Key Provisioning for Group Communication using ACE
draft-ietf-ace-key-groupcomm-17
| Document | Type | Active Internet-Draft (ace WG) | |
|---|---|---|---|
| Authors | Francesca Palombini , Marco Tiloca | ||
| Last updated | 2023-11-29 (Latest revision 2023-10-06) | ||
| Replaces | draft-palombini-ace-key-groupcomm | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews |
IOTDIR Telechat review
by Dave Thaler
Almost ready
GENART Last Call review
by Thomas Fossati
Ready w/issues
TSVART Last Call review
by Vidhi Goel
Ready w/nits
ARTART Last Call review
by Henry Thompson
Ready w/issues
SECDIR Last Call Review due 2023-10-20
Incomplete
OPSDIR Last Call Review due 2023-10-20
Incomplete
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Daniel Migault | ||
| Shepherd write-up | Show Last changed 2022-02-02 | ||
| IESG | IESG state | IESG Evaluation | |
| Action Holder |
Paul Wouters
38
|
||
| Consensus boilerplate | Yes | ||
| Telechat date |
On agenda of 2023-11-30 IESG telechat
Has enough positions to pass. |
||
| Responsible AD | Paul Wouters | ||
| Send notices to | mglt.ietf@gmail.com | ||
| IANA | IANA review state | IANA OK - Actions Needed | |
| IANA expert review state | Expert Reviews OK | ||
| IANA expert review comments | The OAuth Parameters CBOR Mappings, the CoAP Content-Formats, and the OAuth Parameters registrations have been approved. Some comments for the Interface Description (if=) Link Target Attribute Values registration: The registration of this if= type is fundamentally appropriate. However, there are some aspects of its description that could use improvement. * The description uses the term ‘ace group’ to stand for ace.group. That could be confusing, but probably will not lead to interoperability problems. * The description could be pointing to Section 4.1 (of course, a search will also find that section). * Section 4.1 defines a number of resources. Which of these is the one that the link marked if= should point to? One might think that it is the one named /ace-group as an example, and not the individual ones named /ace-group/GROUPNAME. However, the examples in draft-tiloca-core-oscore-discovery indicate otherwise. It would be preferable to make the answer to this question explicit in ace-key-groupcomm (an example for such a link in ace-key-groupcomm would also help). Deferring much of the actual definition of the if= type to draft-tiloca-core-oscore-discovery while already registering it here does not work very well for this specification reader. |
draft-ietf-ace-key-groupcomm-17
ACE Working Group F. Palombini
Internet-Draft Ericsson AB
Intended status: Standards Track M. Tiloca
Expires: 8 April 2024 RISE AB
6 October 2023
Key Provisioning for Group Communication using ACE
draft-ietf-ace-key-groupcomm-17
Abstract
This document defines how to use the Authentication and Authorization
for Constrained Environments (ACE) framework to distribute keying
material and configuration parameters for secure group communication.
Candidate group members acting as Clients and authorized to join a
group can do so by interacting with a Key Distribution Center (KDC)
acting as Resource Server, from which they obtain the keying material
to communicate with other group members. While defining general
message formats as well as the interface and operations available at
the KDC, this document supports different approaches and protocols
for secure group communication. Therefore, details are delegated to
separate application profiles of this document, as specialized
instances that target a particular group communication approach and
define how communications in the group are protected. Compliance
requirements for such application profiles are also specified.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/ace-wg/ace-key-groupcomm.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 8 April 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
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/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Authorization to Join a Group . . . . . . . . . . . . . . . . 11
3.1. Authorization Request . . . . . . . . . . . . . . . . . . 12
3.2. Authorization Response . . . . . . . . . . . . . . . . . 14
3.3. Token Transferring . . . . . . . . . . . . . . . . . . . 16
3.3.1. 'sign_info' Parameter . . . . . . . . . . . . . . . . 18
3.3.2. 'kdcchallenge' Parameter . . . . . . . . . . . . . . 20
4. KDC Functionalities . . . . . . . . . . . . . . . . . . . . . 20
4.1. Interface at the KDC . . . . . . . . . . . . . . . . . . 21
4.1.1. Operations Supported by Clients . . . . . . . . . . . 24
4.1.2. Error Handling . . . . . . . . . . . . . . . . . . . 25
4.2. /ace-group . . . . . . . . . . . . . . . . . . . . . . . 27
4.2.1. FETCH Handler . . . . . . . . . . . . . . . . . . . . 27
4.2.1.1. Retrieve Group Names . . . . . . . . . . . . . . 28
4.3. /ace-group/GROUPNAME . . . . . . . . . . . . . . . . . . 29
4.3.1. POST Handler . . . . . . . . . . . . . . . . . . . . 29
4.3.1.1. Join the Group . . . . . . . . . . . . . . . . . 43
4.3.2. GET Handler . . . . . . . . . . . . . . . . . . . . . 45
4.3.2.1. Retrieve Group Keying Material . . . . . . . . . 46
4.4. /ace-group/GROUPNAME/creds . . . . . . . . . . . . . . . 47
4.4.1. FETCH Handler . . . . . . . . . . . . . . . . . . . . 47
4.4.1.1. Retrieve a Subset of Authentication Credentials in
the Group . . . . . . . . . . . . . . . . . . . . . 50
4.4.2. GET Handler . . . . . . . . . . . . . . . . . . . . . 51
4.4.2.1. Retrieve All Authentication Credentials in the
Group . . . . . . . . . . . . . . . . . . . . . . . 51
4.5. ace-group/GROUPNAME/kdc-cred . . . . . . . . . . . . . . 52
4.5.1. GET Handler . . . . . . . . . . . . . . . . . . . . . 52
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4.5.1.1. Retrieve the KDC's Authentication Credential . . 53
4.6. /ace-group/GROUPNAME/policies . . . . . . . . . . . . . . 55
4.6.1. GET Handler . . . . . . . . . . . . . . . . . . . . . 55
4.6.1.1. Retrieve the Group Policies . . . . . . . . . . . 55
4.7. /ace-group/GROUPNAME/num . . . . . . . . . . . . . . . . 56
4.7.1. GET Handler . . . . . . . . . . . . . . . . . . . . . 56
4.7.1.1. Retrieve the Keying Material Version . . . . . . 57
4.8. /ace-group/GROUPNAME/nodes/NODENAME . . . . . . . . . . . 57
4.8.1. GET Handler . . . . . . . . . . . . . . . . . . . . . 58
4.8.1.1. Retrieve Group and Individual Keying Material . . 59
4.8.2. PUT Handler . . . . . . . . . . . . . . . . . . . . . 61
4.8.2.1. Request to Change Individual Keying Material . . 62
4.8.3. DELETE Handler . . . . . . . . . . . . . . . . . . . 64
4.8.3.1. Leave the Group . . . . . . . . . . . . . . . . . 64
4.9. /ace-group/GROUPNAME/nodes/NODENAME/cred . . . . . . . . 64
4.9.1. POST Handler . . . . . . . . . . . . . . . . . . . . 64
4.9.1.1. Uploading a Authentication Credential Key . . . . 67
5. Removal of a Group Member . . . . . . . . . . . . . . . . . . 68
6. Group Rekeying Process . . . . . . . . . . . . . . . . . . . 69
6.1. Point-to-Point Group Rekeying . . . . . . . . . . . . . . 71
6.2. One-to-Many Group Rekeying . . . . . . . . . . . . . . . 72
6.2.1. Protection of Rekeying Messages . . . . . . . . . . . 74
6.3. Misalignment of Group Keying Material . . . . . . . . . . 77
7. Extended Scope Format . . . . . . . . . . . . . . . . . . . . 77
8. ACE Groupcomm Parameters . . . . . . . . . . . . . . . . . . 79
9. ACE Groupcomm Error Identifiers . . . . . . . . . . . . . . . 83
10. Security Considerations . . . . . . . . . . . . . . . . . . . 85
10.1. Secure Communication in the Group . . . . . . . . . . . 85
10.2. Update of Group Keying Material . . . . . . . . . . . . 86
10.3. Block-Wise Considerations . . . . . . . . . . . . . . . 88
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 88
11.1. Media Type Registrations . . . . . . . . . . . . . . . . 89
11.2. CoAP Content-Formats . . . . . . . . . . . . . . . . . . 89
11.3. OAuth Parameters . . . . . . . . . . . . . . . . . . . . 90
11.4. OAuth Parameters CBOR Mappings . . . . . . . . . . . . . 90
11.5. Interface Description (if=) Link Target Attribute
Values . . . . . . . . . . . . . . . . . . . . . . . . 91
11.6. ACE Groupcomm Parameters . . . . . . . . . . . . . . . . 91
11.7. ACE Groupcomm Key Types . . . . . . . . . . . . . . . . 92
11.8. ACE Groupcomm Profiles . . . . . . . . . . . . . . . . . 93
11.9. ACE Groupcomm Policies . . . . . . . . . . . . . . . . . 94
11.10. Sequence Number Synchronization Methods . . . . . . . . 95
11.11. ACE Groupcomm Errors . . . . . . . . . . . . . . . . . . 95
11.12. ACE Groupcomm Rekeying Schemes . . . . . . . . . . . . . 96
11.13. Expert Review Instructions . . . . . . . . . . . . . . . 97
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 98
12.1. Normative References . . . . . . . . . . . . . . . . . . 98
12.2. Informative References . . . . . . . . . . . . . . . . . 100
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Appendix A. Requirements on Application Profiles . . . . . . . . 103
A.1. Mandatory-to-Address Requirements . . . . . . . . . . . . 103
A.2. Optional-to-Address Requirements . . . . . . . . . . . . 105
Appendix B. Extensibility for Future COSE Algorithms . . . . . . 106
B.1. Format of 'sign_info_entry' . . . . . . . . . . . . . . . 107
Appendix C. Document Updates . . . . . . . . . . . . . . . . . . 108
C.1. Version -16 to -17 . . . . . . . . . . . . . . . . . . . 108
C.2. Version -15 to -16 . . . . . . . . . . . . . . . . . . . 109
C.3. Version -14 to -15 . . . . . . . . . . . . . . . . . . . 109
C.4. Version -13 to -14 . . . . . . . . . . . . . . . . . . . 109
C.5. Version -05 to -13 . . . . . . . . . . . . . . . . . . . 110
C.6. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 110
C.7. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 111
C.8. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 111
C.9. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 112
C.10. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 113
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 113
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 114
1. Introduction
This document builds on the Authentication and Authorization for
Constrained Environments (ACE) framework and defines how to request,
distribute, and renew keying material and configuration parameters to
protect message exchanges in a group communication environment.
Candidate group members acting as Clients and authorized to join a
group can interact with the Key Distribution Center (KDC) acting as
Resource Server and responsible for that group, in order to obtain
the necessary keying material and parameters to communicate with
other group members.
In particular, this document defines the operations and interface
available at the KDC, as well as general message formats for the
interactions between Clients and KDC. At the same time,
communications in the group can rely on different approaches, e.g.,
based on multicast [I-D.ietf-core-groupcomm-bis] or on publish-
subscribe messaging [I-D.ietf-core-coap-pubsub], and can be protected
in different ways.
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Therefore, this document delegates details on the communication and
security approaches used in a group to separate application profiles.
These are specialized instances of this document, targeting a
particular group communication approach and defining how
communications in the group are protected, as well as the specific
keying material and configuration parameters provided to group
members. In order to ensure consistency and aid the development of
such application profiles, this document defines a number of related
compliance requirements (see Appendix A).
New keying material is generated and distributed to the group upon
membership changes (rekeying), if the application requires backward
security (i.e., new group members must be prevented from accessing
communications in the group prior to their joining) and forward
security (i.e., former group members must be prevented from accessing
communications in the group after their leaving).
A group rekeying scheme performs the actual distribution of the new
keying material, by rekeying the current group members when a new
Client joins the group, and the remaining group members when a Client
leaves the group. This can rely on different approaches, including
efficient group rekeying schemes such as [RFC2093], [RFC2094], and
[RFC2627].
Consistently with what is recommeded in the ACE framework, this
document uses CBOR [RFC8949] for data encoding. However, using JSON
[RFC8259] instead of CBOR is possible, by relying on the conversion
method specified in Sections 6.1 and 6.2 of [RFC8949].
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 the ACE framework [RFC9200]
and in the Authorization Information Format (AIF) [RFC9237] to
express authorization information. The terminology for entities
in the considered architecture is defined in OAuth 2.0 [RFC6749].
In particular, this includes Client (C), Resource Server (RS), and
Authorization Server (AS).
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* The terms and concepts described in CoAP [RFC7252]. Unless
otherwise indicated, the term "endpoint" is used here following
its OAuth definition, aimed at denoting resources such as /token
and /introspect at the AS, and /authz-info at the RS. This
document does not use the CoAP definition of "endpoint", which is
"An entity participating in the CoAP protocol".
* The terms and concepts described in CBOR [RFC8949] and COSE
[RFC9052][RFC9053][RFC9338].
A principal interested to participate in group communication as well
as already participating as a group member is interchangeably denoted
as "Client" or "node".
* Group: a set of nodes that share common keying material and
security parameters used to protect their communications with one
another. That is, the term refers to a "security group".
This is not to be confused with an "application group", which has
relevance at the application level and whose members share a
common pool of resources or content. Examples of application
groups are the set of all nodes deployed in a same physical room,
or the set of nodes registered to a pub-sub topic.
The same security group might be associated with multiple
application groups. Also, the same application group can be
associated with multiple security groups. Further details and
considerations on the mapping between the two types of group are
out of the scope of this document.
* Key Distribution Center (KDC): the entity responsible for managing
one or multiple groups, with particular reference to the group
membership and the keying material to use for protecting group
communications.
Furthermore, this document uses "names" or "identifiers" for groups
and nodes. Their different meanings are summarized below.
* Group name: the invariant once established identifier of a group.
It is used in the interactions between Client, AS, and RS to
identify a group. A group name is always unique among the group
names of the existing groups under the same KDC.
* GROUPNAME: the invariant once established text string used in
URIs. GROUPNAME uniquely maps to the group name of a group,
although they do not necessarily coincide.
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* Group identifier: the identifier of the group keying material used
in a group. Unlike group name and GROUPNAME, this identifier
changes over time, when the group keying material is updated.
* Node name: the invariant once established identifier of a node.
It is used in the interactions between Client and RS, as well as
to identify a member of a group. Within the same group, a node
name is always unique among the node names of all the current
members of that group.
* NODENAME: the invariant once established text string used in URIs
to identify a member a group. Its value coincides with the node
name of the associated group member.
This document additionally uses the following terminology:
* Transport profile, to indicate a profile of ACE as per
Section 5.8.4.3 of [RFC9200]. A transport profile specifies the
communication protocol and communication security protocol between
an ACE Client and Resource Server, as well as proof-of-possession
methods, if it supports proof-of-possession access tokens, etc.
Transport profiles of ACE include, for instance, [RFC9203],
[RFC9202], and [RFC9431].
* Application profile, that defines how applications enforce and use
supporting security services they require. These services may
include, for instance, provisioning, revocation, and distribution
of keying material. An application profile may define specific
procedures and message formats.
* Authentication credential, as the set of 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 (CCSs)
[RFC8392], X.509 certificates [RFC5280], and C509 certificates
[I-D.ietf-cose-cbor-encoded-cert].
* Individual keying material: information exclusively pertaining to
a group member, as associated with its group membership and
related to other keying material and parameters used in the group.
For example, this can be a member identifier that is unique within
the group. The specific nature and format of individual keying
material used in a group is defined in application profiles of
this specification. The individual keying material of a group
member is not related to the secure association between that group
member and the KDC.
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Examples throughout this document are expressed in CBOR diagnostic
notation without the tag and value abbreviations.
2. Overview
At a high level, the key provisioning process is separated in two
phases: the first one follows the ACE Framework between Client, AS,
and KDC; the second one is the actual key distribution between Client
and KDC. After the two phases are completed, the Client is able to
participate in the group communication, via a Dispatcher entity.
+------------+ +-----------+
| AS | | KDC |
| | .----->| |
+------------+ / +-----------+
^ /
| /
v / +-----------+
+------------+ / +------------+ |+-----------+
| Client |<-' | Dispatcher | ||+-----------+
| |<------------->| |<------->||| Group |
+------------+ +------------+ ||| members |
+++-----------+
Figure 1: Key Distribution Participants
The following participants (see Figure 1) take part in the
authorization and key distribution.
* Client (C): node that wants to join a group and take part in group
communication with other group memebrs. Within the group, the
Client can have different roles.
* Authorization Server (AS): as per the AS defined in the ACE
Framework, it enforces access policies, and knows if a node is
allowed to join a given group with write and/or read rights.
* Key Distribution Center (KDC): maintains the keying material to
protect group communications, and provides it to Clients
authorized to join a given group. During the first part of the
exchange (Section 3), it takes the role of the RS in the ACE
Framework. During the second part (Section 4), which is not based
on the ACE Framework, it distributes the keying material. In
addition, it provides the latest keying material to group members
when requested or, if required by the application, when membership
changes.
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* Dispatcher: entity through which the Clients communicate with the
group, when sending a message intended to multiple group members.
That is, the Dispatcher distributes such a one-to-many message to
the group members as intended recipients. A single-recipient
message intended to only one group member may be delivered by
alternative means, with no assistance from the Dispatcher.
Examples of a Dispatcher are: the Broker in a pub-sub setting; a
relayer for group communication that delivers group messages as
multiple unicast messages to all group members; an implicit entity
as in a multicast communication setting, where messages are
transmitted to a multicast IP address and delivered on the
transport channel.
If it consists of an explicit entity such as a pub-sub Broker or a
message relayer, the Dispatcher is comparable to an untrusted on-
path intermediary, and as such it is able to read the messages
sent by Clients in the group.
This document specifies a mechanism for:
* Authorizing a Client to join the group (Section 3), and providing
it with the group keying material to communicate with the other
group members (Section 4).
* Allowing a group member to retrieve group keying material
(Section 4.3.2.1 and Section 4.8.1.1).
* Allowing a group member to retrieve authentication credentials of
other group members (Section 4.4.1.1) and to provide an updated
authentication credential (Section 4.9.1.1).
* Allowing a group member to leave the group (Section 4.8.3.1).
* Evicting a group member from the group (Section 5).
* Renewing and re-distributing the group keying material (rekeying),
e.g., upon a membership change in the group (Section 6).
Rekeying the group may result in a temporary misalignment of the
keying material stored by the different group members. Different
situations where this can happen and how they can be handled are
discussed in Section 6.3.
Figure 2 provides a high level overview of the message flow for a
node joining a group. The message flow can be expanded as follows.
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1. The joining node requests an access token from the AS, in order
to access one or more group-membership resources at the KDC and
hence join the associated groups.
This exchange between Client and AS MUST be secured, as specified
by the transport profile of ACE used between Client and KDC.
Based on the response from the AS, the joining node will
establish or continue using a secure communication association
with the KDC.
2. The joining node transfers authentication and authorization
information to the KDC, by transferring the obtained access
token. This is typically achieved by including the access token
in a request sent to the /authz-info endpoint at the KDC.
Once this exchange is completed, the joining node MUST have a
secure communication association established with the KDC, before
joining a group under that KDC.
This exchange and the following secure communications between the
Client and the KDC MUST occur in accordance with the transport
profile of ACE used between Client and KDC, such as the DTLS
transport profile [RFC9202] and OSCORE transport profile
[RFC9203] of ACE.
3. The joining node starts the joining process to become a member of
the group, by sending a request to the related group-membership
resource at the KDC. Based on the application requirements and
policies, the KDC may perform a group rekeying, by generating new
group keying material and distributing it to the current group
members through the rekeying scheme used in the group.
At the end of the joining process, the joining node has received
from the KDC the parameters and group keying material to securely
communicate with the other group members. Also, the KDC has
stored the association between the authorization information from
the access token and the secure session with the joining node.
4. The joining node and the KDC maintain the secure association, to
support possible future communications. These especially include
key management operations, such as retrieval of updated keying
material or participation to a group rekeying process.
5. The joining node can communicate securely with the other group
members, using the keying material provided in step 3.
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C AS KDC Group
| | | Members
/ | | | |
| |--- Authorization Request -->| | |
| | | | |
| |<-- Authorization Response --| | |
(*) < | | | |
| | | | |
| |--- Token Transfer Request ---->| |
| | | |
| |<--- Token Transfer Response-----| |
\ | | | |
| | | |
|--------- Join Request --------->| |
| | | |
| | | -- Group rekeying -->|
| | | (optional) |
|<-------- Join Response ---------| |
| | | |
| | | |
| | | Dispatcher |
| | |
|<======= Secure group communication =========|=========>|
| | |
(*) Defined in the ACE framework
Figure 2: Message Flow Upon New Node's Joining
3. Authorization to Join a Group
This section describes in detail the format of messages exchanged by
the participants when a node requests access to a given group. This
exchange is based on ACE [RFC9200].
As defined in [RFC9200], the Client requests the AS for the
authorization to join the group through the KDC (see Section 3.1).
If the request is approved and authorization is granted, the AS
provides the Client with a proof-of-possession access token and
parameters to securely communicate with the KDC (see Section 3.2).
Communications between the Client and the AS MUST be secured,
according to what is defined by the used transport profile of ACE.
The Content-Format used in the message also depends on the used
transport profile of ACE. For example, it can be application/
ace+cbor for the first two messages and application/cwt for the third
message, which are defined in the ACE framework.
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The transport profile of ACE also defines a number of details such as
the communication and security protocols used with the KDC (see
Appendix C of [RFC9200]).
Figure 3 gives an overview of the exchange described above.
Client AS KDC
| | |
|---- Authorization Request: POST /token ------->| |
| | |
|<--- Authorization Response: 2.01 (Created) ----| |
| | |
|---- Token Transfer Request: POST /authz-info ------->|
| | |
|<--- Token Transfer Response: 2.01 (Created) -------->|
| | |
Figure 3: Message Flow of Join Authorization
3.1. Authorization Request
The Authorization Request sent from the Client to the AS is defined
in Section 5.8.1 of [RFC9200] and MAY contain the following
parameters, which, if included, MUST have format and value as
specified below.
