CoAP Publish-Subscribe Profile for Authentication and Authorization for Constrained Environments (ACE)

CoAP Publish-Subscribe Profile for Authentication and Authorization for Constrained Environments (ACE) Ericsson

francesca.palombini@ericsson.com

Brunel University

csengul@acm.org

RISE AB

marco.tiloca@ri.se

Security ACE Working Group Internet-Draft 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 of 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 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 . Source for this draft and an issue tracker can be found at .

Introduction In a publish-subscribe (Pub-Sub) scenario, devices acting as Publishers and/or Subscribers communicate via a Broker that enforces store-and-forward messaging between those. 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 application group associated with that topic. The specification at defines a Pub-Sub architecture for the Constrained Application Protocol (CoAP) . Building on the Authentication and Authorization for Constrained Environments (ACE) framework , the document defines how to request, distribute, and renew keying material and configuration parameters to protect message exchanges in a group communication environment. That is, candidate group members that act as ACE Clients and are authorized to join a group can interact with a Key Distribution Center (KDC) that acts as ACE Resource Server and is responsible for the group. The KDC provides the necessary keying material and parameters to communicate with other group members. While defines the operations and interface available at the KDC, as well as general message formats for the interactions between Clients and the KDC, it delegates details on the communication and security approaches used in a group to separate application profiles. These are specialized instances of that target a particular group communication approach and define how communications in the group are protected, as well as the specific keying material and configuration parameters provided to group members. This document specifies an application profile of . Message exchanges among the participants as well as message formats and processing follow what is specified in , and they enable secure Pub-Sub communication where a group of Publishers and Subscribers securely communicate through a Broker using CoAP as per . In particular, 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 leverages protocol-specific transport profiles of ACE (e.g., ), in order to achieve communication security, server authentication, and proof of possession of 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 and on keying material provided to the Publishers and Subscribers participating in the same Pub-Sub topic. In particular, source authentication of published topic data is achieved by means of the corresponding Publisher signing such content with its own private key. overviews the relationships between this document and other related documents mentioned above.

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 when, and only when, they appear in all capitals, as shown here. Readers are expected to be familiar with:

The terms and concepts described in the ACE framework for Authentication and Authorization . The terminology for entities in the considered architecture is defined in OAuth 2.0 . In particular, this includes Client, Resource Server (RS), and Authorization Server (AS).
The Authorization Information Format (AIF) used to express authorization information.
The terms and concept related to the message formats and processing specified in , 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 , CBOR , and COSE .
The terms and concepts described in CoAP . Note that 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. The CoAP definition, which is "[a]n entity participating in the CoAP protocol" , is not used in this document.
The terms and concepts of Pub-Sub group communication with CoAP, as described in .

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 as defined in and . Diagnostic notation comments are often used to provide a textual representation of the parameters' keys and values.

Application Profile Overview This document describes how to use and to perform authentication, authorization, and key distribution operations as overviewed in , 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 in turn may have their own sub-topics and so on, thus forming a hierarchy. 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. This profile considers the architecture shown in . 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 . The Key Distribution Center (KDC) also acts as an ACE RS and builds on what is defined for the KDC in . From a high-level point of view, the Clients interact with the KDC in order to join security groups, thereby obtaining the group keying material to protect end-to-end and verify the content published in the associated topics.

Architecture for Pub-Sub with Authorization Server and Key Distribution Center | | | Pub-Sub | | Broker | | Client |<------- (C) ------->| | | | | | +------------+ +------------+ ]]> Both Publishers and Subscribers MUST use the same protocol for interacting with the Broker and participating in Pub-Sub communications. When using the profile defined in this document, such a protocol MUST be CoAP , which is used as described in (REQ22). In particular, when using this application profile, the topic-data resource corresponding to a given topic is hosted precisely at the Broker associated with that topic. Setups where a topic-data resource is possibly hosted at different servers than the Broker are out of the scope of this application profile. 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., for DTLS, or 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 (REQ24). Except for the end-to-end protection of published topic data (see ), all communications between the involved entities (Clients, Broker, KDC, Authorization Server) MUST occur and be secured in accordance with the protocol-specific transport profile of ACE used. For each Client, 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 . During this process, the Client obtains an access token from the AS and uploads it at the Broker, thus providing an evidence of the topics that it is authorised to participate in, and with which permissions. For each Client, 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 (REQ24). This is shown by the interactions A and B in . During this process, the Client obtains an access token from the AS and uploads it at 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, as defined in and further specified in this document. In addition, this profile allows an anonymous Client to perform some of the discovery operations defined in through the Broker, as shown by the interaction O in . That is, an anonymous Client can discover:

The Broker itself, by relying on the resource type "core.ps" (see ); 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 ).

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 , 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 an ACE RS, verifies that the Client is authorised 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 at the Broker. On the other hand, separately for each topic, all the Publishers and Subscribers for that topic have a common, group security association, through which the published topic data 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.

Security Associations between Publisher, Broker, and Subscriber In summary, this profile specifies the following functionalities.

A Client obtains the authorization to participate in a Pub-Sub topic at the Broker with certain permissions. This pertains to operations defined in for taking part in Pub-Sub communication with CoAP.
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 securely 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.
A Client obtains from the KDC the authentication credentials of other group members, and provides or updates the KDC with its own authentication credential.
A Client leaves the group or is removed from the group by the KDC.
The KDC renews and redistributes the group keying material (rekeying), e.g., due to a membership change in the group.

compiles the list of requirements for this application profile of ACE and how they are fulfilled, consistent with the list of requirements defined in .

Getting Authorisation to Join a Pub-Sub Security Group 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.

Authorisation Flow ]| | | | | | | |[----------------- 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 security 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) | | | | | | | ]]> As shown in , after a Client obtains authorisation information for participating in a Pub-Sub topic with name TOPICNAME (see and ), the Client uploads that information at the Broker as per the specific transport profile of ACE used, e.g., or . Following that, the Client can perform the optional discovery of the KDC and security group name at the Broker, by accessing the topic resource corresponding to the topic in question (see ). In order to ensure that the Client can seamlessly access the right topic resource at the Broker, it is RECOMMENDED that a Broker implementing this application profile uses the path /ps/TOPICNAME to host the topic resource for the topic with name TOPICNAME. Alternatively, the Client might not know the right topic resource to target and thus would attempt to access different ones (e.g., based on the result of an early discovery of topic resources, see ), until it finds the right one specifying TOPICNAME as value of the 'topic-name' parameter in the resource representation. Separately, after the Client obtains authorisation information for joining the security group at the KDC (see and ), the Client uploads that information at the KDC (see ). Following that, the Client can join the security group at the KDC and thus obtain the corresponding keying material (see ). Once the steps above have been completed, the Client can take part in the secure group communication for the topic TOPICNAME, e.g., by publishing data to the topic through a request targeting the topic-data resource at the Broker. Note that the overview in shows the specific sequence of steps where the Client: first associates with the Broker for participating in a topic; then discovers the KDC and the name of the security group through the Broker; and finally associates with the KDC, through which it joins the security group. However, if the Client is early aware about how to reach the KDC and about the name of the security group, the Client can instead: first associate with the KDC and join the security group, and then associate with the Broker for participating in the topic. Since 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 can be used instead of CBOR, by relying on the conversion method specified in Sections and of . If JSON is used, the Content-Format of the message has to be specified accordingly. Exact definitions of these exchanges are out of scope for this document.

Topic Discovery at the Broker (Optional) The discovery of a topic at the Broker can be performed by discovering the corresponding topic resource 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 ). Although the links to the topic resources are also specified in the representation of the collection resource at the Broker (see ), the Client is not supposed to access such a resource, as intended for administrative operations that are out of the scope of this document.

AS Discovery at the Broker (Optional) Complementary to what is defined in 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 Unauthorised Resource Request (see ). An example using the CBOR diagnostic notation and CoAP is given below.

AS Request Creation Hints Example

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 authorised to operate. In particular the Client is authorised to retrieve the representation of a topic resource, from which the Client can retrieve information related to the topic in question, as specified in Sections and of . This profile extends the set of CoAP Pub-Sub Topic Properties that is possible to specify within the representation of a topic resource, as originally defined in . In particular, this profile defines the following two topic properties that the Broker can specify in a response to a GET or FETCH request that targets a topic resource (see Sections and of ), with the FETCH request using the Content-Format "application/cbor". Note that, when these topic properties are transported in their respective fields of the response payload, the Content-Format "application/core-pubsub+cbor" defined in is used.

