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: 14 September 2023 Brunel University
M. Tiloca
RISE AB
13 March 2023
Publish-Subscribe Profile for Authentication and Authorization for
Constrained Environments (ACE)
draft-ietf-ace-pubsub-profile-06
Abstract
This document defines an application profile for enabling secure
group communication for a constrained Publish-Subscribe (pub/sub)
scenario, where Publishers and Subscribers communicate through a
broker, using the ACE framework. This profile relies on transport
layer or application layer security 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.
The document describes how to request and provision keying material
for group communication, and protect the content of the pub/sub
client message exchange, focusing mainly on the pub/sub scenarios
using the Constrained Application Protocol (CoAP)
[I-D.ietf-core-coap-pubsub].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 14 September 2023.
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Application Profile Overview . . . . . . . . . . . . . . . . 4
3. Getting Authorisation to Join a Pub/sub security group (A) . 7
3.1. AS Discovery at the Broker (Optional) . . . . . . . . . . 7
3.2. Topic and KDC Discovery at the Broker . . . . . . . . . . 8
3.3. Authorisation Request/Response for the KDC and the
Broker . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.1. Format of Scope . . . . . . . . . . . . . . . . . . . 9
3.4. Authorisation response . . . . . . . . . . . . . . . . . 10
3.5. Token Transfer to KDC . . . . . . . . . . . . . . . . . . 11
4. Client Group Communication Interface at the KDC . . . . . . . 12
4.1. Joining a Security Group . . . . . . . . . . . . . . . . 14
4.1.1. Join Request . . . . . . . . . . . . . . . . . . . . 15
4.1.2. Join Response . . . . . . . . . . . . . . . . . . . . 17
4.1.3. Join Error Handling . . . . . . . . . . . . . . . . . 20
4.2. Other Group Operations through the KDC . . . . . . . . . 21
4.2.1. Querying for Group Information . . . . . . . . . . . 21
4.2.2. Updating Authentication Credentials . . . . . . . . . 21
4.2.3. Removal from a Group . . . . . . . . . . . . . . . . 22
4.2.4. Rekeying a Group . . . . . . . . . . . . . . . . . . 22
5. PubSub Protected Communication (C) . . . . . . . . . . . . . 22
5.1. Using COSE Objects To Protect The Resource
Representation . . . . . . . . . . . . . . . . . . . . . 23
6. Applicability to MQTT PubSub Profile . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
8.1. ACE Groupcomm Profile Registry . . . . . . . . . . . . . 26
8.1.1. CoAP Profile Registration . . . . . . . . . . . . . . 26
8.1.2. MQTT Profile Registration . . . . . . . . . . . . . . 26
8.2. ACE Groupcomm Key Registry . . . . . . . . . . . . . . . 26
8.3. CoRE Resource Type . . . . . . . . . . . . . . . . . . . 27
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8.4. AIF Media-Type Sub-Parameters . . . . . . . . . . . . . . 27
8.5. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 28
8.6. TLS Exporter Labels . . . . . . . . . . . . . . . . . . . 28
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1. Normative References . . . . . . . . . . . . . . . . . . 28
9.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Requirements on Application Profiles . . . . . . . . 32
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
In the publish-subscribe (pub/sub) scenario, devices with limited
reachability communicate via a broker, which enables store-and-
forward messaging between these devices. This document specifies how
to request, distribute and renew the keying material and
configuration parameters to protect message exchanges for pub/sub
communication, using [I-D.ietf-ace-key-groupcomm], which expands from
the ACE framework ([RFC9200]). Message exchanges among the
participants as well as message formats and processing follow the
specifications for provisioning and renewing keying material in group
communication scenarios in [I-D.ietf-ace-key-groupcomm].
The pub/sub communication using the Constrained Application Protocol
(CoAP) [RFC7252] is specified in [I-D.ietf-core-coap-pubsub].This
document gives detailed specifications for CoAP pub/sub, and
describes how it can be applicable to MQTT [MQTT-OASIS-Standard-v5];
similar adaptations can extend to other transport protocols as well.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [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 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.
* The terms and concepts of pub/sub group communication, as
described in [I-D.ietf-core-coap-pubsub].
* The terms and concepts described in CBOR [RFC8949] and COSE
[RFC9052][RFC9053].
A principal interested to participate 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 may be a set
of nodes registered to a pub/sub topic.