* 'scope', specifying the name of the groups that the Client
requests to access, and optionally the roles that the Client
requests to have in those groups.
This parameter is encoded as a CBOR byte string, which wraps a
CBOR array of one or more scope entries. All the scope entries
are specified according to a same format, i.e., either the AIF
format or the textual format defined below.
- If the AIF format is used, each scope entry is encoded as per
[RFC9237]. If a scope entry expresses a set of roles to take
in a group as per this document, the object identifier "Toid"
specifies the group name and MUST be encoded as a CBOR text
string, while the permission set "Tperm" specifies the roles
that the Client wishes to take in the group.
The AIF format is the default format for application profiles
of this specification, and is preferable for those that aim for
a compact encoding of scope. This is desirable especially for
application profiles defining several roles, with the Client
possibly requesting for multiple roles combined.
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Figure 4 shows an example in CDDL notation [RFC8610] where
scope uses the AIF format.
- If the textual format is used, each scope entry is a CBOR array
formatted as follows.
o As first element, the group name, encoded as a CBOR text
string.
o Optionally, as second element, the role or CBOR array of
roles that the Client wishes to take in the group. This
element is optional since roles may have been pre-assigned
to the Client, as associated with its verifiable identity
credentials. Alternatively, the application may have
defined a single, well-known role for the target resource(s)
and audience(s).
Figure 5 shows an example in CDDL notation where scope uses the
textual format, with group name and role identifiers encoded as
CBOR text strings.
It is REQUIRED of application profiles of this specificaton to
specify the exact format and encoding of scope (REQ1). This
includes defining the set of possible roles and their identifiers,
as well as the corresponding encoding to use in the scope entries
according to the used scope format.
If the application profile uses the AIF format, it is also
REQUIRED to register its specific instance of "Toid" and "Tperm",
as well as the corresponding Media Type and Content-Format, as per
the guidelines in [RFC9237] (REQ2).
If the application profile uses the textual format, it MAY
additionally specify CBOR values to use for abbreviating the role
identifiers (OPT1).
* 'audience', with an identifier of the KDC.
As defined in [RFC9200], other additional parameters can be included
if necessary.
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gname = tstr
permissions = uint .bits roles
roles = &(
Requester: 1,
Responder: 2,
Monitor: 3,
Verifier: 4
)
scope_entry = AIF_Generic<gname, permissions>
scope = << [ + scope_entry ] >>
Figure 4: Example of scope using the AIF format
gname = tstr
role = tstr
scope_entry = [ gname , ? ( role / [ 2*role ] ) ]
scope = << [ + scope_entry ] >>
Figure 5: Example of scope using the textual format, with the
group name and role identifiers encoded as text strings
3.2. Authorization Response
The AS processes the Authorization Request as defined in
Section 5.8.2 of [RFC9200], especially verifying that the Client is
authorized to access the specified groups with the requested roles,
or possibly a subset of those.
In case of successful verification, the Authorization Response sent
from the AS to the Client is also defined in Section 5.8.2 of
[RFC9200]. Note that the parameter 'expires_in' MAY be omitted if
the application defines how the expiration time is communicated to
the Client via other means, or if it establishes a default value.
Additionally, when included, the following parameter MUST have the
corresponding values:
* 'scope' has the same format and encoding of 'scope' in the
Authorization Request, defined in Section 3.1. If this parameter
is not present, the granted scope is equal to the one requested in
Section 3.1.
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The proof-of-possession access token (in 'access_token' above) MUST
contain the following parameters:
* a confirmation claim (see for example 'cnf' defined in Section 3.1
of [RFC8747] for CWT);
* an expiration time claim (see for example 'exp' defined in
Section 3.1.4 of [RFC8392] for CWT);
* a scope claim (see for example 'scope' registered in Section 8.14
of [RFC9200] for CWT).
This claim specifies the same access control information as in the
'scope' parameter of the Authorization Response, if the parameter
is present in the message, or as in the 'scope' parameter of the
Authorization Request otherwise.
By default, this claim has the same encoding as the 'scope'
parameter in the Authorization Request, defined in Section 3.1.
Optionally, an alternative extended format of scope defined in
Section 7 can be used. This format explicitly signals the
semantics used to express the actual access control information,
and according to which this has to be parsed. This enables a
Resource Server to correctly process a received access token, also
in case:
- The Resource Server implements a KDC that supports multiple
application profiles of this specification, using different
scope semantics; and/or
- The Resource Server implements further services beyond a KDC
for group communication, using different scope semantics.
If the Authorization Server is aware that this applies to the
Resource Server for which the access token is issued, the
Authorization Server SHOULD use the extended format of scope
defined in Section 7.
The access token MAY additionally contain other claims that the
transport profile of ACE requires, or other optional parameters.
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When receiving an Authorization Request from a Client that was
previously authorized, and for which the AS still stores a valid non-
expired access token, the AS MAY reply with that token. Note that it
is up to application profiles of ACE to make sure that re-posting the
same token does not cause re-use of keying material between nodes
(for example, that is done with the use of random nonces in
[RFC9203]).
3.3. Token Transferring
The Client sends a Token Transfer Request to the KDC, i.e., a CoAP
POST request including the access token and targeting the authz-info
endpoint (see Section 5.10.1 of [RFC9200]).
Note that this request deviates from the one defined in [RFC9200],
since it allows to ask the KDC for additional information concerning
the authentication credentials used in the group to ensure source
authentication, as well as for possible additional group parameters.
The joining node MAY ask for this information from the KDC through
the same Token Transfer Request. In this case, the message MUST have
Content-Format set to application/ace+cbor defined in Section 8.16 of
[RFC9200], and the message payload MUST be formatted as a CBOR map,
which MUST include the access token. The CBOR map MAY additionally
include the following parameter, which, if included, MUST have format
and value as specified below.
* 'sign_info' defined in Section 3.3.1, specifying the CBOR simple
value "null" (0xf6) to request information about the signature
algorithm, the signature algorithm parameters, the signature key
parameters, and the exact format of authentication credentials
used in the groups that the Client has been authorized to join.
Alternatively, such information may be pre-configured on the joining
node, or may be retrieved by alternative means. For example, the
joining node may have performed an early group discovery process and
obtained the link to the associated group-membership resource at the
KDC, together with attributes descriptive of the group configuration
(see, e.g., [I-D.tiloca-core-oscore-discovery]).
After successful verification, the Client is authorized to receive
the group keying material from the KDC and join the group. Hence,
the KDC replies to the Client with a Token Transfer Response, i.e., a
CoAP 2.01 (Created) response.
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The Token Transfer Response MUST have Content-Format "application/
ace+cbor", and its payload is a CBOR map. Note that this deviates
from what is defined in the ACE framework, where the response from
the authz-info endpoint is defined as conveying no payload (see
Section 5.10.1 of [RFC9200]).
If the access token contains a role that requires the Client to send
its own authentication credential to the KDC when joining the group,
the CBOR map MUST include the parameter 'kdcchallenge' defined in
Section 3.3.2, specifying a dedicated challenge N_S generated by the
KDC.
Later, when joining the group (see Section 4.3.1.1), the Client uses
the 'kdcchallenge' value and additional information to build a proof-
of-possession (PoP) input. This is in turn used to compute a PoP
evidence, which the Client also provides to the KDC in order to prove
possession of its own private key (see the 'client_cred_verify'
parameter in Section 4.3.1).
The KDC MUST store the 'kdcchallenge' value associated with the
Client at least until it receives a Join Request from it (see
Section 4.3.1.1), to be able to verify the PoP evidence provided
during the join process, and thus that the Client possesses its own
private key.
The same 'kdcchallenge' value MAY be reused several times by the
Client, to generate a new PoP evidence, e.g., in case the Client
provides the KDC with a new authentication credential while being a
group member (see Section 4.9.1.1), or joins a different group where
it intends to use a different authentication credential. Therefore,
it is RECOMMENDED that the KDC keeps storing the 'kdcchallenge' value
after the first join is processed as well. If the KDC has already
discarded the 'kdcchallenge' value, that will trigger an error
response with a newly generated 'kdcchallenge' value that the Client
can use to restart the join process, as specified in Section 4.3.1.1.
If 'sign_info' is included in the Token Transfer Request, the KDC
SHOULD include the 'sign_info' parameter in the Token Transfer
Response, as per the format defined in Section 3.3.1. Note that the
field 'id' of each 'sign_info_entry' specifies the name, or array of
group names, for which that 'sign_info_entry' applies to. As an
exception, the KDC MAY omit the 'sign_info' parameter in the Token
Transfer Response even if 'sign_info' is included in the Token
Transfer Request, in case none of the groups that the Client is
authorized to join uses signatures to achieve source authentication.
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Note that the CBOR map specified as payload of the 2.01 (Created)
response may include further parameters, e.g., according to the used
transport profile of ACE. Application profiles of this specification
MAY define additional parameters to use within this exchange (OPT2).
Application profiles of this specification MAY define alternative
specific negotiations of parameter values for the signature algorithm
and signature keys, if 'sign_info' is not used (OPT3).
If allowed by the used transport profile of ACE, the Client may
provide the Access Token to the KDC by other means than the Token
Transfer Request. An example is the DTLS transport profile of ACE,
where the Client can provide the access token to the KDC during the
secure session establishment (see Section 3.3.2 of [RFC9202]).
3.3.1. 'sign_info' Parameter
The 'sign_info' parameter is an OPTIONAL parameter of the request and
response messages exchanged between the Client and the authz-info
endpoint at the RS (see Section 5.10.1. of [RFC9200]).
This parameter allows the Client and the RS to exchange information
about a signature algorithm and about authentication credentials to
accordingly use for signature verification. Its exact semantics and
content are application specific.
In this specification and in application profiles building on it,
this parameter is used to exchange information about the signature
algorithm and about authentication credentials to be used with it, in
the groups indicated by the transferred acces token as per its
'scope' claim (see Section 3.2).
When used in the Token Transfer Request sent to the KDC (see
Section 3.3), the 'sign_info' parameter specifies the CBOR simple
value "null" (0xf6). This is done to ask for information about the
signature algorithm and about the authentication credentials used in
the groups that the Client has been authorized to join - or to have a
more restricted interaction as per its granted roles (e.g., the
Client is an external signature verifier).
When used in the following Token Transfer Response from the KDC (see
Section 3.3), the 'sign_info' parameter is a CBOR array of one or
more elements. The number of elements is at most the number of
groups that the Client has been authorized to join - or to have a
more restricted interaction (see above). Each element contains
information about signing parameters and about authentication
credentials for one or more groups, and is formatted as follows.
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* The first element 'id' is a group name or an array of group names,
associated with groups for which the next four elements apply. In
the following, each specified group name is referred to as
'gname'.
* The second element 'sign_alg' is an integer or a text string if
the POST request included the 'sign_info' parameter with value the
CBOR simple value "null" (0xf6), and indicates the signature
algorithm used in the groups identified by the 'gname' values. It
is REQUIRED of the application profiles to define specific values
that this parameter can take (REQ3), selected from the set of
signing algorithms of the COSE Algorithms registry
[COSE.Algorithms].
* The third element 'sign_parameters' is a CBOR array indicating the
parameters of the signature algorithm used in the groups
identified by the 'gname' values. Its content depends on the
value of 'sign_alg'. It is REQUIRED of the application profiles
to define the possible values and structure for the elements of
this parameter (REQ4).
* The fourth element 'sign_key_parameters' is a CBOR array
indicating the parameters of the key used with the signature
algorithm, in the groups identified by the 'gname' values. Its
content depends on the value of 'sign_alg'. It is REQUIRED of the
application profiles to define the possible values and structure
for the elements of this parameter (REQ5).
* The fifth element 'cred_fmt' is either a CBOR integer indicating
the format of authentication credentials used in the groups
identified by the 'gname' values, or has value the CBOR simple
value "null" (0xf6) indicating that the KDC does not act as
repository of authentication credentials for group members. Its
acceptable integer values are taken from the 'Label' column of the
"COSE Header Parameters" registry [COSE.Header.Parameters], with
some of those values also indicating the type of container to use
for exchanging the authentication credentials with the KDC (e.g.,
a chain or bag of certificates). It is REQUIRED of the
application profiles to define specific values to use for this
parameter, consistently with the acceptable formats of
authentication credentials (REQ6).
The CDDL notation [RFC8610] of the 'sign_info' parameter is given
below.
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sign_info = sign_info_req / sign_info_resp
sign_info_req = null ; in the Token Transfer
; Request to the KDC
sign_info_resp = [ + sign_info_entry ] ; in the Token Transfer
; Response from the KDC
sign_info_entry =
[
id : gname / [ + gname ],
sign_alg : int / tstr,
sign_parameters : [ any ],
sign_key_parameters : [ + parameter : any ],
cred_fmt : int / null
]
gname = tstr
This format is consistent with every signature algorithm currently
defined in [RFC9053], i.e., with algorithms that have only the COSE
key type as their COSE capability. Appendix B describes how the
format of each 'sign_info_entry' can be generalized for possible
future registered algorithms having a different set of COSE
capabilities.
3.3.2. 'kdcchallenge' Parameter
The 'kdcchallenge' parameter is an OPTIONAL parameter of response
message returned from the authz-info endpoint at the RS, as defined
in Section 5.10.1 of [RFC9200]. This parameter contains a challenge
generated by the RS and provided to the Client.
In this specification and in application profiles building on it, the
Client may use this challenge to prove possession of its own private
key in the Join Request (see the 'client_cred_verify' parameter in
Section 4.3.1).
4. KDC Functionalities
This section describes the functionalities provided by the KDC, as
related to the provisioning of the keying material as well as to the
group membership management.
In particular, this section defines the interface available at the
KDC; specifies the handlers of each resource provided by the KDC
interface; and describes how Clients interact with those resources to
join a group and to perform additional operations as group members.
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As most important operation after trasferring the access token to the
KDC, the Client can perform a Join Request-Response exchange with the
KDC, by specifying the group it requests to join (see
Section 4.3.1.1). Then, the KDC verifies the access token and that
the Client is authorized to join the specified group. If so, the KDC
provides the Client with the keying material to securely communicate
with the other members of the group.
Later on as a group member, the Client can also rely on the interface
at the KDC to perform additional operations, consistent with the
roles it has in the group.
4.1. Interface at the KDC
The KDC provides its interface by hosting the following resources.
Note that the root url-path "ace-group" used hereafter is a default
name; implementations are not required to use this name, and can
define their own instead. The Interface Description (if=) Link
Target Attribute value "ace.group" is registered in Section 11.5 and
can be used to describe this interface.
If request messages sent to the KDC as well as success response
messages from the KDC include a payload and specify a Content-Format,
those messages MUST have Content-Format set to application/ace-
groupcomm+cbor, defined in Section 11.2. CBOR labels for the message
parameters are defined in Section 8.
* /ace-group : the path of this resource is invariant once the
resource is established, and indicates that this specification is
used. If other applications run on a KDC implementing this
specification and use this same path, those applications will
collide, and a mechanism will be needed to differentiate the
endpoints.
A Client can access this resource in order to retrieve a set of
group names, each corresponding to one of the specified group
identifiers. This operation is described in Section 4.2.1.1.
* /ace-group/GROUPNAME : one such sub-resource to /ace-group is
hosted for each group with name GROUPNAME that the KDC manages,
and contains the symmetric group keying material for that group.
A Client can access this resource in order to join the group with
name GROUPNAME, or later as a group member to retrieve the current
group keying material. These operations are described in
Section 4.3.1.1 and Section 4.3.2.1, respectively.
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If the value of the GROUPNAME URI path and the group name in the
access token scope ('gname' in Section 3.2) are not required to
coincide, the KDC MUST implement a mechanism to map the GROUPNAME
value in the URI to the group name, in order to refer to the
correct group (REQ7).
* /ace-group/GROUPNAME/creds : the path of this resource is
invariant once the resource is established. This resource
contains the authentication credentials of all the members of the
group with name GROUPNAME.
This resource is created only in case the KDC acts as repository
of authentication credentials for group members.
A Client can access this resource in order to retrieve the
authentication credentials of other group members, in addition to
when joining the group. That is, the Client can retrieve the
authentication credentials of all the current group members, or a
subset of them by specifying filter criteria. These operations
are described in Section 4.4.2.1 and Section 4.4.1.1,
respectively.
Clients may be authorized to access this resource even without
being group members, e.g., if authorized to be external signature
verifiers for the group.
* ace-group/GROUPNAME/kdc-cred : the path of this resource is
invariant once the resource is established. This resource
contains the authentication credential of the KDC for the group
with name GROUPNAME.
This resource is created only in case the KDC has an associated
authentication credential and this is required for the correct
group operation. It is REQUIRED of application profiles to define
whether the KDC has such an associated authentication credential
(REQ8).
A Client can interact with this resource in order to retrieve the
current authentication credential of the KDC, in addition to when
joining the group.
Clients may be authorized to access this resource even without
being group members, e.g., if authorized to be external signature
verifiers for the group.
* /ace-group/GROUPNAME/policies : the path of this resource is
invariant once the resource is established. This resource
contains the group policies of the group with name GROUPNAME.
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A Client can access this resource as a group member in order to
retrieve the group policies. This operation is described in
Section 4.6.1.1.
* /ace-group/GROUPNAME/num : the path of this resource is invariant
once the resource is established. This resource contains the
current version number for the symmetric group keying material of
the group with name GROUPNAME.
A Client can access this resource as a group member in order to
retrieve the version number of the keying material currently used
in the group. This operation is described in Section 4.7.1.1.
* /ace-group/GROUPNAME/nodes/NODENAME : one such sub-resource of
/ace-group/GROUPNAME is hosted for each group member of the group
with name GROUPNAME. Each of such resources is identified by the
node name NODENAME of the associated group member, and contains
the group keying material and the individual keying material for
that group member.
A Client as a group member can access this resource in order to
retrieve the current group keying material together with its the
individual keying material; request new individual keying material
to use in the group; and leave the group. These operations are
described in Section 4.8.1.1, Section 4.8.2.1, and
Section 4.8.3.1, respectively.
* /ace-group/GROUPNAME/nodes/NODENAME/cred : the path of this
resource is invariant once the resource is established. This
resource contains the individual authentication credential for the
node with name NODENAME, as group member of the group with name
GROUPNAME.
A Client can access this resource in order to upload at the KDC a
new authentication credential to use in the group. This operation
is described in Section 4.9.1.1.
This resource is not created if the group member does not have an
authentication credential to use in the group, or if the KDC does
not store the authentication credentials of group members.
The KDC is expected to fully provide the interface defined above. It
is otherwise REQUIRED of the application profiles of this
specification to indicate which resources are not hosted, i.e., which
parts of the interface defined in this section are not supported by
the KDC (REQ9). Application profiles of this specification MAY
extend the KDC interface, by defining additional resources and their
handlers.
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It is REQUIRED of the application profiles of this specification to
register a Resource Type for the root url-path (REQ10). This
Resource Type can be used to discover the correct url to access at
the KDC. This Resource Type can also be used at the GROUPNAME sub-
resource, to indicate different application profiles for different
groups.
It is REQUIRED of the application profiles of this specification to
define what specific actions (e.g., CoAP methods) are allowed on each
resource provided by the KDC interface, depending on whether the
Client is a current group member; the roles that a Client is
authorized to take as per the obtained access token (see
Section 3.1); and the roles that the Client has as current group
member (REQ11).
4.1.1. Operations Supported by Clients
It is expected that a Client minimally supports the following set of
primary operations and corresponding interactions with the KDC.
* FETCH request to ace-group/ , in order to retrieve group names
associated with group identifiers.
* POST and GET requests to ace-group/GROUPNAME/ , in order to join a
group (POST) and later retrieve the current group key material as
a group member (GET).
* GET and FETCH requests to ace-group/GROUPNAME/creds , in order to
retrieve the authentication credentials of all the other group
members (GET) or only some of them by filtering (FETCH). While
retrieving authentication credentials remains possible by using
GET requests, retrieval by filtering allows to greatly limit the
size of exchanged messages.
* GET request to ace-group/GROUPNAME/num , in order to retrieve the
current version of the group key material as a group member.
* DELETE request to ace-group/GROUPNAME/nodes/NODENAME , in order to
leave the group.
In addition, some Clients may rather not support the following set of
secondary operations and corresponding interactions with the KDC.
This can be specified, for instance, in compliance documents defining
minimalistic Clients and their capabilities in specific deployments.
In turn, these might also have to consider the used application
profile of this specification.
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* GET request to ace-group/GROUPNAME/kdc-cred , in order to retrieve
the current authentication credential of the KDC, in addition to
when joining the group. This is relevant only if the KDC has an
associated authentication credential and this is required for the
correct group operation.
* GET request to ace-group/GROUPNAME/policies , in order to retrieve
the current group policies as a group member, in addition to when
joining the group.
* GET request to ace-group/GROUPNAME/nodes/NODENAME , in order to
retrieve the current group keying material and individual keying
material. The former can also be retrieved through a GET request
to ace-group/GROUPNAME/ (see above). The latter would not be
possible to re-obtain as a group member.
* PUT request to ace-group/GROUPNAME/nodes/NODENAME , in order to
ask for new individual keying material. The Client would have to
alternatively re-join the group through a POST request to ace-
group/GROUPNAME/ (see above). Furthermore, depending on its roles
in the group or on the application profile of this specification,
the Client might simply not be associated with any individual
keying material.
* POST request to ace-group/GROUPNAME/nodes/NODENAME/cred , in order
to provide the KDC with a new authentication credential. The
Client would have to alternatively re-join the group through a
POST request to ace-group/GROUPNAME/ (see above). Furthermore,
depending on its roles in the group, the Client might simply not
have an associated authentication credential to provide.
It is REQUIRED of application profiles of this specification to
categorize possible newly defined operations for Clients into primary
operations and secondary operations, and to provide accompanying
considerations (REQ12).
4.1.2. Error Handling
Upon receiving a request from a Client, the KDC MUST check that it is
storing a valid access token from that Client. If this is not the
case, the KDC MUST reply with a 4.01 (Unauthorized) error response.