'kdc-uri', with value the URI of the group-membership resource at the 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 at the KDC, before starting the process to join the security group at the KDC. The CBOR abbreviation for this topic property is registered in .
'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. The CBOR abbreviation for this topic property is registered in .

Furthermore, the Resource Type (rt=) Link Target Attribute value "core.ps.gm" is registered in (REQ10) and can be used to describe group-membership resources at the KDC, e.g., by using a CoRE link-format document . 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.

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 and defines only additions or modifications to that specification. Both Authorisation Requests include the following fields (see ):

'scope': Optional. If present, it specifies the following information, depending on the specifically targeted audience. If the audience is the Broker, the scope specifies the names of the topics that the Client wishes to access, together with the corresponding requested permissions. If the audience is the KDC, the scope specifies the names of the security groups that the Client wishes to join, together with the corresponding requested permissions. This parameter is encoded as a CBOR byte string, whose value is the binary encoding of a CBOR array. The format of the encoded scope MUST follow the data model AIF-PUBSUB-GROUPCOMM defined in . The textual format specified in is not used in this application profile (OPT1).
'audience': Required identifier corresponding to either the Broker or the KDC.

Other additional parameters can be included if necessary, as defined in . When using this profile, it is expected that a one-to-one mapping is enforced between the application group and the security group (see ). If this is not the case, the correct access policies for both sets of scopes have to be available to the AS.

Format of Scope Building on , 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) (REQ1). With reference to the generic AIF model = [* [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 data item specifying the name of the group 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 group indicated by "Toid". More specifically, the following applies when, as defined in this document, a scope entry includes a set of permissions for user-related operations performed by a Pub-Sub 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 authorised 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.
  - Admin (0): This operation is reserved for scope entries that express permissions for Administrators of Pub-Sub groups, which are not specified in this document.
  - AppGroup (1): This operation is indicated as wished/authorised when "Toid" specifies the name of an application group (topic), while it is indicated as not wished/authorised when Toid specifies the name of a security group.
  - 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.
  - 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.
  - 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. If a Client wishes to have authorised only the Delete operation on an application group, then the Client does not need to join the corresponding security group, hence it does not need to request an access token for interacting with the KDC. If a Client wishes to have authorised the Delete operation on a security group, then the AS and the KDC ignore the wish for that operation when processing the scope entry in question.
- 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 indicated as not wished/authorised. 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 indicated as authorised), 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 . The following CDDL notation defines a scope entry that uses the AIF-PUBSUB-GROUPCOMM data model and expresses a set of permissions.

Pub-Sub scope using the AIF format 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] ]]> As per the guidelines in , and register the specific instance of "Toid" and "Tperm" as media type parameters and a corresponding Content-Format (REQ2).

Authorization Response The AS replies with an Authorization Response as defined in and , with the following additions:

In the Authorization Response, the AS MUST include the 'expires_in' parameter.
In the Authorization Response, 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 operations that the Client is authorised for that scope entry, encoded as specified in .

Furthermore, the AS MAY use the extended format of scope defined in 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 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 , where ct is the ID assigned to the CoAP Content-Format registered in 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 of this document.

Token Transfer to the KDC The Client transfers its access token to the KDC using one of the methods defined in . This typically includes sending a POST request to the /authz-info endpoint. However, if the DTLS transport profile of ACE is used and the Client uses a symmetric proof-of-possession (PoP) 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 , or in the "identity" field of a PskIdentity within the PreSharedKeyExtension of the ClientHello message when using DTLS 1.3 . 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. 'kdcchallenge' is a challenge N_S generated by the KDC. As to the N_S value, it is RECOMMENDED to be at least 8 bytes long and it is RECOMMENDED to be a random value. Later when joining the group, the Publisher can use the 'kdcchallenge' challenge as part of proving possession of its private key (see ). If a Publisher provides the access token to the KDC through an /authz-info endpoint, the Client MUST support the parameter 'kdcchallenge' (REQ30). 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., information conveyed in the 'sign_info' parameter 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 (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 (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 (REQ5).
'cred_fmt' takes value from the "Label" column of the "COSE Header Parameters" registry (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) , X.509 certificates , and C509 certificates . Future formats would be acceptable to use as long as they comply with the criteria defined above.

This application profile does not define additional parameters to be used in the exchange of Token Transfer Request and Response (OPT2).

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 . The entry of each resource specifies the CoAP methods by means of which it is permitted to access that resource, for nodes with different roles in the group or as non-members. Except for accessing the /ace-group resource with the FETCH method (to retrieve security group names through their group identifiers) and the /ace-group/GROUPNAME resource with the POST method (to join the security group with name GROUPNAME), all other operations are permitted only to current members of the security group with name GROUPNAME (REQ11). The GROUPNAME segment of the URI path MUST match with the group name specified in the scope entry of the scope in the access token (i.e., 'gname' in ) (REQ7). Therefore, a group name has to be consistent with the semantics of URI path segments (see ). Each resource is marked as REQUIRED or OPTIONAL to be hosted at the KDC (REQ9). Resources at the KDC

KDC resource	Description	Operations
/ace-group	REQUIRED. Contains a set of group names, each corresponding to one of the specified group identifiers.	FETCH (All Clients)
/ace-group/GROUPNAME	REQUIRED. Contains symmetric group keying material associated with GROUPNAME.	GET, POST (All Clients)
/ace-group/GROUPNAME/creds	REQUIRED. Contains the authentication credentials of all the Publishers of the group with name GROUPNAME.	GET, FETCH (All Clients)
/ace-group/GROUPNAME/num	REQUIRED. Contains the current version number for the symmetric group keying material of the group with name GROUPNAME.	GET (All Clients)
/ace-group/GROUPNAME/nodes/NODENAME	REQUIRED. Contains the group keying material for that group member NODENAME in GROUPNAME.	GET, DELETE (All Clients). POST (Publishers).
/ace-group/GROUPNAME/nodes/NODENAME/cred	REQUIRED. Contains the authentication credential for NODENAME in the group GROUPNAME.	POST (Publishers)
/ace-group/GROUPNAME/kdc-cred	REQUIRED if the group rekeying scheme used requires the use of a KDC authentication credential. Contains the authentication credential of the KDC for the group with name GROUPNAME.	GET (All Clients)
/ace-group/GROUPNAME/policies	OPTIONAL. Contains the group policies of the group with name GROUPNAME.	GET (All Clients)

The use of these resources follows what is defined in , and only additions or modifications to that specification are defined in this document. With respect to , this document does not define new operations for Clients interacting with the KDC (REQ12). Consistent with what is defined in , some error responses from the KDC can convey error-specific information according to the problem-details format specified in . This application profile does not define additional identifiers of error types as values of the 'error-id' field within the Custom Problem Detail entry 'ace-groupcomm-error' (OPT5).

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 have to 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 that is used as the shared group key to protect the payload of published topic data. The message exchange between the joining node and the KDC follows what is defined in , and only additions or modifications to that specification are defined in this document.

Join Request After establishing a secure communication association with the KDC, the Client sends a Join Request to the KDC as described in . 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 :

'scope': It MUST be present and its value MUST indicate the group that the Client is attempting to join, i.e., the group name, and the permissions that the Client wishes to have in the group. This value corresponds to one scope entry, as defined in .
'get_creds': It MAY be present if the Client wishes to join as a Subscriber 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 .
'cnonce': It specifies a challenge N_C generated by the Client. As to the N_C value, it is RECOMMENDED to be at least 8 bytes long and it is RECOMMENDED to be a random value. Join Requests include a new 'cnonce' at each join attempt.
'client_cred_verify': The use of this parameter is detailed in .

As a Publisher Client has its own authentication credential to use in a group, it MUST support the 'client_cred' and 'client_cred_verify' parameters (REQ30). For this application profile, the 'creds_repo' parameter is not relevant, since the KDC always acts as repository of the Publishers' authentication credentials. Consequently, no encoding is defined for this parameter (OPT6). This application profile does not define the functionalities that are implemented at the resource possibly hosted by the Client at the URI indicated in the 'control_uri' parameter (OPT7), if included in the Join Request.

