Internet DRAFT - draft-ietf-ace-pubsub-profile
draft-ietf-ace-pubsub-profile
ACE Working Group F. Palombini
Internet-Draft Ericsson
Intended status: Standards Track C. Sengul
Expires: 5 September 2024 Brunel University
M. Tiloca
RISE AB
4 March 2024
Publish-Subscribe Profile for Authentication and Authorization for
Constrained Environments (ACE)
draft-ietf-ace-pubsub-profile-09
Abstract
This document defines an application profile of the Authentication
and Authorization for Constrained Environments (ACE) framework, to
enable secure group communication in the Publish-Subscribe (pub/sub)
architecture for the Constrained Application Protocol (CoAP) [draft-
ietf-core-coap-pubsub], where Publishers and Subscribers communicate
through a Broker. This profile relies on protocol-specific transport
profiles of ACE to achieve communication security, server
authentication, and proof-of-possession for a key owned by the Client
and bound to an OAuth 2.0 Access Token. This document specifies the
provisioning and enforcement of authorization information for Clients
to act as Publishers and/or Subscribers, as well as the provisioning
of keying material and security parameters that Clients use for
protecting their communications end-to-end through the Broker.
Note to RFC Editor: Please replace "[draft-ietf-core-coap-pubsub]"
with the RFC number of that document and delete this paragraph.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Authentication and
Authorization for Constrained Environments Working Group mailing list
(ace@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/ace/.
Source for this draft and an issue tracker can be found at
https://github.com/ace-wg/pubsub-profile.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Application Profile Overview . . . . . . . . . . . . . . . . 5
3. Getting Authorisation to Join a Pub/sub security group (A) . 9
3.1. Topic Discovery at the Broker (Optional) . . . . . . . . 11
3.2. AS Discovery at the Broker (Optional) . . . . . . . . . . 11
3.3. KDC Discovery at the Broker (Optional) . . . . . . . . . 11
3.4. Authorisation Request/Response for the KDC and the
Broker . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.4.1. Format of Scope . . . . . . . . . . . . . . . . . . . 13
3.5. Authorisation response . . . . . . . . . . . . . . . . . 15
3.6. Token Transfer to KDC . . . . . . . . . . . . . . . . . . 16
4. Client Group Communication Interface at the KDC . . . . . . . 17
4.1. Joining a Security Group . . . . . . . . . . . . . . . . 20
4.1.1. Join Request . . . . . . . . . . . . . . . . . . . . 20
4.1.2. Join Response . . . . . . . . . . . . . . . . . . . . 23
4.1.3. Join Error Handling . . . . . . . . . . . . . . . . . 27
4.2. Other Group Operations through the KDC . . . . . . . . . 28
4.2.1. Querying for Group Information . . . . . . . . . . . 28
4.2.2. Updating Authentication Credentials . . . . . . . . . 29
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4.2.3. Removal from a Group . . . . . . . . . . . . . . . . 29
4.2.4. Rekeying a Group . . . . . . . . . . . . . . . . . . 29
5. PubSub Protected Communication (C) . . . . . . . . . . . . . 30
5.1. Using COSE Objects To Protect The Resource
Representation . . . . . . . . . . . . . . . . . . . . . 30
5.2. AEAD Nonce . . . . . . . . . . . . . . . . . . . . . . . 33
5.3. Replay Checks . . . . . . . . . . . . . . . . . . . . . . 34
6. Applicability to MQTT PubSub Profile . . . . . . . . . . . . 34
7. Security Considerations . . . . . . . . . . . . . . . . . . . 35
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
8.1. ACE Groupcomm Key Types Registry . . . . . . . . . . . . 36
8.2. ACE Groupcomm Profiles Registry . . . . . . . . . . . . . 36
8.3. CoRE Resource Type . . . . . . . . . . . . . . . . . . . 37
8.4. AIF Media-Type Sub-Parameters . . . . . . . . . . . . . . 37
8.5. CoAP Content-Formats . . . . . . . . . . . . . . . . . . 37
8.6. TLS Exporter Labels . . . . . . . . . . . . . . . . . . . 38
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
9.1. Normative References . . . . . . . . . . . . . . . . . . 38
9.2. Informative References . . . . . . . . . . . . . . . . . 41
Appendix A. Requirements on Application Profiles . . . . . . . . 43
Appendix B. Document Updates . . . . . . . . . . . . . . . . . . 47
B.1. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 47
B.2. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 47
B.3. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 48
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction
In a publish-subscribe (pub/sub) scenario, devices communicate via a
Broker, which enables store-and-forward messaging between these
devices. This effectively enables a form of group communication,
where all the Publishers and Subscribers participating in the same
pub/sub topic are considered members of the same group associated
with that topic.
With a focus on the pub/sub architecture defined in
[I-D.ietf-core-coap-pubsub] for the Constrained Application Protocol
(CoAP) [RFC7252], this document defines an application profile of the
Authentication and Authorization for Constrained Environments (ACE)
framework [RFC9200], which enables pub/sub communication where a
group of Publishers and Subscribers securely communicate through a
Broker using CoAP.
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Building on the message formats and processing defined in
[I-D.ietf-ace-key-groupcomm], this document specifies the
provisioning and enforcement of authorization information for Clients
to act as Publishers and/or Subscribers at the Broker, as well as the
provisioning of keying material and security parameters that Clients
use for protecting end-to-end their communications via the Broker.
In order to protect the pub/sub operations at the Broker as well as
the provisioning of keying material and security parameters, this
profile relies on protocol-specific transport profiles of ACE (e.g.,
[RFC9202], [RFC9203], or [I-D.ietf-ace-edhoc-oscore-profile]) to
achieve communication security, server authentication, and proof-of-
possession for a key owned by the Client and bound to an OAuth 2.0
Access Token.
The content of published messages that are circulated by the Broker
is protected end-to-end between the corresponding Publisher and the
intended Subscribers. To this end, this profile relies on COSE
[RFC9052][RFC9053] and on keying material provided to the Publishers
and Subscribers participating in the same pub/sub topic. In
particular, source authentication of published content is achieved by
means of the corresponding Publisher signing such content with its
own private key.
While this profile focuses on the pub/sub architecture for CoAP, this
document also describes how it can be applicable to MQTT
[MQTT-OASIS-Standard-v5]. Similar adaptations can also extend to
further transport protocols and pub/sub architectures.
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 [RFC9200], and the
Authorization Information Format (AIF) [RFC9237] to express
authorization information. In particular, analogously to
[RFC9200], terminology for entities in the architecture such as
Client (C), Resource Server (RS), and Authorization Server (AS) is
defined in OAuth 2.0 [RFC6749].
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* The terms and concept related to the message formats and
processing, specified in [I-D.ietf-ace-key-groupcomm], for
provisioning and renewing keying material in group communication
scenarios. These include the abbreviations REQx and OPTx denoting
the numbered mandatory-to-address and optional-to-address
requirements, respectively.
* The terms and concepts described in CDDL [RFC8610], CBOR
[RFC8949], and COSE [RFC9052][RFC9053][RFC9338].
* 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 of pub/sub group communication with CoAP,
as described in [I-D.ietf-core-coap-pubsub].
A party interested in participating in group communication as well as
already participating as a group member is interchangeably denoted as
"Client", "pub/sub client", or "node".
* Group: a set of nodes that share common keying material and
security parameters 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 are in this
case the Clients acting as Publishers and/or Subscribers for a
topic.
Examples throughout this document are expressed in CBOR diagnostic
notation without the tag and value abbreviations.
2. Application Profile Overview
This document describes how to use [RFC9200] and
[I-D.ietf-ace-key-groupcomm] to perform authentication,
authorization, and key distribution operations as overviewed in
Section 2 of [I-D.ietf-ace-key-groupcomm], where the considered group
is the security group composed of the pub/sub clients that exchange
end-to-end protected content through the Broker.
Pub/sub clients communicate within their application groups, each of
which is mapped to a topic. Depending on the application, a topic
may consist of one or more sub-topics, which may have their own sub-
topics and so on, thus forming a hierarchy. A security group SHOULD
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be associated with a single application group. However, the same
application group MAY be associated with multiple security groups.
Further details and considerations on the mapping between the two
types of groups are out of the scope of this document.
This profile considers the architecture shown in Figure 1. A Client
can act as a Publisher, or a Subscriber, or both, e.g., by publishing
to some topics and subscribing to others. However, for the
simplicity of presentation, this profile describes Publisher and
Subscriber Clients separately.
Both Publishers and Subscribers act as ACE Clients. The Broker acts
as an ACE RS, and corresponds to the Dispatcher in
[I-D.ietf-ace-key-groupcomm]. The Key Distribution Center (KDC) also
acts as an ACE RS, and builds on what is defined for the KDC in
[I-D.ietf-ace-key-groupcomm]. From a high-level point of view, the
Clients interact with the KDC in order to join security groups and
obtain the group keying material to protect end-to-end and verify the
content published in the associated topics.
+---------------+ +--------------+
| Authorization | | Key |
| Server | | Distribution |
| (AS) | | Center |
| | | (KDC) |
+---------------+ +--------------+
^ ^
| |
+---------(A)----+ |
| |
| +--------------------(B)--------+
v v
+------------+ +------------+
| | <--- (O) ---> | |
| Pub/sub | | Broker |
| Client | <--- (C) ---> | |
| | | |
+------------+ +------------+
Figure 1: Architecture for Pub/sub with Authorization Server and Key
Distribution Center
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Both Publishers and Subscribers MUST use the same pub/sub
communication protocol for their interaction with the Broker. When
using the profile defined in this document, this protocol MUST be
CoAP, which is used as described in [I-D.ietf-core-coap-pubsub].