2. Application Profile Overview
This document describes how to use [I-D.ietf-ace-key-groupcomm] and
[RFC9200] to perform authentication, authorization and key
distribution actions as overviewed in Section 2 of
[I-D.ietf-ace-key-groupcomm], when the considered group is pub/sub
clients belonging to the same security group.
Pub/sub clients communicate within their application groups mapped to
a collection of pub/sub topics. The pub/sub topics may consist of
one or more sub-topics, which may have their own sub-topics, forming
a hierarchy. The applications decide how to map this hierarchy into
different application groups, and a security group SHOULD 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.
The architecture of the scenario is shown in Figure 1. A Client can
act both as a publisher and a subscriber, publishing to some topics,
and subscribing to others. However, for the simplicity of
presentation, this profile describes Publisher and Subscriber Clients
separately. The Broker acts as the ACE RS, and also corresponds to
the Dispatcher in [I-D.ietf-ace-key-groupcomm]. The Clients
communicate with The Key Distribution Center (KDC) to join security
groups, and obtain the group keying material.
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Both Publisher and Subscriber Clients use the same pub/sub
communication protocol and the same transport profile of ACE in their
interaction with the broker. The pub/sub communication protocol
considered in this document is CoAP, as described in
[I-D.ietf-core-coap-pubsub], but the specification can apply to other
pub/sub protocols such as MQTT [MQTT-OASIS-Standard-v5], or other
transport protocols. All clients MUST use CoAP when communicating to
the KDC.
+----------------+ +----------------+
| | | Key |
| Authorization | | Distribution |
| Server | | Center |
| (AS) | | (KDC) |
+----------------+ +----------------+
^ ^
| |
+---------(A)----+ |
| +--------------------(B)--------+
v v
+------------+ +------------+
| | <-- (O) --> | |
| Pub/Sub | | Broker |
| Client | <-- (C)---> | |
| | | |
+------------+ +------------+
Figure 1: Architecture for Pub/Sub with Authorization Server and Key
Distribution Center
All communications between the involved entities MUST be secured.
This profile expects the establishment of a secure connection between
a Client and Broker, using an ACE transport profile such as DTLS
[RFC9202] or OSCORE [RFC9203] (A and C). Once the client establishes
a secure association with KDC with the help of AS, it can request to
join the security groups of its pub/sub topics (A and B), and can
communicate securely with the other group members, using the keying
material provided by the KDC.
(C) corresponds to the exchange between the Client and the Broker,
where the Client sends its access token to the Broker and establishes
a secure connection with the Broker. Depending on the Information
received in (A), the connection set-up may involve, for example, a
DTLS handshake, or other protocols. Depending on the application,
the set up phase may be skipped: for example, if OSCORE is used
directly.
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In addition, this document describes an Optional Discovery though
Broker (O), where an anonymous Clients MAY discover the topic
categories, topics resources, the AS and the KDC from the Broker.
It must be noted that Clients maintain two different security
associations. On the one hand, the Publisher and the Subscriber
clients have a security association with the Broker, which, as the
ACE RS, verifies that the Clients are authorized (Security
Association 1). On the other hand, the Publisher has a security
association with the Subscriber, to protect the publication content
(Security Association 2) while sending it through the broker. The
Security Association 1 is set up using AS and a transport profile of
[RFC9200], the Security Association 2 is set up using AS, KDC and
[I-D.ietf-ace-key-groupcomm].
Given that the publication content is protected, the Broker MAY
accept unauthorised Subscribers. In this case, the Subscriber client
MAY skip setting up Security Association 1 with the Broker and
connect to it as an anonymous client to subscribe to topics of
interest at the Broker.
+------------+ +------------+ +------------+
| | | | | |
| Publisher | | Broker | | Subscriber |
| | | | | |
| | | | | |
+------------+ +------------+ +------------+
: : : : : :
: '------ Security -------' '-----------------------' :
: Association 1 :
'------------------------------- Security --------------'
Association 2
Figure 2: Security Associations between Publisher, Broker,
Subscriber pairs.
In summary, this profile describes how:
1. A Client gets the authorization to join a security group, and
providing it with the group keying material to communicate with
other group members.
2. A Client retrieves group keying material to publish protected
publications to the Broker or read protected publications.
3. A Client retrieves authentication credentials of other group
members, and provides and updates own authentication credentials.
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4. A Client is removed from the group.
5. The KDC renews and redistributes the group keying (rekeying)
material due to membership change in the group.
Appendix 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 Figure 3 provides a high level overview of the message flow
for a node getting authorisation to join a group. This message flow
is expanded in the subsequent sections.