Client Credential 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. If the joining node still provides an authentication credential in the 'client_cred' parameter of the Join Request (see ), the KDC silently ignores that parameter, as well as the related parameter 'client_cred_verify' if present.
The joining node wishes to join as a Publisher and the KDC has not previously acquired an authentication credential of the joining node. Then, the joining node MUST provide a compatible authentication credential in the 'client_cred' parameter of the Join Request (see ).
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 authentication credential as well as the included public key are compatible with the signature algorithm used in the security group and with possible associated parameters, then the 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. If the joining node chooses to do so, then the following applies when composing the Join Request: the value of the 'client_cred' parameter specifies an empty authentication credential, i.e., its value is set to the empty CBOR byte string (0x40); the 'client_cred_verify' parameter is omitted. If the 'client_cred' parameter specifies the empty CBOR byte string (0x40), the KDC silently ignores the 'client_cred_verify' parameter if present.

The joining node MUST provide the KDC with its own authentication credential again, if it has previously provided the KDC with multiple authentication credentials intended for different security groups. If the joining node provides its authentication credential, the KDC performs the consistency checks above and, in the case of success, considers it as the authentication credential associated with the joining node in the group.

Proof of Possession through 'client_cred_verify' The 'client_cred_verify' parameter contains the proof-of-possession (PoP) evidence and is computed by the joining node to prove the possession of its own private key. The PoP evidence is computed as defined below (REQ14). The joining node signs the scope, concatenated with N_S and further concatenated with N_C, by using the private key corresponding to the public key included in the authentication credential that is specified in the 'client_cred' parameter. The N_S may be either of the following:

The challenge received from the KDC in the 'kdcchallenge' parameter of the 2.01 (Created) response to the Token Transfer Request (see ).
If the provisioning of the access token to the KDC has relied on the DTLS profile of ACE and the access token was specified in the "psk_identity" field of the ClientKeyExchange message when using DTLS 1.2 , then N_S is an exporter value computed as defined in (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-pubsub-app" defined in of this document. The same as above holds if TLS 1.2 was used instead of DTLS 1.2, as per .
If the provisioning of the access token to the KDC has relied on the DTLS profile of ACE 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 , then N_S is an exporter value computed as defined in (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 'key_length' in bytes, and the exporter label "EXPORTER-ACE-Sign-Challenge-coap-pubsub-app" defined in of this document. The same as above holds if TLS 1.3 was used instead of DTLS 1.3, as per .
If the Join Request is a retry following 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 or future specifications to define how N_S is computed in further alternative settings.

Join Response Upon receiving the Join Request, the KDC processes it as defined in and returns a success or error response. If the joining node is not requesting to join the group exclusively as a Subscriber and the 'client_cred' parameter does not specify the empty CBOR byte string (0x40), 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 the one included in the authentication credential retrieved from the 'client_cred' parameter of the Join Request. The KDC verifies the signature used as PoP evidence by means of the public key of the joining node, according to the signature algorithm used in the group and possible corresponding parameters. Instead, if the joining node is not requesting to join the group exclusively as a Subscriber and the 'client_cred' parameter specifies the empty CBOR byte string (0x40), the KDC verifies that it is storing exactly one eligible authentication credential for the joining node (e.g., of the format accepted in the group). In the case that an error occurs when processing the Join Request, the KDC and the joining node follow what is defined in , complemented by what is defined in of the present document. In the case of success, the KDC replies with a Join Response, whose payload is formatted as a CBOR map and MUST contain the following fields as per :

'gkty': this field specifies the key type "Group_PubSub_Keying_Material" (REQ18) registered in for the 'key' parameter.
'key': this field specifies the keying material to use for secure communication in the group (REQ17). This field has as value a CBOR map that includes the following parameters.
- 'group_key': this parameter is identified by the CBOR unsigned integer 0 used as map key. Its value is a COSE_Key object as defined in and conveying the group key to use in the security group. The COSE_Key object MUST contain the following parameters:
  - 'kty', with value 4 (Symmetric).
  - 'alg', with value the identifier of the AEAD algorithm used in the security group. The value is taken from the "Value" column of the "COSE Algorithms" registry .
  - 'Base IV', with value the Base Initialization Vector (Base IV) to use in the security group with this group key.
  - 'k', with value the symmetric encryption key to use as group key.
  - 'kid', with value the identifier of the COSE_Key object, hence of the group key. 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. Consistent with the CBOR type of the 'kid' parameter, the Gid of the security group is encoded as a CBOR byte string (REQ13).
- 'group_SenderId': this parameter is identified by the CBOR unsigned integer 1 used as map key. Its value is the Client's Sender ID encoded as a CBOR byte string. This parameter 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 ). 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.
- 'cred_fmt': this parameter is identified by the CBOR unsigned integer 2 used as map key. Its value specifies the format of authentication credentials used in the group and is taken from the "Label" column of the "COSE Header Parameters" registry . At the time of writing this specification, acceptable formats of authentication credentials are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs) , X.509 certificates , and C509 certificates . 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': this parameter is identified by the CBOR unsigned integer 3 used as map key. Its value specifies the signature algorithm used to sign messages in the group and is taken from the "Value" column of the "COSE Algorithms" registry .
- 'sign_params': this parameter is identified by the CBOR unsigned integer 4 used as map key. Its value specifies the parameters of the signature algorithm and is encoded as a CBOR array including the following two elements:
  - '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 .
  - '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 .
'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 (REQ19).
'creds', which MUST be present if the 'get_creds' parameter was present in the Join Request and MUST NOT be present otherwise. It specifies the authentication credentials of all the Publishers in the security group.
'peer_roles', which MAY be omitted even if 'creds' is present, since: i) each authentication credential conveyed in the 'creds' parameter is associated with a Client authorised to be Publisher in the group; and ii) it is irrelevant whether such a Client is also authorised to be Subscriber in the group. If 'creds' is not present, 'peer_roles' MUST NOT be present.
'peer_identifiers', which MUST be present if 'creds' is also present and MUST NOT be present otherwise. The identifiers are the Publisher Sender IDs, whose corresponding authentication credentials are specified in the 'creds' parameter (REQ25).
'kdc_cred', which MUST be present if the group rekeying scheme used requires the use of a KDC authentication credential. It 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. It is encoded as a CBOR byte string, with value a nonce N_KDC generated by the KDC. As to the N_KDC value, it is RECOMMENDED to be at least 8 bytes long and it is RECOMMENDED to be a random value.
'kdc_cred_verify', which MUST be present if 'kdc_cred' is present. It is encoded as a CBOR byte string, with value the PoP evidence that the KDC computes to prove the possession of its own private key. 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 as the default rekeying scheme in the group (OPT9). In any other case, the 'group_rekeying' parameter MUST be included.

The 'group_policies' parameter is not expected to be included in the Join Response (REQ20). This application profile does not define the functionalities that are possibly implemented at the resource hosted by the Client at the URI indicated in the 'control_group_uri' parameter (OPT10), if included in the Join Response. 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 at the KDC. 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 used in the group is either: the one retrieved from the 'client_cred' parameter of the Join Request, if the parameter did not specify the empty CBOR byte string (0x40); or, otherwise, the one that the KDC acquired from previous interactions with the joining node and is currently storing. The KDC associates the Client's authentication credential 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. If the application requires backward security, the KDC MUST generate updated security parameters and group keying material, and provide those to the current group members upon the new node's joining (see ). In such a case, the joining node is not able to access secure communication in the Pub-Sub group that occurred prior to its joining. Upon receiving the Join Response, the joining node retrieves the KDC's authentication credential from the 'kdc_cred' parameter (if present). The joining node MUST verify the signature used as PoP evidence, which is specified by the 'kdc_cred_verify' parameter of the Join Response (REQ21).

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 joining node is not requesting to join the group exclusively as a Subscriber and the 'client_cred' parameter is not present in the Join Request.
The joining node is not requesting to join the group exclusively as a Subscriber, the 'client_cred' parameter does not specify the empty CBOR byte string (0x40), and any of the following conditions holds:
- The value of the 'client_cred' parameter is not an eligible authentication credential (e.g., it is not of the format accepted in the group) (OPT8).
- The 'client_cred_verify' parameter is not present in the Join Request.
The joining node is not requesting to join the group exclusively as a Subscriber, the 'client_cred' parameter specifies the empty CBOR byte string (0x40), and any of the following conditions holds:
- The KDC does not store an eligible authentication credential for the joining node (e.g., of the format accepted in the group).
- The KDC stores multiple eligible authentication credentials for the joining node (e.g., of the format accepted in the group).
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 .