What is specified in this document can apply to other pub/sub
protocols such as MQTT [MQTT-OASIS-Standard-v5], or to further
transport protocols.
All Publishers and Subscribers MUST use CoAP when communicating with
the KDC.
Furthermore, both Publishers and Subscribers MUST use the same
transport profile of ACE (e.g., [RFC9202] for DTLS; or [RFC9203] or
[I-D.ietf-ace-edhoc-oscore-profile] for OSCORE) in their interaction
with the Broker. In order to reduce the number of libraries that
Clients have to support, it is RECOMMENDED that the same transport
profile of ACE is used also for the interaction between the Clients
and the KDC.
All communications between the involved entities MUST be secured.
The Client and the Broker MUST have a secure communication
association, which they establish with the help of the AS and using a
transport profile of ACE. This is shown by the interactions A and C
in Figure 1. During this process, the Client obtains an Access Token
from the AS and uploads it to the Broker, thus providing an evidence
of the topics that it is authorized to participate in, and with which
permissions.
The Client and the KDC MUST have a secure communication association,
which they also establish with the help of the AS and using a
transport profile of ACE. This is shown by the interactions A and B
in Figure 1. During this process, the Client obtains an Access Token
from the AS and uploads it to the KDC, thus providing an evidence of
the security groups that it can join, as associated with the topics
of interest at the Broker. Based on the permissions specified in the
Access Token, the Client can request the KDC to join a security
group, after which the Client obtains from the KDC the keying
material to use for communicating with the other group members. This
builds on the process for joining security groups with ACE defined in
[I-D.ietf-ace-key-groupcomm] and further specified in this document.
In addition, this profile allows an anonymous Client to perform some
of the discovery operations defined in Section 2.3 of
[I-D.ietf-core-coap-pubsub] through the Broker, as shown by the
interaction O in Figure 1. That is, an anonymous Client can
discover:
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* the Broker itself, by relying on the resource type "core.ps" (see
Section 2.3.1 of [I-D.ietf-core-coap-pubsub]); and
* topics of interest at the Broker (i.e., the corresponding topic
resources hosted at the Broker), by relying on the resource type
"core.ps.conf" (see Section 2.3.2 of [I-D.ietf-core-coap-pubsub]).
However, an anonymous Client is not allowed to access topic resources
at the Broker and obtain from those any additional information or
metadata about the corresponding topic (e.g., the topic status, the
URI of the topic-data resource where to publish or subscribe for that
topic, or the URI to the KDC).
As highlighted in Figure 2, each Client maintains two different
security associations pertaining to the pub/sub group communication.
On the one hand, the Client has a pairwise security association with
the Broker, which, as the ACE RS, verifies that the Client is
authorized to perform data operations (i.e., publish, subscribe,
read, delete) on a certain set of topics (Security Association 1).
As discussed above, this security association is set up with the help
of the AS and using a transport profile of ACE, when the Client
obtains the Access Token to upload to the Broker.
On the other hand, separately for each topic, all the Publisher and
Subscribers for that topic have a common, group security association,
through which the published content sent through the Broker is
protected end-to-end (Security Association 2). As discussed above,
this security association is set up and maintained as the different
Clients request the KDC to join the security group, upon which they
obtain from the KDC the corresponding group keying material to use
for protecting end-to-end and verifying the content of their pub/sub
group communication.
+------------+ +------------+ +------------+
| | | | | |
| Publisher | | Broker | | Subscriber |
| | | | | |
| | | | | |
+------------+ +------------+ +------------+
: : : : : :
: : : : : :
: '----- Security ------' '------ Security ------' :
: Association 1 Association 1 :
: :
'---------------------- Security ------------------------'
Association 2
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Figure 2: Security Associations between Publisher, Broker, and
Subscriber.
In summary, this profile specifies the following functionalities.
1. A Client obtains the authorization to participate in a pub/sub
topic at the Broker with certain permissions. This pertains
operations defined in [I-D.ietf-core-coap-pubsub] for taking part
in pub/sub communication with CoAP.
2. A Client obtains the authorization to join a security group with
certain permissions. This allows the Client to obtain from the
KDC the group keying material for communicating with other group
members, i.e., to protect end-to-end and verify the content
published at the Broker on topics associated with the security
group.
3. A Client obtains from the KDC the authentication credentials of
other group members, and provides or updates the KDC with its
authentication credential.
4. A Client leaves the group or is removed from the group by the
KDC.
5. The KDC renews and redistributes the group keying material
(rekeying), e.g., due to a membership change in the group.
Appendix A lists the specifications on this application profile of
ACE, based on the requirements defined in Appendix A of
[I-D.ietf-ace-key-groupcomm].
3. Getting Authorisation to Join a Pub/sub security group (A)
Figure 3 provides a high level overview of the message flow for a
Client getting authorisation to join a group. Square brackets denote
optional steps. The message flow is expanded in the subsequent
sections.
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Client Broker AS KDC
| | | |
|[<-------- Discovery of Topic Resource -------->]| | |
| | | |
|[--------------- Resource Request ------------->]| | |
|[<--------------- AS Information ---------------]| | |
| | | |
| | | |
|----- Authorisation Request (Audience: Broker) ------>| |
|<---- Authorisation Response (Audience: Broker) ------| |
| | | |
| | | |
|------ Upload of authorisation information ----->| | |
|<----- Establishment of secure association ----->| | |
| | | |
| | | |
|[<-- Discovery of KDC and name of sec. group -->]| | |
| | | |
| | | |
|------- Authorisation Request (Audience: KDC) ------->| |
|<------ Authorisation Response (Audience: KDC) -------| |
| | | |
| | | |
|--------- Upload of authorisation information ------------->|
|<-------- Establishment of secure association ------------->|
| | | |
| | | |
|----- Request to join the security group for the topic ---->|
|<-------- Keying material for the security group -----------|
| | | |
| | | |
|--------------- Resource Request --------------->| | |
| (Publication/Subscription to the topic) | | |
| | | |
Figure 3: Authorisation Flow
Since [RFC9200] recommends the use of CoAP and CBOR, this document
describes the exchanges assuming that CoAP and CBOR are used.
However, using HTTP instead of CoAP is possible, by leveraging the
corresponding parameters and methods. Analogously, JSON [RFC8259]
can be used instead of CBOR, using the conversion method specified in
Sections 6.1 and 6.2 of [RFC8949]. In case JSON is used, the
Content-Format of the message has to be changed accordingly. Exact
definitions of these exchanges are out of scope for this document.
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3.1. Topic Discovery at the Broker (Optional)
The discovery of a topic at the Broker can be performed by
discovering the corresponding topic resources hosted at the Broker.
For example, the Client can send a lookup request to /.well-known/
core at the Broker, specifying as lookup criterion the resource type
"core.ps.conf" (see Section 2.3.2 of [I-D.ietf-core-coap-pubsub]).
Although the links to the topic resources are also specified in the
representation of the collection resource at the Broker (see
Section 2.4 of [I-D.ietf-core-coap-pubsub]), the Client is not
supposed to access such a resource, as intended for administrative
operations that are out of the scope of this document.
3.2. AS Discovery at the Broker (Optional)
Complementary to what is defined in Section 5.1 of [RFC9200] for AS
discovery, the Broker MAY send the address of the AS to the Client in
the 'AS' parameter of the AS Request Creation Hints, as a response to
an Unauthorized Resource Request (see Section 5.2 of [RFC9200]). An
example using CBOR diagnostic notation and CoAP is given below:
4.01 Unauthorized
Content-Format: application/ace+cbor
Payload:
{
"AS": "coaps://as.example.com/token"
}
Figure 4: AS Request Creation Hints Example
3.3. KDC Discovery at the Broker (Optional)
Once a Client has obtained an Access Token from the AS and
accordingly established a secure association with the Broker, the
Client has the permission to access the topic resources at the Broker
that pertain to the topics on which the Client is authorized to
operate.
In particular the Client is authorized to retrieve the representation
of a topic resource, from which the Client can retrieve information
related to the topic in question, as specified in Section 2.5 of
[I-D.ietf-core-coap-pubsub].
This profile extends the set of CoAP Pub/sub Parameters that is
possible to specify within the representation of a topic resource, as
originally defined in Section 3 of [I-D.ietf-core-coap-pubsub]. In
particular, this profile defines the following two parameters that
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the Broker can specify in a response from a topic resource (see
Section 2.5 of [I-D.ietf-core-coap-pubsub]). Note that, when these
parameters are transported in their respective fields of the message
payload, the Content-Format application/core-pubsub+cbor defined in
[I-D.ietf-core-coap-pubsub] MUST be used.
* 'kdc_uri', with value the URI of the group membership resource at
KDC, where Clients can send a request to join the security group
associated with the topic in question. The URI is encoded as a
CBOR text string. Clients will have to obtain an Access Token
from the AS to upload to the KDC, before starting the process to
join the security group at the KDC.
* 'sec_gp', specifying the name of the security group associated
with the topic in question, as a stable and invariant identifier.
The name of the security group is encoded as a CBOR text string.
Furthermore, the Resource Type (rt=) Link Target Attribute value
"core.ps.gm" is registered in Section 8.3 (REQ10), and can be used to
describe group-membership resources at KDC, e.g., by using a link-
format document [RFC6690]. As an alternative to the discovery
approach defined above and provided by the Broker, applications can
use this common resource type to discover links to group-membership
resources at the KDC for joining security groups associated with pub/
sub topics.
3.4. Authorisation Request/Response for the KDC and the Broker
A Client sends two Authorisation Requests to the AS, targeting two
different audiences, i.e., the Broker and the KDC.