Client Broker AS
| [--Resource Request (CoAP/MQTT or other)-->] | |
| [<----AS Information (CoAP/MQTT or other)--] | |
|[<--Topic Collection/Resource/KDC Discovery->]|
| |
| --Broker Authorisation Req (CoAP/HTTP or other)-->|
| <---Authorisation Response (CoAP/HTTP or other) --|
| |
| --KDC Authorisation Req (CoAP/HTTP or other)----->|
| <---Authorisation Response (CoAP/HTTP or other) --|
Figure 3: Authorisation Flow
Since [RFC9200] recommends the use of CoAP and CBOR, this document
describes the exchanges assuming CoAP and CBOR are used. However,
using HTTP instead of CoAP is possible, using 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
or Media Type of the message has to be changed accordingly. Exact
definition of these exchanges are considered out of scope for this
document.
3.1. AS Discovery at the Broker (Optional)
Complementary to what is defined in [RFC9200] (Section 5.1) for AS
discovery, the Broker MAY send the address of the AS to the Client in
the 'AS' parameter in the AS Information as a response to an
Unauthorized Resource Request (Section 5.2). An example using CBOR
diagnostic notation and CoAP is given below:
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4.01 Unauthorized
Content-Format: application/ace-groupcomm+cbor
{"AS": "coaps://as.example.com/token"}
Figure 4: AS Information example
3.2. Topic and KDC Discovery at the Broker
A Broker can offer a topic discovery entry point to enable clients to
find topics of interest. The resource entry point thus represents a
collection of related resources as specified in [RFC6690] and is
identified by the resource type "core.ps.coll". A topic collection
is a group of topic configuration resources that define topic
properties and are identified by the resource type "core.ps.conf".
An anonymous pub/sub client MAY request a collection of the topics
present in the broker by making a CoAP GET request to the collection
URI. An anonymous pub/sub client MAY read the configuration of a
topic by making a CoAP GET request to the topic configuration URI.
(ToDo: Consider a discovery token to be consumed by the Broker for
topic collection, and topic configuration?)
(ToDo: Instead of defining "core.ps.gm", need to extend Topic
Configuration Representation in core-coap-pubsub to include KDC?)
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 and its sub-resources at Broker, e.g., by using
a link-format document [RFC6690]. Applications can use this common
resource type to discover links to group-membership resources for
joining pub/sub groups.
3.3. Authorisation Request/Response for the KDC and the Broker
The Client sends two Authorisation Requests to the AS for two
audiences: the Broker and the KDC, respectively. AS handles
authorisation requests for topics a Client is allowed to Publish or
Subscribe to the Broker, corresponding to an application group. The
client sends a request to the KDC to join the security group(s)
corresponding to those application groups to be able protect the
message content with the group key.
Communications between the Client and the AS MUST be secured,
according to what is defined by the used transport profile of ACE.
This section builds on Section 3 of [I-D.ietf-ace-key-groupcomm] and
defined only additions or modifications to that specification.
Both Authorisation Requests include the following fields (Section 3.1
of [I-D.ietf-ace-key-groupcomm]):
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* 'scope': Optional. If present, specifies the name of the topic
groups, that the Client requests to access. This parameter is a
CBOR byte string that encodes a CBOR array, whose format SHOULD
follow the data model AIF-PUBSUB-GROUPCOMM defined below.
* 'audience': Required identifier corresponding to either the KDC or
the Broker.
Other additional parameters can be included if necessary, as defined
in [RFC9200].
For the Broker, the scope represents pub/sub topics i.e., the
application group, and for the KDC, the scope represents the
corresponding security group. This document expects a one-to-one
mapping between the application group and the security group, and the
client uses the same scope for both requests. If there is not a one-
to-one mapping, the client MUST ask for the correct scopes in its
Authorization Requests, and the correct policies regarding both sets
of scopes MUST be available to the AS. How the client discovers the
(application group, security group) association is out of scope of
this document.
3.3.1. Format of Scope
The 'scope' parameter SHOULD follow the AIF format (REQ1). However,
if the ACE transport profile, supports another 'scope' format, then
implementations MAY use this format.
Based on the generic AIF model
AIF-Generic<Toid, Tperm> = [* [Toid, Tperm]]
The value of the CBOR byte string used as the scope encodes the CBOR
array [* [Toid, Tperm]], where each [Toid, Tperm] element corresponds
to one scope entry.