Unless the request targets the /ace-group resource, the KDC MUST
check that it is storing a valid access token from that Client such
that:
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* The scope specified in the access token includes a scope entry
related to the group name GROUPNAME associated with the targeted
resource; and
* The set of roles specified in that scope entry allows the Client
to perform the requested operation on the targeted resource
(REQ11).
In case the KDC stores a valid access token but the verifications
above fail, the KDC MUST reply with a 4.03 (Forbidden) error
response. This response MAY be an AS Request Creation Hints, as
defined in Section 5.3 of [RFC9200], in which case the Content-Format
MUST be set to application/ace+cbor.
If the request is not formatted correctly (e.g., required fields are
not present or are not encoded as expected), the handler MUST reply
with a 4.00 (Bad Request) error response.
If the request includes unknown or non-expected fields, the handler
MUST silently ignore them and continue processing the request.
Application profiles of this specification MAY define optional or
mandatory payload formats for specific error cases (OPT4).
Some error responses from the KDC can have Content-Format set to
application/ace-groupcomm+cbor. In such a case, the paylod of the
response MUST be a CBOR map, which includes the following fields.
* 'error', with value a CBOR integer specifying the error occurred
at the KDC. The value is taken from the "Value" column of the
"ACE Groupcomm Errors" registry defined in Section 11.11 of this
specification. This field MUST be present.
* 'error_description', with value a CBOR text string specifying a
human-readable diagnostic description of the error occurred at the
KDC, written in English. The diagnostic text is intended for
software engineers as well as for device and network operators, in
order to aid debugging and provide context for possible
intervention. The diagnostic message SHOULD be logged by the KDC.
This field MAY be present, and it is unlikely relevant in an
unattended setup where human intervention is not expected.
The 'error' and 'error_description' fields are defined as OPTIONAL to
support for Clients (see Section 8). A Client supporting the 'error'
parameter and able to understand the specified error may use that
information to determine what actions to take next.
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Section 9 of this specification defines an initial set of error
identifiers, as possible values for the 'error' field. Application
profiles of this specification inherit this initial set of error
identifiers and MAY define additional value (OPT5).
4.2. /ace-group
This resource implements the FETCH handler.
4.2.1. FETCH Handler
The FETCH handler receives group identifiers and returns the
corresponding group names and GROUPNAME URIs.
The handler expects a request with payload formatted as a CBOR map,
which MUST contain the following fields:
* 'gid', whose value is encoded as a CBOR array, containing one or
more group identifiers. The exact encoding of group identifier
MUST be specified by the application profile (REQ13). The Client
indicates that it wishes to receive the group names of all the
groups having these identifiers.
The handler identifies the groups that are secured by the keying
material identified by those group identifiers.
If all verifications succeed, the handler replies with a 2.05
(Content) response, whose payload is formatted as a CBOR map that
MUST contain the following fields:
* 'gid', whose value is encoded as a CBOR array, containing zero or
more group identifiers. The handler indicates that those are the
identifiers it is sending group names for. This CBOR array is a
subset of the 'gid' array in the FETCH request.
* 'gname', whose value is encoded as a CBOR array, containing zero
or more group names. The elements of this array are encoded as
text strings. Each element of index i of this CBOR array
corresponds to the element of group identifier i in the 'gid'
array.
* 'guri', whose value is encoded as a CBOR array, containing zero or
more URIs, each indicating a group-membership resource. The
elements of this array are encoded as text strings. Each element
of index i of this CBOR array corresponds to the element of group
identifier i in the 'gid' array.
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If the KDC does not find any group associated with the specified
group identifiers, the handler returns a response with payload
formatted as a CBOR byte string of zero length.
Note that the KDC only verifies that the node is authorized by the AS
to access this resource. Nodes that are not members of the group but
are authorized to do signature verification on the group messages may
be allowed to access this resource, if the application needs it.
4.2.1.1. Retrieve Group Names
In case the joining node only knows the group identifier of the group
it wishes to join or about which it wishes to get update information
from the KDC, the node can contact the KDC to request the
corresponding group name and group-membership resource URI. The node
can request several group identifiers at once. It does so by sending
a CoAP FETCH request to the /ace-group endpoint at the KDC formatted
as defined in Section 4.2.1.
Figure 6 gives an overview of the exchanges described above, and
Figure 7 shows an example.
Client KDC
| |
|------------ Group Name and URI Retrieval Request: -------->|
| FETCH /ace-group |
| |
|<-- Group Name and URI Retrieval Response: 2.05 (Content) --|
| |
Figure 6: Message Flow of Group Name and URI Retrieval Request-
Response
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Request:
Header: FETCH (Code=0.05)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation):
{ "gid": [01, 02] }
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation):
{ "gid": [01, 02], "gname": ["group1", "group2"],
"guri": ["ace-group/g1", "ace-group/g2"] }
Figure 7: Example of Group Name and URI Retrieval Request-Response
4.3. /ace-group/GROUPNAME
This resource implements the POST and GET handlers.
4.3.1. POST Handler
The POST handler processes the Join Request sent by a Client to join
a group, and returns a Join Response as successful result of the
joining process (see Section 4.3.1.1). At a high level, the POST
handler adds the Client to the list of current group members, adds
the authentication credential of the Client to the list of the group
members' authentication credentials, and returns the symmetric group
keying material for the group identified by GROUPNAME.
The handler expects a request with payload formatted as a CBOR map,
which MAY contain the following fields, which, if included, MUST have
format and value as specified below.
* 'scope', with value the specific group that the Client is
attempting to join, i.e., the group name, and the roles it wishes
to have in the group. This value is a CBOR byte string wrapping
one scope entry, as defined in Section 3.1.
* 'get_creds', if the Client wishes to receive the authentication
credentials of the current group members from the KDC. This
parameter may be included in the Join Request if the KDC stores
the authentication credentials of the group members, while it is
not useful to include it if the Client obtains those
authentication credentials through alternative means, e.g., from
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the AS. Note that including this parameter might result in a
following Join Response of large size, which can be inconvenient
for resource-constrained devices.
If the Client wishes to retrieve the authentication credentials of
all the current group members, the 'get_creds' parameter MUST
encode the CBOR simple value "null" (0xf6). Otherwise, the
'get_creds' parameter MUST encode a non-empty CBOR array,
containing the following three elements formatted as defined
below.
- The first element, namely 'inclusion_flag', encodes the CBOR
simple value "true" (0xf5). That is, the Client indicates that
it wishes to receive the authentication credentials of all
group members having their node identifier specified in the
third element of the 'get_creds' array, namely 'id_filter' (see
below).
- The second element, namely 'role_filter', is a non-empty CBOR
array. Each element of the array contains one role or a
combination of roles for the group identified by GROUPNAME.
That is, when the Join Request includes a non-Null 'get_creds'
parameter, the Client filters authentication credentials based
on node identifiers.
In particular, the Client indicates that it wishes to retrieve
the authentication credentials of all the group members having
any of the single roles, or at least all of the roles indicated
in any combination of roles. For example, the array ["role1",
"role2+role3"] indicates that the Client wishes to receive the
authentication credentials of all group members that have at
least "role1" or at least both "role2" and "role3".
- The third element, namely 'id_filter', is an empty CBOR array.
That is, when the Join Request includes a non-Null 'get_creds'
parameter, the Client does not filter authentication
credentials based on node identifiers.
In fact, when first joining the group, the Client is not
expected or capable to express a filter based on node
identifiers of other group members. Instead, when already a
group member and sending a Join Request to re-join, the Client
is not expected to include the 'get_creds' parameter in the
Join Request altogether, since it can rather retrieve
authentication credentials associated with specific group
identifiers as defined in Section 4.4.1.1.
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The CDDL definition [RFC8610] of 'get_creds' is given in Figure 8,
using as example encoding: node identifier encoded as a CBOR byte
string; role identifier encoded as a CBOR text string, and
combination of roles encoded as a CBOR array of roles.
Note that, for this handler, 'inclusion_flag' is always set to
true, the array of roles 'role_filter' is always non-empty, while
the array of node identifiers 'id_filter' is always empty.
However, this is not necessarily the case for other handlers using
the 'get_creds' parameter.
inclusion_flag = bool
role = tstr
comb_role = [ 2*role ]
role_filter = [ *(role / comb_role) ]
id = bstr
id_filter = [ *id ]
get_creds = null / [ inclusion_flag, role_filter, id_filter]
Figure 8: CDLL definition of get_creds, using as example node
identifier encoded as bstr and role as tstr
* 'client_cred', encoded as a CBOR byte string, with value the
original binary representation of the Client's authentication
credential. This parameter is used if the KDC is managing
(collecting from/distributing to the Client) the authentication
credentials of the group members, and if the Client's role in the
group will require for it to send messages to one or more group
members. It is REQUIRED of the application profiles to define the
specific formats that are acceptable to use for authentication
credentials in the group (REQ6).
* 'cnonce', encoded as a CBOR byte string, and including a dedicated
nonce N_C generated by the Client. This parameter MUST be
present.
* 'client_cred_verify', encoded as a CBOR byte string. This
parameter MUST be present if the 'client_cred' parameter is
present and no authentication credential associated with the
Client's token can be retrieved for that group.
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This parameter contains a proof-of-possession (PoP) evidence
computed by the Client over the following PoP input: the scope
(encoded as CBOR byte string), concatenated with N_S (encoded as
CBOR byte string) concatenated with N_C (encoded as CBOR byte
string), where:
- scope is the CBOR byte string either specified in the 'scope'
parameter above, if present, or as a default scope that the
handler is expected to understand, if omitted.
- N_S is the challenge received from the KDC in the
'kdcchallenge' parameter of the 2.01 (Created) response to the
Token Transfer Request (see Section 3.3), encoded as a CBOR
byte string.
- N_C is the nonce generated by the Client and specified in the
'cnonce' parameter above, encoded as a CBOR byte string.
An example of PoP input to compute 'client_cred_verify' using CBOR
encoding is given in Figure 9.
A possible type of PoP evidence is a signature, that the Client
computes by using its own private key, whose corresponding public
key is specified in the authentication credential carried in the
'client_cred' parameter. Application profiles of this
specification MUST specify the exact approaches used to compute
the PoP evidence to include in 'client_cred_verify', and MUST
specify which of those approaches is used in which case (REQ14).
If the token was not provided to the KDC through a Token Transfer
Request (e.g., it is used directly to validate TLS instead), it is
REQUIRED of the specific application profile to define how the
challenge N_S is generated (REQ15).
* 'creds_repo', which can be present if the format of the Client's
authentication credential in the 'client_cred' parameter is a
certificate. In such a case, this parameter has as value the URI
of the certificate. This parameter is encoded as a CBOR text
string. Alternative specific encodings of this parameter MAY be
defined in applications of this specification (OPT6).
* 'control_uri', with value a full URI, encoded as a CBOR text
string. A default url-path is /ace-group/GROUPNAME/node, although
implementations can use different ones instead. The URI MUST NOT
have url-path ace-group/GROUPNAME.
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If 'control_uri' is specified in the Join Request, the Client acts
as a CoAP server and hosts a resource at this specific URI. The
KDC MAY use this URI to send CoAP requests to the Client (acting
as CoAP server in this exchange), for example for one-to-one
provisioning of new group keying material when performing a group
rekeying (see Section 4.8.1.1), or to inform the Client of its
removal from the group Section 5.
In particular, this resource is intended for communications
concerning exclusively the group whose group name GROUPNAME is
specified in the 'scope' parameter. If the KDC does not implement
mechanisms using this resource for that group, it can ignore this
parameter. Other additional functionalities of this resource MAY
be defined in application profiles of this specifications (OPT7).
scope, N_S, and N_C expressed in CBOR diagnostic notation:
scope = h'826667726f7570316673656e646572'
N_S = h'018a278f7faab55a'
N_C = h'25a8991cd700ac01'
scope, N_S, and N_C as CBOR encoded byte strings:
scope = 0x4f826667726f7570316673656e646572
N_S = 0x48018a278f7faab55a
N_C = 0x4825a8991cd700ac01
PoP input:
0x4f 826667726f7570316673656e646572
48 018a278f7faab55a 48 25a8991cd700ac01
Figure 9: Example of PoP input to compute 'client_cred_verify'
using CBOR encoding
If the request does not include a 'scope' field, the KDC is expected
to understand with what roles the Client is requesting to join the
group. For example, as per the access token, the Client might have
been granted access to the group with only one role. If the KDC
cannot determine which exact scope should be considered for the
Client, it MUST reply with a 4.00 (Bad Request) error response.
The handler considers the scope specified in the access token
associated with the Client, and checks the scope entry related to the
group with name GROUPNAME associated with the endpoint. In
particular, the handler checks whether the set of roles specified in
that scope entry includes all the roles that the Client wishes to
have in the group as per the Join Request. If this is not the case,
the KDC MUST reply with a 4.03 (Forbidden) error response.
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If the KDC manages the group members' authentication credentials, the
handler checks if one is included in the 'client_cred' field. If so,
the KDC retrieves the authentication credential and performs the
following actions.
* If the access token was provided through a Token Transfer Request
(see Section 3.3) but the KDC cannot retrieve the 'kdcchallenge'
associated with this Client (see Section 3.3), the KDC MUST reply
with a 4.00 Bad Request error response, which MUST also have
Content-Format application/ace-groupcomm+cbor. The payload of the
error response is a CBOR map including a newly generated
'kdcchallenge' value. This is specified in the 'kdcchallenge'
parameter.
* The KDC checks the authentication credential to be valid for the
group identified by GROUPNAME. That is, it checks that the
authentication credential has the format used in the group, is
intended for the public key algorithm used in the group, and is
aligned with the possible associated parameters used in the group.
If this verification fails, the handler MUST reply with a 4.00
(Bad Request) error response. The response MUST have Content-
Format set to application/ace-groupcomm+cbor and is formatted as
defined in Section 4. The value of the 'error' field MUST be set
to 2 ("Authentication credential incompatible with the group
configuration").
* The KDC verifies the PoP evidence contained in the
'client_cred_verify' field. Application profiles of this
specification MUST specify the exact approaches used to verify the
PoP evidence, and MUST specify which of those approaches is used
in which case (REQ14).
If the PoP evidence does not pass verification, the handler MUST
reply with a 4.00 (Bad Request) error response. The response MUST
have Content-Format set to application/ace-groupcomm+cbor and is
formatted as defined in Section 4. The value of the 'error' field
MUST be set to 3 ("Invalid Proof-of-Possession evidence").
If no authentication credential is included in the 'client_cred'
field, the handler checks if an authentication credential is already
associated with the received access token and to the group identified
by GROUPNAME (see also Section 4.3.1.1). Note that the same joining
node may use different authentication credentials in different
groups, and all those authentication credentials would be associated
with the same access token.
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If an eligible authentication credential for the Client is neither
present in the 'client_cred' field nor retrieved from the stored ones
at the KDC, it is RECOMMENDED that the handler stops the processing
and replies with a 4.00 (Bad Request) error response. Applications
profiles MAY define alternatives (OPT8).
If, regardless the reason, the KDC replies with a 4.00 (Bad Request)
error response, this response MAY have Content-Format set to
application/ace-groupcomm+cbor and have a CBOR map as payload. For
instance, the CBOR map can include a 'sign_info' parameter formatted
as 'sign_info_res' defined in Section 3.3.1, with the 'cred_fmt'
element set to the CBOR simple value "null" (0xf6) if the Client sent
its own authentication credential and the KDC is not set to store
authentication credentials of the group members.
If all the verifications above succeed, the KDC proceeds as follows.
First, only in case the Client is not already a group member, the
handler performs the following actions:
* The handler adds the Client to the list of current members of the
group.
* The handler assigns a name NODENAME to the Client, and creates a
sub-resource to /ace-group/GROUPNAME at the KDC, i.e., "/ace-
group/GROUPNAME/nodes/NODENAME".
* The handler associates the node identifier NODENAME to the access
token and the secure session for the Client.
Then, the handler performs the following actions.
* If the KDC manages the group members' authentication credentials:
- The handler associates the retrieved Client's authentication
credential to the tuple composed of the node name NODENAME, the
group name GROUPNAME, and the received access token.
- The handler adds the retrieved Client's authentication
credential to the stored list of authentication credentials
stored for the group identified by GROUPNAME. If such list
already includes an authentication credential for the Client,
but a different authentication credential is specified in the
'client_cred' field, then the handler MUST replace the old
authentication credential in the list with the one specified in
the 'client_cred' field.
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* If the application requires backward security or if the used
application profile prescribes so, the KDC MUST generate new group
keying material and securely distribute it to the current group
members (see Section 6).
* The handler returns a successful Join Response as defined below,
containing the symmetric group keying material; the group
policies; and the authentication credentials of the current
members of the group, if the KDC manages those and the Client
requested them.
The Join Response MUST have response code 2.01 (Created) if the
Client has been added to the list of group members in this join
exchange (see above), or 2.04 (Changed) otherwise, i.e., if the
Client is re-joining the group without having left it.
The Join Response message MUST include the Location-Path CoAP option,
specifying the URI path to the sub-resource associated with the
Client, i.e., "/ace-group/GROUPNAME/nodes/NODENAME".
The Join Response message MUST have Content-Format application/ace-
groupcomm+cbor. The payload of the response is formatted as a CBOR
map, which MUST contain the following fields and values.
* 'gkty', identifying the key type of the 'key' parameter. The set
of values can be found in the "Key Type" column of the "ACE
Groupcomm Key Types" registry. Implementations MUST verify that
the key type matches the application profile being used, if
present, as registered in the "ACE Groupcomm Key Types" registry.
* 'key', containing the keying material for the group communication,
or information required to derive it.
* 'num', containing the version number of the keying material for
the group communication, formatted as an integer. This is a
strictly monotonic increasing field. The application profile MUST
define the initial version number (REQ16).
The exact format of the 'key' value MUST be defined in applications
of this specification (REQ17), as well as values of 'gkty' accepted
by the application (REQ18). Additionally, documents specifying the
key format MUST register it in the "ACE Groupcomm Key Types" registry
defined in Section 11.7, including its name, type, and application
profile to be used with.
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+----------+----------------+---------+-------------+------------+
| Name | Key Type Value | Profile | Description | Reference |
+----------+----------------+---------+-------------+------------+
| Reserved | 0 | | This value | [RFC-XXXX] |
| | | | is reserved | |
+----------+----------------+---------+-------------+------------+
Figure 10: ACE Groupcomm Key Types
Note to RFC Editor: In Figure 10, please replace "[RFC-XXXX]" with
the RFC number of this specification and delete this paragraph.
The response SHOULD contain the following parameter:
* 'exp', with value the expiration time of the keying material for
the group communication, encoded as a CBOR unsigned integer. This
field contains a numeric value representing the number of seconds
from 1970-01-01T00:00:00Z UTC until the specified UTC date/time,
ignoring leap seconds, analogous to what is specified for
NumericDate in Section 2 of [RFC7519]. Group members MUST NOT use
the keying material after the time indicated in this field, and
they can retrieve the new group keying material from the KDC.
Optionally, the response MAY contain the following parameters, which,
if included, MUST have format and value as specified below.
* 'ace_groupcomm_profile', with value a CBOR integer that MUST be
used to uniquely identify the application profile for group
communication. Applications of this specification MUST register
an application profile identifier and the related value for this
parameter in the "ACE Groupcomm Profiles" registry (REQ19).
+----------+------------------------+------------+------------+
| Name | Description | CBOR Value | Reference |
+----------+------------------------+------------+------------+
| Reserved | This value is reserved | 0 | [RFC-XXXX] |
+----------+------------------------+------------+------------+
Figure 11: ACE Groupcomm Profiles
Note to RFC Editor: In Figure 11, please replace "[RFC-XXXX]" with
the RFC number of this specification and delete this paragraph.
* 'creds', MUST be present if 'get_creds' was present in the
request, otherwise it MUST NOT be present. This parameter is a
CBOR array specifying the authentication credentials of the group
members, i.e., of all of them or of the ones selected according to
the 'get_creds' parameter in the request. In particular, each
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element of the array is a CBOR byte string, with value the
original binary representation of a group member's authentication
credential. It is REQUIRED of the application profiles to define
the specific formats of authentication credentials that are
acceptable to use in the group (REQ6).
* 'peer_roles', SHOULD be present if 'creds' is also present,
otherwise it MUST NOT be present. This parameter is a CBOR array
of n elements, with n the number of authentication credentials
included in the 'creds' parameter (at most the number of members
in the group). The i-th element of the array specifies the role
(or CBOR array of roles) that the group member associated with the
i-th authentication credential in 'creds' has in the group. In
particular, each array element is encoded as the role element of a
scope entry, as defined in Section 3.1.
This parameter MAY be omitted if the Client can rely on other
means to unambiguously gain knowledge of the role of each group
member whose associated authentication credential is specified in
the 'creds' parameter. For example, all such group members may
have the same role in the group joined by the Client, and such a
role can be unambiguously assumed by the Client (e.g., based on
what is defined in the used application profile of this
specification). As another example, each and every of the
authentication credentials specified in the 'creds' parameter can
indicate the role(s) that the corresponding group member has in
the group joined by the Client.
When receiving the authentication credential of a Client in the
'client_cred' parameter of a Join Request (see Section 4.3.1.1) or
of an Authentication Credential Update Request (see
Section 4.9.1.1), the KDC is not expected to check that the
authentication credential indicates the role(s) that the Client
can have or has in the group in question. When preparing a Join
Response, the KDC can decide about including or not the
'peer_roles' parameter depending on the specific set of
authentication credentials specified in the 'creds' parameter of
that Join Response.
* 'peer_identifiers', MUST be present if 'creds' is also present,
otherwise it MUST NOT be present. This parameter is a CBOR array
of n elements, with n the number of authentication credentials
included in the 'creds' parameter (at most the number of members
in the group). The i-th element of the array specifies the node
identifier that the group member associated with the i-th
authentication credential in 'creds' has in the group. In
particular, the i-th array element is encoded as a CBOR byte
string, with value the node identifier of the group member.
