordering of
messages coming from a single sender. Like in OSCORE [RFC8613], a
recipient endpoint can determine the relative order of requests or
responses from another sender endpoint by means of their Partial
IV. It is not required to determine ordering of messages from
different senders.
Appendix B. List of Use Cases
Group Communication for CoAP [I-D.ietf-core-groupcomm-bis] provides
the necessary background for multicast-based CoAP communication, with
particular reference to low-power and lossy networks (LLNs) and
resource constrained environments. The interested reader is
encouraged to first read [I-D.ietf-core-groupcomm-bis] to understand
the non-security related details. This section discusses a number of
use cases that benefit from secure group communication, and refers to
the three types of groups from Appendix A. Specific security
requirements for these use cases are discussed in Appendix A.
* Lighting control: consider a building equipped with IP-connected
lighting devices, switches, and border routers. The lighting
devices acting as servers are organized into application groups
and CoAP groups, according to their physical location in the
building. For instance, lighting devices in a room or corridor
can be configured as members of a single application group and
corresponding CoAP group. Those lighting devices together with
the switches acting as clients in the same room or corridor can be
configured as members of the corresponding security group.
Switches are then used to control the lighting devices by sending
on/off/dimming commands to all lighting devices in the CoAP group,
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while border routers connected to an IP network backbone (which is
also multicast-enabled) can be used to interconnect routers in the
building. Consequently, this would also enable logical groups to
be formed even if devices with a role in the lighting application
may be physically in different subnets (e.g., on wired and
wireless networks). Connectivity between lighting devices may be
realized, for instance, by means of IPv6 and (border) routers
supporting 6LoWPAN [RFC4944][RFC6282]. Group communication
enables synchronous operation of a set of connected lights,
ensuring that the light preset (e.g., dimming level or color) of a
large set of luminaires are changed at the same perceived time.
This is especially useful for providing a visual synchronicity of
light effects to the user. As a practical guideline, events
within a 200 ms interval are perceived as simultaneous by humans,
which is necessary to ensure in many setups. Devices may reply
back to the 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 set of lighting
devices. In more advanced lighting control use cases, an 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 set of lighting devices. Especially in
professional lighting scenarios, the roles of client and server
are configured by the lighting commissioner, and devices strictly
follow those roles.
* Integrated building control: enabling Building Automation and
Control Systems (BACSs) to control multiple heating, ventilation
and air-conditioning units to predefined presets. Controlled
units can be organized into application groups and CoAP groups in
order to reflect their physical position in the building, e.g.,
devices in the same room can be configured as members of a single
application group and corresponding CoAP group. As a practical
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
execution of the requested operation (e.g., OK, failure, error)
and their current operational status.
* Software and firmware updates: software and firmware updates often
comprise quite a large amount of data. This can overload a Low-
power and Lossy Network (LLN) that is otherwise typically used to
deal with only small amounts of data, on an infrequent base.
Rather than sending software and firmware updates as unicast
messages to each individual device, multicasting such updated data
to a larger set of devices at once displays a number of benefits.
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For instance, it can significantly reduce the network load and
decrease the overall time latency for propagating this data to all
devices. Even if the complete whole update process itself is
secured, securing the individual messages is important, in case
updates consist of relatively large amounts of data. In fact,
checking individual received data piecemeal for tampering avoids
that devices store large amounts of partially corrupted data and
that they detect tampering hereof only after all data has been
received. Devices receiving software and firmware updates are
expected to possibly reply back, in order to provide a feedback
about the execution of the update operation (e.g., OK, failure,
error) and their current operational status.
* Parameter and configuration update: by means of multicast
communication, it is possible to update the settings of a set of
similar devices, both simultaneously and efficiently. Possible
parameters are related, for instance, to network load management
or network access control. Devices receiving parameter and
configuration updates are expected to possibly reply back, to
provide a feedback about the execution of the update operation
(e.g., OK, failure, error) and their current operational status.
* Commissioning of Low-power and Lossy Network (LLN) systems: a
commissioning device is responsible for querying all devices in
the local network or a selected subset of them, in order to
discover their presence, and be aware of their capabilities,
default configuration, and operating conditions. Queried devices
displaying similarities in their capabilities and features, or
sharing a common physical location can be configured as members of
a single application group and corresponding CoAP group. Queried
devices are expected to reply back to the commissioning device, in
order to notify their presence, and provide the requested
information and their current operational status.