As to the former, the AS handles Authorisation Requests related to a
topic for which the Client is allowed to perform topic data
operations at the Broker, as corresponding to an application group.
As to the latter, the AS handles Authorization Requests for security
groups that the Client is allowed to join, in order to obtain the
group keying material for protecting end-to-end and verifying the
content of exchanged messages on the associated pub/sub topics.
This section builds on Section 3 of [I-D.ietf-ace-key-groupcomm] and
defines only additions or modifications to that specification.
Both Authorisation Requests include the following fields (see
Section 3.1 of [I-D.ietf-ace-key-groupcomm]):
* 'scope': Optional. If present, it specifies the following
information, depending on the specifically targeted audience.
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If the audience is the Broker, the scope specifies the name of the
topics that the Client wishes to access, together with the
corresponding permissions. If the audience is the KDC, the scope
specifies the name of the security groups that the Client wishes
to join, together with the corresponding permissions.
This parameter is encoded as a CBOR byte string, whose value is
the binary encoding of a CBOR array. The format MUST follow the
data model AIF-PUBSUB-GROUPCOMM defined in Section 3.4.1.
* 'audience': Required identifier corresponding to either the Broker
or the KDC.
Other additional parameters can be included if necessary, as defined
in [RFC9200].
When using this profile, it is expected that a one-to-one mapping is
enforced between the application group and the security group (see
Section 2). If this is not the case, the correct access policies for
both sets of scopes have to be available to the AS.
3.4.1. Format of Scope
Building on Section 3.1 of [I-D.ietf-ace-key-groupcomm], this section
defines the exact format and encoding of scope used in this profile.
To this end, this profile uses the Authorization Information Format
(AIF) [RFC9237] (REQ1). With reference to the generic AIF model
AIF-Generic<Toid, Tperm> = [* [Toid, Tperm]]
the value of the CBOR byte string used as scope encodes the CBOR
array [* [Toid, Tperm]], where each [Toid, Tperm] element corresponds
to one scope entry.
Furthermore, this document defines the new AIF data model AIF-PUBSUB-
GROUPCOMM that this profile MUST use to format and encode scope
entries.
In particular, the following holds for each scope entry.
The object identifier ("Toid") is specialized as a CBOR item
specifying the name of the groups pertaining to the scope entry.
The permission set ("Tperm") is specialized as a CBOR unsigned
integer with value R, specifying the permissions that the Client
wishes to have in the groups indicated by "Toid".
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More specifically, the following applies when, as defined in this
document, a scope entry includes as set of permissions for user-
related operations performed by a pubsub Client.
* The object identifier ("Toid") is a CBOR text string, specifying
the name of one application group (topic) or of the corresponding
security group to which the scope entry pertains.
* The permission set ("Tperm") is a CBOR unsigned integer, whose
value R specifies the operations that the Client wishes to or has
been authorized to perform on the resources at the Broker
associated with the application group (topic) indicated by "Toid",
or on the resources at the KDC associated with the security group
indicated by "Toid" (REQ1). The value R is computed as follows.
- Each operation (i.e., permission detail) in the permission set
is converted into the corresponding numeric identifier X taken
from the following set.
o Admin (0): This operation is reserved for scope entries that
express permissions for Administrators of pub/sub groups.
o AppGroup (1): This operation is signaled as wished/
authorized when "Toid" specifies the name of an application
group (topic), while it is signaled as not wished/authorized
when Toid specifies the name of a security group.
o Publish (2): This operation concerns the publication of data
to the topic in question, performed by means of a PUT
request sent by a Publisher Client to the corresponding
topic-data resource at the Broker.
o Read (3): This operation concerns both: i) the subscription
at the topic-data resource for the topic in question at the
Broker, performed by means of a GET request with the CoAP
Observe Option set to 0 and sent by a Subscriber Client; and
ii) the simple reading of the latest data published to the
topic in question, performed by means of a simple GET
request sent to the same topic-data resource.
o Delete (4): This operation concerns the deletion of the
topic-data resource for the topic in question at the Broker,
performed by means of a DELETE request sent to that
resource.
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- The set of N numeric identifiers is converted into the single
value R, by taking two to the power of each numeric identifier
X_1, X_2, ..., X_N, and then computing the inclusive OR of the
binary representations of all the power values.
Since this application profile considers user-related operations,
the "Admin" operation is signaled as not wished/authorized. That
is, the scope entries MUST have the least significant bit of
"Tperm" set to 0.
If the "Toid" of a scope entry in an access token specifies the name
of an application group (i.e., the "AppGroup" operation is signaled
as authorized), the Client has also the permission to retrieve the
configuration of the application group (topic) whose name is
indicated by "Toid", by sending a GET or FETCH request to the
corresponding topic resource at the Broker.
The specific interactions between the Client and the Broker are
defined in [I-D.ietf-core-coap-pubsub].
The following CDDL [RFC8610] notation defines a scope entry that uses
the AIF-PUBSUB-GROUPCOMM data model and expresses a set of
permissions.
AIF-PUBSUB-GROUPCOMM = AIF-Generic<pubsub-group, pubsub-perm>
pubsub-group = tstr ; name of pub/sub topic or of
; the associated security group
pubsub-perm = uint .bits pubsub-perm-details
pubsub-perm-details = &(
Admin: 0,
AppGroup: 1
Publish: 2,
Read: 3,
Delete: 4
)
scope_entry = [pubsub-group, pubsub-perm]
Figure 5: Pub/sub scope using the AIF format
3.5. Authorisation response
The AS responds with an Authorization Response to each request,
containing claims, as defined in Section 5.8.2 of [RFC9200] and
Section 3.2 of [I-D.ietf-ace-key-groupcomm] with the following
additions:
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* The AS MUST include the 'expires_in' parameter. Other means for
the AS to specify the lifetime of Access Tokens are out of the
scope of this document.
* The AS MUST include the 'scope' parameter, when the value included
in the Access Token differs from the one specified by the Client
in the Authorization Request. In such a case, the second element
of each scope entry specifies the set of interactions that the
Client is authorized for that scope entry, encoded as specified in
Section 3.4.
Furthermore, the AS MAY use the extended format of scope defined in
Section 7 of [I-D.ietf-ace-key-groupcomm] for the 'scope' claim of
the Access Token. In such a case, the AS MUST use the CBOR tag with
tag number TAG_NUMBER, associated with the CoAP Content-Format CF_ID
for the media type application/aif+cbor registered in Section 8.5 of
this document (REQ28).
Note to RFC Editor: In the previous paragraph, please replace
"TAG_NUMBER" with the CBOR tag number computed as TN(ct) in
Section 4.3 of [RFC9277], where ct is the ID assigned to the CoAP
Content-Format registered in Section 8.5 of this document. Then,
please replace "CF_ID" with the ID assigned to that CoAP Content-
Format. Finally, please delete this paragraph.
This indicates that the binary encoded scope follows the scope
semantics defined for this application profile in Section 3.4.1 of
this document.
3.6. Token Transfer to KDC
The Client transfers its access token to the KDC using one of the
methods defined in the Section 3.3 of [I-D.ietf-ace-key-groupcomm].
This typically includes sending a POST request to the authz-info
endpoint. However, if the DTLS transport profile of ACE [RFC9202] is
used and the Client uses a symmetric proof-of-possession key in the
DTLS handshake, the Client MAY provide the access token to the KDC in
the "psk_identity" field of the DTLS ClientKeyExchange message when
using DTLS 1.2 [RFC6347], or in the "identity" field of a PskIdentity
within the PreSharedKeyExtension of the ClientHello message when
using DTLS 1.3 [RFC9147]. In addition to that, the following
applies.
In the Token Transfer Response to the Publishers, i.e., the Clients
whose scope of the access token includes the "Publish" permission for
at least one scope entry, the KDC MUST include the parameter
'kdcchallenge' in the CBOR map. 'kdcchallange' is a challenge N_S
generated by the KDC, and is RECOMMENDED to be an 8-byte long random
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nonce. Later when joining the group, the Publisher can use the
'kdcchallenge' as part of proving possession of its private key (see
[I-D.ietf-ace-key-groupcomm]). If a Publisher provides the access
token to the KDC through an authz-info endpoint, the Client MUST
support the parameter 'kdcchallenge'.
If 'sign_info' is included in the Token Transfer Request, the KDC
SHOULD include the 'sign_info' parameter in the Token Transfer
Response. Note that the joining node may have obtained such
information by alternative means e.g., the 'sign_info' may have been
pre-configured (OPT3).
The following applies for each element 'sign_info_entry'.
* 'sign_alg' MUST take its value from the "Value" column of one of
the recommended algorithms in the "COSE Algorithms" registry
[IANA.cose_algorithms] (REQ3).
* 'sign_parameters' is a CBOR array. Its format and value are the
same of the COSE capabilities array for the algorithm indicated in
'sign_alg' under the "Capabilities" column of the "COSE
Algorithms" registry [IANA.cose_algorithms] (REQ4).
* 'sign_key_parameters' is a CBOR array. Its format and value are
the same of the COSE capabilities array for the COSE key type of
the keys used with the algorithm indicated in 'sign_alg', as
specified for that key type in the "Capabilities" column of the
"COSE Key Types" registry [IANA.cose_key-type] (REQ5).
* 'cred_fmt' takes value from the "Label" column of the "COSE Header
Parameters" registry [IANA.cose_header-parameters] (REQ6).