This document defines the new AIF specific data model AIF-PUBSUB-
GROUPCOMM, that this profile SHOULD use to format and encode scope
entries.
* The object identifier ("Toid") is a CBOR text string, specifying
the topic name for the scope entry.
* The permission set ("Tperm") is a CBOR unsigned integer with
value, specifying the Client role, based on the operations the
Client can execute on Topic Data in the group. The set of numbers
representing the permissions is converted into a single number by
taking two to the power of each method number and computing the
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inclusive OR of the binary representations of all the power
values. The roles a Client is allowed are Publish (1), Subscribe
(or Read) (2) and Delete (3). An Admin(0) role is also defined,
which is reserved for expressing permissions for Administrators of
Pub/Sub groups. For Pub/Sub client communication, the scope entry
MUST NOT include the Admin permission i.e., the least significant
bit of "Tperm" always set to 0.
AIF-PUBSUB-GROUPCOMM = AIF-Generic<pubsub-topic, pubsub-perm>
pubsub-topic = tstr ; Pub/sub topic name
; (the associated security group)
pubsub-perm = uint . bits pubsub-roles
pubsub-roles = &(
Admin: 0,
Pub: 1,
Sub: 2,
Delete: 3
)
scope_entry = [pubsub-topic, pubsub-perm]
Figure 5: Pub/Sub scope using the AIF format
3.4. 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:
* 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 joining
node in the Authorization Request. In such a case, the second
element of each scope entry MUST be present, and specifies the set
of roles that the joining node is actually authorized to take in
for that scope entry, encoded as specified in Section 3.3.
ToDo: Extend the authorisation response to describe the token
returned, and do a MUST on the Audience claim to indicate the
response is for KDC or Broker?
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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 3.3.1 of
this document.
3.5. Token Transfer to KDC
After receiving a token from the AS, the Client transfers the token
to the KDC using one of the methods defined Section 3.3
[I-D.ietf-ace-key-groupcomm]. This typically includes sending a POST
request to the authz-info endpoint. However, if using the DTLS
transport profile of ACE [RFC9202] 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 DTLS ClientKeyExchange message.
In addition to that, the following applies.
In the token transfer response to the Publisher Clients, i.e., the
Clients whose scope of the access token includes the "Pub" role, 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 a 8-byte long random nonce. Later when joining the
group, the Publisher Client can use the 'kdcchallenge' as part of
proving possession of its private key (see
[I-D.ietf-ace-key-groupcomm]). If a Publisher Client 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'.
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* 'sign_alg' MUST take 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
The Clients uses the following KDC resources to enable group
communication:
+=================================+====================+============+
| 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) |
| | symmetric group | |
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| | keying material | |
| | associated with | |
| | GROUPNAME | |
+---------------------------------+--------------------+------------+
| /ace-group/GROUPNAME/creds | Required. | GET, FETCH |
| | Contains the | (All) |
| | authentication | |
| | credentials of | |
| | all the | |
| | Publisher | |
| | members of the | |
| | group with name | |
| | GROUPNAME | |
+---------------------------------+--------------------+------------+
| /ace-group/GROUPNAME/num | Required. | GET (All) |
| | Contains the | |
| | current version | |
| | number for the | |
| | symmetric group | |
| | keying material | |
| | of the group | |
| | with name | |
| | GROUPNAME | |
+---------------------------------+--------------------+------------+
| /ace-group/GROUPNAME/nodes/ | Required. | GET, |
| NODENAME | Contains the | DELETE |
| | group keying | (All). |
| | material for | PUT not |
| | that group | supported. |
| | member NODENAME | |
| | in GROUPNAME. | |
+---------------------------------+--------------------+------------+
| /ace- | Required. | POST (Pub) |
| group/GROUPNAME/nodes/NODENAME/ | Authentication | |
| cred | credential for | |
| | NODENAME in the | |
| | group GROUPNAME | |
+---------------------------------+--------------------+------------+
| /ace-group/GROUPNAME/kdc-cred | MUST be hosted | GET (All) |
| | if a group re- | |
| | keying mechanism | |
| | is used. | |
| | Contains the | |
| | authentication | |
| | credential of | |
| | the KDC for the | |
| | group with name | |
| | GROUPNAME. | |
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+---------------------------------+--------------------+------------+
| /ace-group/GROUPNAME/policies | Optional. | |
| | Contains the | |
| | group policies | |
| | of the group | |
| | with | |
+---------------------------------+--------------------+------------+
| name GROUPNAME. | GET (All) | |
+---------------------------------+--------------------+------------+
Table 1
Note that the use of these resources follows what is defined in
[I-D.ietf-ace-key-groupcomm] applies, and only additions or
modifications to that specification are defined in this document.