* Emergency multicast: a particular emergency-related information
(e.g., natural disaster) is generated and multicast by an
emergency notifier, and relayed to multiple devices. The latter
may reply back to the emergency notifier, in order to provide
their feedback and local information related to the ongoing
emergency. This kind of setups should additionally rely on a
fault-tolerant multicast algorithm, such as Multicast Protocol for
Low-Power and Lossy Networks (MPL).
Appendix C. Example of Group Identifier Format
This section provides an example of how the Group Identifier (Gid)
can be specifically formatted. That is, the Gid can be composed of
two parts, namely a Group Prefix and a Group Epoch.
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For each group, the Group Prefix is constant over time and is
uniquely defined in the set of all the groups associated with the
same Group Manager. The choice of the Group Prefix for a given
group's Security Context is application specific. 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
incremented by 1 each time new keying material, together with a new
Gid, is distributed to the group in order to establish a new Security
Context (see Section 3.2).
As an example, a 3-byte Gid can be composed of: i) a 1-byte Group
Prefix '0xb1' interpreted as a raw byte string; and ii) a 2-byte
Group Epoch interpreted as an unsigned integer ranging from 0 to
65535. Then, after having established the Common Context 61532 times
in the group, its Gid will assume value '0xb1f05c'.
Using an immutable Group Prefix for a group with a Group Manager that
cannot reassign Gid values (see Section 3.2) limits the total number
of rekeying instances. With a Group Manager that does reassign Gid
values, it limits the maximum active number of rekeying instances
that a CoAP observation [RFC7641] can persist through. In either
case, the group epoch size needs to be chosen depending on the
expected rate of rekeying instances.
As discussed in Section 13.6, if endpoints are deployed in multiple
groups managed by different non-synchronized Group Managers, it is
possible that Group Identifiers of different groups coincide at some
point in time. In this case, a recipient has to handle coinciding
Group Identifiers, and has to try using different Security Contexts
to process an incoming message, until the right one is found and the
message is correctly verified. Therefore, it is favorable that Group
Identifiers from different Group Managers have a size that result in
a small probability of collision. How small this probability should
be is up to system designers.
Appendix D. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
D.1. Version -19 to -20
* Change Controller for the target attribute "gosc" set to "IETF".
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D.2. Version -18 to -19
* Unified presentation of handling of multiple responses.
* Added Rikard Hoeglund as Contributor.
D.3. Version -17 to -18
* Changed document title.
* Possible use with CoAP-mappable HTTP.
* Added Common Context parameter "Authentication Credential Format".
* Renamed "Group Encryption Key" to "Signature Encryption Key".
Consistent fixes in its derivation.
* Renamed "Signature Encryption Algorithm" to "Group Encryption
Algorithm".
* Ensured a single Common IV, also when the two encryption
algorithms have different AEAD nonce sizes.
* Guidelines on the Pairwise Key Agreement Algorithm and derivation
of the Diffie-Hellman secret.
* The possible use of a mode follows from the set parameters.
* The Group Manager is always present; 'gm_cred' in the external_aad
cannot be null anymore.
* The authentication credential of the Group Manager can have a
different format than that of the group members'.
* Set-up of new endpoints moved to document body.
* The encrypted countersignature is a result of the header
compression, not of COSE.
* Revised examples of compressed and non-compressed COSE object.
* Removed excessive requirements on group rekeying scheduling.
* More considerations on the strictness of group key management.
* Clearer alternatives on retaining an old Security Context.
* Revised used of terminology on freshness.
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* Clarifications, fixes and editorial improvements.
D.4. Version -16 to -17
* Definition and registration of the target attribute "gosc".
* Reference update and editorial fixes.
D.5. Version -15 to -16
* Clients "SHOULD" use the group mode for one-to-many requests.
* Handling of multiple non-notification responses.
* Revised presentation of security properties.
* Improved listing of operations defined for the group mode that are
inherited by the pairwise mode.
* Editorial improvements.
D.6. Version -14 to -15
* Updated references and editorial fixes.
D.7. Version -13 to -14
* Replaced "node" with "endpoint" where appropriate.
* Replaced "owning" with "storing" (of keying material).
* Distinction between "authentication credential" and "public key".
* Considerations on storing whole authentication credentials.
* Considerations on Denial of Service.
* Recycling of Group IDs by tracking the "Birth Gid" of each group
member is now optional to support and use for the Group Manager.
* Fine-grained suppression of error responses.
* Changed section title "Mandatory-to-Implement Compliance
Requirements" to "Implementation Compliance".