Acceptable values denote a format of authentication credential
that MUST explicitly provide the public key as well as the
comprehensive set of information related to the public key
algorithm, including, e.g., the used elliptic curve (when
applicable). Acceptable formats of authentication credentials
include CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs)
[RFC8392], X.509 certificates [RFC7925] and C509 certificates
[I-D.ietf-cose-cbor-encoded-cert]. Future formats would be
acceptable to use as long as they comply with the criteria defined
above.
4. Client Group Communication Interface at the KDC
In order to enable secure group communication for the pub/sub
clients, the KDC provides the resources listed in Table 1. Each
resource is marked as REQUIRED or OPTIONAL to be hosted at the KDC.
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+=================================+=================+===============+
| KDC resource | Description | Operations |
+=================================+=================+===============+
| /ace-group | REQUIRED. | FETCH (All |
| | Contains a set | Clients) |
| | of group | |
| | names, each | |
| | corresponding | |
| | to one of the | |
| | specified | |
| | group | |
| | identifiers. | |
+---------------------------------+-----------------+---------------+
| /ace-group/GROUPNAME | REQUIRED. | GET, POST |
| | Contains | (All Clients) |
| | symmetric | |
| | group keying | |
| | material | |
| | associated | |
| | with | |
| | GROUPNAME. | |
+---------------------------------+-----------------+---------------+
| /ace-group/GROUPNAME/creds | REQUIRED. | GET, FETCH |
| | Contains the | (All Clients) |
| | authentication | |
| | credentials of | |
| | all the | |
| | Publishers of | |
| | the group with | |
| | name | |
| | GROUPNAME. | |
+---------------------------------+-----------------+---------------+
| /ace-group/GROUPNAME/num | REQUIRED. | GET (All |
| | Contains the | Clients) |
| | current | |
| | version number | |
| | for the | |
| | symmetric | |
| | group keying | |
| | material of | |
| | the group with | |
| | name | |
| | GROUPNAME. | |
+---------------------------------+-----------------+---------------+
| /ace-group/GROUPNAME/nodes/ | REQUIRED. | GET, DELETE |
| NODENAME | Contains the | (All |
| | group keying | Clients). |
| | material for | PUT |
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| | that group | (Publishers). |
| | member | |
| | NODENAME in | |
| | GROUPNAME. | |
+---------------------------------+-----------------+---------------+
| /ace- | REQUIRED. | POST |
| group/GROUPNAME/nodes/NODENAME/ | Authentication | (Publishers) |
| cred | credential for | |
| | NODENAME in | |
| | the group | |
| | GROUPNAME. | |
+---------------------------------+-----------------+---------------+
| /ace-group/GROUPNAME/kdc-cred | REQUIRED if a | GET (All |
| | group re- | Clients) |
| | keying | |
| | mechanism is | |
| | used. | |
| | Contains the | |
| | authentication | |
| | credential of | |
| | the KDC for | |
| | the group with | |
| | name | |
| | GROUPNAME. | |
+---------------------------------+-----------------+---------------+
| /ace-group/GROUPNAME/policies | OPTIONAL. | GET (All |
| | Contains the | Clients) |
| | group policies | |
| | of the group | |
| | with name | |
| | GROUPNAME. | |
+---------------------------------+-----------------+---------------+
Table 1: Resources at the KDC
The use of these resources follows what is defined in
[I-D.ietf-ace-key-groupcomm], and only additions or modifications to
that specification are defined in this document.
Consistent with what is defined in [I-D.ietf-ace-key-groupcomm], some
error responses from the KDC can convey error-specific information
according to the problem-details format specified in [RFC9290].
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4.1. Joining a Security Group
This section describes the interactions between a Client and the KDC
to join a security group. Source authentication of a message sent
within the group is ensured by means of a digital signature embedded
in the message. Subscribers must be able to retrieve Publishers'
authentication credentials from a trusted repository, to verify
source authentication of received messages. Hence, on joining a
security group, a Publisher is expected to provide its own
authentication credential to the KDC.
On a successful join, the Clients receive from the KDC the symmetric
COSE Key used as shared group key to protect the payload of a
published topic data.
The message exchange between the joining node and the KDC follows
what is defined in Section 4.3.1.1 of [I-D.ietf-ace-key-groupcomm]
and only additions or modifications to that specification are defined
in this document.
Client KDC
|----- Join Request (CoAP) ------>|
| |
|<-----Join Response (CoAP) ------|
Figure 6: Join Flow
4.1.1. Join Request
After establishing a secure communication association with the KDC,
the Client sends a Join Request to the KDC as described in
Section 4.3 of [I-D.ietf-ace-key-groupcomm]. More specifically, the
Client sends a POST request to the /ace-group/GROUPNAME endpoint,
with Content-Format "application/ace-groupcomm+cbor". The payload
contains the following information formatted as a CBOR map, which
MUST be encoded as defined in Section 4.3.1 of
[I-D.ietf-ace-key-groupcomm]:
* 'scope': It MUST be present and specify the group that the Client
is attempting to join, i.e., the group name, and the permissions
it wishes to have in the group. This value corresponds to one
scope entry, as defined in Section 3.4.1.
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* 'get_creds': It MAY be present if the Client wishes to join as a
Subcriber and wants to retrieve the public keys of all the
Publishers upon joining. Otherwise, this parameter MUST NOT be
present. If the parameter is present, the parameter MUST encode
the CBOR simple value null (0xf6). Note that the parameter
'role_filter' is not necessary, as the KDC returns the
authentication credentials of Publishers by default.
* 'client_cred': The use of this parameter is detailed in
Section 4.1.1.1.
* 'cnonce': It specifies a dedicated nonce N_C generated by the
Client. It is RECOMMENDED to use use an 8-byte long random nonce.
Join Requests MUST include a new 'cnonce' at each join attempt.
* 'client_cred_verify': The use of this parameter is detailed in
Section 4.1.1.2.
As a Publisher Client has its own authentication credential to use in
a group, it MUST support the client_cred', 'cnonce', and
'client_cred_verify' parameters.
4.1.1.1. Client Credentials in 'client_cred'
One of the following cases can occur when a new Client attempts to
join a security group.
* The joining node is not a Publisher, i.e., it is not going to send
data to the application group. In this case, the joining node is
not required to provide its own authentication credential to the
KDC. In case the joining node still provides an authentication
credential in the 'client_cred' parameter of the Join Request (see
Section 4.1.1), the KDC silently ignores that parameter, as well
as the related parameter 'client_cred_verify'.
* The joining node wishes to join as a Publisher, and:
- The KDC already acquired the authentication credential of the
joining node either during a past group joining process, or
when establishing a secure communication association using
asymmetric proof-of-possession keys. If the joining node's
proof-of-possession key is compatible with the signature
algorithm used in the security group and with possible
associated parameters, then the corresponding authentication
credential can be used in the group. In this case, the joining
node MAY choose not to provide again its authentication
credential to the KDC in order to limit the size of the Join
Request.
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- The KDC has not acquired an authentication credential. Then,
the joining node MUST provide a compatible authentication
credential in the 'client_cred' parameter of the Join Request
(see Section 4.1.1).
Finally, the joining node MUST provide its authentication credential
again if it has provided the KDC with multiple authentication
credentials during past joining processes intended for different
security groups. If the joining node provides its authentication
credential, the KDC performs the consistency checks above and, in
case of success, considers it as the authentication credential
associated with the joining node in the group.
4.1.1.2. Proof-of-Possession
The 'client_cred_verify' parameter contains the proof-of-possession
evidence, and is computed as defined below (REQ14).
The Publisher signs the scope, concatenated with N_S and concatenated
with N_C, using the private key corresponding to the public key in
the 'client_cred' parameter.
The N_S may be either:
* The challenge received from the KDC in the 'kdcchallenge'
parameter of the 2.01 (Created) response to the Token Transfer
Request (see Section 3.6).
* If the provisioning of the access token to the KDC has relied on
the DTLS profile of ACE [RFC9202], and the access token was
specified in the "psk_identity" field of the ClientKeyExchange
message when using DTLS 1.2 [RFC6347], then N_S is an exporter
value computed as defined in Section 4 of [RFC5705] (REQ15).
Specifically, N_S is exported from the DTLS session between the
joining node and the KDC, using an empty context value (i.e., a
context value of zero-length), 32 as length value in bytes, and
the exporter label "EXPORTER-ACE-Sign-Challenge-coap-group-pubsub-
app" defined in Section 8.6 of this document.
* If the provisioning of the access token to the KDC has relied on
the DTLS profile of ACE [RFC9202], and the access token was
specified in the "identity" field of a PskIdentity within the
PreSharedKeyExtension of the ClientHello message when using DTLS
1.3 [RFC9147], then N_S is an exporter value computed as defined
in Section 7.5 of [RFC8446] (REQ15).
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Specifically, N_S is exported from the DTLS session between the
joining node and the KDC, using an empty 'context_value' (i.e., a
'context_value' of zero length), 32 as 'key_length' in bytes, and
the exporter label "EXPORTER-ACE-Sign-Challenge-coap-group-pubsub-
app" defined in Section 8.6 of this document.
* If the Join Request is a retry in response to an error response
from the KDC, which included a new 'kdcchallenge' parameter, then
N_S MUST be the new value from this parameter.
It is up to applications to define how N_S is computed in further
alternative settings.
4.1.2. Join Response
On receiving the Join Request, the KDC processes the request as
defined in Section 4.3.1 of [I-D.ietf-ace-key-groupcomm], and returns
a success or error response.