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* 'group_policies', with value a CBOR map, whose entries specify how
the group handles specific management aspects. These include, for
instance, approaches to achieve synchronization of sequence
numbers among group members. The elements of this field are
registered in the "ACE Groupcomm Policies" registry. This
specification defines the three elements "Sequence Number
Synchronization Methods", "Key Update Check Interval", and
"Expiration Delta", which are summarized in Figure 12.
Application profiles that build on this document MUST specify the
exact content format and default value of included map entries
(REQ20).
+--------------+-------+----------+----------------------+------------+
| Name | CBOR | CBOR | Description | Reference |
| | label | type | | |
+--------------+-------+----------+----------------------+------------+
| Sequence | TBD | tstr/int | Method for recipient | [RFC-XXXX] |
| Number | | | group members to | |
| Synchroniza- | | | synchronize with | |
| tion Method | | | sequence numbers of | |
| | | | of sender group | |
| | | | members. Its value | |
| | | | is taken from the | |
| | | | 'Value' column of | |
| | | | the Sequence Number | |
| | | | Synchronization | |
| | | | Method registry | |
+--------------+-------+----------+----------------------+------------+
| Key Update | TBD | int | Polling interval in | [RFC-XXXX] |
| Check | | | seconds, for group | |
| Interval | | | members to check at | |
| | | | the KDC if the | |
| | | | latest group keying | |
| | | | material is the one | |
| | | | that they store | |
+--------------+-------+----------+----------------------+------------+
| Expiration | TBD | uint | Number of seconds | [RFC-XXXX] |
| Delta | | | from 'exp' until a | |
| | | | UTC date/time, after | |
| | | | which group members | |
| | | | MUST stop using the | |
| | | | group keying | |
| | | | material that they | |
| | | | store to decrypt | |
| | | | incoming messages | |
+--------------+-------+----------|----------------------|------------+
Figure 12: ACE Groupcomm Policies
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Note to RFC Editor: In Figure 12, please replace all occurrences of
"[RFC-XXXX]" with the RFC number of this specification and delete
this paragraph.
* 'kdc_cred', encoded as a CBOR byte string, with value the original
binary representation of the KDC's authentication credential.
This parameter is used if the KDC has an associated authentication
credential and this is required for the correct group operation.
It is REQUIRED of application profiles to define whether the KDC
has an authentication credential and if this has to be provided
through the 'kdc_cred' parameter (REQ8).
In such a case, the KDC's authentication credential MUST have the
same format used for the authentication credentials of the group
members. It is REQUIRED of the application profiles to define the
specific formats that are acceptable to use for the authentication
credentials in the group (REQ6).
* 'kdc_nonce', encoded as a CBOR byte string, and including a
dedicated nonce N_KDC generated by the KDC. This parameter MUST
be present if the 'kdc_cred' parameter is present.
* 'kdc_cred_verify', encoded as a CBOR byte string. This parameter
MUST be present if the 'kdc_cred' parameter is present.
This parameter contains a proof-of-possession (PoP) evidence
computed by the KDC over the following PoP input: the nonce N_C
(encoded as CBOR byte string) concatenated with the nonce N_KDC
(encoded as CBOR byte string), where:
- N_C is the nonce generated by the Client and specified in the
'cnonce' parameter of the Join Request, encoded as a CBOR byte
string.
- N_KDC is the nonce generated by the KDC and specified in the
'kdc_nonce' parameter, encoded as a CBOR byte string.
An example of PoP input to compute 'kdc_cred_verify' using CBOR
encoding is given in Figure 14.
A possible type of PoP evidence is a signature, that the KDC
computes by using its own private key, whose corresponding public
key is specified in the authentication credential carried in the
'kdc_cred' parameter. Application profiles of this specification
MUST specify the exact approaches used by the KDC to compute the
PoP evidence to include in 'kdc_cred_verify', and MUST specify
which of those approaches is used in which case (REQ21).
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* 'rekeying_scheme', identifying the rekeying scheme that the KDC
uses to provide new group keying meterial to the group members.
This parameter is encoded as a CBOR integer, whose value is taken
from the "Value" column of the "ACE Groupcomm Rekeying Schemes"
registry defined in Section 11.12 of this specification.
+-------+----------------+-------------------------------+------------+
| Value | Name | Description | Reference |
+-------+----------------+-------------------------------+------------+
| 0 | Point-to-Point | The KDC individually targets | [RFC-XXXX] |
| | | each node to rekey, using the | |
| | | pairwise secure communication | |
| | | association with that node | |
+-------+----------------+-------------------------------+------------+
Figure 13: ACE Groupcomm Rekeying Schemes
Application profiles of this specification MAY define a default group
rekeying scheme, to refer to in case the 'rekeying_scheme' parameter
is not included in the Join Response (OPT9).
Note to RFC Editor: In Figure 13, please replace "[RFC-XXXX]" with
the RFC number of this specification and delete this paragraph.
* 'mgt_key_material', encoded as a CBOR byte string and containing
the specific administrative keying material that the joining node
requires in order to participate in the group rekeying process
performed by the KDC. This parameter MUST NOT be present if the
'rekeying_scheme' parameter is not present and the application
profile does not specify a default group rekeying scheme to use in
the group. Some simple rekeying scheme may not require specific
administrative keying material to be provided, e.g., the basic
"Point-to-Point" group rekeying scheme (see Section 6.1).
In more advanced group rekeying schemes, the administrative keying
material can be composed of multiple keys organized, for instance,
into a logical tree hierarchy, whose root key is the only
administrative key shared by all the group members. In such a
case, each group member is exclusively associated with one leaf
key in the hierarchy, and stores only the administrative keys from
the associated leaf key all the way up along the path to the root
key. That is, different group members can be provided with a
different subset of the overall administrative keying material.
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It is expected from separate documents to define how the advanced
group rekeying scheme possibly indicated in the 'rekeying_scheme'
parameter is used by an application profile of this specification.
This includes defining the format of the administrative keying
material to specify in 'mgt_key_material', consistently with the
group rekeying scheme and the application profile in question.
* 'control_group_uri', with value a full URI, encoded as a CBOR text
string. The URI MUST specify addressing information intended to
reach all the members in the group. For example, this can be a
multicast IP address, optionally together with a port number that,
if omitted, defaults to 5683, i.e., the default port number for
the "coap" URI scheme (see Section 6.1 of [RFC7252]). The URI
MUST include GROUPNAME in the url-path. A default url-path is
/ace-group/GROUPNAME, although implementations can use different
ones instead. The URI MUST NOT have url-path ace-group/GROUPNAME/
node.
If 'control_group_uri' is included in the Join Response, the
Clients supporting this parameter act as CoAP servers, host a
resource at this specific URI, and listen to the specified
addressing information.
The KDC MAY use this URI to send one-to-many CoAP requests to the
Client group members (acting as CoAP servers in this exchange),
for example for one-to-many provisioning of new group keying
material when performing a group rekeying (see Section 4.8.1.1),
or to inform the Clients of their removal from the group
Section 5.
In particular, this resource is intended for communications
concerning exclusively the group whose group name GROUPNAME is
specified in the 'scope' parameter. If the KDC does not implement
mechanisms using this resource for that group, it can ignore this
parameter. Other additional functionalities of this resource MAY
be defined in application profiles of this specifications (OPT10).
N_C and N_KDC expressed in CBOR diagnostic notation:
N_C = h'25a8991cd700ac01'
N_KDC = h'cef04b2aa791bc6d'
N_C and N_KDC as CBOR encoded byte strings:
N_C = 0x4825a8991cd700ac01
N_KDC = 0x48cef04b2aa791bc6d
PoP input:
0x48 25a8991cd700ac01 48 cef04b2aa791bc6d
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Figure 14: Example of PoP input to compute 'kdc_cred_verify'
using CBOR encoding
After sending the Join Response, the KDC MUST store the N_C value
specified in the 'cnonce' parameter of the Join Request, as a
'clientchallenge' value associated with the Client. If, as a group
member, the Client later sends a GET request to the /ace-
group/GROUPNAME/kdc-cred resource for retrieving the latest KDC's
authentication credential (see Section 4.5.1), then the KDC is able
to use the stored 'clientchallenge' for computing a PoP evidence to
include in the response sent to the Client, hence proving the
possession of its own private key.
If the Join Response includes the 'kdc_cred_verify' parameter, the
Client verifies the conveyed PoP evidence and considers the group
joining unsuccessful in case of failed verification. Application
profiles of this specification MUST specify the exact approaches used
by the Client to verify the PoP evidence in 'kdc_cred_verify', and
MUST specify which of those approaches is used in which case (REQ21).
Specific application profiles that build on this document MUST
specify the communication protocol that members of the group use to
communicate with each other (REQ22) and how exactly the keying
material is used to protect the group communication (REQ23).
4.3.1.1. Join the Group
Figure 15 gives an overview of the join exchange between the Client
and the KDC, when the Client first joins a group, while Figure 16
shows an example.
Client KDC
| |
|-------- Join Request: POST /ace-group/GROUPNAME ------>|
| |
|<------------ Join Response: 2.01 (Created) ----------- |
| Location-Path = "/ace-group/GROUPNAME/nodes/NODENAME" |
Figure 15: Message Flow of the Join Request-Response
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Request:
Header: POST (Code=0.02)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation,
with AUTH_CRED and POP_EVIDENCE being CBOR byte strings):
{ "scope": << [ "group1", ["sender", "receiver"] ] >> ,
"get_creds": [true, ["sender"], []], "client_cred": AUTH_CRED,
"cnonce": h'25a8991cd700ac01', "client_cred_verify": POP_EVIDENCE }
Response:
Header: Created (Code=2.01)
Content-Format: "application/ace-groupcomm+cbor"
Location-Path: "kdc.example.com"
Location-Path: "g1"
Location-Path: "nodes"
Location-Path: "c101"
Payload (in CBOR diagnostic notation,
with KEY being a CBOR byte strings):
{ "gkty": 13, "key": KEY, "num": 12, "exp": 1609459200,
"creds": [ AUTH_CRED_1, AUTH_CRED_2 ],
"peer_roles": ["sender", ["sender", "receiver"]],
"peer_identifiers": [ ID1, ID2 ] }
Figure 16: Example of First Join Request-Response for Group Joining
If not previously established, the Client and the KDC MUST first
establish a pairwise secure communication channel (REQ24). This can
be achieved, for instance, by using a transport profile of ACE. The
join exchange MUST occur over that secure channel. The Client and
the KDC MAY use that same secure channel to protect further pairwise
communications that must be secured.
The secure communication protocol is REQUIRED to establish the secure
channel between the Client and the KDC by using the proof-of-
possession key bound to the access token. As a result, the proof-of-
possession to bind the access token to the Client is performed by
using the proof-of-possession key bound to the access token for
establishing secure communication between the Client and the KDC.
To join the group, the Client sends a CoAP POST request to the /ace-
group/GROUPNAME endpoint at the KDC, where GROUPNAME is the group
name of the group to join, formatted as specified in Section 4.3.1.
This group name is the same as in the scope entry corresponding to
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that group, specified in the 'scope' parameter of the Authorization
Request/Response, or it can be retrieved from it. Note that, in case
of successful joining, the Client will receive the URI to retrieve
individual keying material and to leave the group in the Location-
Path option of the response.
If the node is joining a group for the first time and the KDC
maintains the authentication credentials of the group members, the
Client is REQUIRED to send its own authentication credential and
proof-of-possession (PoP) evidence in the Join Request (see the
'client_cred' and 'client_cred_verify' parameters in Section 4.3.1).
The request is accepted only if both the authentication credential is
provided and the PoP evidence is successfully verified.
If a node re-joins a group as authorized by the same access token and
using the same authentication credential, it can omit the
authentication credential and the PoP evidence, or just the PoP
evidence, from the Join Request. Then, the KDC will be able to
retrieve the node's authentication credential associated with the
access token for that group. If the authentication credential has
been discarded, the KDC replies with 4.00 (Bad Request) error
response, as specified in Section 4.3.1. If a node re-joins a group
but wants to update its own authentication credential, it needs to
include both its authentication credential and the PoP evidence in
the Join Request like when it joined the group for the first time.
4.3.2. GET Handler
The GET handler returns the symmetric group keying material for the
group identified by GROUPNAME.
The handler expects a GET request.
In addition to what is defined in Section 4.1.2, the handler verifies
that the Client is a current member of the group. If the
verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
response. The response MUST have Content-Format set to application/
ace-groupcomm+cbor and is formatted as defined in Section 4. The
value of the 'error' field MUST be set to 0 ("Operation permitted
only to group members").
If all verifications succeed, the handler replies with a 2.05
(Content) response containing the symmetric group keying material.
The payload of the response is formatted as a CBOR map which MUST
contain the parameters 'gkty', 'key', and 'num' specified in
Section 4.3.1.
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Each of the following parameters specified in Section 4.3.1 MUST also
be included in the payload of the response, if they are included in
the payload of the Join Responses sent for the group:
'rekeying_scheme', 'mgt_key_material'.
The payload MAY also include the parameters 'ace_groupcomm_profile'
and 'exp' parameters specified in Section 4.3.1.
4.3.2.1. Retrieve Group Keying Material
A node in the group can contact the KDC to retrieve the current group
keying material, by sending a CoAP GET request to the /ace-group/
GROUPNAME endpoint at the KDC, where GROUPNAME is the group name.
Figure 17 gives an overview of the join exchange between the Client
and the KDC, when the Client first joins a group, while Figure 18
shows an example.
Client KDC
| |
|----- Key Distribution Request: GET /ace-group/GROUPNAME ------>|
| |
|<----------- Key Distribution Response: 2.05 (Content) --------- |
| |
Figure 17: Message Flow of Key Distribution Request-Response
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Payload: -
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation,
with KEY being a CBOR byte strings):
{ "gkty": 13, "key": KEY, "num": 12 }
Figure 18: Example of Key Distribution Request-Response
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4.4. /ace-group/GROUPNAME/creds
This resource implements the GET and FETCH handlers.
4.4.1. FETCH Handler
The FETCH handler receives identifiers of group members for the group
identified by GROUPNAME and returns the authentication credentials of
such group members.
The handler expects a request with payload formatted as a CBOR map,
that MUST contain the following field.
* 'get_creds', whose value is encoded as in Section 4.3.1 with the
following modifications.
- The arrays 'role_filter' and 'id_filter' MUST NOT both be
empty, i.e., in CBOR diagnostic notation: [ bool, [ ], [ ] ].
If the 'get_creds' parameter has such a format, the request
MUST be considered malformed, and the KDC MUST reply with a
4.00 (Bad Request) error response.
Note that a group member can retrieve the authentication
credentials of all the current group members by sending a GET
request to the same KDC resource instead (see Section 4.4.2.1).
- The element 'inclusion_flag' encodes the CBOR simple value
"true" (0xf5) if the third element 'id_filter' specifies an
empty CBOR array, or if the Client wishes to receive the
authentication credentials of the nodes having their node
identifier specified in 'id_filter' (i.e, selection by
inclusive filtering). Instead, this element encodes the CBOR
simple value "false" (0xf4) if the Client wishes to receive the
authentication credentials of the nodes not having the node
identifiers specified in the third element 'id_filter' (i.e.,
selection by exclusive filtering).
- The array 'role_filter' can be empty, if the Client does not
wish to filter the requested authentication credentials based
on the roles of the group members.
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- The array 'id_filter' contains zero or more node identifiers of
group members, for the group identified by GROUPNAME. The
Client indicates that it wishes to receive the authentication
credentials of the nodes having or not having these node
identifiers, in case the 'inclusion_flag' element encodes the
CBOR simple value "true" (0xf5) or "false" (0xf4),
respectively. The array 'id_filter' may be empty, if the
Client does not wish to filter the requested authentication
credentials based on the node identifiers of the group members.
Note that, in case the 'role_filter' array and the 'id_filter' array
are both non-empty:
* If the 'inclusion_flag' encodes the CBOR simple value "true"
(0xf5), the handler returns the authentication credentials of
group members whose roles match with 'role_filter' and/or having
their node identifier specified in 'id_filter'.
* If the 'inclusion_flag' encodes the CBOR simple value "false"
(0xf4), the handler returns the authentication credentials of
group members whose roles match with 'role_filter' and, at the
same time, not having their node identifier specified in
'id_filter'.
The specific format of authentication credentials as well as
identifiers, roles, and combination of roles of group members MUST be
specified by application profiles of this specification (REQ1, REQ6,
REQ25).
The handler identifies the authentication credentials of the current
group members for which either:
* the role identifier matches with one of those indicated in the
request; note that the request can contain a "combination of
roles", where the handler select all group members who have all
roles included in the combination.
* the node identifier matches with one of those indicated in the
request.
If all verifications succeed, the handler returns a 2.05 (Content)
message response with payload formatted as a CBOR map, containing
only the following parameters from Section 4.3.1.
* 'num', which encodes the version number of the current group
keying material.
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* 'creds', which encodes the list of authentication credentials of
the selected group members.
* 'peer_roles', which encodes the role (or CBOR array of roles) that
each of the selected group members has in the group.
This parameter SHOULD be present and it MAY be omitted, according
to the same criteria defined for the Join Response (see
Section 4.3.1).
* 'peer_identifiers', which encodes the node identifier that each of
the selected group members has in the group.
The specific format of authentication credentials as well as of node
identifiers of group members is specified by the application profile
(REQ6, REQ25).
If the KDC does not store any authentication credential associated
with the specified node identifiers, the handler returns a response
with payload formatted as a CBOR byte string of zero length.
The handler MAY enforce one of the following policies, in order to
handle possible node identifiers that are included in the 'id_filter'
element of the 'get_creds' parameter of the request but are not
associated with any current group member. Such a policy MUST be
specified by the application profile (REQ26).
* The KDC silently ignores those node identifiers.
* The KDC retains authentication credentials of group members for a
given amount of time after their leaving, before discarding them.
As long as such authentication credentials are retained, the KDC
provides them to a requesting Client.
If the KDC adopts this policy, the application profile MUST also
specify the amount of time during which the KDC retains the
authentication credential of a former group member after its
leaving, possibly on a per-member basis.
Note that this resource handler only verifies that the node is
authorized by the AS to access this resource. Nodes that are not
members of the group but are authorized to do signature verifications
on the group messages may be allowed to access this resource, if the
application needs it.
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4.4.1.1. Retrieve a Subset of Authentication Credentials in the Group
In case the KDC maintains the authentication credentials of group
members, a node in the group can contact the KDC to request the
authentication credentials, roles, and node identifiers of a
specified subset of group members, by sending a CoAP FETCH request to
the /ace-group/GROUPNAME/creds endpoint at the KDC, where GROUPNAME
is the group name, and formatted as defined in Section 4.4.1.
Figure 19 gives an overview of the exchange mentioned above, while
Figure 20 shows an example of such an exchange.
Client KDC
| |
| Authentication Credential Request: |
|-------------------------------------------------------->|
| FETCH /ace-group/GROUPNAME/creds |
| |
|<-- Authentication Credential Response: 2.05 (Created) --|
| |
Figure 19: Message Flow of Authentication Credential Request-
Response to Obtain the Authentication Credentials of Specific
Group Members
Request:
Header: FETCH (Code=0.05)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "pub-key"
Content-Format: "application/ace-groupcomm+cbor"
Payload:
{ "get_creds": [true, [], [ ID3 ]] }
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation):
{ "creds": [ AUTH_CRED_3 ],
"peer_roles": [ "receiver" ],
"peer_identifiers": [ ID3 ] }
Figure 20: Example of Authentication Credential Request-Response
to Obtain the Authentication Credentials of Specific Group
Members
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4.4.2. GET Handler
The handler expects a GET request.
If all verifications succeed, the KDC replies with a 2.05 (Content)
response as in the FETCH handler in Section 4.4.1, but specifying in
the payload the authentication credentials of all the group members,
together with their roles and node identifiers.
The parameter 'peer_roles' SHOULD be present in the payload of the
response and it MAY be omitted, according to the same criteria
defined for the Join Response (see Section 4.3.1).
4.4.2.1. Retrieve All Authentication Credentials in the Group
In case the KDC maintains the authentication credentials of group
members, a group or an external signature verifier can contact the
KDC to request the authentication credentials, roles, and node
identifiers of all the current group members, by sending a CoAP GET
request to the /ace-group/GROUPNAME/creds endpoint at the KDC, where
GROUPNAME is the group name.
Figure 21 gives an overview of the message exchange, while Figure 22
shows an example of such an exchange.
Client KDC
| |
| Authentication Credential Request: |
|-------------------------------------------------------->|
| GET /ace-group/GROUPNAME/creds |
| |
|<-- Authentication Credential Response: 2.05 (Content) --|
| |
Figure 21: Message Flow of Authentication Credential Request-
Response to Obtain the Authentication Credentials of all the
Group Members
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Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "pub-key"
Payload: -
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation):
{ "num": 5,
"creds": [ AUTH_CRED_1, AUTH_CRED_2, AUTH_CRED_3 ],
"peer_roles": ["sender", ["sender", "receiver"], "receiver"],
"peer_identifiers": [ ID1, ID2, ID3 ] }
Figure 22: Example of Authentication Credential Request-Response
to Obtain the Authentication Credentials of all the Group Members
4.5. ace-group/GROUPNAME/kdc-cred
This resource implements a GET handler.
4.5.1. GET Handler
The handler expects a GET request.
If all verifications succeed, the handler returns a 2.05 (Content)
message containing the KDC's authentication credential together with
a proof-of-possession (PoP) evidence. The response MUST have
Content-Format set to application/ace-groupcomm+cbor. The payload of
the response is a CBOR map, which includes the following fields.
* The 'kdc_cred' parameter, specifying the KDC's authentication
credential. This parameter is encoded like the 'kdc_cred'
parameter in the Join Response (see Section 4.3.1).
* The 'kdc_nonce' parameter, specifying a nonce generated by the
KDC. This parameter is encoded like the 'kdc_nonce' parameter in
the Join Response (see Section 4.3.1).