* "Challenge-Response Synchronization" moved to the document body.
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* RFC 7641 and draft-ietf-core-echo-request-tag as normative
references.
* Clarifications and editorial improvements.
D.8. Version -12 to -13
* Fixes in the derivation of the Group Encryption Key.
* Added Mandatory-to-Implement compliance requirements.
* Changed UCCS to CCS.
D.9. Version -11 to -12
* No mode of operation is mandatory to support.
* Revised parameters of the Security Context, COSE object and
external_aad.
* Revised management of keying material for the Group Manager.
* Informing of former members when rekeying the group.
* Admit encryption-only algorithms in group mode.
* Encrypted countersignature through a keystream.
* Added public key of the Group Manager as key material and
protected data.
* Clarifications about message processing, especially notifications.
* Guidance for message processing of external signature checkers.
* Updated derivation of pairwise keys, with more security
considerations.
* Termination of ongoing observations as client, upon leaving or
before re-joining the group.
* Recycling Group IDs by tracking the "Birth Gid" of each group
member.
* Expanded security and privacy considerations about the group mode.
* Removed appendices on skipping signature verification and on COSE
capabilities.
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* Fixes and editorial improvements.
D.10. Version -10 to -11
* Loss of Recipient Contexts due to their overflow.
* Added diagram on keying material components and their relation.
* Distinction between anti-replay and freshness.
* Preservation of Sender IDs over rekeying.
* Clearer cause-effect about reset of SSN.
* The GM provides public keys of group members with associated
Sender IDs.
* Removed 'par_countersign_key' from the external_aad.
* One single format for the external_aad, both for encryption and
signing.
* Presence of 'kid' in responses to requests protected in pairwise
mode.
* Inclusion of 'kid_context' in notifications following a group
rekeying.
* Pairwise mode presented with OSCORE as baseline.
* Revised examples with signature values.
* Decoupled growth of clients' Sender Sequence Numbers and loss of
synchronization for server.
* Sender IDs not recycled in the group under the same Gid.
* Processing and description of the Group Flag bit in the OSCORE
option.
* Usage of the pairwise mode for multicast requests.
* Clarifications on synchronization using the Echo option.
* General format of context parameters and external_aad elements,
supporting future registered COSE algorithms (new Appendix).
* Fixes and editorial improvements.
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D.11. Version -09 to -10
* Removed 'Counter Signature Key Parameters' from the Common
Context.
* New parameters in the Common Context covering the DH secret
derivation.
* New countersignature header parameter from draft-ietf-cose-
countersign.
* Stronger policies non non-recycling of Sender IDs and Gid.
* The Sender Sequence Number is reset when establishing a new
Security Context.
* Added 'request_kid_context' in the aad_array.
* The server can respond with 5.03 if the client's public key is not
available.
* The observer client stores an invariant identifier of the group.
* Relaxed storing of original 'kid' for observer clients.
* Both client and server store the 'kid_context' of the original
observation request.
* The server uses a fresh PIV if protecting the response with a
Security Context different from the one used to protect the
request.
* Clarifications on MTI algorithms and curves.
* Removed optimized requests.
* Overall clarifications and editorial revision.
D.12. Version -08 to -09
* Pairwise keys are discarded after group rekeying.
* Signature mode renamed to group mode.
* The parameters for countersignatures use the updated COSE
registries. Newly defined IANA registries have been removed.
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* Pairwise Flag bit renamed as Group Flag bit, set to 1 in group
mode and set to 0 in pairwise mode.
* Dedicated section on updating the Security Context.
* By default, sender sequence numbers and replay windows are not
reset upon group rekeying.
* An endpoint implementing only a silent server does not support the
pairwise mode.
* Separate section on general message reception.
* Pairwise mode moved to the document body.
* Considerations on using the pairwise mode in non-multicast
settings.
* Optimized requests are moved as an appendix.
* Normative support for the signature and pairwise mode.
* Revised methods for synchronization with clients' sender sequence
number.
* Appendix with example values of parameters for countersignatures.
* Clarifications and editorial improvements.
D.13. Version -07 to -08
* Clarified relation between pairwise mode and group communication
(Section 1).
* Improved definition of "silent server" (Section 1.1).
* Clarified when a Recipient Context is needed (Section 2).
* Signature checkers as entities supported by the Group Manager
(Section 2.3).
* Clarified that the Group Manager is under exclusive control of Gid
and Sender ID values in a group, with Sender ID values under each
Gid value (Section 2.3).