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, with value 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) error response to a Client that sends a Join Request to join the security group as Publisher, if there are currently no Sender IDs available to assign. The response MUST have Content-Format set to "application/concise-problem-details+cbor" and is formatted as defined in . Within the Custom Problem Detail entry 'ace-groupcomm-error', the value of the 'error-id' field MUST be set to 4 ("No available individual keying material").

Other Group Operations through the KDC

Obtaining Latest Information on the Group, Group Keying Material, and Sender ID A Client can access the following resources at the KDC, in order to retrieve latest information about the group or the group keying material.

'/ace-group': All Clients can send a FETCH request to retrieve a set of group names associated with the 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 ) for which the group name and the URI to the group-membership resource are provided in the returned response.
'/ace-group/GROUPNAME': All group member Clients can send a GET request to retrieve the symmetric group keying material of the group with the name GROUPNAME. The KDC processes the Key Distribution Request according to . The Key Distribution Response is formatted as defined in , with the following additions.
- The 'key' field is formatted as defined in of this document, with the difference that it does not include the 'group_SenderId' parameter.
- The 'exi' field MUST be present.
- The 'ace_groupcomm_profile' field MUST be present and has value "coap_group_pubsub_app" (PROFILE_TBD).
Upon receiving the Key Distribution Response, the requesting group member retrieves the updated security parameters and group keying material. If they differ from the currently stored ones, then the group member uses the received ones thereafter for the security processing of published topic data, i.e., for encryption operations when acting as Publisher and for decryption operations when acting as Subscriber. This application profile does not specify policies that instruct group members to retain messages and for how long, if those messages are unsuccessfully decrypted (OPT11).
'/ace-group/GROUPNAME/creds': The KDC acts as a repository of authentication credentials for the Publishers that are members 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: i) each authentication credential conveyed in the 'creds' parameter is associated with a Client authorised to be Publisher in the group; and ii) it is irrelevant whether such a Client is also authorised to be Subscriber in the group. If 'creds' is not present, 'peer_roles' MUST NOT be present.
'/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.
'/ace-group/GROUPNAME/nodes/NODENAME': A group member can send a Key Distribution Request to the KDC by sending a GET request to this resource to retrieve the latest group keying material as well as its Sender ID that it has in the group (if Publisher). The KDC processes the Key Distribution Request according to . The Key Distribution Response is formatted as defined in , with the following additions.
- The 'key' field is formatted as defined in of this document. If the requesting group member is not a Publisher Client, then the 'key' field does not include the 'group_SenderId' parameter.
- The 'exi' field MUST be present.
Upon receiving the Key Distribution Response, the group member retrieves the updated security parameters, group keying material, and Sender ID (if the 'key' field includes the 'group_SenderId' parameter). If they differ from the currently stored ones, then the group member uses the received ones thereafter for the security processing of published topic data, i.e., for encryption operations when acting as Publisher and for decryption operations when acting as Subscriber. This application profile does not specify policies that instruct group members to retain messages and for how long, if those messages are unsuccessfully decrypted (OPT11).

Requesting a New Sender ID A Publisher group member with node name NODENAME can at some point exhaust its Sender Sequence Numbers used for protecting its published topic data (see ). When this happens, the Publisher MUST send a Key Renewal Request message to the KDC, as per . That is, it sends a CoAP POST request to the endpoint /ace-group/GROUPNAME/nodes/NODENAME at the KDC. Upon receiving the Key Renewal Request, the KDC processes it as defined in , with the addition that the KDC takes one of the following actions.

The KDC rekeys the group. That is, the KDC generates new group keying material for that group (see ) and replies to the Publisher with a group rekeying message as defined in , providing the new group keying material. Then, the KDC rekeys the rest of the group, as discussed in . The KDC SHOULD perform a group rekeying if one is already scheduled to occur within a time frame that is acceptably short, as per application-specific policies at the KDC. For instance, a group rekeying could be already upcoming in accordance with an application-specific rekeying period or scheduling, or as a reaction to a recent change in the group membership. If a group rekeying is not already scheduled to occur within an acceptably short time frame, the KDC SHOULD NOT rekey the group when receiving a Key Renewal Request (OPT12).
The KDC determines and assigns a new Sender ID for the Publisher (REQ27), and it replies with a Key Renewal Response formatted as defined in . The CBOR map in the response payload includes only the parameter 'group_SenderId' registered in , which specifies the new Sender ID of the Publisher encoded as a CBOR byte string (REQ27). The KDC MUST assign a new Sender ID that has not been used in the group since the latest time when the current Gid value was assigned to the group (i.e., since the latest group rekeying, see ). The KDC MUST return a 5.03 (Service Unavailable) error response if there are currently no Sender IDs available to assign in the group. The response MUST have Content-Format set to "application/concise-problem-details+cbor" and is formatted as defined in . Within the Custom Problem Detail entry 'ace-groupcomm-error', the value of the 'error-id' field MUST be set to 4 ("No available individual keying material").

Updating Authentication Credentials A Publisher group member 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, the Publisher sends a CoAP POST request to its associated sub-resource /ace-group/GROUPNAME/nodes/NODENAME/cred at the KDC (see ). Following a successful processing of the request, the KDC replaces the stored authentication credential of this Client for the group GROUPNAME with the one specified in the request. This application profile does not specify a method for the group member to provide other group members with the identifier of its new authentication credential (OPT13).

Leaving a Group A group member with node name NODENAME can actively request to leave the security group with name GROUPNAME. To this end, the Client sends a CoAP DELETE request to the associated sub-resource /ace-group/GROUPNAME/nodes/NODENAME at the KDC (see ). The KDC can also remove a group member due to any of the reasons described in .

Group Rekeying Process Rekeying a group consists in the KDC generating and distributing a new symmetric key, which is used as group key from then on to protect the publication of topic data with COSE (see ). The KDC MUST perform a group rekeying as described in . Reasons that can trigger a group rekeying include a change in the group membership, or the current group keying material approaching its expiration time. In addition, the KDC MAY rekey the group for other reasons, e.g., according to an application-specific rekeying period or scheduling. Upon generating the new group key and before starting its distribution:

The KDC MUST increment the version number of the group keying material.
The KDC MUST generate a new Group Identifier (Gid) for the group. This is used as the identifier of the new group key, when providing it to the current group members through the group rekeying and to Clients (re-)joining the security group thereafter (see ). That is, the value of the new Gid is specified by the 'kid' parameter of the COSE_Key Object that is used to encode the new group key.

When rekeying the group, the KDC MUST preserve the current value of the Sender ID of each member in that group. The default rekeying scheme is "Point-to-Point" (see ), according to which the KDC individually targets each node to rekey, using the pairwise secure communication association with that node. In particular, a group rekeying message MUST have Content-Format set to "application/ace-groupcomm+cbor" and have the same format used for the Join Response message defined in , with the following differences:

Within the 'key' field, only the parameter 'group_key' is present.
The fields 'kdc_cred' ,'kdc_nonce', 'kdc_cred_verify', and 'group_rekeying' are not present.
The fields 'creds' and 'peer_identifiers' SHOULD be present, if the group rekeying is performed due to one or multiple Clients joining the group as Publishers. Following the same semantics used in the Join Response message, the two parameters specify the authentication credentials and Sender IDs of such Clients. Like in the Join Response message, the 'peer_roles' parameter MAY be omitted.

This application profile does not define additional information to include in rekeying messages for the "Point-to-Point" group rekeying scheme (OPT14).

Pub-Sub Protected Communication In the diagram shown in , (D) corresponds to the publication on a topic at the Broker, by using a CoAP PUT request. The Publisher protects the published topic data end-to-end for the Subscribers by using COSE (), as detailed in . In the same diagram, (E) corresponds to the subscription of a Subscriber to the same topic, by means of a CoAP GET request with the Observe option set to 0 (register) , as per . Finally, (F) corresponds to the Observe notification response from the Broker to the Subscriber, where the published topic data is conveyed as originally protected end-to-end with COSE by the Publisher.