* The 'kdc_cred_verify' parameter, specifying a PoP evidence
computed by the KDC over the following PoP input: the nonce N_C
(encoded as CBOR byte string) concatenated with the nonce N_KDC
(encoded as CBOR byte string), where:
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- N_C is the nonce generated by the Client that was specified in
the 'cnonce' parameter of the Join Request, and that the KDC
stored as 'clientchallenge' value associated with this Client
after sending the corresponding Join Response (see
Section 4.3.1). This nonce is encoded as a CBOR byte string.
- N_KDC is the nonce generated by the KDC and specified in the
'kdc_nonce' parameter, encoded as a CBOR byte string.
An example of PoP input to compute 'kdc_cred_verify' using CBOR
encoding is given in Figure 23.
The PoP evidence is computed by means of the same method used for
computing the PoP evidence that was included in the Join Response
for this Client (see Section 4.3.1).
Application profiles of this specification MUST specify the exact
approaches used by the KDC to compute the PoP evidence to include
in 'kdc_cred_verify', and MUST specify which of those approaches
is used in which case (REQ21).
N_C and N_KDC expressed in CBOR diagnostic notation:
N_C = h'25a8991cd700ac01'
N_KDC = h'0b7db12aaff56da3'
N_C and N_KDC as CBOR encoded byte strings:
N_C = 0x4825a8991cd700ac01
N_KDC = 0x480b7db12aaff56da3
PoP input:
0x48 25a8991cd700ac01 48 0b7db12aaff56da3
Figure 23: Example of PoP input to compute 'kdc_cred_verify'
using CBOR encoding
4.5.1.1. Retrieve the KDC's Authentication Credential
In case the KDC has an associated authentication credential as
required for the correct group operation, a group member or an
external signature verifier can contact the KDC to request the KDC's
authentication credential, by sending a CoAP GET request to the /ace-
group/GROUPNAME/kdc-cred endpoint at the KDC, where GROUPNAME is the
group name.
Upon receiving the 2.05 (Content) response, the Client retrieves the
KDC's authentication credential from the ’kdc_cred’ parameter, and
MUST verify the proof-of-possession (PoP) evidence specified in the
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'kdc_cred_verify' parameter. In case of successful verification of
the PoP evidence, the Client MUST store the obtained KDC's
authentication credential and replace the currently stored one.
The PoP evidence is verified by means of the same method used when
processing the Join Response (see Section 4.3.1). Application
profiles of this specification MUST specify the exact approaches used
by the Client to verify the PoP evidence in 'kdc_cred_verify', and
MUST specify which of those approaches is used in which case (REQ21).
Figure 24 gives an overview of the exchange described above, while
Figure 25 shows an example.
Group
Member KDC
| |
| KDC Authentication Credential Request |
|------------------------------------------------------------>|
| GET ace-group/GROUPNAME/gm-pub-key |
| |
|<-- KDC Authentication Credential Response: 2.05 (Content) --|
| |
Figure 24: Message Flow of KDC Authentication Credential Request-
Response to Obtain the Authentication Credential of the KDC
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "kdc-pub-key"
Payload: -
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation, with AUTH_CRED_KDC
and POP_EVIDENCE being CBOR byte strings):
{
"kdc_nonce": h'0b7db12aaff56da3',
"kdc_cred": AUTH_CRED_KDC,
"kdc_cred_verify": POP_EVIDENCE
}
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Figure 25: Example of KDC Authentication Credential Request-
Response to Obtain the Authentication Credential of the KDC
4.6. /ace-group/GROUPNAME/policies
This resource implements the GET handler.
4.6.1. GET Handler
The handler expects a GET request.
In addition to what is defined in Section 4.1.2, the handler verifies
that the Client is a current member of the group. If the
verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
response. The response MUST have Content-Format set to application/
ace-groupcomm+cbor and is formatted as defined in Section 4. The
value of the 'error' field MUST be set to 0 ("Operation permitted
only to group members").
If all verifications succeed, the handler replies with a 2.05
(Content) response containing the list of policies for the group
identified by GROUPNAME. The payload of the response is formatted as
a CBOR map including only the parameter 'group_policies' defined in
Section 4.3.1 and specifying the current policies in the group. If
the KDC does not store any policy, the payload is formatted as a
zero-length CBOR byte string.
The specific format and meaning of group policies MUST be specified
in the application profile (REQ20).
4.6.1.1. Retrieve the Group Policies
A node in the group can contact the KDC to retrieve the current group
policies, by sending a CoAP GET request to the /ace-group/GROUPNAME/
policies endpoint at the KDC, where GROUPNAME is the group name, and
formatted as defined in Section 4.6.1
Figure 26 gives an overview of the exchange described above, while
Figure 27 shows an example.
Client KDC
| |
|-- Policies Request: GET ace-group/GROUPNAME/policies -->|
| |
|<----------- Policies Response: 2.05 (Content) ----------|
| |
Figure 26: Message Flow of Policies Request-Response
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Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "policies"
Payload: -
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload(in CBOR diagnostic notation):
{ "group_policies": {"exp-delta": 120} }
Figure 27: Example of Policies Request-Response
4.7. /ace-group/GROUPNAME/num
This resource implements the GET handler.
4.7.1. GET Handler
The handler expects a GET request.
In addition to what is defined in Section 4.1.2, the handler verifies
that the Client is a current member of the group. If the
verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
response. The response MUST have Content-Format set to application/
ace-groupcomm+cbor and is formatted as defined in Section 4. The
value of the 'error' field MUST be set to 0 ("Operation permitted
only to group members").
If all verifications succeed, the handler returns a 2.05 (Content)
message containing an integer that represents the version number of
the symmetric group keying material. This number is incremented on
the KDC every time the KDC updates the symmetric group keying
material, before the new keying material is distributed. This number
is stored in persistent storage.
The payload of the response is formatted as a CBOR integer.
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4.7.1.1. Retrieve the Keying Material Version
A node in the group can contact the KDC to request information about
the version number of the symmetric group keying material, by sending
a CoAP GET request to the /ace-group/GROUPNAME/num endpoint at the
KDC, where GROUPNAME is the group name, formatted as defined in
Section 4.7.1. In particular, the version is incremented by the KDC
every time the group keying material is renewed, before it's
distributed to the group members.
Figure 28 gives an overview of the exchange described above, while
Figure 29 shows an example.
Client KDC
| |
|---- Version Request: GET ace-group/GROUPNAME/num ---->|
| |
|<--------- Version Response: 2.05 (Content) -----------|
| |
Figure 28: Message Flow of Version Request-Response
Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "num"
Payload: -
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload(in CBOR diagnostic notation):
13
Figure 29: Example of Version Request-Response
4.8. /ace-group/GROUPNAME/nodes/NODENAME
This resource implements the GET, PUT, and DELETE handlers.
In addition to what is defined in Section 4.1.2, each of the handlers
performs the following two verifications.
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* The handler verifies that the Client is a current member of the
group. If the verification fails, the KDC MUST reply with a 4.03
(Forbidden) error response. The response MUST have Content-Format
set to application/ace-groupcomm+cbor and is formatted as defined
in Section 4. The value of the 'error' field MUST be set to 0
("Operation permitted only to group members").
* The handler verifies that the node name of the Client is equal to
NODENAME used in the url-path. If the verification fails, the
handler replies with a 4.03 (Forbidden) error response.
4.8.1. GET Handler
The handler expects a GET request.
If all verifications succeed, the handler replies with a 2.05
(Content) response containing both the group keying material and the
individual keying material for the Client, or information enabling
the Client to derive it. The payload of the response is formatted as
a CBOR map. The format for the group keying material is the same as
defined in the response of Section 4.3.2. The specific format of
individual keying material for group members, or of the information
to derive it, and corresponding CBOR label, MUST be specified in the
application profile (REQ27) and registered in Section 11.6.
Optionally, the KDC can make the sub-resource at ace-
group/GROUPNAME/nodes/NODENAME also Observable [RFC7641] for the
associated node. In case the KDC removes that node from the group
without having been explicitly asked for it, this allows the KDC to
send an unsolicited 4.04 (Not Found) response to the node as a
notification of eviction from the group (see Section 5).
Note that the node could have been observing also the resource at
ace-group/GROUPNAME, in order to be informed of changes in the keying
material. In such a case, this method would result in largely
overlapping notifications received for the resource at ace-group/
GROUPNAME and the sub-resource at ace-group/GROUPNAME/nodes/NODENAME.
In order to mitigate this, a node that supports the No-Response
option [RFC7967] can use it when starting the observation of the sub-
resource at ace-group/GROUPNAME/nodes/NODENAME. In particular, the
GET observation request can also include the No-Response option, with
value set to 2 (Not interested in 2.xx responses).
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4.8.1.1. Retrieve Group and Individual Keying Material
When any of the following happens, a node MUST stop using the stored
group keying material to protect outgoing messages, and SHOULD stop
using it to decrypt and verify incoming messages.
* Upon expiration of the keying material, according to what is
indicated by the KDC with the 'exp' parameter (e.g., in a Join
Response), or to a pre-configured value.
* Upon receiving a notification of revoked/renewed keying material
from the KDC, possibly as part of an update of the keying material
(rekeying) triggered by the KDC.
* Upon receiving messages from other group members without being
able to retrieve the keying material to correctly decrypt them.
This may be due to rekeying messages previously sent by the KDC,
that the Client was not able to receive or decrypt.
In either case, if it wants to continue participating in the group
communication, the Client has to request the latest keying material
from the KDC. To this end, the Client sends a CoAP GET request to
the /ace-group/GROUPNAME/nodes/NODENAME endpoint at the KDC,
formatted as specified in Section 4.8.1. The Client can request the
latest keying material from the KDC before the currently stored, old
keying material reaches its expiration time.
Note that policies can be set up, so that the Client sends a Key
Distribution Request to the KDC only after a given number of received
messages could not be decrypted (because of failed decryption
processing or inability to retrieve the necessary keying material).
It is application dependent and pertaining to the particular message
exchange (e.g., [I-D.ietf-core-oscore-groupcomm]) to set up these
policies for instructing Clients to retain incoming messages and for
how long (OPT11). This allows Clients to possibly decrypt such
messages after getting updated keying material, rather than just
consider them non valid messages to discard right away.
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After having failed to decrypt messages from another group member and
having sent a Key Distribution Request to the KDC, the Client might
end up retrieving the same, latest group keying material that it
already stores. In such a case, multiple failed decryptions might be
due to the message sender and/or the KDC that have changed their
authentication credential. Hence, the Client can retrieve such
latest authentication credentials, by sending to the KDC an
Authentication Credential Request (see Section 4.4.1.1 and
Section 4.4.2.1) and a KDC Authentication Credential Request (see
Section 4.5.1.1), respectively.
The Client can also send to the KDC a Key Distribution Request
without having been triggered by a failed decryption of a message
from another group member, if the Client wants to be sure that it
currently stores the latest group keying material. If that is the
case, the Client will receive from the KDC the same group keying
material it already stores.
Figure 30 gives an overview of the exchange described above, while
Figure 31 shows an example.
Client KDC
| |
|------------------ Key Distribution Request: --------------->|
| GET ace-group/GROUPNAME/nodes/NODENAME |
| |
|<-------- Key Distribution Response: 2.05 (Content) ---------|
| |
Figure 30: Message Flow of Key Distribution Request-Response
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Request:
Header: GET (Code=0.01)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "nodes"
Uri-Path: "c101"
Payload: -
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation,
with KEY and IND_KEY being CBOR byte strings,
and "ind-key" being the profile-specified label
for individual keying material):
{ "gkty": 13, "key": KEY, "num": 12, "ind-key": IND_KEY }
Figure 31: Example of Key Distribution Request-Response
4.8.2. PUT Handler
The PUT handler processes requests from a Client that asks for new
individual keying material, as required to process messages exchanged
in the group.
The handler expects a PUT request with empty payload.
In addition to what is defined in Section 4.1.2 and at the beginning
of Section 4.8, the handler verifies that this operation is
consistent with the set of roles that the Client has in the group
(REQ11). If the verification fails, the KDC MUST reply with a 4.00
(Bad Request) error response. The response MUST have Content-Format
set to application/ace-groupcomm+cbor and is formatted as defined in
Section 4. The value of the 'error' field MUST be set to 1 ("Request
inconsistent with the current roles").
If the KDC is currently not able to serve this request, i.e., to
generate new individual keying material for the requesting Client,
the KDC MUST reply with a 5.03 (Service Unavailable) error response.
The response MUST have Content-Format set to application/ace-
groupcomm+cbor and is formatted as defined in Section 4. The value
of the 'error' field MUST be set to 4 ("No available node
identifiers").
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If all verifications succeed, the handler replies with a 2.05
(Content) response containing newly generated, individual keying
material for the Client. The payload of the response is formatted as
a CBOR map. The specific format of newly-generated individual keying
material for group members, or of the information to derive it, and
corresponding CBOR label, MUST be specified in the application
profile (REQ27) and registered in Section 11.6.
The typical successful outcome consists in replying with newly
generated, individual keying material for the Client, as defined
above. However, application profiles of this specification MAY also
extend this handler in order to achieve different akin outcomes
(OPT12), for instance:
* Not providing the Client with newly generated, individual keying
material, but rather rekeying the whole group, i.e., providing all
the current group members with newly generated group keying
material.
* Both providing the Client with newly generated, individual keying
material, as well as rekeying the whole group, i.e., providing all
the current group members with newly generated group keying
material.
In either case, the handler may specify the new group keying material
as part of the 2.05 (Content) response.
Note that this handler is not intended to accommodate requests from a
group member to trigger a group rekeying, whose scheduling and
execution is an exclusive prerogative of the KDC (see also related
security considerations in Section 10.2).
4.8.2.1. Request to Change Individual Keying Material
A Client may ask the KDC for new, individual keying material. For
instance, this can be due to the expiration of such individual keying
material, or to the exhaustion of AEAD nonces, if an AEAD encryption
algorithm is used for protecting communications in the group. An
example of individual keying material can simply be an individual
encryption key associated with the Client. Hence, the Client may ask
for a new individual encryption key, or for new input material to
derive it.
To this end, the Client performs a Key Renewal Request-Response
exchange with the KDC, i.e., it sends a CoAP PUT request to the /ace-
group/GROUPNAME/nodes/NODENAME endpoint at the KDC, where GROUPNAME
is the group name and NODENAME is its node name, and formatted as
defined in Section 4.8.1.
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Figure 32 gives an overview of the exchange described above, while
Figure 33 shows an example.
Client KDC
| |
|------------------ Key Renewal Request: -------------->|
| PUT ace-group/GROUPNAME/nodes/NODENAME |
| |
|<-------- Key Renewal Response: 2.05 (Content) --------|
| |
Figure 32: Message Flow of Key Renewal Request-Response
Request:
Header: PUT (Code=0.03)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "nodes"
Uri-Path: "c101"
Payload: -
Response:
Header: Content (Code=2.05)
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation, with IND_KEY being a
CBOR byte string, and "ind-key" being the profile-specified
label for individual keying material):
{ "ind-key": IND_KEY }
Figure 33: Example of Key Renewal Request-Response
Note that there is a difference between the Key Renewal Request in
this section and the Key Distribution Request in Section 4.8.1.1.
The former asks the KDC for new individual keying material, while the
latters asks the KDC for the current group keying material together
with the current individual keying material.
As discussed in Section 4.8.2, application profiles of this
specification may define alternative outcomes for the Key Renewal
Request-Response exchange (OPT12), where the provisioning of new
individual keying material is replaced by or combined with the
execution of a whole group rekeying.
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4.8.3. DELETE Handler
The DELETE handler removes the node identified by NODENAME from the
group identified by GROUPNAME.
The handler expects a DELETE request with empty payload.
In addition to what is defined in Section 4.1.2, the handler verifies
that the Client is a current member of the group. If the
verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
response. The response MUST have Content-Format set to application/
ace-groupcomm+cbor and is formatted as defined in Section 4. The
value of the 'error' field MUST be set to 0 ("Operation permitted
only to group members").
If all verification succeeds, the handler performs the actions
defined in Section 5 and replies with a 2.02 (Deleted) response with
empty payload.
4.8.3.1. Leave the Group
A Client can actively request to leave the group. In this case, the
Client sends a CoAP DELETE request to the endpoint /ace-
group/GROUPNAME/nodes/NODENAME at the KDC, where GROUPNAME is the
group name and NODENAME is its node name, formatted as defined in
Section 4.8.3
Note that, after having left the group, the Client may wish to join
it again. Then, as long as the Client is still authorized to join
the group, i.e., the associated access token is still valid, the
Client can request to re-join the group directly to the KDC (see
Section 4.3.1.1), without having to retrieve a new access token from
the AS.
4.9. /ace-group/GROUPNAME/nodes/NODENAME/cred
This resource implements the POST handler.
4.9.1. POST Handler
The POST handler is used to replace the stored authentication
credential of this Client (identified by NODENAME) with the one
specified in the request at the KDC, for the group identified by
GROUPNAME.
The handler expects a POST request with payload as specified in
Section 4.3.1, with the difference that it includes only the
parameters 'client_cred', 'cnonce', and 'client_cred_verify'.
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The PoP evidence included in the 'client_cred_verify' parameter is
computed in the same way considered in Section 4.3.1 and defined by
the specific application profile (REQ14), by using the following to
build the PoP input: i) the same scope specified by the Client in the
'scope' parameter of the original Join Request; ii) the latest N_S
value stored by the Client; iii) a new N_C nonce generated by the
Client and specified in the parameter 'client_cred' of this request.
An example of PoP input to compute 'client_cred_verify' using CBOR
encoding is given in Figure 34.
It is REQUIRED of the application profiles to define the specific
formats of authentication credentials that are acceptable to use in
the group (REQ6).
In addition to what is defined in Section 4.1.2 and at the beginning
of Section 4.8, the handler verifies that this operation is
consistent with the set of roles that the node has in the group. If
the verification fails, the KDC MUST reply with a 4.00 (Bad Request)
error response. The response MUST have Content-Format set to
application/ace-groupcomm+cbor and is formatted as defined in
Section 4. The value of the 'error' field MUST be set to 1 ("Request
inconsistent with the current roles").
If the KDC cannot retrieve the 'kdcchallenge' associated with this
Client (see Section 3.3), the KDC MUST reply with a 4.00 (Bad
Request) error response, which MUST also have Content-Format
application/ace-groupcomm+cbor. The payload of the error response is
a CBOR map including a newly generated 'kdcchallenge' value. This is
specified in the 'kdcchallenge' parameter. In such a case the KDC
MUST store the newly generated value as the 'kdcchallenge' value
associated with this Client, replacing the currently stored value (if
any).
Otherwise, the handler checks that the authentication credential
specified in the 'client_cred' field is valid for the group
identified by GROUPNAME. That is, the handler checks that the
authentication credential is encoded according to the format used in
the group, is intended for the public key algorithm used in the
group, and is aligned with the possible associated parameters used in
the group. If that cannot be successfully verified, the handler MUST
reply with a 4.00 (Bad Request) error response. The response MUST
have Content-Format set to application/ace-groupcomm+cbor and is
formatted as defined in Section 4. The value of the 'error' field
MUST be set to 2 ("Authentication Credential incompatible with the
group configuration").
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Otherwise, the handler verifies the PoP evidence contained in the
'client_cred_verify' field of the request, by using the
authentication credential specified in the 'client_cred' field, as
well as the same way considered in Section 4.3.1 and defined by the
specific application profile (REQ14). If the PoP evidence does not
pass verification, the handler MUST reply with a 4.00 (Bad Request)
error response. The response MUST have Content-Format set to
application/ace-groupcomm+cbor and is formatted as defined in
Section 4. The value of the 'error' field MUST be set to 3 ("Invalid
Proof-of-Possession evidence").
If all verifications succeed, the handler performs the following
actions.
* The handler associates the authentication credential from the
'client_cred' field of the request with the node identifier
NODENAME, as well as with the access token associated with the
node identified by NODENAME.
* In the stored list of group members' authentication credentials
for the group identified by GROUPNAME, the handler replaces the
authentication credential of the node identified by NODENAME with
the authentication credential specified in the 'client_cred' field
of the request.
Then, the handler replies with a 2.04 (Changed) response, which does
not include a payload.
scope, N_S, and N_C expressed in CBOR diagnostic notation:
scope = h'826667726f7570316673656e646572'
N_S = h'018a278f7faab55a'
N_C = h'0446baefc56111bf'
scope, N_S, and N_C as CBOR encoded byte strings:
scope = 0x4f826667726F7570316673656E646572
N_S = 0x48018a278f7faab55a
N_C = 0x480446baefc56111bf
PoP input:
0x4f 826667726f7570316673656e646572
48 018a278f7faab55a 48 0446baefc56111bf
Figure 34: Example of PoP input to compute 'client_cred_verify'
using CBOR encoding
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4.9.1.1. Uploading a Authentication Credential Key
In case the KDC maintains the authentication credentials of group
members, a node in the group can contact the KDC to upload a new
authentication credential to use in the group, and to replace the
currently stored one.
To this end, the Client performs an Authentication Credential Update
Request-Response exchange with the KDC, i.e., it sends a CoAP POST
request to the /ace-group/GROUPNAME/nodes/NODENAME/cred endpoint at
the KDC, where GROUPNAME is the group name and NODENAME is its node
name.
The request is formatted as specified in Section 4.9.1.
Figure Figure 35 gives an overview of the exchange described above,
while Figure 36 shows an example.