* Mitigation policies in case of recycled 'kid' values
(Section 2.4).
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* More generic exhaustion (not necessarily wrap-around) of sender
sequence numbers (Sections 2.5 and 10.11).
* Pairwise key considerations, as to group rekeying and Sender
Sequence Numbers (Section 3).
* Added reference to static-static Diffie-Hellman shared secret
(Section 3).
* Note for implementation about the external_aad for signing
(Sectino 4.3.2).
* Retransmission by the application for group requests over
multicast as Non-confirmable (Section 7).
* A server MUST use its own Partial IV in a response, if protecting
it with a different context than the one used for the request
(Section 7.3).
* Security considerations: encryption of pairwise mode as
alternative to group-level security (Section 10.1).
* Security considerations: added approach to reduce the chance of
global collisions of Gid values from different Group Managers
(Section 10.5).
* Security considerations: added implications for block-wise
transfers when using the signature mode for requests over unicast
(Section 10.7).
* Security considerations: (multiple) supported signature algorithms
(Section 10.13).
* Security considerations: added privacy considerations on the
approach for reducing global collisions of Gid values
(Section 10.15).
* Updates to the methods for synchronizing with clients' sequence
number (Appendix E).
* Simplified text on discovery services supporting the pairwise mode
(Appendix G.1).
* Editorial improvements.
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D.14. Version -06 to -07
* Updated abstract and introduction.
* Clarifications of what pertains a group rekeying.
* Derivation of pairwise keying material.
* Content re-organization for COSE Object and OSCORE header
compression.
* Defined the Pairwise Flag bit for the OSCORE option.
* Supporting CoAP Observe for group requests and responses.
* Considerations on message protection across switching to new
keying material.
* New optimized mode based on pairwise keying material.
* More considerations on replay protection and Security Contexts
upon key renewal.
* Security considerations on Group OSCORE for unicast requests, also
as affecting the usage of the Echo option.
* Clarification on different types of groups considered
(application/security/CoAP).
* New pairwise mode, using pairwise keying material for both
requests and responses.
D.15. Version -05 to -06
* Group IDs mandated to be unique under the same Group Manager.
* Clarifications on parameter update upon group rekeying.
* Updated external_aad structures.
* Dynamic derivation of Recipient Contexts made optional and
application specific.
* Optional 4.00 response for failed signature verification on the
server.
* Removed client handling of duplicated responses to multicast
requests.
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* Additional considerations on public key retrieval and group
rekeying.
* Added Group Manager responsibility on validating public keys.
* Updates IANA registries.
* Reference to RFC 8613.
* Editorial improvements.
D.16. Version -04 to -05
* Added references to draft-dijk-core-groupcomm-bis.
* New parameter Counter Signature Key Parameters (Section 2).
* Clarification about Recipient Contexts (Section 2).
* Two different external_aad for encrypting and signing
(Section 3.1).
* Updated response verification to handle Observe notifications
(Section 6.4).
* Extended Security Considerations (Section 8).
* New "Counter Signature Key Parameters" IANA Registry
(Section 9.2).
D.17. Version -03 to -04
* Added the new "Counter Signature Parameters" in the Common Context
(see Section 2).
* Added recommendation on using "deterministic ECDSA" if ECDSA is
used as countersignature algorithm (see Section 2).
* Clarified possible asynchronous retrieval of keying material from
the Group Manager, in order to process incoming messages (see
Section 2).
* Structured Section 3 into subsections.
* Added the new 'par_countersign' to the aad_array of the
external_aad (see Section 3.1).
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* Clarified non reliability of 'kid' as identity identifier for a
group member (see Section 2.1).
* Described possible provisioning of new Sender ID in case of
Partial IV wrap-around (see Section 2.2).
* The former signature bit in the Flag Byte of the OSCORE option
value is reverted to reserved (see Section 4.1).
* Updated examples of compressed COSE object, now with the sixth
less significant bit in the Flag Byte of the OSCORE option value
set to 0 (see Section 4.3).
* Relaxed statements on sending error messages (see Section 6).
* Added explicit step on computing the countersignature for outgoing
messages (see Sections 6.1 and 6.3).
* Handling of just created Recipient Contexts in case of
unsuccessful message verification (see Sections 6.2 and 6.4).
* Handling of replied/repeated responses on the client (see
Section 6.4).
* New IANA Registry "Counter Signature Parameters" (see
Section 9.1).