4.1. Joining a Security Group
This section describes the interactions between the joining node and
the KDC to join a pub/sub group. Source authentication of a message
sent within the pub/sub group is ensured by means of a digital
signature embedded in the message. Subscribers must be able to
retrieve Publishers' authentication credential from a trusted
repository, to verify source authenticity of received messages.
Hence, on joining a pub/sub group, a Publisher node is expected to
provide its own authentication credential to the KDC.
On a successful join, the Clients receive the symmetric COSE Key
received from the KDC to protect the payload of a published topic
data.
The message exchange between the joining node and the KDC follows
what's 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
|----- Joining Request (CoAP) ------>|
| |
|<-----Joining Response (CoAP) ------|
Figure 6: Join Flow
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4.1.1. Join Request
After establishing a secure communication, 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 MUST contain
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': Required. MUST be set to the specific group that the
Client is attempting to join, i.e., the group name, and the roles
it wishes to have in the group. This value corresponds to one
scope entry, as defined in Section 3.3.1.
* 'get_creds': Optional, present if the Subcriber Client wants to
retrieve the public keys of all the Publisher Clients 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 no 'role_filter' is necessary as
KDC returns the authentication credentials of Publisher Clients by
default.
* 'client_cred': The use of this parameter is detailed in
Section 4.1.1.1.
* 'cnonce': Optional, MUST be present if 'client_cred' is present.
It is a dedicated nonce N_C generated by the Client. It is
RECOMMENDED to use a 8-byte long random nonce. Join Requests MUST
include a new 'cnonce' at each join attempt.
* 'client_cred_verify': Optional, MUST be present if 'client_cred'
is present. 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 client_cred', 'cnonce', 'client_cred_verify'
parameters.
4.1.1.1. Client Credentials-'client_cred'
One of the following cases can occur when a new node attempts to join
a pub/sub group.
* The joining node requests to join the group exclusively as a
Subscriber or for Delete, i.e., it is not going to send messages
to the group. In this case, the joining node is not required to
provide its own authentication credential to the KDC. In case the
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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
parameters 'cnonce' and 'client_cred_verify'.
* The joining node has a Publisher role, and
- the KDC already acquired the authentication credential of the
joining node either during a past group joining process, or
during establishing a secure communication association, and the
joining node and the KDC use a symmetric proof-of-possession
key. If the authentication credential and the proof-of-
possession key are compatible with the signature or ECDH
algorithm, and possible associated parameters, then the key can
be used for the authentication credential in pub/sub groups.
In this case, the joining node MAY choose not to provide again
its own authentication credential to the KDC, in order to limit
the size of the Join Request.
- the KDC hasn't 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 own authentication
credential again if it has provided the KDC with multiple
authentication credentials during past joining processes intended for
different pub/sub groups. If the joining node provides its own
authentication credential, the KDC performs consistency checks as per
Section 4.1.1 and, in case of success, considers it as the
authentication credential associated with the joining node in the
pub/sub 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' paramater.
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 3.5).
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* If the Publisher Client used a symmetric proof-of-possession key
in the DTLS handshake [RFC9202] with the KDC, then it is an
exporter value computed as defined in Section 7.5 of [RFC8446].
Specifically, N_S is exported from the DTLS session between the
joining node and the KDC, using an empty 'context_value', 32 bytes
as 'key_length', 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, N_S
MUST be this new challenge parameter.
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 may
return a success or error response.
If 'client_cred' field is present, the KDC verifies signature in the
'client_cred_verify'. 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 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.
For a Publisher Client, the KDC assigns an available Sender ID that
has not been used in the group. The KDC MUST NOT assign a Sender ID
to the joining node if the node doesn't have a Publisher role. The
Sender ID MUST be unique, and MAY be short. ToDo: SenderID Size from
groupcomm oscore? - the maximum length of Sender ID in bytes equals
the length of the AEAD nonce minus 6; for AES-CCM-16-64-128 the
maximum length of Sender ID is 7 bytes.
In the case of any join request error, the KDC and the Client
attempting the join follow the procedure defined in Section 4.1.3.
In the case of success, the Client is added to the list of current
members, if not already a member. The Client is assigned a NODENAME
and a sub-resource /ace-group/GROUPNAME/nodes/NODENAME. NODENAME is
associated to the access token and secure session of the Client.