Client KDC
| |
|----------- Authentication Credential Update Request: --------->|
| POST ace-group/GROUPNAME/nodes/NODENAME/cred |
| |
|<-- Authentication Credential Update Response: 2.04 (Changed) --|
| |
Figure 35: Message Flow of Authentication Credential Update
Request-Response
Request:
Header: POST (Code=0.02)
Uri-Host: "kdc.example.com"
Uri-Path: "ace-group"
Uri-Path: "g1"
Uri-Path: "nodes"
Uri-Path: "c101"
Uri-Path: "pub-key"
Content-Format: "application/ace-groupcomm+cbor"
Payload (in CBOR diagnostic notation, with AUTH_CRED
and POP_EVIDENCE being CBOR byte strings):
{ "client_cred": AUTH_CRED, "cnonce": h'0446baefc56111bf',
"client_cred_verify": POP_EVIDENCE }
Response:
Header: Changed (Code=2.04)
Payload: -
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Figure 36: Example of Authentication Credential Update Request-
Response
Additionally, after updating its own authentication credential, a
group member MAY send a number of requests including an identifier of
the updated authentication credential, to notify other group members
that they have to retrieve it. How this is done depends on the group
communication protocol used, and therefore is application profile
specific (OPT13).
5. Removal of a Group Member
A Client identified by NODENAME may be removed from a group
identified by GROUPNAME where it is a member, for example due to the
following reasons.
1. The Client explicitly asks to leave the group, as defined in
Section 4.8.3.1.
2. The node has been found compromised or is suspected so.
3. The Client's authorization to be a group member with the current
roles is not valid anymore, i.e., the access token has expired or
has been revoked. If the AS provides token introspection (see
Section 5.9 of [RFC9200]), the KDC can optionally use it and
check whether the Client is still authorized.
In either case, the KDC performs the following actions.
* The KDC removes the Client from the list of current members of the
group.
* In case of forced eviction, i.e., for cases 2 and 3 above, the KDC
deletes the authentication credential of the removed Client, if it
acts as repository of authentication credentials for group
members.
* If the removed Client is registered as an observer of the group-
membership resource at ace-group/GROUPNAME, the KDC removes the
Client from the list of observers of that resource.
* If the sub-resource nodes/NODENAME was created for the removed
Client, the KDC deletes that sub-resource.
In case of forced eviction, i.e., for cases 2 and 3 above, the KDC
MAY explicitly inform the removed Client, by means of the
following methods.
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- If the evicted Client implements the 'control_uri' resource
specified in Section 4.3.1, the KDC sends a DELETE request,
targeting the URI specified in the 'control_uri' parameter of
the Join Request (see Section 4.3.1).
- If the evicted Client is observing its associated sub-resource
at ace-group/GROUPNAME/nodes/NODENAME (see Section 4.8.1), the
KDC sends an unsolicited 4.04 (Not Found) error response, which
does not include the Observe option and indicates that the
observed resource has been deleted (see Section 3.2 of
[RFC7641]).
The response MUST have Content-Format set to application/ace-
groupcomm+cbor and is formatted as defined in Section 4. The
value of the 'error' field MUST be set to 5 ("Group membership
terminated").
* If the application requires forward security or the used
application profile requires so, the KDC MUST generate new group
keying material and securely distribute it to all the current
group members except the leaving node (see Section 6).
6. Group Rekeying Process
A group rekeying is started and driven by the KDC. The KDC is not
intended to accommodate explicit requests from group members to
trigger a group rekeying. That is, the scheduling and execution of a
group rekeying is an exclusive prerogative of the KDC. Reasons that
can trigger a group rekeying are a change in the group membership,
the current group keying material approaching its expiration time, or
a regularly scheduled update of the group keying material.
The KDC can perform a group rekeying before the current group keying
material expires, unless it is acceptable or there are reasons to
temporarily pause secure communications in the group, following the
expiration of the current keying material.
The KDC MUST increment the version number NUM of the current keying
material, before distributing the newly generated keying material
with version number NUM+1 to the group. Once completed the group
rekeying, the KDC MUST delete the old keying material and SHOULD
store the newly distributed keying material in persistent storage.
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Distributing the new group keying material requires the KDC to send
multiple rekeying messages to the group members. Depending on the
rekeying scheme used in the group and the reason that has triggered
the rekeying process, each rekeying message can be intended to one or
multiple group members, hereafter referred to as target group
members. The KDC MUST support at least the "Point-to-Point" group
rekeying scheme in Section 6.1 and MAY support additional ones.
Each rekeying message MUST have Content-Format set to application/
ace-groupcomm+cbor and its payload formatted as a CBOR map, which
MUST include at least the information specified in the Key
Distribution Response message (see Section 4.3.2), i.e., the
parameters 'gkty', 'key', and 'num' defined in Section 4.3.1. The
CBOR map MAY include the parameter 'exp', as well as the parameter
'mgt_key_material' specifying new administrative keying material for
the target group members, if relevant for the used rekeying scheme.
A rekeying message may include additional information, depending on
the rekeying scheme used in the group, the reason that has triggered
the rekeying process, and the specific target group members. In
particular, if the group rekeying is performed due to one or multiple
Clients that have joined the group and the KDC acts as repository of
authentication credentials of the group members, then a rekeying
message MAY also include the authentication credentials that those
Clients use in the group, together with the roles and node identifier
that the corresponding Client has in the group. It is RECOMMENDED to
specify this information by means of the parameters 'creds',
'peer_roles', and 'peer_identifiers', like done in the Join Response
message (see Section 4.3.1).
The complete format of a rekeying message, including the encoding and
content of the 'mgt_key_material' parameter, has to be defined in
separate specifications aimed at profiling the used rekeying scheme
in the context of the used application profile of this specification.
As a particular case, an application profile of this specification
MAY define additional information to include in rekeying messages for
the "Point-to-Point" group rekeying scheme in Section 6.1 (OPT14).
Consistently with the used group rekeying scheme, the actual delivery
of rekeying messages can occur through different approaches, as
discussed in the following Section 6.1 and Section 6.2.
The possible, temporary misalignment of the keying material stored by
the different group members due to a group rekeying is discussed in
Section 6.3. Further security considerations related to the group
rekeying process are compiled in Section 10.2.
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6.1. Point-to-Point Group Rekeying
This approach consists in the KDC sending one individual rekeying
message to each target group member. In particular, the rekeying
message is protected by means of the security association between the
KDC and the target group member in question, as per the used
application profile of this specification and the used transport
profile of ACE.
This is the approach taken by the basic "Point-to-Point" group
rekeying scheme, that the KDC can explicitly signal in the Join
Response (see Section 4.3.1), through the 'rekeying_scheme' parameter
specifying the value 0.
When taking this approach in the group identified by GROUPNAME, the
KDC can practically deliver the rekeying messages to the target group
members in different, co-existing ways.
* The KDC SHOULD make the ace-group/GROUPNAME resource Observable
[RFC7641]. Thus, upon performing a group rekeying, the KDC can
distribute the new group keying material through individual
notification responses sent to the target group members that are
also observing that resource.
In case the KDC deletes the group (and thus deletes the ace-group/
GROUPNAME resource), relying on CoAP Observe as discussed above
also allows the KDC to send an unsolicited 4.04 (Not Found)
response to each observer group member, as a notification of group
termination. The response MUST have Content-Format set to
application/ace-groupcomm+cbor and is formatted as defined in
Section 4. The value of the 'error' field MUST be set to 6
("Group deleted").
* If a target group member specified a URI in the 'control_uri'
parameter of the Join Request upon joining the group (see
Section 4.3.1), the KDC can provide that group member with the new
group keying material by sending a unicast POST request to that
URI.
A Client that does not plan to observe the ace-group/GROUPNAME
resource at the KDC SHOULD provide a URI in the 'control_uri'
parameter of the Join Request upon joining the group.
If the KDC has to send a rekeying message to a target group member,
but this did not include the 'control_uri' parameter in the Join
Request and is not a registered observer for the ace-group/GROUPNAME
resource, then that target group member would not be able to
participate to the group rekeying. Later on, after having repeatedly
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failed to successfully exchange secure messages in the group, that
group member can retrieve the current group keying material from the
KDC, by sending a GET request to ace-group/GROUPNAME or ace-
group/GROUPNAME/nodes/NODENAME (see Section 4.3.2 and Section 4.8.1,
respectively).
6.2. One-to-Many Group Rekeying
This section provides high-level recommendations on how the KDC can
rekey a group by means of a more efficient and scalable group
rekeying scheme, e.g., [RFC2093][RFC2094][RFC2627]. That is, each
rekeying message might be, and likely is, intended to multiple target
group members, and thus can be delivered to the whole group, although
possible to decrypt only for the actual target group members.
This yields an overall lower number of rekeying messages, thus
potentially reducing the overall time required to rekey the group.
On the other hand, it requires the KDC to provide and use additional
administrative keying material to protect the rekeying messages, and
to additionally sign them to ensure source authentication (see
Section 6.2.1). Typically, this pays off in large-scale groups,
where the introduced performance overhead is less than the one
experienced by rekeying the group in a point-to-point fashion (see
Section 6.1).
The exact set of rekeying messages to send, their content and format,
the administrative keying material to use to protect them, as well as
the set of target group members depend on the specific group rekeying
scheme, and are typically affected by the reason that has triggered
the group rekeying. Details about the data content and format of
rekeying messages have to be defined by separate documents profiling
the use of the group rekeying scheme, in the context of the used
application profile of this specification.
When one of these group rekeying schemes is used, the KDC provides a
number of related information to a Client joining the group in the
Join Response message (see Section 4.3.1). In particular,
'rekeying_scheme' identifies the rekeying scheme used in the group
(if no default can be assumed); 'control_group_uri', if present,
specifies a URI with a multicast address where the KDC will send the
rekeying messages for that group; 'mgt_key_material' specifies a
subset of the administrative keying material intended for that
particular joining Client to have, as used to protect the rekeying
messages sent to the group when intended also to that joining Client.
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Rekeying messages can be protected at the application layer, by using
COSE and the administrative keying material as prescribed by the
specific group rekeying scheme (see Section 6.2.1). After that, the
delivery of protected rekeying messages to the intended target group
members can occur in different ways, such as the following ones.
* Over multicast - In this case, the KDC simply sends a rekeying
message as a CoAP request addressed to the multicast URI specified
in the 'control_group_uri' parameter of the Join Response (see
Section 4.3.1).
If a particular rekeying message is intended to a single target
group member, the KDC may alternatively protect the message using
the security association with that group member, and deliver the
message like when using the "Point-to-Point" group rekeying scheme
(see Section 6.1).
* Through a pub-sub communication model - In this case, the KDC acts
as publisher and publishes each rekeying message to a specific
"rekeying topic", which is associated with the group and is hosted
at a broker server. Following their group joining, the group
members subscribe to the rekeying topic at the broker, thus
receiving the group rekeying messages as they are published by the
KDC.
In order to make such message delivery more efficient, the
rekeying topic associated with a group can be further organized
into subtopics. For instance, the KDC can use a particular
subtopic to address a particular set of target group members
during the rekeying process, as possibly aligned to a similar
organization of the administrative keying material (e.g., a key
hierarchy).
The setup of rekeying topics at the broker as well as the
discovery of the topics at the broker for group members are
application specific. A possible way is for the KDC to provide
such information in the Join Response message (see Section 4.3.1),
by means of a new parameter analogous to 'control_group_uri' and
specifying the URI(s) of the rekeying topic(s) that a group member
has to subscribe to at the broker.
Regardless the specifically used delivery method, the group rekeying
scheme can perform a possible roll-over of the administrative keying
material through the same sent rekeying messages. Actually, such a
roll-over occurs every time a group rekeying is performed upon the
leaving of group members, which have to be excluded from future
communications in the group.
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From a high level point of view, each group member stores only a
subset of the overall administrative keying material, obtained upon
joining the group. Then, when a group rekeying occurs:
* Each rekeying message is protected by using a (most convenient)
key from the administrative keying material such that: i) the used
key is not stored by any node leaving the group, i.e., the key is
safe to use and does not have to be renewed; and ii) the used key
is stored by all the target group members, that indeed have to be
provided with new group keying material to protect communications
in the group.
* Each rekeying message includes not only the new group keying
material intended to all the rekeyed group members, but also any
new administrative keys that: i) are pertaining to and supposed to
be stored by the target group members; and ii) had to be updated
since leaving group members store the previous version.
Further details depend on the specific rekeying scheme used in the
group.
6.2.1. Protection of Rekeying Messages
When using a group rekeying scheme relying on one-to-many rekeying
messages, the actual data content of each rekeying message is
prepared according to what the rekeying scheme prescribes.
Then, the KDC can protect the rekeying message as defined below. The
used encryption algorithm which SHOULD be the same one used to
protect communications in the group. The method defined below
assumes that the following holds for the management keying material
specified in the 'mgt_key_material' parameter of the Join Response
(see Section 4.3.1).
* The included symmetric encryption keys are accompanied by a
corresponding and unique key identifier assigned by the KDC.
* A Base IV is also included, with the same size of the AEAD nonce
considered by the encryption algorithm to use.
First, the KDC computes a COSE_Encrypt0 object as follows.
* The encryption key to use is selected from the administrative
keying material, as defined by the rekeying scheme used in the
group.
* The plaintext is the actual data content of the rekeying message.
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* The Additional Authenticated Data (AAD) is empty, unless otherwise
specified by separate documents profiling the use of the group
rekeying scheme.
* Since the KDC is the only sender of rekeying messages, the AEAD
nonce can be computed as follows, where NONCE_SIZE is the size in
bytes of the AEAD nonce. Separate documents profiling the use of
the group rekeying scheme may define alternative ways to compute
the AEAD nonce.
The KDC considers the following values.
- COUNT, as a 2-byte unsigned integer associated with the used
encryption key. Its value is set to 0 when starting to perform
a new group rekeying instance, and is incremented after each
use of the encryption key.
- NEW_NUM, as the version number of the new group keying material
to distribute in this rekeying instance, left-padded with
zeroes to exactly NONCE_SIZE - 2 bytes.
Then, the KDC computes a Partial IV as the byte string
concatenation of COUNT and NEW_NUM, in this order. Finally, the
AEAD nonce is computed as the XOR between the Base IV and the
Partial IV.
In order to comply with the security requirements of AEAD
encryption algorithms, the KDC MUST NOT use the same pair (AEAD
encryption key, AEAD nonce). For example, this includes not using
the same encryption key from the administrative keying material
more than 2^16 times during the same rekeying instance.
* The protected header of the COSE_Encrypt0 object MUST include the
following parameters.
- 'alg', specifying the used encryption algorithm.
- 'kid', specifying the identifier of the encryption key from the
administrative keying material used to protect this rekeying
message.
* The unprotected header of the COSE_Encrypt0 object MUST include
the 'Partial IV' parameter, with value the Partial IV computed
above.
In order to ensure source authentication, each rekeying message
protected with the administrative keying material MUST be signed by
the KDC. To this end, the KDC computes a countersignature of the
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COSE_Encrypt0 object, as described in Sections 3.2 and 3.3 of
[RFC9338]. In particular, the following applies when computing the
countersignature.
* The Countersign_structure contains the context text string
"CounterSignature0".
* The private key of the KDC is used as signing key.
* The payload is the ciphertext of the COSE_Encrypt0 object.
* The Additional Authenticated Data (AAD) is empty, unless otherwise
specified by separate documents profiling the use of a group
rekeying scheme.
* The protected header of the signing object MUST include the
parameter 'alg', specifying the used signature algorithm.
If source authentication of messages exchanged in the group is also
ensured by means of signatures, then rekeying messages MUST be signed
using the same signature algorithm and related parameters. Also, the
KDC's authentication credential including the public key to use for
signature verification MUST be provided in the Join Response through
the 'kdc_cred' parameter, together with the corresponding proof-of-
possession (PoP) evidence in the 'kdc_cred_verify' parameter.
If source authentication of messages exchanged in the group is not
ensured by means of signatures, then the KDC MUST provide its
authentication credential together with a corresponding PoP evidence
as part of the management keying material specified in the
'mgt_key_material' parameter of the Join Response (see
Section 4.3.1). It is RECOMMENDED to specify this information by
using the same format and encoding used for the parameters
'kdc_cred', 'kdc_nonce', and 'kdc_cred_verify' in the Join Response.
It is up to separate documents profiling the use of the group
rekeying scheme to specify such details.
After that, the KDC specifies the computed countersignature in the
'COSE_Countersignature0' header parameter of the COSE_Encrypt0
object.
Finally, the KDC specifies the COSE_Encrypt0 object as payload of a
CoAP request, which is sent to the target group members as per the
used message delivery method.
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6.3. Misalignment of Group Keying Material
A group member can receive a message shortly after the group has been
rekeyed, and new keying material has been distributed by the KDC. In
the following two cases, this may result in misaligned keying
material between the group members.
In the first case, the sender protects a message using the old group
keying material. However, the recipient receives the message after
having received the new group keying material, hence not being able
to correctly process it. A possible way to limit the impact of this
issue is to preserve the old, recent group keying material for a
maximum amount of time defined by the application, during which it is
used solely for processing incoming messages. By doing so, the
recipient can still temporarily process received messages also by
using the old, retained group keying material. Note that a former
(compromised) group member can take advantage of this by sending
messages protected with the old, retained group keying material.
Therefore, a conservative application policy should not admit the
storage of old group keying material. Eventually, the sender will
have obtained the new group keying material too, and can possibly re-
send the message protected with such keying material.
In the second case, the sender protects a message using the new group
keying material, but the recipient receives that message before
having received the new group keying material. Therefore, the
recipient would not be able to correctly process the message and
hence discards it. If the recipient receives the new group keying
material shortly after that and the application at the sender
endpoint performs retransmissions, the former will still be able to
receive and correctly process the message. In any case, the
recipient should actively ask the KDC for the latest group keying
material according to an application-defined policy, for instance
after a given number of unsuccessfully decrypted incoming messages.
7. Extended Scope Format
This section defines an extended format of binary encoded scope,
which additionally specifies the semantics used to express the same
access control information from the corresponding original scope.
As also discussed in Section 3.2, this enables a Resource Server to
unambiguously process a received access token, also in case the
Resource Server runs multiple applications or application profiles
that involve different scope semantics.
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The extended format is intended only for the 'scope' claim of access
tokens, for the cases where the claim takes as value a CBOR byte
string. That is, the extended format does not apply to the 'scope'
parameter included in ACE messages, i.e., the Authorization Request
and Authorization Response exchanged between the Client and the
Authorization Server (see Sections 5.8.1 and 5.8.2 of [RFC9200]), the
AS Request Creation Hints message from the Resource Server (see
Section 5.3 of [RFC9200]), and the Introspection Response from the
Authorization Server (see Section 5.9.2 of [RFC9200]).
The value of the 'scope' claim following the extended format is
composed as follows. Given the original scope using a semantics SEM
and encoded as a CBOR byte string, the corresponding extended scope
consists of the same CBOR byte string enclosed by a CBOR tag
[RFC8949], whose tag number identifies the semantics SEM.
The resulting tagged CBOR byte string is used as value of the 'scope'
claim of the access token.
Figure 37 and Figure 38 build on the examples in Section 3.2, and
show the corresponding extended scopes.
gname = tstr
permissions = uint .bits roles
roles = &(
Requester: 1,
Responder: 2,
Monitor: 3,
Verifier: 4
)
scope_entry = AIF_Generic<gname, permissions>
scope = << [ + scope_entry ] >>
extended_scope = #6.TAG_FOR_THIS_SEMANTICS(scope)
Figure 37: Example CDLL definition of scope, using the default
Authorization Information Format
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gname = tstr
role = tstr
scope_entry = [ gname , ? ( role / [ 2*role ] ) ]
scope = << [ + scope_entry ] >>
extended_scope = #6.TAG_FOR_THIS_SEMANTICS(scope)
Figure 38: CDLL definition of scope, using as example group name
encoded as tstr and role as tstr
The usage of the extended scope format is not limited to application
profiles of this specification or to applications based on group
communication. Rather, it is generally applicable to any application
and application profile where access control information in the
access token is expressed as a binary encoded scope.
Applications and application profiles using the extended format of
scope have to specify which CBOR tag from [CBOR.Tags] is used for
identifying the scope semantics, or to register a new CBOR tag if a
suitable one does not exist already (REQ28). In case there is an
already existing, suitable CBOR tag, a new CBOR tag should not be
registered in order to avoid codepoint squatting.
If the binary encoded scope uses a semantics associated with a
registered CoAP Content-Format [RFC7252][CoAP.Content.Formats], then
a suitable CBOR tag associated with that CoAP Content-Format would
already be registered, as defined in Section 4.3 of [RFC9277].
This is especially relevant when the binary encoded scope uses the
AIF format. That is, it is expected that the definition of an AIF
specific data model comes together with the registration of CoAP
Content-Formats for the relevant combinations of its Toid and Tperm
values. As discussed above, this yields the automatic registration
of the CBOR tags associated with those CoAP Content-Formats.
8. ACE Groupcomm Parameters
This specification defines a number of parameters used during the
second part of the message exchange, after the exchange of Token
Transfer Request and Response. The table below summarizes them, and
specifies the CBOR key to use instead of the full descriptive name.
Note that the media type application/ace-groupcomm+cbor MUST be used
when these parameters are transported in the respective message
fields.
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+-----------------------+------+---------------------+------------+
| Name | CBOR | CBOR Type | Reference |
| | Key | | |
+-----------------------+------+---------------------+------------+
| error | TBD | int | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| error_description | TBD | tstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| gid | TBD | array | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| gname | TBD | array of tstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| guri | TBD | array of tstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| scope | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| get_creds | TBD | array / | [RFC-XXXX] |
| | | Simple value "null" | |
+-----------------------+------+---------------------+------------+
| client_cred | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| cnonce | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| client_cred_verify | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| creds_repo | TBD | tstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| control_uri | TBD | tstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| gkty | TBD | int / tstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| key | TBD | See the "ACE | [RFC-XXXX] |
| | | Groupcomm Key | |
| | | Types" registry | |
+-----------------------+------+---------------------+------------+
| num | TBD | int | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| ace_groupcomm_profile | TBD | int | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| exp | TBD | int | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| creds | TBD | array | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| peer_roles | TBD | array | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| peer_identifiers | TBD | array | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| group_policies | TBD | map | [RFC-XXXX] |
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+-----------------------+------+---------------------+------------+
| kdc_cred | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| kdc_nonce | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| kdc_cred_verify | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| rekeying_scheme | TBD | int | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| mgt_key_material | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| control_group_uri | TBD | tstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| sign_info | TBD | array | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
| kdcchallenge | TBD | bstr | [RFC-XXXX] |
+-----------------------+------+---------------------+------------+
Figure 39: ACE Groupcomm Parameters
Note to RFC Editor: In Figure 39, please replace all occurrences of
"[RFC-XXXX]" with the RFC number of this specification and delete
this paragraph.