If the 'client_cred' parameter is present, the KDC verifies the
signature in the 'client_cred_verify' parameter. As PoP input, the
KDC uses the value of the 'scope' parameter from the Join Request as
a CBOR byte string, concatenated with N_S encoded as a CBOR byte
string, concatenated with N_C encoded as a CBOR byte string. As
public key of the joining node, the KDC uses either the one included
in the authentication credential retrieved from the 'client_cred'
parameter of the Join Request or the one from the already stored
authentication credential from previous interactions with the joining
node. The KDC verifies the PoP evidence, which is a signature, by
using the public key of the joining node, as well as the signature
algorithm used in the group and possible corresponding parameters.
In the case of any join request error, the KDC and the Client
attempting to join follow the procedure defined in Section 4.1.3.
In case of success, the KDC responds with a Join Response, whose
payload formatted as a CBOR map MUST contain the following fields as
per Section 4.3.1 of [I-D.ietf-ace-key-groupcomm]:
* 'gkty': the key type "Group_PubSub_Keying_Material" (REQ18)
registered in Section 8.1 for the 'key' parameter.
* 'key': The keying material for group communication includes the
following parameters (REQ17):
- 'cred_fmt', specifying the format of authentication credentials
used in the group. This parameter takes its value from the
"Label" column of the "COSE Header Parameters" registry
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[IANA.cose_header-parameters]. At the time of writing this
specification, acceptable formats of authentication credentials
are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs)
[RFC8392], X.509 certificates [RFC7925] and C509 certificates
[I-D.ietf-cose-cbor-encoded-cert]. Further formats may be
available in the future, and would be acceptable to use as long
as they comply with the criteria defined above (REQ6).
- 'sign_alg', specifying the Signature Algorithm used to sign
messages in the group. This parameter takes values from the
"Value" column of the "COSE Algorithms" registry
[IANA.cose_algorithms].
- 'sign_params', specifying the parameters of the Signature
Algorithm. This parameter is a CBOR array, which includes the
following two elements:
o 'sign_alg_capab' is a CBOR array, with 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 [IANA.cose_algorithms].
o 'sign_key_type_capab' is a CBOR array, with the same format
and value of the COSE capabilities array for the COSE key
type of the keys used with the Signature Algorithm indicated
in 'sign_alg', as specified for that key type in the
"Capabilities" column of the "COSE Key Types" registry
[IANA.cose_key-type].
- 'group_key', specifying a COSE_Key object as defined in
[RFC9052] and conveying the group key to use in the security
group. The COSE_Key object MUST contain the following
parameters:
o 'kty', with value 4 (Symmetric).
o 'alg', with value the identifier of the AEAD algorithm used
in the security group.
o 'Base IV', with value the Base Initialization Vector (Base
IV) to use in the security group with this group key.
o 'k', with value the symmetric encryption key to use as group
key.
o 'kid', with value the identifier of the COSE_Key object,
hence of the group key.
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This value is used as Group Identifier (Gid) of the security
group, as long as the present key is used as group key in
the security group.
- 'group_SenderId', specifying the Client's Sender ID encoded as
a CBOR byte string. This field MUST be included if the Client
is joining the security group as a Publisher, and MUST NOT be
included otherwise. A Publisher Client MUST support the
'group_SenderId' parameter (REQ29).
The Sender ID MUST be unique within the security group. The
KDC MUST only assign an available Sender ID that has not been
used in the security group since the last time when the current
Gid value was assigned to the group (i.e., since the latest
group rekeying, see Section 4.2.4). The KDC MUST NOT assign a
Sender ID to the joining node if the node is not joining the
group as a Publisher.
The Sender ID can be short in length. Its maximum length in
bytes is the length in bytes of the AEAD nonce for the AEAD
algorithm, minus 6. This means that, when using AES-CCM-
16-64-128 as AEAD algorithm in the security group, the maximum
length of Sender IDs is 7 bytes.
* 'num', specifying the version number of the keying material
specified in the 'key' field. The initial value of the version
number MUST be set to 0 upon creating the group (REQ16).
* 'exi', which MUST be present.
* 'ace-groupcomm-profile', which MUST be present and has value
"coap_group_pubsub_app" (PROFILE_TBD), which is registered in
Section 8.2 (REQ19).
* 'creds', which MUST be present if the 'get_creds' parameter was
present. Otherwise, it MUST NOT be present. The KDC provides the
authentication credentials of all the Publishers in the security
group.
* 'peer_roles', which MAY be omitted even if 'creds' is present,
since each authentication credential conveyed in the 'creds'
parameter: i) is associated with a Client authorized to be
Publisher in the group; and ii) plays no role if such Client was
also authorized to be Subscriber in the group. If 'creds' is not
present, 'peer_roles' MUST NOT be present.
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* 'peer_identifiers', which MUST be present if 'creds' is also
present. Otherwise, it MUST NOT be present. The identifiers are
the Publisher Sender IDs whose authentication credential is
specified in the 'creds' parameter (REQ25).
* 'kdc_cred', which MUST be present if group re-keying is used, and
is encoded as a CBOR byte string, with value the original binary
representation of the KDC's authentication credential (REQ8).
* 'kdc_nonce', which MUST be present if 'kdc_cred' is present, and
is encoded as a CBOR byte string, and including a dedicated nonce
N_KDC generated by the KDC. For N_KDC, it is RECOMMENDED to use
an 8-byte long random nonce.
* 'kdc_cred_verify', which MUST be present if 'kdc_cred' is present,
and is encoded as a CBOR byte string. The KDC MUST compute the
specified PoP evidence as a signature by using the signature
algorithm used in the group, as well as its own private key
associated with the authentication credential specified in the
'kdc_cred' parameter (REQ21).
* 'group_rekeying', which MAY be omitted if the KDC uses the "Point-
to-Point" group rekeying scheme registered in Section 11.13 of
[I-D.ietf-ace-key-groupcomm] as the default rekeying scheme in the
group (OPT9). In any other case, the 'group_rekeying' parameter
MUST be included.
After sending a successful Join Response, the KDC adds the Client to
the list of current members of the security group, if that Client is
not already a group member. Also, the Client is assigned a name
NODENAME and a sub-resource /ace-group/GROUPNAME/nodes/NODENAME.
Furthermore, the KDC associates NODENAME with the Client's access
token and with the secure communication association that the KDC has
with the Client. If the Client is a Publisher, its authentication
credential is also associated with the tuple containing NODENAME,
GROUPNAME, the current Gid, the newly assigned Publisher's Sender ID,
and the Client's access token. The KDC MUST keep this association
updated over time.
Note that, as long as the secure communication association between
the Client and the KDC persists, the same Client re-joining the group
is recognized by the KDC by virtue of such a secure communication
association. As a consequence, the re-joining Client keeps the same
NODENAME and the associated subresource /ace-group/GROUPNAME/nodes/
NODENAME. Also, if the Client is a Publisher, it receives a new
Sender ID according to the same criteria defined above.
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If the application requires backward security, the KDC MUST generate
updated security parameters and group keying material, and provide it
to the current group members upon the new node's joining (see
Section 4.2.4). In such a case, the joining node is not able to
access secure communication in the pub/sub group prior its joining.
Upon receiving the Join Response, the joining node retrieves the
KDC's authentication credential from the 'kdc_cred' parameter. The
joining node MUST verify the proof-of-possession (PoP) evidence,
which is a signature, specified in the 'kdc_cred_verify' parameter of
the Join Response (REQ21).
4.1.3. Join Error Handling
The KDC MUST reply with a 4.00 (Bad Request) error response (OPT4) to
the Join Request in the following cases:
* The 'client_cred' parameter is present in the Join Request and its
value is not an eligible authentication credential (e.g., it is
not of the format accepted in the group) (OPT8).
* The 'client_cred' parameter is present but does not include the
'cnonce' and 'client_cred_verify' parameters.
* The 'client_cred' parameter is not present while the joining node
is not going to join the group exclusively as a Subscriber, and
any of the following conditions holds:
- The KDC does not store an eligible authentication credential
(e.g., of the format accepted in the group) for the joining
node.
- The KDC stores multiple eligible authentication credentials
(e.g., of the format accepted in the group) for the joining
node.
* The 'scope' parameter is not present in the Join Request, or it is
present and specifies any set of permissions not included in the
list defined in Section 3.4.1.
A 4.00 (Bad Request) error response from the KDC to the joining node
MAY have content format application/ace-groupcomm+cbor and contain a
CBOR map as payload.
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The CBOR map MAY include the 'kdcchallenge' parameter. If present,
this parameter is a CBOR byte string, which encodes a newly generated
'kdcchallenge' value that the Client can use when preparing a new
Join Request. In such a case, the KDC MUST store the newly generated
value as the 'kdcchallenge' value associated with the joining node,
which replaces the currently stored value (if any).
Upon receiving the Join Response, if 'kdc_cred' is present but the
Client cannot verify the PoP evidence, the Client MUST stop
processing the Join Response and MAY send a new Join Request to the
KDC.
The KDC MUST return a 5.03 (Service Unavailable) response to a Client
that sends a Join Request to join the security group as Publisher, in
case there are currently no Sender IDs available to assign.
4.2. Other Group Operations through the KDC
4.2.1. Querying for Group Information
* '/ace-group': All Clients can send a FETCH request to retrieve a
set of group names associated with their group identifiers
specified in the request payload. Each element of the CBOR array
'gid' is a CBOR byte string (REQ13), which encodes the Gid of the
group (see Section 4.1.2) for which the group name and the URI to
the group-membership resource are provided.
* '/ace-group/GROUPNAME': All Clients can use GET requests to
retrieve the symmetric group keying material of the group with the
name GROUPNAME. The value of the GROUPNAME URI path and the group
name in the access token scope ('gname') MUST coincide.