Publishers' client credentials are also associated with tuple
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containing NODENAME, GROUPNAME, sender ID and access token. The KDC
responds with a Join Response with response code 2.01 (Created) if
the Client has been added to the list of group members, and 2.04
(Changed) otherwise (e.g., if the Client is re-joining). The
Content-Format is "application/ace-groupcomm+cbor". The payload
(formatted as a CBOR map) MUST contain the following fields from the
Join Response and encode them as defined in Section 4.3.1 of
[I-D.ietf-ace-key-groupcomm]:
* 'gkty': the key type "Group_PubSub_COSE_Key" for the 'key'
parameter defined in Section 8.2 of this document.
* 'key': The keying material for group communication includes
'group_SenderId' if the Client is a Publisher, and a "COSE_Key".
The "COSE_Key" object is defined in [RFC9052] [RFC9053] and
contains:
- 'kty' with value 4 (symmetric)
- 'kid' with value defined by the KDC
- 'alg' with value defined by the KDC
- 'Base IV' with value defined by the KDC
- 'k', the value of the symmetric key (REQ17)
* 'num': MUST be initialised to 0 as the version number of the
keying material.
* 'exp', MUST be present.
* 'ace-groupcomm-profile' parameter MUST be present and has value
"coap_group_pubsub_app" (PROFILE_TBD), which is defined in
Section 8.1.1 of this document.
* 'creds', 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 Publisher Clients in the
group.
* 'peer_roles' MUST be present if 'creds' is also present.
Otherwise, it MUST NOT be present. (ToDo: Requested a change for
this, and see how the Groupcomm draft is updated.)
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* 'peer_identifiers' 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 (REQ 25).
* 'kdc_cred', MUST be present if group re-keying is used, and
encoded as a CBOR byte string, with value the original binary
representation of the KDC's authentication credential (REQ8).
* 'kdc_nonce', MUST be present, if 'kdc_cred' is present and 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 a
8-byte long random nonce.
* 'kdc_cred_verify' MUST be present, if 'kdc_cred' is present and
encoded as a CBOR byte string. The PoP evidence is computed over
the nonce N_KDC, which is specified in the 'kdc_nonce' parameter
and taken as PoP input. KDC MUST compute the 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': MAY be omitted, if the KDC uses the "Point-to-
Point" group rekeying scheme registered in Section 11.12 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.
To generate the keying material, the KDC starts at the same Base IV
and Partial IV, and different keys are derived for each sender, based
on their Sender ID, sent as the 'group_SenderId' inside the 'key'
parameter. A Publisher Client MUST support 'group_SenderId'
parameter (REQ29).
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 pubsub 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).
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4.1.3. Join Error Handling
The KDC MUST reply with a 4.00 (Bad Request) error response 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 both
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 roles not included in the role
list as defined in Section 3.3.1.
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. 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, possibly replacing the currently
stored value.
On 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 Group Manager MUST return a 5.03 (Service Unavailable) response
to a Publisher's join request in case there are currently no Sender
IDs available.
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4.2. Other Group Operations through the KDC
4.2.1. Querying for Group Information
* '/ace-group': All Clients send FETCH requests to retrieve a set of
group names associated with their group identifiers. Each element
of the CBOR array 'gid' is a CBOR byte string (REQ13), which
encodes the Gid of the group for which the group name and the URI
to the group-membership resource are provided. ToDo: Support or
not?
* '/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': KDC acts as a repository of
authentication credentials for Publisher Clients. The Subscriber
Clients of the group use GET/FETCH requests to retrieve the
authentication credentials of all or subset of the group members
of the group with name GROUPNAME. The KDC silently ignores the
Sender IDs included in the 'get_creds' parameter of the request
that are not associated with any current group member (REQ26).
* '/ace-group/GROUPNAME/num': All group member Clients use GET
requests 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 use GET
requests to retrieve the current authentication credential of the
KDC.
4.2.2. Updating Authentication Credentials
A Publisher Client can contact the KDC to upload a new authentication
credential to use in the group, and replace the currently stored one.
To this end, it sends a CoAP POST request to the /ace-
group/GROUPNAME/nodes/NODENAME/cred. The KDC replaces the stored
authentication credential of this Client (identified by NODENAME)
with the one specified in the request at the KDC, for the group
identified by GROUPNAME.