Secure Pub-Sub Communication between Publisher and Subscriber | | | | | | | | | | | Publisher | | Broker | <--- (E) --- | Subscriber | | | | | | | | | | | ---- (F) --> | | | | | | | | +-----------+ +--------+ +------------+ ]]> provides a more detailed example of such a secure Pub-Sub communication. All the messages exchanged between a Client and the Broker are protected with the secure communication association between that Client and the Broker. In addition, COSE is used to protect end-to-end the published topic data, which is conveyed in a PUT request from the Publisher to the topic-data resource at the Broker and in a successful response (typically 2.05 (Content)) from that resource to a Subscriber. The example also shows a delete operation, where the Publisher deletes the topic-data resource by sending a CoAP DELETE request to the URI of that resource. In the case of success, the Broker replies with a 2.02 (Deleted) response. Consequently, the Broker also unsubscribes all the Clients subscribed to that topic-data resource, by removing them from the list of observers and sending them a final 4.04 (Not Found) error response as per .

Using COSE to Protect the Published Topic Data The Publisher uses the symmetric COSE Key received from the KDC to protect the payload of the Publish operation (see ). Specifically, the Publisher creates a COSE_Encrypt0 object by means of the COSE Key currently used as group key (REQ23). 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. Also, the Publisher uses its private key corresponding to the public key provided to the KDC, in order to countersign the COSE_Encrypt0 object as specified in (REQ23). 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 KDC (see ) or from its local storage. Then, the Subscriber uses that public key to verify the countersignature. In the 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 the 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 with 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 the result of a group rekeying (see ). The Publisher MUST increment its Sender Sequence Number value by 1, after having completed an encryption operation by means of the current group key. When the Publisher exhausts its Sender Sequence Numbers, the Publisher MUST NOT protect further topic data by using the current group key while still retaining its current Sender ID, and it MUST send a Key Renewal Request message to the KDC (see ). This will result in the KDC rekeying the group and distributing a new group key, or in the KDC providing the Publisher with a new Sender ID. The Publisher MUST reset its Sender Sequence Number to 0 upon receiving a new Sender ID from the KDC.
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 the value of the 'group_SenderId' parameter of the Join Response (see ).
- The 'signature' field, with value the countersignature.
The countersignature is computed as defined in , 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 , by using the current group key as encryption key, the AEAD nonce computed as defined in , the topic data to publish as plaintext, and the Enc_structure populated as follows:

'context' takes "Encrypt0".
'protected' takes the serialization of the protected parameter 'alg' from the 'protected' field of the COSE_Encrypt0 object.
'external_aad is not supplied.

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 ). The AEAD nonce for producing or consuming the COSE_Encrypt0 object is constructed as defined below and also shown in .

Left-pad the Partial IV (PIV) with zeroes to exactly 5 bytes.
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.
Concatenate the size of the ID_PIV (a single byte S) with the padded ID_PIV and the padded PIV.
XOR the result from the previous step with the Base IV.

AEAD Nonce Construction <-- 5 bytes --> +---+-------------------+--------+---------+-----+ | S | padding | ID_PIV | padding | PIV |-----+ +---+-------------------+--------+---------+-----+ | | <---------------- nonce length ----------------> | +------------------------------------------------+ | | Base IV |-->(XOR) +------------------------------------------------+ | | <---------------- nonce length ----------------> | +------------------------------------------------+ | | Nonce |<----+ +------------------------------------------------+ ]]> The construction above only supports AEAD algorithms that use nonces with length equal to 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 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 and ), 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 reuses the same AEAD nonce with the same group key, nor any two Publishers use the same AEAD nonce with the same group key.

Replay Checks In order to protect from replay of published topic data, every Subscriber maintains a Replay Window for each different Publisher in the same group. It is RECOMMENDED that the Replay Window has a default size of 32. Upon receiving a topic data published by a given Publisher P, the Subscriber retrieves the Sender ID of P conveyed as 'kid' in the 'Countersignature version 2' parameter of the COSE_Encrypt0 object (see ) and determines the Replay Window W_P associated with P. The Subscriber MUST verify that, according to W_P, the Sender Sequence Number SN_P specified by the 'Partial IV' parameter of the COSE_Encrypt0 object has not been received before from P. If the verification above fails, the Subscriber MUST stop processing the COSE_Encrypt0 object conveying the topic data. If the value of SN_P is strictly smaller than the currently smallest value in W_P, then the Subscriber MUST stop processing the COSE_Encrypt0 object. If the verification above succeeds, the Subscriber proceeds with processing the COSE_Encrypt0 object, by verifying the countersignature from P using P's public key as well as by decrypting and verifying the COSE_Encrypt0 object using the group key. If both operations succeed, the Subscriber updates W_P as follows:

If SN_P is strictly greater than the currently largest value in W_P, then W_P is updated in order to set SN_P as its largest value.
SN_P is marked to denote that it has been received.

The operation of validating the 'Partial IV' and updating the Replay Window MUST be atomic. Upon installing a new group key (e.g., due to a group rekeying performed by the KDC, see ) or upon receiving published topic data from a given Publisher for the first time, the Subscriber initializes the Replay Window corresponding to that Publisher, i.e., the smallest value of the Replay Window is set to 0.

Operational Considerations This section compiles the operational considerations that hold for this document.

Logging When performing its normal operations, the KDC is expected to produce and store timestamped logs about the following:

Any event that has resulted in the KDC sending an error response, as a reply to a request received at any of the resources exported by the interface specified in this document. The logged information contains a description of the error occurred in the context of the present application profile, together with a description of the event related to the error and relevant metadata about the Client that has sent the request. For instance, possible metadata include: addressing information of the Client; when applicable, the Sender ID that is assigned to the Client in the group; when applicable, (an identifier of) the authentication credential of the Client (i.e., that the Client uses in the group or has used to authenticate itself to the KDC when establishing their secure communication association). Note that, if the error response uses the format problem-details defined in , then the optional "detail" entry in the response payload is meant to convey the diagnostic description of the error, which is meant to be part of the log entry for this event. This is consistent with , which states that the diagnostic description of the error should be logged.
Any event consisting in a successfully performed operation that is triggered by a request received at any of the resources exported by the interface specified in this document. Such events include:
- A Client joining or re-joining a group.
- The upload of a new authentication credential for use within the group.
- The acquisition of a new Sender ID for use within the group.
- A Client leaving a group.
The logged information contains a description of the operation performed in the context of the present application profile, together with relevant metadata about the Client that has sent the request. For instance, possible metadata include: addressing information of the Client; when applicable, the Sender ID that is assigned to the Client in the group; when applicable, (an identifier of) the authentication credential of the Client (i.e., that the Client uses in the group or has used to authenticate itself to the KDC when establishing their secure communication association).
The execution and successful/unsuccessful completion of a group rekeying instance. The logged information includes:
- The reason for the group rekeying (e.g., scheduled/periodic occurrence, group joining of a new member, group leaving of a current member).
- A description of the group rekeying operations performed (e.g., a list of steps performed throughout the rekeying process).
- The outcome of the group rekeying instance.
- In the case of success, the version number of the newly established group keying material and the newly established Group Identifier (Gid).
The addition of a group member to the group or the eviction of a group member from the group. The logged information also contains relevant metadata about the Client that has been added to or removed from the group. For instance, possible metadata include: addressing information of the Client; when applicable, the Sender ID that is currently (was latest) assigned to the Client added to (removed from) the group; when applicable, (an identifier of) the authentication credential of the Client added to or removed from the group (i.e., that the Client uses in the group or has used to authenticate itself to the KDC when establishing their secure communication association).
The creation, (re-)configuration, or termination of a group.

In addition to what is compiled above, the KDC could log additional information. Further details about what the KDC logs, with what granularity, and based on what triggering events and conditions are application-specific and left to operators to define. The KDC MUST NOT log any secret or confidential information pertaining to a group, such as:

The group key used in the group as symmetric encryption key.
The Base Initialization Vector (Base IV) to use in the security group with the group key.
The private key associated with the KDC’s authentication credential used in the group, if any.
Rekeying messages that are exchanged in the group.
If applicable, administrative keying material used to protect the group rekeying process.