* '/ace-group/GROUPNAME/creds': The KDC acts as a repository of
authentication credentials for the Publishers that are member of
the security group with name GROUPNAME. The members of the group
that are Subscribers can send GET/FETCH requests to this resource
in order to retrieve the authentication credentials of all or a
subset of the group members that are Publishers. The KDC silently
ignores the Sender IDs included in the 'get_creds' parameter of
the request that are not associated with any current Publisher
group member (REQ26).
The response from the KDC MAY omit the parameter 'peer_roles',
since each authentication credential conveyed in the 'creds'
parameter: i) is associated with a Client authorized to be
Publisher in the group; and ii) plays no role if such Client was
also authorized to be Subscriber in the group. If 'creds' is not
present, 'peer_roles' MUST NOT be present.
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* '/ace-group/GROUPNAME/num': All group member Clients can send a
GET request to this resource in order to retrieve the current
version number for the symmetric group keying material of the
group with name GROUPNAME.
* '/ace-group/GROUPNAME/kdc-cred': All group member Clients can send
a GET request to this resource in order to retrieve the current
authentication credential of the KDC.
4.2.2. Updating Authentication Credentials
A Publisher with node name NODENAME can contact the KDC to upload a
new authentication credential to use in the security group with name
GROUPNAME, and replace the currently stored one. To this end, it
sends a CoAP POST request to its associated sub-resource /ace-
group/GROUPNAME/nodes/NODENAME/cred. The KDC replaces the stored
authentication credential of this Client for the group GROUPNAME with
the one specified in the request.
4.2.3. Removal from a Group
A Client with node name NODENAME can actively request to leave the
security group with name GROUPNAME. In this case, the Client sends a
CoAP DELETE request to the associated sub-resource /ace-
group/GROUPNAME/nodes/NODENAME at the KDC. The KDC can also remove a
group member due to any of the reasons described in Section 5 of
[I-D.ietf-ace-key-groupcomm].
4.2.4. Rekeying a Group
The KDC MUST trigger a group rekeying as described in Section 6 of
[I-D.ietf-ace-key-groupcomm] due to a change in the group membership
or the current group keying material approaching its expiration time.
The KDC MAY trigger regularly scheduled update of the group keying
material.
Upon generating the new group keying material and before starting its
distribution, the KDC MUST increment the version number of the group
keying material. The KDC MUST preserve the current value of the
Sender ID of each member in that group. The KDC MUST also generate a
new Group Identifier (Gid) for the group, and use it as identifier of
the new group key provided through the group rekeying and hereafter
to Clients (re-)joining the security group (see Section 4.1.2).
The default rekeying scheme is Point-to-Point (see Section 6.1 of
[I-D.ietf-ace-key-groupcomm]), where the KDC individually targets
each node to rekey, using the pairwise secure communication
association with that node.
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If the group rekeying is performed due to one or multiple Clients
joining the group as Publishers, then a rekeying message includes the
Sender IDs and authentication credentials that those clients are
going to use in the group. This information is specified by means of
the parameters 'creds' and 'peer_identifiers', like done in the Join
Response message. In the same spirit, the 'peer_roles' parameter MAY
be omitted.
5. PubSub Protected Communication (C)
+------------+ +------------+ +------------+
| | | | | |
| Publisher | ----(D)---> | Broker | | Subscriber |
| | | | <----(E)---- | |
| | | | -----(F)---> | |
+------------+ +------------+ +------------+
Figure 7: Secure communication between Publisher and Subscriber
(D) corresponds to the publication of a topic on the Broker, using a
CoAP PUT. The publication (the resource representation) is protected
with COSE ([RFC9052][RFC9053]) by the Publisher. The (E) message is
the subscription of the Subscriber, and uses a CoAP GET with the
Observe option set to 0 (zero) [RFC7641], as per
[I-D.ietf-core-coap-pubsub]. The (F) message is the response from
the Broker, where the publication is protected with COSE by the
Publisher. (ToDo: Add Delete to the flow?)
Publisher Broker Subscriber
| --- PUT /topic -----> | |
| protected with COSE | |
| | <--- GET /topic ----- |
| | Observe:0 |
| | |
| | ---- Response ------> |
| | protected with COSE |
Figure 8: Example of protected communication for CoAP
5.1. Using COSE Objects To Protect The Resource Representation
The Publisher uses the symmetric COSE Key received from the KDC to
protect the payload of the Publish operation (see Section 4.3 of
[I-D.ietf-core-coap-pubsub]). Specifically, the Publisher creates a
COSE_Encrypt0 object [RFC9052][RFC9053] by means of the COSE Key
currently used as group key. The encryption algorithm and Base IV to
use are specified by the 'alg' and 'Base IV' parameters of the COSE
Key, together with its key identifier in the 'kid' parameter.
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Also, the Publisher uses its private key corresponding to the public
key sent to the KDC for countersigning the COSE_Encrypt0 object as
specified in [RFC9338]. The countersignature is specified in the
'Countersignature version 2' parameter, within the 'unprotected'
field of the COSE_Encrypt0 object.
Finally, the Publisher sends the COSE_Encrypt0 object conveying the
countersignature to the Broker, as payload of the PUT request sent to
the topic-data of the topic targeted by the Publish operation.
Upon receiving a response from the topic-data resource at the Broker,
the Subscriber uses the 'kid' parameter from the 'Countersignature
version 2' parameter within the 'unprotected' field of the
COSE_Encrypt0 object, in order to retrieve the Publisher's public key
from the Broker or from its local storage. Then, the Subscriber uses
that public key to verify the countersignature.
In case of successful verification, the Subscriber uses the 'kid'
parameter from the 'unprotected' field of the COSE_Encrypt0 object,
in order to retrieve the COSE Key used as current group key from its
local storage. Then, the Subscriber uses that group key to verify
and decrypt the COSE_Encrypt0 object. In case of successful
verification, the Subscriber delivers the received topic data to the
application.
The COSE_Encrypt0 object is constructed as follows.
The 'protected' field MUST include:
* The 'alg' parameter, with value the identifier of the AEAD
algorithm specified in the 'alg' parameter of the COSE Key used as
current group key.
The 'unprotected' field MUST include:
* The 'kid' parameter, with the same value specified in the 'kid'
parameter of the COSE Key used as current group key. This value
represents the current Group ID (Gid) of the security group
associated with the application group (topic).
* The 'Partial IV' parameter, with value set to the current Sender
Sequence Number of the Publisher. All leading bytes of value zero
SHALL be removed when encoding the Partial IV, except in the case
of Partial IV value 0, which is encoded to the byte string 0x00.
The Publisher MUST initialize the Sender Sequence Number to 0 upon
joining the security group, and MUST reset it to 0 upon receiving
a new group key as result of a group rekeying (see Section 4.2.4).
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The Publisher MUST increment its Sender Sequence Number value by
1, after having completed an encryption operation by means of the
current group key.
* The 'Countersignature version 2' parameter, specifying the
countersignature of the COSE_Encrypt0 object. In particular:
- The 'protected' field includes the 'alg' parameter, with value
the identifier of the Signature Algorithm used in the security
group.
- The 'unprotected' field includes the 'kid' parameter, with
value the Publisher's Sender ID that the Publisher obtained
from the KDC when joining the security group, as value of the
'group_SenderId' parameter of the Join Response (see
Section 4.1.2).
- The 'signature' field, with value the countersignature.
The countersignature is computed as defined in [RFC9338], by using
the private key of the Publisher as signing key, and by means of
the Signature Algorithm used in the group. The fields of the
Countersign_structure are populated as follows:
- 'context' takes "CounterSignature".
- 'body_protected' takes the serialized parameters from the
'protected' field of the COSE_Encrypt0 object, i.e., the 'alg'
parameter.
- 'sign_protected' takes the serialized parameters from the
'protected' field of the 'Countersignature version 2'
parameter, i.e., the 'alg' parameter.
- 'external_aad is not supplied.
- 'payload' is the ciphertext of the COSE_Encrypt0 object (see
below).
The 'ciphertext' field specifies the ciphertext computed over the
topic data to publish. The ciphertext is computed as defined in
[RFC9052][RFC9053], by using the current group key as encryption key,
the AEAD Nonce computed as defined in Section 5.2, the topic data to
publish as plaintext, and the Enc_structure populated as follows:
* 'context' takes "Encrypt0".
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* 'protected' takes the serialization of the protected parameter
'alg' from the 'protected' field of the COSE_Encrypt0 object.
* 'external_aad is not supplied.
5.2. AEAD Nonce
This section defines how to generate the AEAD nonce used for
encrypting and decrypting the COSE_Encrypt0 object protecting the
published topic data. This construction is analogous to that used to
generate the AEAD nonce in the OSCORE security protocol (see
Section 5.2 of [RFC8613]).
The AEAD nonce for producing or consuming the COSE_Encrypt0 object is
constructed as defined below and also shown in Figure 9.
1. Left-pad the Partial IV (PIV) with zeroes to exactly 5 bytes.
2. Left-pad the Sender ID of the Publisher that generated the
Partial IV (ID_PIV) with zeroes to exactly the nonce length of
the AEAD algorithm minus 6 bytes.
3. Concatenate the size of the ID_PIV (a single byte S) with the
padded ID_PIV and the padded PIV.