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4.2.3. Removal from a Group
A Client can actively request to leave the group. In this case, the
Client sends a CoAP DELETE request to the endpoint /ace-
group/GROUPNAME/nodes/NODENAME at the KDC, where GROUPNAME is the
group name and NODENAME is its node name. 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
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.
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.
Default rekeying scheme is Point-to-point (Section 6.1 of
[I-D.ietf-ace-key-groupcomm]), where KDC individually targets each
node to rekey, using the pairwise secure communication association
with that node.
If the group rekeying is performed due to one or multiple Publisher
Clients that have joined the group, then a rekeying message includes
sender IDs, and authentication credentials that those Clients use in
the group, together with their roles. This information is specified
by means of the parameters 'creds', 'peer_roles' and
'peer_identifiers', like done in the Join Response message.
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
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the subscription of the Subscriber, and uses a CoAP GET with the
Observe option set to 0 (zero) [I-D.ietf-core-coap-pubsub]. The
subscription MAY be unprotected. 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 (Section 4.3 of
[I-D.ietf-core-coap-pubsub]). Specifically, the COSE Key is used to
create a COSE_Encrypt0 object with an AEAD algorithm specified by the
KDC. The AEAD key lengths, AEAD nonce length, and maximum Sender
Sequence Number (Partial IV) are algorithm dependent.
The Publisher uses the private key corresponding to the public key
sent to the KDC to countersign the COSE Object as specified in
[RFC9052] [RFC9053]. The payload is replaced by the COSE object
before the publication is sent to the Broker.
The Subscriber uses the 'kid' in the 'countersignature' field in the
COSE object to retrieve the right public key to verify the
countersignature. It then uses the symmetric key received from KDC
to verify and decrypt the publication received in the payload from
the Broker (in the case of CoAP the publication is received by the
CoAP Notification).
The COSE object is constructed in the following way (as described in
[RFC9052] [RFC9053]).
The protected Headers MUST contain:
* alg, an AEAD algorithm specified by the KDC, the same as received
in the symmetric COSE Key
The unprotected Headers MUST contain:
* kid, with the value the same as in the symmetric COSE Key received
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* the Partial IV, with value a Sender Sequence Number that is
incremented for every message sent. 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 IV, generated following the construction in Section 5.2 of
[RFC8613] using the sender ID, Partial IV, and Base IV from the
symmetric COSE Key received.
* the counter signature
- the algorithm (protected),
- the kid, the sender ID (unprotected)
- the signature computed as specified in [RFC9052] [RFC9053].
* The ciphertext, computed over the plaintext that MUST contain the
message payload.
The 'external_aad' is an empty string.
The encryption and decryption operations are described in [RFC9052]
[RFC9053].
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
pair-wise rekeying initiated by the KDC.
Authorisation Server (AS) Discovery is defined in Section 2.2.6.1 of
[I-D.ietf-ace-mqtt-tls-profile] 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.
Differently for MQTT, the Client sends an authorisation request to
the AS using AIF-MQTT data model for representing the requested
scopes is described in Section 3 of the
[I-D.ietf-ace-mqtt-tls-profile]. In the authorisation response, the
'profile' claim is set to "mqtt_pubsub_app" as defined in
Section 8.1.2.
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Both Publisher and Subscriber Clients MUST authorise to the Broker
with their respective tokens (described in
[I-D.ietf-ace-mqtt-tls-profile]) i.e., anonymous Subscribers are not
supported in the profile. A Publisher Client sends PUBLISH messages
for a given topic and protects the payload with the corresponding key
for the associated security group. The Broker validates the PUBLISH
message by verifying its topic in the stored token. A Subscriber
Client may send SUBSCRIBE messages with one or multiple topic
filters. A topic filter may correspond to multiple topics. The
Broker validates the SUBSCRIBE message by checking the stored token
for the Client. 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, when the Publisher and Subscriber use
asymmetric crypto, 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-03] 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-03] 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. In this setting, caching of
publications on the Broker is still allowed.
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
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8.1. ACE Groupcomm Profile Registry
The following registrations are done for the "ACE Groupcomm Profile"
Registry following the procedure specified in
[I-D.ietf-ace-key-groupcomm].
Note to RFC Editor: Please replace all occurrences of "[[This
document]]" with the RFC number of this specification and delete this
paragraph.
8.1.1. CoAP Profile Registration
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 Key: TBD
Reference: [[This document]]
8.1.2. MQTT Profile Registration
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 Key: TBD
Reference: [[This document]]
8.2. ACE Groupcomm Key Registry
The following registrations are done for the "ACE Groupcomm Key
Types" defined in Section 11.7 of [I-D.ietf-ace-key-groupcomm].