It is up to the application to specify for how long a log entry is retained from the time of its creation and until its deletion. Different retention policies could be enforced for different groups. For a given group, the oldest log entries are expected to be those deleted first, and different retention policies could be enforced depending on whether the group currently exists or has been deleted. It is out of the scope of this document what specific semantics and data model are used by the KDC for producing and processing the logs. Specific semantics and data models can be defined by applications and future specifications. The KDC is expected to make the logs that it produces available for secure access by authorized external management applications and operators. In particular, logged information could be retrieved in the following ways.

By accessing logs at the KDC through polling. This can occur in an occasional, regular, or event-driven way.
Through notifications sent by the KDC according to an operator-defined frequency.
Through notifications asynchronously sent by the KDC, throttling them in order to prevent congestion and duplication and to not create attack vectors.

Some of the logged information can be privacy-sensitive. This especially holds for the metadata about a Client, i.e., addressing information of the Client and, when applicable, (an identifier of) the authentication credential of the Client (i.e., that the Client uses in the group or has used to authenticate itself to the KDC when establishing their secure communication association). If external management applications and operators obtain such metadata, they become able to track a given Client, as to its interactions with one or multiple KDCs and its membership in groups under such KDC(s). Therefore, the logged information that is effectively provided to external management applications and operators SHOULD be redacted by the KDC, by omitting any privacy-sensitive information element that could enable or facilitate the impairment of Clients' privacy, e.g., by tracking Clients across different groups and different KDCs. Exceptions could apply, e.g., if the KDC can verify that the management application or operator in question is specifically authorized to obtain such privacy-sensitive information and appropriately entitled to obtain it according to enforced privacy policies. It is out of the scope of this document to provide operational considerations about the production, storage, processing, and sharing of logs at the Broker.

Administration of Groups It is out of the scope of this document how groups are created, (re-)configured, and terminated at the KDC. Specific methods, tools, and data models to perform such operations and administer groups at the KDC can be defined by applications and future specifications. It is out of the scope of this document to provide operational considerations about how application groups (topics) are created, (re-)configured, and terminated at the Broker, as well as about the store-and-forward of published topic data via the Broker.

Access Control Building on the ACE framework and the foundation provided in , this application profile enforces access control for Clients that interact with the interface at the KDC specified in this document and with the interface at the Broker specified in . In particular, the granularity of such access control takes into account the resource specifically targeted at the KDC or at the Broker, the operation requested by sending a request to that resource, and the specific role(s) that the requesting Client is authorized to have according to its corresponding access token uploaded at the KDC or at the Broker. Furthermore, the interactions that a Client has with the KDC and with the Broker are secured as per the specific transport profile of ACE used (e.g., and ).

Security Considerations Security considerations for this profile are inherited from , the ACE framework for Authentication and Authorization , and the specific transport profile of ACE used, such as and . The following security considerations also apply for this profile. Consistent with the intended group-confidentiality model, each Client in a security group is able to decrypt the data published in the topic(s) associated with that group, by using the symmetric group key that is shared with all the other group members. At the same time, source authentication of the published topic data is achieved by means of a digital signature, which the Publisher of the data in question computes with its private key and embeds in the published topic data. This ensures integrity of the published topic data as well as its origin, thus preventing a group member from impersonating another one. To this end, both Publishers and Subscribers rely on asymmetric cryptography, while Subscribers have to be able to access the public keys of the Publishers to a specific topic in order to verify the signature over the published topic data. This might make the message exchange quite heavy for small constrained devices. The Broker is only trusted with verifying that a Publisher is authorised to publish on a certain topic and with distributing that data only to the Subscribers authorised to obtain it. However, the Broker is not trusted with the published topic data in itself, which the Broker cannot read or modify as it does not have access to the group key required for decrypting the data. With respect to the reuse of challenges for proof-of-possession input, the same considerations apply as in , with the difference that the KDC of the present document is considered instead of the Group Manager defined therein. Access tokens might have to be revoked before their expiration time. provides a list of possible circumstances where this can happen, and it specifies a method that an Authorization Server can use in order to notify the KDC, the Broker, and the Clients about pertaining access tokens that have been revoked but are not expired yet. Aligned with , the KDC performs a group rekeying when one or more members leave the group, thus preserving forward security. In particular, Clients can be excluded from future communications related to a topic, by appropriately rekeying the group associated with the topic in question. According to the specific application requirements, the KDC can also rekey the group upon a new node's joining, in the case that backward security has also to be preserved (see ). The KDC can also rekey the group for further reasons, e.g., according to an application-specific rekeying period or scheduling.

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.

ACE Groupcomm Key Types IANA is asked to register the following entry in the "ACE Groupcomm Key Types" registry within the "Authentication and Authorization for Constrained Environments (ACE)" registry group.

Name: Group_PubSub_Keying_Material
Key Type Value: GROUPCOMM_KEY_TBD (suggested value: 2)
Profile: coap_group_pubsub_app ( of [RFC-XXXX]).
Description: Encoded as described in of [RFC-XXXX].
References: , , [RFC-XXXX]

ACE Groupcomm Profiles IANA is asked to register the following entries in the "ACE Groupcomm Profiles" registry , within the "Authentication and Authorization for Constrained Environments (ACE)" registry group.

Name: coap_group_pubsub_app
Description: Application profile to provision keying material for participating in group communication based on the Pub-Sub architecture for CoAP and protected with COSE .
CBOR Value: TBD (suggested value: 2)
Reference: [RFC-XXXX]

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.

CoAP Pubsub Topic Properties IANA is asked to add the following entries to the "CoAP Pubsub Topic Properties" registry within the "Constrained RESTful Environments (CoRE) Parameters" registry group, which is defined by .

Name: kdc-uri
CBOR Key: TBD (range 0-255)
CBOR Type: tstr
Reference: [RFC-XXXX]

Name: sec-gp
CBOR Key: TBD (range 0-255)
CBOR Type: tstr
Reference: [RFC-XXXX]

AIF Media-Type Sub-Parameters For the media-types "application/aif+cbor" and "application/aif+json" defined in , IANA is requested to register the following entries for the two media-type parameters Toid and Tperm, in the respective sub-registry defined in within the "MIME Media Type Sub-Parameter" registry group.

Parameter: Toid
Name: pubsub-topic
Description/Specification: Name of one application group (topic) or of a corresponding security group.
Reference: [RFC-XXXX]

Parameter: Tperm
Name: pubsub-perm
Description/Specification: Permissions pertaining to application groups (topics) or to corresponding security groups.
Reference: [RFC-XXXX]

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.

Content Type: application/aif+cbor;toid=pubsub-topic;tperm=pubsub-perm
Content Coding: -
ID: 299 (suggested)
Reference: [RFC-XXXX]

Content Type: application/aif+json;toid=pubsub-topic;tperm=pubsub-perm
Content Coding: -
ID: 300 (suggested)
Reference: [RFC-XXXX]

TLS Exporter Labels IANA is asked to register the following entry in the "TLS Exporter Labels" registry within the "Transport Layer Security (TLS) Parameters" registry group, which is defined in and updated in .

Value: EXPORTER-ACE-Sign-Challenge-coap-pubsub-app
DTLS-OK: Y
Recommended: N
Reference: of [RFC-XXXX]