D.18. Version -02 to -03
* Revised structure and phrasing for improved readability and better
alignment with draft-ietf-core-object-security.
* Added discussion on wrap-Around of Partial IVs (see Section 2.2).
* Separate sections for the COSE Object (Section 3) and the OSCORE
Header Compression (Section 4).
* The countersignature is now appended to the encrypted payload of
the OSCORE message, rather than included in the OSCORE Option (see
Section 4).
* Extended scope of Section 5, now titled " Message Binding,
Sequence Numbers, Freshness and Replay Protection".
* Clarifications about Non-confirmable messages in Section 5.1
"Synchronization of Sender Sequence Numbers".
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* Clarifications about error handling in Section 6 "Message
Processing".
* Compacted list of responsibilities of the Group Manager in
Section 7.
* Revised and extended security considerations in Section 8.
* Added IANA considerations for the OSCORE Flag Bits Registry in
Section 9.
* Revised Appendix D, now giving a short high-level description of a
new endpoint set-up.
D.19. Version -01 to -02
* Terminology has been made more aligned with RFC7252 and draft-
ietf-core-object-security: i) "client" and "server" replace the
old "multicaster" and "listener", respectively; ii) "silent
server" replaces the old "pure listener".
* Section 2 has been updated to have the Group Identifier stored in
the 'ID Context' parameter defined in draft-ietf-core-object-
security.
* Section 3 has been updated with the new format of the Additional
Authenticated Data.
* Major rewriting of Section 4 to better highlight the differences
with the message processing in draft-ietf-core-object-security.
* Added Sections 7.2 and 7.3 discussing security considerations
about uniqueness of (key, nonce) and collision of group
identifiers, respectively.
* Minor updates to Appendix A.1 about assumptions on multicast
communication topology and group size.
* Updated Appendix C on format of group identifiers, with practical
implications of possible collisions of group identifiers.
* Updated Appendix D.2, adding a pointer to draft-palombini-ace-key-
groupcomm about retrieval of nodes' public keys through the Group
Manager.
* Minor updates to Appendix E.3 about Challenge-Response
synchronization of sequence numbers based on the Echo option from
draft-ietf-core-echo-request-tag.
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D.20. Version -00 to -01
* Section 1.1 has been updated with the definition of group as
"security group".
* Section 2 has been updated with:
- Clarifications on establishment/derivation of Security
Contexts.
- A table summarizing the the additional context elements
compared to OSCORE.
* Section 3 has been updated with:
- Examples of request and response messages.
- Use of CounterSignature0 rather than CounterSignature.
- Additional Authenticated Data including also the signature
algorithm, while not including the Group Identifier any longer.
* Added Section 6, listing the responsibilities of the Group
Manager.
* Added Appendix A (former section), including assumptions and
security objectives.
* Appendix B has been updated with more details on the use cases.
* Added Appendix C, providing an example of Group Identifier format.
* Appendix D has been updated to be aligned with draft-palombini-
ace-key-groupcomm.
Acknowledgments
The authors sincerely thank Christian Amsüss, Stefan Beck, Rolf Blom,
Carsten Bormann, Esko Dijk, Martin Gunnarsson, Klaus Hartke, Richard
Kelsey, Dave Robin, Jim Schaad, Ludwig Seitz, Peter van der Stok, and
Erik Thormarker for their feedback and comments.
The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; the H2020 projects SIFIS-Home
(Grant agreement 952652) and ARCADIAN-IoT (Grant agreement
101020259); the SSF project SEC4Factory under the grant RIT17-0032;
and the EIT-Digital High Impact Initiative ACTIVE.
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Contributors
Rikard Höglund
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: rikard.hoglund@ri.se
Authors' Addresses
Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: marco.tiloca@ri.se
Göran Selander
Ericsson AB
Torshamnsgatan 23
SE-16440 Stockholm Kista
Sweden
Email: goran.selander@ericsson.com
Francesca Palombini
Ericsson AB
Torshamnsgatan 23
SE-16440 Stockholm Kista
Sweden
Email: francesca.palombini@ericsson.com
John Preuß Mattsson
Ericsson AB
Torshamnsgatan 23
SE-16440 Stockholm Kista
Sweden
Email: john.mattsson@ericsson.com
Tiloca, et al. Expires 5 March 2024 [Page 113]
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Jiye Park
Universitaet Duisburg-Essen
Schuetzenbahn 70
45127 Essen
Germany
Email: ji-ye.park@uni-due.de
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