Note to RFC Editor: Please replace all occurrences of "[[This
document]]" with the RFC number of this specification and delete this
paragraph.
Name: Group_PubSub_COSE_Key
Key Type Value: GROUPCOMM_KEY_TBD
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Profile: coap_group_pubsub_app, defined in Section 8.1.1 of this
document.
Description: COSE_Key object
References: [RFC9052] [RFC9053], [[This document]]
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 a Broker.
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: [[This document]]
* Parameter: Tperm
* Name: pubsub-perm
* Description/Specification: Permissions corresponding to the roles
in pub/sub group
*Reference: [[This document]]
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8.5. CoAP Content-Format
IANA is asked to register the following entries to the "CoAP Content-
Formats" registry within the "Constrained RESTful Environments (CoRE)
Parameters" registry group.
* Media Type: application/aif+cbor;Toid="pubsub-
topic",Tperm="pubsub-perm"
* Encoding: -
* ID: 294 (suggested)
* Reference: [RFC-XXXX]
* Media Type: application/aif+json;Toid="pubsub-
topic",Tperm="pubsub-perm"
* Encoding: -
* 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] (Section XXX)
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-16, 5 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-key-
groupcomm-16>.
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[I-D.ietf-core-coap-pubsub]
Koster, M., Keränen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", Work in Progress, Internet-Draft, draft-ietf-
core-coap-pubsub-11, 7 November 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-pubsub-11>.
[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-05, 10 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cbor-encoded-cert-05>.
[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>.
[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>.
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[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/rfc/rfc7252>.
[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>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/rfc/rfc8613>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
[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>.
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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>.
9.2. Informative References
[I-D.draft-ietf-ace-revoked-token-notification-03]
Tiloca, M., Seitz, L., 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-03, 24
October 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-ace-revoked-token-notification-03>.
[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-ace-mqtt-tls-profile]
Sengul, C. and A. Kirby, "Message Queuing Telemetry
Transport (MQTT)-TLS profile of Authentication and
Authorization for Constrained Environments (ACE)
Framework", Work in Progress, Internet-Draft, draft-ietf-
ace-mqtt-tls-profile-17, 23 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
mqtt-tls-profile-17>.
[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>.
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[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>.
[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>.
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 3.3.1.
* REQ2: If the AIF format of 'scope' is used, register its specific
instance of "Toid" and "Tperm" as Media Type parameters and a
corresponding Content-Format, as per the guidelines in
[RFC9237].: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].
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* 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.
* 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
left optional, see Section 4 of this document.
* REQ10: Register a Resource Type for the root url-path, which is
used to discover the correct url to access at the KDC : the
Resource Type (rt=) Link Target Attribute value "core.ps.gm" is
registered in Section Section 8.3. ToDo: This possibly will not
stay as the final method, KDC discovery done differently through
topic discovery?
* 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,
see Section 4.2.1.
* REQ14: Specify the approaches used to compute and verify the PoP
evidence to include in 'client_cred_verify', and which of those
approaches is used in which case: See Section 4.1.1.2 in this
document.
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* 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 in this document.
* REQ16: Define the initial value of the 'num' parameter: The
initial value MUST be set to 0.
* REQ17: Specify the format of the 'key' parameter: See
Section 4.1.2.
* REQ18: Specify the acceptable values of the 'gkty' parameter:
Group_PubSub_COSE_Key, see Section 8.2.
* REQ19: Specify and register the application profile identifier:
coap_group_pubsub_app, see Section 8.1.1.
* 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: see Section 4.1.2.
* REQ22: Specify the communication protocol 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.: 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 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.
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* 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 3.4 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 client_cred', 'cnonce',
'client_cred_verify' parameters; see Section 4.1.1. A Publisher
Client that provides the token to the KDC, through the authz-info
endpoint, MUST support the parameter 'kdcchallenge'; see
Section 3.5.
* 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 3.5.
* 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' field in an error response from the KDC:
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.
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* 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.
* 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.
Acknowledgments
The author wishes to thank Ari Keränen, John Preuß Mattsson, Ludwig
Seitz, Göran Selander, and Jim Schaad 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
Francesca Palombini
Ericsson
Email: francesca.palombini@ericsson.com
Cigdem Sengul
Brunel University
Email: csengul@acm.org
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Marco Tiloca
RISE AB
Email: marco.tiloca@ri.se
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