The KDC is expected to support all the parameters above. Instead, a
Client can support only a subset of such parameters, depending on the
roles it expects to take in the joined groups or on other conditions
defined in application profiles of this specification.
In the following, the parameters are categorized according to the
support expected by Clients. That is, a Client that supports a
parameter is able to: i) use and specify it in a request message to
the KDC; and ii) understand and process it if specified in a response
message from the KDC. It is REQUIRED of application profiles of this
specification to sort their newly defined parameters according to the
same categorization (REQ29).
Note that the actual use of a parameter and its inclusion in a
message depends on the specific exchange, the specific Client and
group involved, as well as what is defined in the used application
profile of this specification.
A Client MUST support the following parameters.
* 'scope', 'gkty', 'key', 'num', 'exp', 'gid', 'gname', 'guri',
'creds', 'peer_identifiers', 'ace_groupcomm_profile',
'control_uri', 'rekeying_scheme'.
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A Client SHOULD support the following parameter.
* 'get_creds'. That is, not supporting this parameter would yield
the inconvenient and undesirable behavior where: i) the Client
does not ask for the other group members' authentication
credentials upon joining the group (see Section 4.3.1.1); and ii)
later on as a group member, the Client only retrieves the
authentication credentials of all group members (see
Section 4.4.2.1).
A Client MAY support the following optional parameters. Application
profiles of this specification MAY define that Clients must or should
support these parameters instead (OPT15).
* 'error', 'error_description'.
The following conditional parameters are relevant only if specific
conditions hold. It is REQUIRED of application profiles of this
specification to define whether Clients must, should, or may support
these parameters, and under which circumstances (REQ30).
* 'client_cred', 'cnonce', 'client_cred_verify'. These parameters
are relevant for a Client that has an authentication credential to
use in a joined group.
* 'kdcchallenge'. This parameter is relevant for a Client that has
an authentication credential to use in a joined group and that
provides the access token to the KDC through a Token Transfer
Request (see Section 3.3).
* 'creds_repo'. This parameter is relevant for a Client that has an
authentication credential to use in a joined group and that makes
it available from a key repository different than the KDC.
* 'group_policies'. This parameter is relevant for a Client that is
interested in the specific policies used in a group, but it does
not know them or cannot become aware of them before joining that
group.
* 'peer_roles'. This parameter is relevant for a Client that has to
know about the roles of other group members, especially when
retrieving and handling their corresponding authentication
credentials.
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* 'kdc_nonce', 'kdc_cred', 'kdc_cred_verify'. These parameters are
relevant for a Client that joins a group for which, as per the
used application profile of this specification, the KDC has an
associated authentication credential and this is required for the
correct group operation.
* 'mgt_key_material'. This parameter is relevant for a Client that
supports an advanced rekeying scheme possibly used in the group,
such as based on one-to-many rekeying messages sent over IP
multicast.
* 'control_group_uri'. This parameter is relevant for a Client that
supports the hosting of local resources each associated with a
group (hence acting as CoAP server) and the reception of one-to-
many requests sent to those resources by the KDC (e.g., over IP
multicast), targeting multiple members of the corresponding group.
Examples of related management operations that the KDC can perform
by this means are the eviction of group members and the execution
of a group rekeying process through an advanced rekeying scheme,
such as based on one-to-many rekeying messages.
9. ACE Groupcomm Error Identifiers
This specification defines a number of values that the KDC can
include as error identifiers, in the 'error' field of an error
response with Content-Format application/ace-groupcomm+cbor.
+-------+---------------------------------------------+
| Value | Description |
+-------+---------------------------------------------+
| 0 | Operation permitted only to group members |
+-------+---------------------------------------------+
| 1 | Request inconsistent with the current roles |
+-------+---------------------------------------------+
| 2 | Authentication credential incompatible with |
| | the group configuration |
+-------+---------------------------------------------+
| 3 | Invalid proof-of-possession evidence |
+-------+---------------------------------------------+
| 4 | No available node identifiers |
+-------+---------------------------------------------+
| 5 | Group membership terminated |
+-------+---------------------------------------------+
| 6 | Group deleted |
+-------+---------------------------------------------+
Figure 40: ACE Groupcomm Error Identifiers
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A Client supporting the 'error' parameter (see Section 4.1.2 and
Section 8) and able to understand the specified error may use that
information to determine what actions to take next. If it is
included in the error response and supported by the Client, the
'error_description' parameter may provide additional context.
In particular, the following guidelines apply, and application
profiles of this specification can define more detailed actions for
the Client to take when learning that a specific error has occurred.
* In case of error 0, the Client should stop sending the request in
question to the KDC. Rather, the Client should first join the
targeted group. If it has not happened already, this first
requires the Client to obtain an appropriate access token
authorizing access to the group and provide it to the KDC.
* In case of error 1, the Client as a group member should re-join
the group with all the roles needed to perform the operation in
question. This might require the Client to first obtain a new
access token and provide it to the KDC, if the current access
token does not authorize to take those roles in the group. For
operations admitted to a Client which is not a group member (e.g.,
an external signature verifier), the Client should first obtain a
new access token authorizing to also have the missing roles.
* In case of error 2, the Client has to obtain or self-generate a
different asymmetric key pair, as aligned to the publick key
algorithms and parameters used in the targeted group. After that,
the Client should provide the KDC with its new authentication
credential, consistent with the format used in the targeted group
and including the new public key.
* In case of error 3, the Client should ensure to be computing its
proof-of-possession evidence by correctly using the parameters and
procedures defined in the used application profile of this
specification. In an unattended setup, it might be not possible
for a Client to autonomously diagnose the error and take an
effective next action to address it.
* In case of error 4, the Client should wait for a certain (pre-
configured) amount of time, before trying re-sending its request
to the KDC.
* In case of error 5, the Client may try joining the group again.
This might require the Client to first obtain a new access token
and provide it to the KDC, e.g., if the current access token has
expired.
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* In case of error 6, the Client should clean up its state regarding
the group, just like if it has left the group with no intention to
re-join it.
10. Security Considerations
Security considerations are inherited from the ACE framework
[RFC9200], and from the specific transport profile of ACE used
between the Clients and the KDC, e.g., [RFC9202] and [RFC9203].
Furthermore, the following security considerations apply.
10.1. Secure Communication in the Group
When a group member receives a message from a certain sender for the
first time since joining the group, it needs to have a mechanism in
place to avoid replayed messages and to assert their freshness, e.g.,
Appendix B.1.2 of [RFC8613] or Section 10 of
[I-D.ietf-core-oscore-groupcomm]. Such a mechanism aids the
recipient group member also in case it has rebooted and lost the
security state used to protect previous group communications with
that sender.
By its nature, the KDC is invested with a large amount of trust,
since it acts as generator and provider of the symmetric keying
material used to protect communications in each of its groups. While
details depend on the specific communication and security protocols
used in the group, the KDC is in the position to decrypt messages
exchanged in the group as if it was also a group member, as long as
those are protected through commonly shared group keying material.
A compromised KDC would thus put the attacker in the same position,
which also means that:
* The attacker can generate and control new group keying material,
hence possibly rekeying the group and evicting certain group
members as part of a broader attack.
* The attacker can actively participate to communications in a group
even without been authorized to join it, and can allow further
unauthorized entities to do so.
* The attacker can build erroneous associations between node
identifiers and group members' authentication credentials.
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On the other hand, as long as the security protocol used in the group
ensures source authentication of messages (e.g., by means of
signatures), the KDC is not able to impersonate group members since
it does not have their private keys.
Further security considerations are specific of the communication and
security protocols used in the group, and thus have to be provided by
those protocols and complemented by the application profiles of this
specification using them.
10.2. Update of Group Keying Material
The KDC can generate new group keying material and provide it to the
group members (rekeying) through the rekeying scheme used in the
group, as discussed in Section 6.
In particular, the KDC must renew the group keying material latest
upon its expiration. Before then, the KDC may also renew the group
keying material on a regular or periodical fashion.
Unless otherwise defined by an application profile of this
specification, the KDC SHOULD renew the group keying material upon a
group membership change. In particular, since the minimum number of
group members is one, the KDC SHOULD provide also a Client joining an
empty group with new keying material never used before in that group.
Similarly, the KDC SHOULD provide new group keying material also to a
Client that remains the only member in the group after the leaving of
other group members.
Note that the considerations in Section 10.1 about dealing with
replayed messages still hold, even in case the KDC rekeys the group
upon every single joining of a new group member. However, if the KDC
has renewed the group keying material upon a group member's joining,
and the time interval between the end of the rekeying process and
that member's joining is sufficiently small, then that group member
is also on the safe side, since it would not accept replayed messages
protected with the old group keying material previous to its joining.
Once a joining node has obtained the new, latest keying material
through a Join Response from the KDC (see Section 4.3.1.1), the
joining node becomes able to read any message that was exchanged in
the group and protected with that keying material. This is the case
if the KDC provides the current group members with the new, latest
keying material before completing the joining procedure. However,
the joining node is not able to read messages exchanged in the group
and protected with keying material older than the one provided in the
Join Response, i.e., having a strictly lower version number NUM.
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A node that has left the group should not expect any of its outgoing
messages to be successfully processed, if received by other nodes in
the group after its leaving, due to a possible group rekeying
occurred before the message reception.
The KDC may enforce a rekeying policy that takes into account the
overall time required to rekey the group, as well as the expected
rate of changes in the group membership. That is, the KDC may not
rekey the group at each and every group membership change, for
instance if members' joining and leaving occur frequently and
performing a group rekeying takes too long. Instead, the KDC might
rekey the group after a minimum number of group members have joined
or left within a given time interval, or after a maximum amount of
time since the last group rekeying was completed, or yet during
predictable network inactivity periods.
However, this would result in the KDC not constantly preserving
backward and forward security in the group. That is:
* Newly joining group members would be able to access the keying
material used before their joining, and thus they could access
past group communications if they have recorded old exchanged
messages. This might still be acceptable for some applications
and in situations where the new group members are freshly deployed
through strictly controlled procedures.
* The leaving group members would remain able to access upcoming
group communications, as protected with the current keying
material that has not been updated. This is typically
undesirable, especially if the leaving group member is compromised
or suspected to be, and it might have an impact or compromise the
security properties of the protocols used in the group to protect
messages exchanged among the group members.
The KDC should renew the group keying material in case it has
rebooted, even in case it stores the whole group keying material in
persistent storage. This assumes that the secure associations with
the current group members as well as any administrative keying
material required to rekey the group are also stored in persistent
storage.
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However, if the KDC relies on Observe notifications to distribute the
new group keying material, the KDC would have lost all the current
ongoing Observations with the group members after rebooting, and the
group members would continue using the old group keying material.
Therefore, the KDC will rather rely on each group member asking for
the new group keying material (see Section 4.3.2.1 and
Section 4.8.1.1), or rather perform a group rekeying by actively
sending rekeying messages to group members as discussed in Section 6.
The KDC needs to have a mechanism in place to detect DoS attacks from
nodes repeatedly performing actions that might trigger a group
rekeying. Such actions can include leaving and/or re-joining the
group at high rates, or often asking the KDC for new indidivual
keying material. Ultimately, the KDC can resort to removing these
nodes from the group and (temperorarily) preventing them from joining
the group again.
The KDC also needs to have a congestion control mechanism in place,
in order to avoid network congestion upon distributing new group
keying material. For example, CoAP and Observe give guidance on such
mechanisms, see Section 4.7 of [RFC7252] and Section 4.5.1 of
[RFC7641].
10.3. Block-Wise Considerations
If the Block-Wise CoAP options [RFC7959] are used, and the keying
material is updated in the middle of a Block-Wise transfer, the
sender of the blocks just changes the group keying material to the
updated one and continues the transfer. As long as both sides get
the new group keying material, updating the group keying material in
the middle of a transfer will not cause any issue. Otherwise, the
sender will have to transmit the message again, when receiving an
error message from the recipient.
Compared to a scenario where the transfer does not use Block-Wise,
depending on how fast the group keying material is changed, the group
members might consume a larger amount of the network bandwidth by
repeatedly resending the same blocks, which might be problematic.
11. IANA Considerations
This document has the following actions for IANA.
Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
with the RFC number of this specification and delete this paragraph.
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11.1. Media Type Registrations
This specification registers the 'application/ace-groupcomm+cbor'
media type for messages of the protocols defined in this document
following the ACE exchange and carrying parameters encoded in CBOR.
This registration follows the procedures specified in [RFC6838].
Type name: application
Subtype name: ace-groupcomm+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Must be encoded as a CBOR map containing the
parameters defined in [RFC-XXXX].
Security considerations: See Section 10 of [RFC-XXXX].
Interoperability considerations: N/A
Published specification: [RFC-XXXX]
Applications that use this media type: The type is used by
Authorization Servers, Clients, and Resource Servers that support the
ACE groupcomm framework as specified in [RFC-XXXX].
Fragment identifier considerations: N/A
Additional information: N/A
Person & email address to contact for further information: ACE WG
mailing list (ace@ietf.org) or IETF Applications and Real-Time Area
(art@ietf.org)
Intended usage: COMMON
Restrictions on usage: None
Author/Change controller: IETF
Provisional registration: No
11.2. CoAP Content-Formats
IANA is asked to register the following entry to the "CoAP Content-
Formats" registry within the "CoRE Parameters" registry group.
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Media Type: application/ace-groupcomm+cbor
Encoding: -
ID: TBD
Reference: [RFC-XXXX]
11.3. OAuth Parameters
IANA is asked to register the following entries in the "OAuth
Parameters" registry following the procedure specified in
Section 11.2 of [RFC6749].
* Parameter name: sign_info
* Parameter usage location: client-rs request, rs-client response
* Change Controller: IETF
* Specification Document(s): [RFC-XXXX]
* Parameter name: kdcchallenge
* Parameter usage location: rs-client response
* Change Controller: IETF
* Specification Document(s): [RFC-XXXX]
11.4. OAuth Parameters CBOR Mappings
IANA is asked to register the following entries in the "OAuth
Parameters CBOR Mappings" registry following the procedure specified
in Section 8.10 of [RFC9200].
* Name: sign_info
* CBOR Key: TBD (range -256 to 255)
* Value Type: Simple value "null" / array
* Reference: [RFC-XXXX]
* Name: kdcchallenge
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* CBOR Key: TBD (range -256 to 255)
* Value Type: Byte string
* Reference: [RFC-XXXX]
11.5. Interface Description (if=) Link Target Attribute Values
IANA is asked to register the following entry in the "Interface
Description (if=) Link Target Attribute Values" registry within the
"CoRE Parameters" registry group.
* Attribute Value: ace.group
* Description: The 'ace group' interface is used to provision keying
material and related information and policies to members of a
group using the ACE framework.
* Reference: [RFC-XXXX]
11.6. ACE Groupcomm Parameters
This specification establishes the "ACE Groupcomm Parameters" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
The registry has been created to use the "Expert Review" registration
procedure [RFC8126]. Expert review guidelines are provided in
Section 11.13. Values in this registry are covered by different
registration policies as indicated. It should be noted that, in
addition to the expert review, some portions of the registry require
a specification, potentially a Standards Track RFC, to be supplied as
well.
The columns of this registry are:
* Name: This is a descriptive name that enables easier reference to
the item. The name MUST be unique. It is not used in the
encoding.
* CBOR Key: This is the value used as CBOR key of the item. These
values MUST be unique. The value can be a positive integer, a
negative integer, or a text string. Different ranges of values
use different registration policies [RFC8126]. Integer values
from -256 to 255 as well as text strings of length 1 are
designated as "Standards Action With Expert Review". Integer
values from -65536 to -257 and from 256 to 65535, as well as text
strings of length 2 are designated as "Specification Required".
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Integer values greater than 65535 as well as text strings of
length greater than 2 are designated as "Expert Review". Integer
values less than -65536 are marked as "Private Use".
* CBOR Type: This contains the CBOR type of the item, or a pointer
to the registry that defines its type, when that depends on
another item.
* Reference: This contains a pointer to the public specification for
the item.
This registry has been initially populated with the values in
Figure 39.
11.7. ACE Groupcomm Key Types
This specification establishes the "ACE Groupcomm Key Types" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
The registry has been created to use the "Expert Review" registration
procedure [RFC8126]. Expert review guidelines are provided in
Section 11.13. Values in this registry are covered by different
registration policies as indicated. It should be noted that, in
addition to the expert review, some portions of the registry require
a specification, potentially a Standards Track RFC, to be supplied as
well.
The columns of this registry are:
* Name: This is a descriptive name that enables easier reference to
the item. The name MUST be unique. It is not used in the
encoding.
* Key Type Value: This is the value used to identify the keying
material. These values MUST be unique. The value can be a
positive integer, a negative integer, or a text string. Different
ranges of values use different registration policies [RFC8126].
Integer values from -256 to 255 as well as text strings of length
1 are designated as "Standards Action With Expert Review".
Integer values from -65536 to -257 and from 256 to 65535, as well
as text strings of length 2 are designated as "Specification
Required". Integer values greater than 65535 as well as text
strings of length greater than 2 are designated as "Expert
Review". Integer values less than -65536 are marked as "Private
Use".
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* Profile: This field may contain one or more descriptive strings of
application profiles to be used with this item. The values should
be taken from the Name column of the "ACE Groupcomm Profiles"
registry.
* Description: This field contains a brief description of the keying
material.
* Reference: This contains a pointer to the public specification for
the format of the keying material, if one exists.
This registry has been initially populated with the value in
Figure 10.
11.8. ACE Groupcomm Profiles
This specification establishes the "ACE Groupcomm Profiles" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
The registry has been created to use the "Expert Review" registration
procedure [RFC8126]. Expert review guidelines are provided in
Section 11.13. Values in this registry are covered by different
registration policies as indicated. It should be noted that, in
addition to the expert review, some portions of the registry require
a specification, potentially a Standards Track RFC, to be supplied as
well.
The columns of this registry are:
* Name: The name of the application profile, to be used as value of
the profile attribute.
* Description: Text giving an overview of the application profile
and the context it is developed for.
* CBOR Value: CBOR abbreviation for the name of this application
profile. These values MUST be unique. The value can be a
positive integer or a negative integer. Different ranges of
values use different registration policies [RFC8126]. Integer
values from -256 to 255 are designated as "Standards Action With
Expert Review". Integer values from -65536 to -257 and from 256
to 65535 are designated as "Specification Required". Integer
values greater than 65535 are designated as "Expert Review".
Integer values less than -65536 are marked as "Private Use".
* Reference: This contains a pointer to the public specification of
the abbreviation for this application profile, if one exists.
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This registry has been initially populated with the value in
Figure 11.
11.9. ACE Groupcomm Policies
This specification establishes the "ACE Groupcomm Policies" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
The registry has been created to use the "Expert Review" registration
procedure [RFC8126]. Expert review guidelines are provided in
Section 11.13. Values in this registry are covered by different
registration policies as indicated. It should be noted that, in
addition to the expert review, some portions of the registry require
a specification, potentially a Standards Track RFC, to be supplied as
well.
The columns of this registry are:
* Name: The name of the group communication policy.
* CBOR label: The value to be used to identify this group
communication policy. These values MUST be unique. The value can
be a positive integer, a negative integer, or a text string.
Different ranges of values use different registration policies
[RFC8126]. Integer values from -256 to 255 as well as text
strings of length 1 are designated as "Standards Action With
Expert Review". Integer values from -65536 to -257 and from 256
to 65535, as well as text strings of length 2 are designated as
"Specification Required". Integer values greater than 65535 as
well as text strings of length greater than 2 are designated as
"Expert Review". Integer values less than -65536 are marked as
"Private Use".
* CBOR type: the CBOR type used to encode the value of this group
communication policy.
* Description: This field contains a brief description for this
group communication policy.
* Reference: This field contains a pointer to the public
specification providing the format of the group communication
policy, if one exists.
This registry has been initially populated with the values in
Figure 12.
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11.10. Sequence Number Synchronization Methods
This specification establishes the "Sequence Number Synchronization
Methods" IANA registry within the "Authentication and Authorization
for Constrained Environments (ACE)" registry group.
The registry has been created to use the "Expert Review" registration
procedure [RFC8126]. Expert review guidelines are provided in
Section 11.13. Values in this registry are covered by different
registration policies as indicated. It should be noted that, in
addition to the expert review, some portions of the registry require
a specification, potentially a Standards Track RFC, to be supplied as
well.
The columns of this registry are:
* Name: The name of the sequence number synchronization method.
* Value: The value to be used to identify this sequence number
synchronization method. These values MUST be unique. The value
can be a positive integer, a negative integer, or a text string.
Different ranges of values use different registration policies
[RFC8126]. Integer values from -256 to 255 as well as text
strings of length 1 are designated as "Standards Action With
Expert Review". Integer values from -65536 to -257 and from 256
to 65535, as well as text strings of length 2 are designated as
"Specification Required". Integer values greater than 65535 as
well as text strings of length greater than 2 are designated as
"Expert Review". Integer values less than -65536 are marked as
"Private Use".
* Description: This field contains a brief description for this
sequence number synchronization method.
* Reference: This field contains a pointer to the public
specification describing the sequence number synchronization
method.
11.11. ACE Groupcomm Errors
This specification establishes the "ACE Groupcomm Errors" IANA
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group.
The registry has been created to use the "Expert Review" registration
procedure [RFC8126]. Expert review guidelines are provided in
Section 11.13. Values in this registry are covered by different
registration policies as indicated. It should be noted that, in
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addition to the expert review, some portions of the registry require
a specification, potentially a Standards Track RFC, to be supplied as
well.