4. XOR the result from the previous step with the Base IV.
<- nonce length minus 6 B -> <-- 5 bytes -->
+---+-------------------+--------+---------+-----+
| S | padding | ID_PIV | padding | PIV |-----+
+---+-------------------+--------+---------+-----+ |
|
<---------------- nonce length ----------------> |
+------------------------------------------------+ |
| Base IV |-->(XOR)
+------------------------------------------------+ |
|
<---------------- nonce length ----------------> |
+------------------------------------------------+ |
| Nonce |<----+
+------------------------------------------------+
Figure 9: AEAD Nonce Construction
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The construction above only supports AEAD algorithms that use nonces
with length equal or greater than 7 bytes. At the same time, it
makes it easy to verify that the nonces will be unique when used with
the the same group key, even though this is shared and used by all
the Publishers in the security group. In fact:
* Since Publisher's Sender IDs are unique within the security group
and they are not reassigned until a group rekeying occurs (see
Section 4.1.2 and Section 4.2.4), two Publisher Clients cannot
share the same tuple (S, padded ID_PIV) by construction.
* Since a Publisher increments by 1 its Sender Sequence Number after
each use that it makes of the current group key, the Publisher
never reuses the same tuple (S, padded ID_PIV, padded PIV)
together with the same group key.
* Therefore neither the same Publisher nor any two Publishers use
the same AEAD nonce with the same group key.
5.3. Replay Checks
This section defines how a Subscriber Client checks whether the topic
data conveyed in a received message from the Broker is a replay.
TBD
6. Applicability to MQTT PubSub Profile
The steps MQTT clients go through would be similar to the CoAP
clients, and the payload of the MQTT PUBLISH messages will be
protected using COSE. The MQTT clients needs to use CoAP to
communicate to the KDC, to join security groups, and be part of the
pairwise rekeying initiated by the KDC.
Authorisation Server (AS) Discovery is defined in Section 2.4.1 of
[RFC9431] for MQTT v5 clients (and not supported for MQTT v3
clients). $SYS/ has been widely adopted as a prefix to topics that
contain server-specific information or control APIs, and may be used
for topic and KDC discovery.
When the Client sends an authorisation request to the AS using the
AIF-PUBSUB-GROUPCOMM data model in the authorisation response, the
'profile' claim is set to "mqtt_pubsub_app" as defined in
Section 8.2.
Both Publishers and Subscribers MUST authorise to the Broker with
their respective tokens, as described in [RFC9431]. A Publisher
sends PUBLISH messages for a given topic and protects the payload
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with the corresponding key for the associated security group. A
Subscriber may send SUBSCRIBE messages with one or multiple topic
filters. A topic filter may correspond to multiple topics. The
Broker forwards all PUBLISH messages to all authorised Subscribers,
including the retained messages.
7. Security Considerations
All the security considerations in [I-D.ietf-ace-key-groupcomm]
apply.
In the profile described above, the Publishers and Subscribers use
asymmetric cryptography, which would make the message exchange quite
heavy for small constrained devices. Moreover, all Subscribers must
be able to access the public keys of all the Publishers to a specific
topic to verify the publications.
Even though access tokens have expiration times, an access token may
need to be revoked before its expiration time (see
[I-D.draft-ietf-ace-revoked-token-notification] for a list of
possible circumstances). Clients can be excluded from future
publications through re-keying for a certain topic. This could be
set up to happen on a regular basis, for certain applications. How
this could be done is out of scope for this work. The method
described in [I-D.draft-ietf-ace-revoked-token-notification] MAY be
used to allow an Authorization Server to notify the KDC about revoked
access tokens.
The Broker is only trusted with verifying that the Publisher is
authorized to publish, but is not trusted with the publications
itself, which it cannot read nor modify.
With respect to the reusage of nonces for Proof-of-Possession input,
the same considerations apply as in the
[I-D.ietf-ace-key-groupcomm-oscore].
TODO: expand on security and privacy considerations
8. IANA Considerations
Note to RFC Editor: Please replace "[RFC-XXXX]" with the RFC number
of this document and delete this paragraph.
This document has the following actions for IANA.
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8.1. ACE Groupcomm Key Types Registry
IANA is asked to register the following entry in the "ACE Groupcomm
Key Types" registry defined in Section 11.8 of
[I-D.ietf-ace-key-groupcomm].
* Name: Group_PubSub_Keying_Material
* Key Type Value: GROUPCOMM_KEY_TBD
* Profile: coap_group_pubsub_app, defined in Section 8.2 of this
document.
* Description: Encoded as described in the Section 4.1.2 of this
document.
* References: [RFC9052], [RFC9053], [RFC-XXXX]
8.2. ACE Groupcomm Profiles Registry
IANA is asked to register the following entries in the "ACE Groupcomm
Profiles" registry defined in Section 11.9 of
[I-D.ietf-ace-key-groupcomm].
* Name: coap_group_pubsub_app
* Description: Profile for delegating client authentication and
authorization for publishers and subscribers in a CoAP pub/sub
setting scenario in a constrained environment.
* CBOR Value: TBD
* Reference: [RFC-XXXX]
* Name: mqtt_pubsub_app
* Description: Profile for delegating client authentication and
authorization for publishers and subscribers in a MQTT pub/sub
setting scenario in a constrained environment.
* CBOR Value: TBD
* Reference: [RFC-XXXX]
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8.3. CoRE Resource Type
IANA is asked to register the following entry in the "Resource Type
(rt=) Link Target Attribute Values" registry within the "Constrained
Restful Environments (CoRE) Parameters" registry group.
* Value: "core.ps.gm"
* Description: Group-membership resource for pub/sub communication.
* Reference: [RFC-XXXX]
Clients can use this resource type to discover a group membership
resource at the KDC.
8.4. AIF Media-Type Sub-Parameters
For the media-types application/aif+cbor and application/aif+json
defined in Section 5.1 of [RFC9237], IANA is requested to register
the following entries for the two media-type parameters Toid and
Tperm, in the respective sub-registry defined in Section 5.2 of
[RFC9237] within the "MIME Media Type Sub-Parameter" registry group.
* Parameter: Toid
* Name: pubsub-topic
* Description/Specification: Pub/sub topic name, corresponding to
the security group
* Reference: [RFC-XXXX]
* Parameter: Tperm
* Name: pubsub-perm
* Description/Specification: Permissions corresponding to the topic
data interactions in the pub/sub group
* Reference: [RFC-XXXX]
8.5. CoAP Content-Formats
IANA is asked to register the following entries to the "CoAP Content-
Formats" registry within the "Constrained RESTful Environments (CoRE)
Parameters" registry group.
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* Content Type: application/aif+cbor;Toid="pubsub-
topic",Tperm="pubsub-perm"
* Content Coding: -
* ID: 294 (suggested)
* Reference: [RFC-XXXX]
* Content Type: application/aif+json;Toid="pubsub-
topic",Tperm="pubsub-perm"
* Content Coding: -
* ID: 295 (suggested)
* Reference: [RFC-XXXX]
8.6. TLS Exporter Labels
IANA is asked to register the following entry to the "TLS Exporter
Labels" registry defined in Section 6 of [RFC5705] and updated in
Section 12 of [RFC8447].
* Value: EXPORTER-ACE-Sign-Challenge-coap-group-pubsub-app
* DTLS-OK: Y
* Recommended: N
* Reference: [RFC-XXXX]
9. References
9.1. Normative References
[I-D.ietf-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", Work in Progress, Internet-
Draft, draft-ietf-ace-key-groupcomm-18, 16 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-key-
groupcomm-18>.
[I-D.ietf-core-coap-pubsub]
Jimenez, J., Koster, M., and A. Keränen, "A publish-
subscribe architecture for the Constrained Application
Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
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ietf-core-coap-pubsub-13, 20 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-pubsub-13>.
[IANA.cose_algorithms]
IANA, "COSE Algorithms",
<https://www.iana.org/assignments/cose>.
[IANA.cose_header-parameters]
IANA, "COSE Header Parameters",
<https://www.iana.org/assignments/cose>.
[IANA.cose_key-type]
IANA, "COSE Key Types",
<https://www.iana.org/assignments/cose>.
[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>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/rfc/rfc5705>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/rfc/rfc6347>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/rfc/rfc6690>.
[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>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/rfc/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/rfc/rfc7641>.
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[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/rfc/rfc7925>.
[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>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/rfc/rfc8447>.
[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>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
[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>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
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[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>.
[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>.
[RFC9290] Fossati, T. and C. Bormann, "Concise Problem Details for
Constrained Application Protocol (CoAP) APIs", RFC 9290,
DOI 10.17487/RFC9290, October 2022,
<https://www.rfc-editor.org/rfc/rfc9290>.
[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>.
9.2. Informative References
[I-D.draft-ietf-ace-revoked-token-notification]
Tiloca, M., Palombini, F., Echeverria, S., and G. Lewis,
"Notification of Revoked Access Tokens in the
Authentication and Authorization for Constrained
Environments (ACE) Framework", Work in Progress, Internet-
Draft, draft-ietf-ace-revoked-token-notification-06, 2
June 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-ace-revoked-token-notification-06>.
[I-D.ietf-ace-edhoc-oscore-profile]
Selander, G., Mattsson, J. P., Tiloca, M., and R. Höglund,
"Ephemeral Diffie-Hellman Over COSE (EDHOC) and Object
Security for Constrained Environments (OSCORE) Profile for
Authentication and Authorization for Constrained
Environments (ACE)", Work in Progress, Internet-Draft,
draft-ietf-ace-edhoc-oscore-profile-03, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
edhoc-oscore-profile-03>.
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[I-D.ietf-ace-key-groupcomm-oscore]
Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", Work in Progress, Internet-
Draft, draft-ietf-ace-key-groupcomm-oscore-16, 6 March
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
ace-key-groupcomm-oscore-16>.
[I-D.ietf-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-09, 4 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cbor-encoded-cert-09>.
[MQTT-OASIS-Standard-v5]
Banks, A., Briggs, E., Borgendale, K., and R. Gupta,
"OASIS Standard MQTT Version 5.0", 2017,
<http://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-
v5.0-os.html>.