References Normative References COSE Algorithms IANA COSE Key Types IANA COSE Header Parameters IANA ACE Groupcomm Key Types IANA ACE Groupcomm Profiles IANA Resource Type (rt=) Link Target Attribute Values IANA CoAP Content-Formats IANA TLS Exporter Labels IANA A publish-subscribe architecture for the Constrained Application Protocol (CoAP) Ericsson Dogtiger Labs Ericsson This document describes a publish-subscribe architecture for the Constrained Application Protocol (CoAP), extending the capabilities of CoAP communications for supporting endpoints with long breaks in connectivity and/or up-time. CoAP clients publish on and subscribe to a topic via a corresponding topic resource at a CoAP server acting as broker. Uniform Resource Identifier (URI): Generic Syntax A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] The Transport Layer Security (TLS) Protocol Version 1.2 This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] Keying Material Exporters for Transport Layer Security (TLS) A number of protocols wish to leverage Transport Layer Security (TLS) to perform key establishment but then use some of the keying material for their own purposes. This document describes a general mechanism for allowing that. [STANDARDS-TRACK] Datagram Transport Layer Security Version 1.2 This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document updates DTLS 1.0 to work with TLS version 1.2. [STANDARDS-TRACK] Constrained RESTful Environments (CoRE) Link Format This specification defines Web Linking using a link format for use by constrained web servers to describe hosted resources, their attributes, and other relationships between links. Based on the HTTP Link Header field defined in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carried as a payload and is assigned an Internet media type. "RESTful" refers to the Representational State Transfer (REST) architecture. A well-known URI is defined as a default entry point for requesting the links hosted by a server. [STANDARDS-TRACK] The OAuth 2.0 Authorization Framework The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 1.0 protocol described in RFC 5849. [STANDARDS-TRACK] The Constrained Application Protocol (CoAP) The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. Observing Resources in the Constrained Application Protocol (CoAP) The Constrained Application Protocol (CoAP) is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients and provides eventual consistency between the state observed by each client and the actual resource state at the server. Transport Layer Security (TLS) / Datagram Transport Layer Security (DTLS) Profiles for the Internet of Things A common design pattern in Internet of Things (IoT) deployments is the use of a constrained device that collects data via sensors or controls actuators for use in home automation, industrial control systems, smart cities, and other IoT deployments. This document defines a Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) 1.2 profile that offers communications security for this data exchange thereby preventing eavesdropping, tampering, and message forgery. The lack of communication security is a common vulnerability in IoT products that can easily be solved by using these well-researched and widely deployed Internet security protocols. CBOR Web Token (CWT) CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. The Transport Layer Security (TLS) Protocol Version 1.3 This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. IANA Registry Updates for TLS and DTLS This document describes a number of changes to TLS and DTLS IANA registries that range from adding notes to the registry all the way to changing the registration policy. These changes were mostly motivated by WG review of the TLS- and DTLS-related registries undertaken as part of the TLS 1.3 development process. This document updates the following RFCs: 3749, 5077, 4680, 5246, 5705, 5878, 6520, and 7301. Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. Concise Binary Object Representation (CBOR) The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. CBOR Object Signing and Encryption (COSE): Structures and Process Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. CBOR Object Signing and Encryption (COSE): Initial Algorithms Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. The Datagram Transport Layer Security (DTLS) Protocol Version 1.3 This document specifies version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection / non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. Authentication and Authorization for Constrained Environments Using the OAuth 2.0 Framework (ACE-OAuth) This specification defines a framework for authentication and authorization in Internet of Things (IoT) environments called ACE-OAuth. The framework is based on a set of building blocks including OAuth 2.0 and the Constrained Application Protocol (CoAP), thus transforming a well-known and widely used authorization solution into a form suitable for IoT devices. Existing specifications are used where possible, but extensions are added and profiles are defined to better serve the IoT use cases. An Authorization Information Format (AIF) for Authentication and Authorization for Constrained Environments (ACE) Information about which entities are authorized to perform what operations on which constituents of other entities is a crucial component of producing an overall system that is secure. Conveying precise authorization information is especially critical in highly automated systems with large numbers of entities, such as the Internet of Things. This specification provides a generic information model and format for representing such authorization information, as well as two variants of a specific instantiation of that format for use with Representational State Transfer (REST) resources identified by URI path. On Stable Storage for Items in Concise Binary Object Representation (CBOR) This document defines a stored ("file") format for Concise Binary Object Representation (CBOR) data items that is friendly to common systems that recognize file types, such as the Unix file(1) command. Concise Problem Details for Constrained Application Protocol (CoAP) APIs This document defines a concise "problem detail" as a way to carry machine-readable details of errors in a Representational State Transfer (REST) response to avoid the need to define new error response formats for REST APIs for constrained environments. The format is inspired by, but intended to be more concise than, the problem details for HTTP APIs defined in RFC 7807. CBOR Object Signing and Encryption (COSE): Countersignatures Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. CBOR Object Signing and Encryption (COSE) defines a set of security services for CBOR. This document defines a countersignature algorithm along with the needed header parameters and CBOR tags for COSE. This document updates RFC 9052. Extension of the Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE) to Transport Layer Security (TLS) This document updates "Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE)" (RFC 9202) by specifying that the profile applies to TLS as well as DTLS. Key Provisioning for Group Communication Using Authentication and Authorization for Constrained Environments (ACE) This document defines how to use the Authentication and Authorization for Constrained Environments (ACE) framework to distribute keying material and configuration parameters for secure group communication. Candidate group members that act as Clients and are authorized to join a group can do so by interacting with a Key Distribution Center (KDC) acting as the Resource Server, from which they obtain the keying material to communicate with other group members. While defining general message formats as well as the interface and operations available at the KDC, this document supports different approaches and protocols for secure group communication. Therefore, details are delegated to separate application profiles of this document as specialized instances that target a particular group communication approach and define how communications in the group are protected. Compliance requirements for such application profiles are also specified. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Object Security for Constrained RESTful Environments (OSCORE) This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. The JavaScript Object Notation (JSON) Data Interchange Format JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE) This specification defines a profile of the Authentication and Authorization for Constrained Environments (ACE) framework that allows constrained servers to delegate client authentication and authorization. The protocol relies on DTLS version 1.2 or later for communication security between entities in a constrained network using either raw public keys or pre-shared keys. A resource-constrained server can use this protocol to delegate management of authorization information to a trusted host with less-severe limitations regarding processing power and memory. The Object Security for Constrained RESTful Environments (OSCORE) Profile of the Authentication and Authorization for Constrained Environments (ACE) Framework This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Object Security for Constrained RESTful Environments (OSCORE) to provide communication security and proof-of-possession for a key owned by the client and bound to an OAuth 2.0 access token. Notification of Revoked Access Tokens in the Authentication and Authorization for Constrained Environments (ACE) Framework This document specifies a method of the Authentication and Authorization for Constrained Environments (ACE) framework, which allows an authorization server to notify clients and resource servers (i.e., registered devices) about revoked access tokens. As specified in this document, the method allows clients and resource servers (RSs) to access a Token Revocation List (TRL) on the authorization server by using the Constrained Application Protocol (CoAP), with the possible additional use of resource observation. Resulting (unsolicited) notifications of revoked access tokens complement alternative approaches such as token introspection, while not requiring additional endpoints on clients and RSs. CBOR Encoded X.509 Certificates (C509 Certificates) Ericsson AB Ericsson AB University of Glasgow RISE AB IN Groupe NIO This document specifies a CBOR encoding of X.509 certificates. The resulting certificates are called C509 certificates. The CBOR encoding supports a large subset of RFC 5280 and common certificate profiles, and it is extensible. Two types of C509 certificates are defined. One type is an invertible CBOR re-encoding of DER-encoded X.509 certificates with the signature field copied from the DER encoding. The other type is identical except that the signature is computed over the CBOR encoding instead of the DER encoding, thereby avoiding the use of ASN.1. Both types of certificates have the same semantics as X.509 while providing comparable size reduction. This document also specifies CBOR-encoded data structures for certification requests and certification request templates, new COSE headers, as well as a TLS certificate type and a file format for C509. This document updates RFC 6698 by extending the TLSA selectors registry to include C509 certificates. Key Management for Group Object Security for Constrained RESTful Environments (Group OSCORE) Using Authentication and Authorization for Constrained Environments (ACE) RISE AB Ericsson AB This document defines an application profile of the Authentication and Authorization for Constrained Environments (ACE) framework, to request and provision keying material in group communication scenarios that are based on the Constrained Application Protocol (CoAP) and are secured with Group Object Security for Constrained RESTful Environments (Group OSCORE). This application profile delegates the authentication and authorization of Clients, which join an OSCORE group through a Resource Server acting as Group Manager for that group. This application profile leverages protocol-specific transport profiles of ACE to achieve communication security, server authentication, and proof of possession of a key owned by the Client and bound to an OAuth 2.0 access token.

Requirements on Application Profiles This section lists how this application profile of ACE addresses the requirements defined in .