The columns of this registry are:
* Value: The value to be used to identify the error. These values
MUST be unique. The value can be a positive integer or a negative
integer. Different ranges of values use different registration
policies [RFC8126]. Integer values from -256 to 255 are
designated as "Standards Action With Expert Review". Integer
values from -65536 to -257 and from 256 to 65535 are designated as
"Specification Required". Integer values greater than 65535 are
designated as "Expert Review". Integer values less than -65536
are marked as "Private Use".
* Description: This field contains a brief description of the error.
* Reference: This field contains a pointer to the public
specification defining the error, if one exists.
This registry has been initially populated with the values in
Figure 40. The Reference column for all of these entries refers to
this document.
11.12. ACE Groupcomm Rekeying Schemes
This specification establishes the "ACE Groupcomm Rekeying Schemes"
IANA registry within the "Authentication and Authorization for
Constrained Environments (ACE)" registry group.
The registry has been created to use the "Expert Review" registration
procedure [RFC8126]. Expert review guidelines are provided in
Section 11.13. Values in this registry are covered by different
registration policies as indicated. It should be noted that, in
addition to the expert review, some portions of the registry require
a specification, potentially a Standards Track RFC, to be supplied as
well.
The columns of this registry are:
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* Value: The value to be used to identify the group rekeying scheme.
These values MUST be unique. The value can be a positive integer
or a negative integer. Different ranges of values use different
registration policies [RFC8126]. Integer values from -256 to 255
are designated as "Standards Action With Expert Review". Integer
values from -65536 to -257 and from 256 to 65535 are designated as
"Specification Required". Integer values greater than 65535 are
designated as "Expert Review". Integer values less than -65536
are marked as "Private Use".
* Name: The name of the group rekeying scheme.
* Description: This field contains a brief description of the group
rekeying scheme.
* Reference: This field contains a pointer to the public
specification defining the group rekeying scheme, if one exists.
This registry has been initially populated with the value in
Figure 13.
11.13. Expert Review Instructions
The IANA Registries established in this document are defined as
expert review. This section gives some general guidelines for what
the experts should be looking for, but they are being designated as
experts for a reason so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
* Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered and that the point is likely to be used in deployments.
The zones tagged as private use are intended for testing purposes
and closed environments, code points in other ranges should not be
assigned for testing.
* Specifications are required for the standards track range of point
assignment. Specifications should exist for specification
required ranges, but early assignment before a specification is
available is considered to be permissible. When specifications
are not provided, the description provided needs to have
sufficient information to identify what the point is being used
for.
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* Experts should take into account the expected usage of fields when
approving point assignment. The fact that there is a range for
Standards Track documents does not mean that a Standards Track
document cannot have points assigned outside of that range. The
length of the encoded value should be weighed against how many
code points of that length are left, the size of device it will be
used on, and the number of code points left that encode to that
size.
12. References
12.1. Normative References
[CBOR.Tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<https://www.iana.org/assignments/cbor-tags/cbor-
tags.xhtml>.
[CoAP.Content.Formats]
IANA, "CoAP Content-Formats",
<https://www.iana.org/assignments/core-parameters/core-
parameters.xhtml#content-formats>.
[COSE.Algorithms]
IANA, "COSE Algorithms",
<https://www.iana.org/assignments/cose/
cose.xhtml#algorithms>.
[COSE.Header.Parameters]
IANA, "COSE Header Parameters",
<https://www.iana.org/assignments/cose/cose.xhtml#header-
parameters>.
[COSE.Key.Types]
IANA, "COSE key Types",
<https://www.iana.org/assignments/cose/cose.xhtml#key-
type>.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and J. Park, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-20, 2
September 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-oscore-groupcomm-20>.
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[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>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/rfc/rfc6749>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/rfc/rfc6838>.
[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>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/rfc/rfc7967>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[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>.
[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>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/rfc/rfc8747>.
[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>.
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[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>.
[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>.
[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>.
[RFC9237] Bormann, C., "An Authorization Information Format (AIF)
for Authentication and Authorization for Constrained
Environments (ACE)", RFC 9237, DOI 10.17487/RFC9237,
August 2022, <https://www.rfc-editor.org/rfc/rfc9237>.
[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>.
12.2. Informative References
[I-D.ietf-core-coap-pubsub]
Koster, M., Keränen, A., and J. Jimenez, "A publish-
subscribe architecture for the Constrained Application
Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
ietf-core-coap-pubsub-12, 13 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-pubsub-12>.
[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>.
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[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.tiloca-core-oscore-discovery]
Tiloca, M., Amsüss, C., and P. Van der Stok, "Discovery of
OSCORE Groups with the CoRE Resource Directory", Work in
Progress, Internet-Draft, draft-tiloca-core-oscore-
discovery-14, 8 September 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-discovery-14>.
[RFC2093] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Specification", RFC 2093,
DOI 10.17487/RFC2093, July 1997,
<https://www.rfc-editor.org/rfc/rfc2093>.
[RFC2094] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Architecture", RFC 2094,
DOI 10.17487/RFC2094, July 1997,
<https://www.rfc-editor.org/rfc/rfc2094>.
[RFC2627] Wallner, D., Harder, E., and R. Agee, "Key Management for
Multicast: Issues and Architectures", RFC 2627,
DOI 10.17487/RFC2627, June 1999,
<https://www.rfc-editor.org/rfc/rfc2627>.
[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>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/rfc/rfc7519>.
[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>.
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[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>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/rfc/rfc8259>.
[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>.
[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>.
[RFC9202] Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
L. Seitz, "Datagram Transport Layer Security (DTLS)
Profile for Authentication and Authorization for
Constrained Environments (ACE)", RFC 9202,
DOI 10.17487/RFC9202, August 2022,
<https://www.rfc-editor.org/rfc/rfc9202>.
[RFC9203] Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
"The Object Security for Constrained RESTful Environments
(OSCORE) Profile of the Authentication and Authorization
for Constrained Environments (ACE) Framework", RFC 9203,
DOI 10.17487/RFC9203, August 2022,
<https://www.rfc-editor.org/rfc/rfc9203>.
[RFC9277] Richardson, M. and C. Bormann, "On Stable Storage for
Items in Concise Binary Object Representation (CBOR)",
RFC 9277, DOI 10.17487/RFC9277, August 2022,
<https://www.rfc-editor.org/rfc/rfc9277>.
[RFC9431] Sengul, C. and A. Kirby, "Message Queuing Telemetry
Transport (MQTT) and Transport Layer Security (TLS)
Profile of Authentication and Authorization for
Constrained Environments (ACE) Framework", RFC 9431,
DOI 10.17487/RFC9431, July 2023,
<https://www.rfc-editor.org/rfc/rfc9431>.
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Appendix A. Requirements on Application Profiles
This section lists the requirements on application profiles of this
specification, for the convenience of application profile designers.
A.1. Mandatory-to-Address Requirements
* REQ1: Specify the format and encoding of 'scope'. This includes
defining the set of possible roles and their identifiers, as well
as the corresponding encoding to use in the scope entries
according to the used scope format (see Section 3.1).
* REQ2: If the AIF format of 'scope' is used, register its specific
instance of "Toid" and "Tperm" as Media Type parameters and a
corresponding Content-Format, as per the guidelines in [RFC9237].
* REQ3: If used, specify the acceptable values for 'sign_alg' (see
Section 3.3).
* REQ4: If used, specify the acceptable values for 'sign_parameters'
(see Section 3.3).
* REQ5: If used, specify the acceptable values for
'sign_key_parameters' (see Section 3.3).
* REQ6: Specify the acceptable formats for authentication
credentials and, if used, the acceptable values for 'cred_fmt'
(see Section 3.3).
* REQ7: If the value of the GROUPNAME URI path and the group name in
the access token scope (gname in Section 3.2) are not required to
coincide, specify the mechanism to map the GROUPNAME value in the
URI to the group name (see Section 4.1).
* REQ8: Define whether the KDC has an authentication credential and
if this has to be provided through the 'kdc_cred' parameter, see
Section 4.3.1.
* REQ9: Specify if any part of the KDC interface as defined in this
document is not supported by the KDC (see Section 4.1).
* REQ10: Register a Resource Type for the root url-path, which is
used to discover the correct url to access at the KDC (see
Section 4.1).
* REQ11: Define what specific actions (e.g., CoAP methods) are
allowed on each resource provided by the KDC interface, depending
on whether the Client is a current group member; the roles that a
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Client is authorized to take as per the obtained access token (see
Section 3.1); and the roles that the Client has as current group
member.
* REQ12: Categorize possible newly defined operations for Clients
into primary operations expected to be minimally supported and
secondary operations, and provide accompanying considerations (see
Section 4.1.1).
* REQ13: Specify the encoding of group identifier (see
Section 4.2.1).
* REQ14: Specify the approaches used to compute and verify the PoP
evidence to include in 'client_cred_verify', and which of those
approaches is used in which case (see Section 4.3.1).
* REQ15: Specify how the nonce N_S is generated, if the token is not
provided to the KDC through the Token Transfer Request to the
authz-info endpoint (e.g., if it is used directly to validate TLS
instead).
* REQ16 Define the initial value of the 'num' parameter (see
Section 4.3.1).
* REQ17: Specify the format of the 'key' parameter (see
Section 4.3.1).
* REQ18: Specify the acceptable values of the 'gkty' parameter (see
Section 4.3.1).
* REQ19: Specify and register the application profile identifier
(see Section 4.3.1).
* REQ20: If used, specify the format and content of 'group_policies'
and its entries. Specify the policies default values (see
Section 4.3.1).
* REQ21: Specify the approaches used to compute and verify the PoP
evidence to include in 'kdc_cred_verify', and which of those
approaches is used in which case (see Section 4.3.1).
* REQ22: Specify the communication protocol the members of the group
must use (e.g., multicast CoAP).
* REQ23: Specify the security protocol the group members must use to
protect their communication (e.g., group OSCORE). This must
provide encryption, integrity, and replay protection.
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* REQ24: Specify how the communication is secured between Client and
KDC. Optionally, specify transport profile of ACE [RFC9200] to
use between Client and KDC (see Section 4.3.1.1).
* REQ25: Specify the format of the identifiers of group members (see
Section 4.3.1).
* REQ26: Specify policies at the KDC to handle ids that are not
included in 'get_creds' (see Section 4.4.1).
* REQ27: Specify the format of newly-generated individual keying
material for group members, or of the information to derive it,
and corresponding CBOR label (see Section 4.8.1).
* REQ28: Specify which CBOR tag is used for identifying the
semantics of binary scopes, or register a new CBOR tag if a
suitable one does not exist already (see Section 7).
* REQ29: Categorize newly defined parameters according to the same
criteria of Section 8.
* REQ30: Define whether Clients must, should, or may support the
conditional parameters defined in Section 8, and under which
circumstances.
A.2. Optional-to-Address Requirements
* OPT1: Optionally, if the textual format of 'scope' is used,
specify CBOR values to use for abbreviating the role identifiers
in the group (see Section 3.1).
* OPT2: Optionally, specify the additional parameters used in the
exchange of Token Transfer Request and Response (see Section 3.3).
* OPT3: Optionally, specify the negotiation of parameter values for
signature algorithm and signature keys, if 'sign_info' is not used
(see Section 3.3).
* OPT4: Optionally, specify possible or required payload formats for
specific error cases.
* OPT5: Optionally, specify additional identifiers of error types,
as values of the 'error' field in an error response from the KDC.
* OPT6: Optionally, specify the encoding of 'creds_repo' if the
default is not used (see Section 4.3.1).
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* OPT7: Optionally, specify the functionalities implemented at the
'control_uri' resource hosted at the Client, including message
exchange encoding and other details (see Section 4.3.1).
* OPT8: Optionally, specify the behavior of the handler in case of
failure to retrieve an authentication credential for the specific
node (see Section 4.3.1).
* OPT9: Optionally, define a default group rekeying scheme, to refer
to in case the 'rekeying_scheme' parameter is not included in the
Join Response (see Section 4.3.1).
* OPT10: Optionally, specify the functionalities implemented at the
'control_group_uri' resource hosted at the Client, including
message exchange encoding and other details (see Section 4.3.1).
* OPT11: Optionally, specify policies that instruct Clients to
retain messages and for how long, if they are unsuccessfully
decrypted (see Section 4.8.1.1). This makes it possible to
decrypt such messages after getting updated keying material.
* OPT12: Optionally, specify for the KDC to perform group rekeying
(together or instead of renewing individual keying material) when
receiving a Key Renewal Request (see Section 4.8.2.1).
* OPT13: Optionally, specify how the identifier of a group member's
authentication credential is included in requests sent to other
group members (see Section 4.9.1.1).
* OPT14: Optionally, specify additional information to include in
rekeying messages for the "Point-to-Point" group rekeying scheme
(see Section 6).
* OPT15: Optionally, specify if Clients must or should support any
of the parameters defined as optional in this specification (see
Section 8).
Appendix B. Extensibility for Future COSE Algorithms
As defined in Section 8.1 of [RFC9053], future algorithms can be
registered in the "COSE Algorithms" registry [COSE.Algorithms] as
specifying none or multiple COSE capabilities.
To enable the seamless use of such future registered algorithms, this
section defines a general, agile format for each 'sign_info_entry' of
the 'sign_info' parameter in the Token Transfer Response, see
Section 3.3.1.
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If any of the currently registered COSE algorithms is considered,
using this general format yields the same structure of
'sign_info_entry' defined in this document, thus ensuring backward
compatibility.
B.1. Format of 'sign_info_entry'
The format of each 'sign_info_entry' (see Section 3.3.1) is
generalized as follows.
* 'sign_parameters' includes N >= 0 elements, each of which is a
COSE capability of the signature algorithm indicated in
'sign_alg'.
In particular, 'sign_parameters' has the same format and value of
the COSE capabilities array for the signature algorithm indicated
in 'sign_alg', as specified for that algorithm in the
'Capabilities' column of the "COSE Algorithms" registry
[COSE.Algorithms].
* 'sign_key_parameters' is replaced by N elements 'sign_capab', each
of which is a CBOR array.
The i-th 'sign_capab' array (i = 0, ..., N-1) is the array of COSE
capabilities for the algorithm capability specified in
'sign_parameters'[i].
In particular, each 'sign_capab' array has the same format and
value of the COSE capabilities array for the algorithm capability
specified in 'sign_parameters'[i].
Such a COSE capabilities array is currently defined for the
algorithm capability COSE key type, in the "Capabilities" column
of the "COSE Key Types" registry [COSE.Key.Types].
sign_info_entry =
[
id : gname / [ + gname ],
sign_alg : int / tstr,
sign_parameters : [ * alg_capab : any ],
* sign_capab : [ * capab : any ],
cred_fmt : int / null
]
gname = tstr
Figure 41: 'sign_info_entry' with general format
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Appendix C. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
C.1. Version -16 to -17
* Expanded definition of "Dispatcher".
* Added definition of "Individual keying material" to the
terminology.
* Early definition of "backward security" and "forward security".
* Clarified high-level breakdown of the key provisioning process in
two phases.
* Fixed the CDDL definition of 'sign_info_entry'.
* Clarified meaning of the 'cred_fmt' and 'exp' parameters.
* Clarified that invariance applies to resources paths, not to
resources.
* Relaxed rule about including the 'peer_roles' parameter.
* Ensured that the KDC always has a Client-side challenge for
computing a proof-of-possession evidence.
* More guidelines for group members that fail to decrypt messages.
* Fetching the latest keying material can happen before the old,
stored one expires.
* Renewing the current keying material can happen before it expires.
* Moved up the discussion on misalignment of group keying material.
* Expanded security considerations on group rekeying for joining
nodes.
* Revised size of integer for building AEAD nonces for group
rekeying.
* Added reserved value to the "ACE Groupcomm Profiles" IANA
registry.
* Revised the future-ready generalization of 'sign_info_entry'.
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* Revised and fixed IANA considerations.
* Fixes and editorial improvements.
C.2. Version -15 to -16
* Distinction between authentication credentials and public keys.
* Consistent renaming of parameters and URI paths.
* Updated format of scope entries when using AIF.
* Updated signaling of semantics for binary encoded scopes.
* Editorial fixes.
C.3. Version -14 to -15
* Fixed nits.
C.4. Version -13 to -14
* Clarified scope and goal of the document in abstract and
introduction.
* Overall clarifications on semantics of operations and parameters.
* Major restructuring in the presentation of the KDC interface.
* Revised error handling, also removing redundant text.
* Imported parameters and KDC resource about the KDC's public key
from draft-ietf-ace-key-groupcomm-oscore.
* New parameters 'group_rekeying_scheme' and 'control_group_uri'.
* Provided example of administrative keying material transported in
'mgt_key_material'.
* Reasoned categorization of parameters, as expected support by ACE
Clients.
* Reasoned categorization of KDC functionalities, as minimally/
optional to support for ACE Clients.
* Guidelines on enhanced error responses using 'error' and
'error_description'.
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* New section on group rekeying, discussing at a high-level a basic
one-to-one approach and possible one-to-many approaches.
* Revised and expanded security considerations, also about the KDC.
* Updated list of requirements for application profiles.
* Several further clarifications and editorial improvements.
C.5. Version -05 to -13
* Incremental revision of the KDC interface.
* Removed redundancy in parameters about signature algorithm and
signature keys.
* Node identifiers always indicated with 'peer_identifiers'.
* Format of public keys changed from raw COSE Keys to be
certificates, CWTs or CWT Claims Set (CCS). Adapted parameter
'pub_key_enc'.
* Parameters and functionalities imported from draft-ietf-key-
groupcomm-oscore where early defined.
* Possible provisioning of the KDC's Diffie-Hellman public key in
response to the Token transferring to /authz-info.
* Generalized proof-of-possession evidence, to be not necessarily a
signature.
* Public keys of group members may be retrieved filtering by role
and/or node identifier.
* Enhanced error handling with error code and error description.
* Extended "typed" format for the 'scope' claim, optional to use.
* Editorial improvements.
C.6. Version -04 to -05
* Updated uppercase/lowercase URI segments for KDC resources.
* Supporting single Access Token for multiple groups/topics.
* Added 'control_uri' parameter in the Join Request.
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* Added 'peer_roles' parameter to support legal requesters/
responders.
* Clarification on stopping using owned keying material.
* Clarification on different reasons for processing failures,
related policies, and requirement OPT11.
* Added a KDC sub-resource for group members to upload a new public
key.
* Possible group rekeying following an individual Key Renewal
Request.
* Clarified meaning of requirement REQ3; added requirement OPT12.
* Editorial improvements.
C.7. Version -03 to -04
* Revised RESTful interface, as to methods and parameters.
* Extended processing of Join Request, as to check/retrieval of
public keys.
* Revised and extended profile requirements.
* Clarified specific usage of parameters related to signature
algorithms/keys.
* Included general content previously in draft-ietf-ace-key-
groupcomm-oscore
* Registration of media type and content format application/ace-
group+cbor
* Editorial improvements.
C.8. Version -02 to -03
* Exchange of information on the signature algorithm and related
parameters, during the Token POST (Section 3.3).
* Restructured KDC interface, with new possible operations
(Section 4).
* Client PoP signature for the Join Request upon joining
(Section 4.1.2.1).
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* Revised text on group member removal (Section 5).
* Added more profile requirements (Appendix A).
C.9. Version -01 to -02
* Editorial fixes.
* Distinction between transport profile and application profile
(Section 1.1).
* New parameters 'sign_info' and 'pub_key_enc' to negotiate
parameter values for signature algorithm and signature keys
(Section 3.3).
* New parameter 'type' to distinguish different Key Distribution
Request messages (Section 4.1).
* New parameter 'client_cred_verify' in the Key Distribution Request
to convey a Client signature (Section 4.1).
* Encoding of 'pub_keys_repos' (Section 4.1).
* Encoding of 'mgt_key_material' (Section 4.1).
* Improved description on retrieval of new or updated keying
material (Section 6).
* Encoding of 'get_pub_keys' in Public Key Request (Section 7.1).
* Extended security considerations (Sections 10.1 and 10.2).
* New "ACE Public Key Encoding" IANA registry (Section 11.2).
* New "ACE Groupcomm Parameters" IANA registry (Section 11.3),
populated with the entries in Section 8.
* New "Ace Groupcomm Request Type" IANA registry (Section 11.4),
populated with the values in Section 9.
* New "ACE Groupcomm Policy" IANA registry (Section 11.7) populated
with two entries "Sequence Number Synchronization Method" and "Key
Update Check Interval" (Section 4.2).
* Improved list of requirements for application profiles
(Appendix A).
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C.10. Version -00 to -01
* Changed name of 'req_aud' to 'audience' in the Authorization
Request (Section 3.1).
* Defined error handling on the KDC (Sections 4.2 and 6.2).
* Updated format of the Key Distribution Response as a whole
(Section 4.2).
* Generalized format of 'pub_keys' in the Key Distribution Response
(Section 4.2).
* Defined format for the message to request leaving the group
(Section 5.2).
* Renewal of individual keying material and methods for group
rekeying initiated by the KDC (Section 6).
* CBOR type for node identifiers in 'get_pub_keys' (Section 7.1).
* Added section on parameter identifiers and their CBOR keys
(Section 8).
* Added request types for requests to a Join Response (Section 9).
* Extended security considerations (Section 10).
* New IANA registries "ACE Groupcomm Key registry", "ACE Groupcomm
Profile registry", "ACE Groupcomm Policy registry" and "Sequence
Number Synchronization Method registry" (Section 11).
* Added appendix about requirements for application profiles of ACE
on group communication (Appendix A).
Acknowledgments
The following individuals were helpful in shaping this document:
Christian Amsüss, Carsten Bormann, Rikard Höglund, Ben Kaduk, Watson
Ladd, John Preuß Mattsson, Daniel Migault, Jim Schaad, Ludwig Seitz,
Göran Selander, Cigdem Sengul, Peter van der Stok, and Paul Wouters.
The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; by the H2020 project SIFIS-Home
(Grant agreement 952652); and by the EIT-Digital High Impact
Initiative ACTIVE.
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Authors' Addresses
Francesca Palombini
Ericsson AB
Torshamnsgatan 23
SE-16440 Stockholm Kista
Sweden
Email: francesca.palombini@ericsson.com
Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: marco.tiloca@ri.se
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