[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>.
[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>.
[RFC9431] Sengul, C. and A. Kirby, "Message Queuing Telemetry
Transport (MQTT) and Transport Layer Security (TLS)
Profile of Authentication and Authorization for
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Constrained Environments (ACE) Framework", RFC 9431,
DOI 10.17487/RFC9431, July 2023,
<https://www.rfc-editor.org/rfc/rfc9431>.
Appendix A. Requirements on Application Profiles
This section lists the specifications on this profile based on the
requirements defined in Appendix A of [I-D.ietf-ace-key-groupcomm].
* REQ1: Specify the format and encoding of 'scope': see
Section 3.4.1.
* 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]:
see Section 8.4.
* REQ3: If used, specify the acceptable values for 'sign_alg':
values from the "Value" column of the "COSE Algorithms" registry
[IANA.cose_algorithms].
* REQ4: If used, specify the acceptable values for
'sign_parameters': format and values from the COSE algorithm
capabilities as specified in the "COSE Algorithms" registry
[IANA.cose_algorithms].
* REQ5: If used, specify the acceptable values for
'sign_key_parameters': its format and value are the same of the
COSE capabilities array for the COSE key type of the keys used
with the algorithm indicated in 'sign_alg', as specified for that
key type in the "Capabilities" column of the "COSE Key Types"
registry [IANA.cose_key-type].
* REQ6: Specify the acceptable formats for authentication
credentials and, if used, the acceptable values for 'cred_fmt':
acceptable formats explicitly provide the public key as well as
the comprehensive set of information related to the public key
algorithm. Takes value from the "Label" column of the "COSE
Header Parameters" registry [IANA.cose_header-parameters].
* REQ7: If the value of the GROUPNAME URI path and the group name in
the access token scope (gname) are not required to coincide,
specify the mechanism to map the GROUPNAME value in the URI to the
group name: not applicable; a perfect matching is required.
* REQ8: Define whether the KDC has an authentication credential and
if this has to be provided through the 'kdc_cred' parameter:
optional, see Section 4.1.2 of this document.
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* REQ9: Specify if any part of the KDC interface as defined in
[I-D.ietf-ace-key-groupcomm] is not supported by the KDC: some are
left optional, see Section 4 of this document.
* REQ10: Register a Resource Type for the group-membership resource,
which is used to discover the correct URL for sending a Join
Request to the KDC: the Resource Type (rt=) Link Target Attribute
value "core.ps.gm" is registered in Section Section 8.3.
* 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
Client is authorized to take as per the obtained access token; and
the roles that the Client has as current group member: see
Section 4 of this document.
* REQ12: Categorize possible newly defined operations for Clients
into primary operations expected to be minimally supported and
secondary operations, and provide accompanying considerations:
none added.
* REQ13: Specify the encoding of group identifier: CBOR byte string,
with value used also to identify the current group key used in the
security group (see Section 4.1.2).
* 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 4.1.1.2.
* 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): see Section 4.1.1.2.
* REQ16: Define the initial value of the 'num' parameter: the
initial value MUST be set to 0 (see Section 4.1.2).
* REQ17: Specify the format of the 'key' parameter and register a
corresponding entry in the "ACE Groupcomm Key Types" IANA
registry: see Section 4.1.2 and Section 8.1.
* REQ18: Specify the acceptable values of the 'gkty' parameter:
Group_PubSub_Keying_Material, see Section 4.1.2.
* REQ19: Specify and register the application profile identifier:
coap_group_pubsub_app, see Section 4.1.2 and Section 8.2.
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* REQ20: If used, specify the format and content of 'group_policies'
and its entries. Specify the policies default values: ToDo.
* 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. If external signature verifiers
are supported, specify how those provide a nonce to the KDC to be
used for computing the PoP evidence: see Section 4.1.2.
* REQ22: Specify the communication protocol that the members of the
group must use: CoAP [RFC7252], and for pub/sub communication
[I-D.ietf-core-coap-pubsub].
* REQ23: Specify the security protocol the group members must use to
protect their communication. This must provide encryption,
integrity and replay protection: a symmetric COSE Key is used to
create a COSE_Encrypt0 object with an AEAD algorithm specified by
the KDC.
* REQ24: Specify how the communication is secured between Client and
KDC. Optionally, specify transport profile of ACE [RFC9200] to
use between Client and KDC: ACE transport profile such as for DTLS
[RFC9202] or OSCORE [RFC9203].
* REQ25: Specify the format of the identifiers of group members: the
Sender ID defined in Section 4.1.2.
* REQ26: Specify policies at the KDC to handle ids that are not
included in 'get_creds': see Section 4.2.1.
* REQ27: Specify the format of newly-generated individual keying
material for group members, or of the information to derive it,
and corresponding CBOR label: not applicable.
* 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 3.5 and
Section 8.5 of this document.
* REQ29: Categorize newly defined parameters according to the same
criteria of Section 8 of [I-D.ietf-ace-key-groupcomm]: a Publisher
Client MUST support 'group_SenderId' in 'key'; see Section 4.1.2.
* REQ30: Define whether Clients must, should or may support the
conditional parameters defined in Section 8 of
[I-D.ietf-ace-key-groupcomm], and under which circumstances: a
Publisher Client MUST support the client_cred', 'cnonce', and
'client_cred_verify' parameters; see Section 4.1.1. A Publisher
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Client that provides the token to the KDC, through the authz-info
endpoint, MUST support the parameter 'kdcchallenge'; see
Section 3.6.
* OPT1: Optionally, if the textual format of 'scope' is used,
specify CBOR values to use for abbreviating the role identifiers
in the group: no.
* OPT2: Optionally, specify the additional parameters used in the
exchange of Token Transfer Request and Response: no.
* OPT3: Optionally, specify the negotiation of parameter values for
signature algorithm and signature keys, if 'sign_info' is not
used: see Section 3.6.
* OPT4: Optionally, specify possible or required payload formats for
specific error cases: see Section 4.1.3.
* OPT5: Optionally, specify additional identifiers of error types,
as values of the 'error-id' field within the Custom Problem Detail
entry 'ace-groupcomm-error': no.
* OPT6: Optionally, specify the encoding of 'creds_repo' if the
default is not used: no.
* OPT7: Optionally, specify the functionalities implemented at the
'control_uri' resource hosted at the Client, including message
exchange encoding and other details: no.
* OPT8: Optionally, specify the behavior of the handler in case of
failure to retrieve an authentication credential for the specific
node: The KDC MUST reply with a 4.00 (Bad Request) error response
to the Join Request; see Section 4.1.3.
* OPT9: Optionally, define a default group rekeying scheme, to refer
to in case the 'rekeying_scheme' parameter is not included in the
Join Response: the "Point-to-Point" rekeying scheme registered in
Section 11.12 of [I-D.ietf-ace-key-groupcomm].
* OPT10: Optionally, specify the functionalities implemented at the
'control_group_uri' resource hosted at the Client, including
message exchange encoding and other details: no.
* OPT11: Optionally, specify policies that instruct Clients to
retain messages and for how long, if they are unsuccessfully
decrypted: no.
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* OPT12: Optionally, specify for the KDC to perform group rekeying
(together or instead of renewing individual keying material) when
receiving a Key Renewal Request: ToDo.
* OPT13: Optionally, specify how the identifier of a group member's
authentication credential is included in requests sent to other
group members: no.
* OPT14: Optionally, specify additional information to include in
rekeying messages for the "Point-to-Point" group rekeying scheme:
ToDo.
* OPT15: Optionally, specify if Clients must or should support any
of the parameters defined as optional in
[I-D.ietf-ace-key-groupcomm]: no.
Appendix B. Document Updates
B.1. Version -08 to -09
This section is to be removed before publishing as an RFC.
* Improved terminology section.
* Generalized scope format for future, admin-related extensions.
* Improved definition of permissions in the format of scope.
* Clarified alternative computing of N_S Challenge when DTLS is
used.
* Use of the parameter 'exi' in the Join Response.
* Use of RFC 9290 instead of the custom format of error responses.
* Fixed construction of the COSE_Encrypt0 object.
* Fixed use of the resource type "core.ps.gm".
* Updated formulation of profile requirements.
* Clarification and editorial improvements.
B.2. Version -07 to -08
* Revised presentation of the scope format.
* Revised presentation of the Join Request-Response exchange.
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* The 'cnonce' parameter must be present in the Join Request.
* The 'kid' of the group key is used as Group Identifier.
* Relaxed inclusion of the 'peer_roles' parameter.
* More detailed description of the encryption and signing
operations.
* Defined construction of the AEAD nonce.
* Clarifications and editorial improvements.
B.3. Version -06 to -07
* Revised abstract and introduction.
* Clarified use of "application groups".
* Revised use of protocols and transport profiles with Broker and
KDC.
* Revised overview of the profile and its security associations.
* Revised presentation of authorization flow.
* Subscribers cannot be anonymous anymore.
* Revised scope definition.
* Revised Join Response.
* Revised COSE countersignature, COSE encrypt objects.
* Further clarifications, fixes and editorial improvements.
Acknowledgments
The authors wish to thank Ari Keränen, John Preuß Mattsson, Jim
Schaad, Ludwig Seitz, and Göran Selander for the useful discussion
and reviews that helped shape this document.
The work on this document has been partly supported by the H2020
project SIFIS-Home (Grant agreement 952652).
Authors' Addresses
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Francesca Palombini
Ericsson
Email: francesca.palombini@ericsson.com
Cigdem Sengul
Brunel University
Email: csengul@acm.org
Marco Tiloca
RISE AB
Email: marco.tiloca@ri.se
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