Mandatory-to-Address Requirements

REQ1: Specify the format and encoding of scope. This includes defining the set of possible roles and their identifiers, as well as the corresponding encoding to use in the scope entries according to the used scope format: see .
REQ2: If scope uses AIF, register its specific instance of "Toid" and "Tperm" as media type parameters and a corresponding Content-Format, as per the guidelines in : see and .
REQ3: If used, specify the acceptable values for the 'sign_alg' parameter: values from the "Value" column of the "COSE Algorithms" registry .
REQ4: If used, specify the acceptable values and structure for the 'sign_parameters' parameter: values and structure from the COSE algorithm capabilities as specified in the "COSE Algorithms" registry .
REQ5: If used, specify the acceptable values and structure for the 'sign_key_parameters' parameter: values and structure from the COSE key type capabilities as specified in the "COSE Key Types" registry .
REQ6: Specify the acceptable formats for authentication credentials and, if applicable, the acceptable values for the 'cred_fmt' parameter: acceptable formats explicitly provide the public key as well as the comprehensive set of information related to the public key algorithm (see and ). Consistent acceptable values for 'cred_fmt' are taken from the "Label" column of the "COSE Header Parameters" registry .
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, since a perfect matching is required.
REQ8: Define whether the KDC has an authentication credential as required for the correct group operation and if this has to be provided through the 'kdc_cred' parameter: optional, see .
REQ9: Specify if any part of the KDC interface as defined in is not supported by the KDC: part of the KDC interface is optional to support, see .
REQ10: Register a Resource Type for the group-membership resources, 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 .
REQ11: Define what specific actions (e.g., CoAP methods) are allowed on each resource accessible through the KDC interface, depending on: whether the Client is a current group member; the roles that a Client is authorised to take as per the obtained access token; and the roles that the Client has as a current group member: see .
REQ12: Categorize possible newly defined operations for Clients into primary operations expected to be minimally supported and secondary operations, and provide accompanying considerations: no new operations are defined.
REQ13: Specify the encoding of group identifiers: CBOR byte string, with value used also to identify the current group key used in the security group (see and ).
REQ14: Specify the approaches used to compute and verify the PoP evidence to include in the 'client_cred_verify' parameter and which of those approaches is used in which case: see .
REQ15: Specify how N_S is generated, if the access token is not provided to the KDC through the Token Transfer Request sent to the /authz-info endpoint (e.g., the access token is instead transferred during the establishment of a secure communication association): see .
REQ16: Define the initial value of the version number for the group keying material: the initial value MUST be set to 0 (see ).
REQ17: Specify the format of the group keying material that is conveyed in the 'key' parameter: see .
REQ18: Specify the acceptable values of the 'gkty' parameter. For each of them, register a corresponding entry in the "ACE Groupcomm Key Types" IANA registry if such an entry does not exist already: Group_PubSub_Keying_Material, see and .
REQ19: Specify and register the application profile identifier: coap_group_pubsub_app (see and ).
REQ20: If used, specify the format and default values of the entries of the CBOR map to include in the 'group_policies' parameter: not applicable.
REQ21: Specify the approaches used to compute and verify the PoP evidence to include in the 'kdc_cred_verify' parameter 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 .
REQ22: Specify the communication protocol that members of the group use to communicate with each other (e.g., CoAP for group communication): CoAP , used for Pub-Sub communications as defined in .
REQ23: Specify the security protocol that members of the group use to protect the group communication (e.g., Group OSCORE). This must provide encryption, integrity, and replay protection: Publishers in a group use a symmetric group key to protect published topic data as a COSE_Encrypt0 object, per the AEAD algorithm specified by the KDC. A Publisher also produces a COSE countersignature of the COSE_Encrypt0 object by using its private key, per the signature algorithm specified by the KDC.
REQ24: Specify how the communication is secured between the Client and the KDC. Optionally, specify a transport profile of ACE to use between the Client and the KDC: ACE transport profile such as for DTLS or OSCORE .
REQ25: Specify the format of the node identifiers of group members: the Sender ID defined in .
REQ26: Specify policies at the KDC to handle node identifiers that are included in the 'get_creds' parameter but are not associated with any current group member: see .
REQ27: Specify the format of (newly generated) individual keying material for group members or of the information to derive such keying material, as well as the corresponding CBOR map key that has to be registered in the "ACE Groupcomm Parameters" registry: see .
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 and .
REQ29: Categorize newly defined parameters according to the same criteria of : a Publisher Client MUST support 'group_SenderId' in 'key'; see .
REQ30: Define whether Clients must, should, or may support the conditional parameters defined in and under which circumstances: a Publisher Client MUST support the client_cred' and 'client_cred_verify' parameters (see ). A Publisher Client that provides the access token to the KDC through the /authz-info endpoint MUST support the parameter 'kdcchallenge' (see ).

Optional-to-Address Requirements

OPT1: Optionally, if the textual format of scope is used, specify CBOR values to use for abbreviating the role identifiers in the group: not applicable.
OPT2: Optionally, specify the additional parameters used in the exchange of Token Transfer Request and Response: none are defined.
OPT3: Optionally, specify the negotiation of parameter values for signature algorithm and signature keys, if the 'sign_info' parameter is not used: see .
OPT4: Optionally, specify possible or required payload formats for specific error cases: see .
OPT5: Optionally, specify additional identifiers of error types as values of the 'error-id' field within the Custom Problem Detail entry 'ace-groupcomm-error': none are defined.
OPT6: Optionally, specify the encoding of the 'creds_repo' parameter if the default one is not used: no encoding is defined.
OPT7: Optionally, specify the functionalities implemented at the resource hosted by the Client at the URI indicated in the 'control_uri' parameter, including the encoding of exchanged messages and other details: no functionalities are defined.
OPT8: Optionally, specify the behavior of the POST handler of group-membership resources, for the case when it fails to retrieve an authentication credential for the specific Client: The KDC MUST reply with a 4.00 (Bad Request) error response to the Join Request (see ).
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 .
OPT10: Optionally, specify the functionalities implemented at the resource hosted by the Client at the URI indicated in the 'control_group_uri' parameter, including the encoding of exchanged messages and other details: no functionalities are defined.
OPT11: Optionally, specify policies that instruct Clients to retain messages and for how long, if those are unsuccessfully decrypted: no such policies are specified.
OPT12: Optionally, specify for the KDC to perform a group rekeying when receiving a Key Renewal Request, together with or instead of renewing individual keying material: the KDC SHOULD perform a group rekeying if one is already scheduled to occur within an acceptably short time frame, otherwise it SHOULD NOT (see ).
OPT13: Optionally, specify how the identifier of a group member's authentication credential is included in requests sent to other group members: no such method is defined.
OPT14: Optionally, specify additional information to include in rekeying messages for the "Point-to-Point" group rekeying scheme (see ): no additional information is defined.

Document Updates

Version -04 to -05

Alignment with draft-ietf-core-coap-pubsub-21:
- Generalized response code in successful responses from the Broker.
- Content-Format of certain messages to/from the Broker.
- Referred section numbers.
IANA registration request for the CoAP Pubsub Topic Properties 'kdc-uri' and 'sec-gp'.
Editorial fixes.

Version -03 to -04

Minor fixes in the CDDL grammar of the scope entry.
Explicitly mentioned that topic-data resources are hosted at the Broker.
More consistent use of the terms "nonce" and "challenge".
Moved up the statement of compliance with REQ7.
Clarified that group names are consistent with the semantics of URI path segments.
Added references to IANA registries.
Editorial fixes.

Version -02 to -03

Updated introduction: clarified relationships between this document and related documents.
Ensured consistency with RFC 9594 when using an optimized Join Request for re-joining a group (presence of the 'client_cred' parameter).
Added the "Operational Considerations" section.
Clarifications:
- Secure communications required as per the transport profile of ACE used.
- Explicitly mentioned the rationale for computing a proof-of-possession (PoP) evidence.
- Reasons for and flexibility of group rekeying.
Editorial fixes and improvements.

Version -01 to -02

Clarified high-level description of the authorisation flow.
Clarified relation between a group rekeying and a Key Renewal Request.
Editorial fixes and improvements.

Version -00 to -01

Clarified recommendations about randomness and size of nonces.
Clarified generation of N_S if TLS is used with the KDC instead of DTLS.
Added missing references to REQ and OPT requirements.
Updated IANA considerations:
- Suggested values for IANA registrations.
- Renamed TLS Exporter Label.
- Fixed content types in the CoAP Content-Formats registrations.
Updated references.
Editorial fixes and improvements.

Version -00

Imported content from draft-ietf-ace-pubsub-profile-11.
Removed content about MQTT.
New document title.

Acknowledgments The authors wish to thank , , , , , and for the useful discussion and reviews that helped shape this document. This work was supported by the Sweden's Innovation Agency VINNOVA within the EUREKA CELTIC-NEXT projects CRITISEC and CYPRESS; and by the H2020 project SIFIS-Home (Grant agreement 952652).