]> 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 for a key owned by the Client and bound to an OAuth 2.0 Access Token. This document specifies the provisioning and enforcement of authorization information for Clients to act as Publishers and/or Subscribers, as well as the provisioning of keying material and security parameters that Clients use for protecting their communications end-to-end through the Broker. Note to RFC Editor: Please replace "[draft-ietf-core-coap-pubsub]" with the RFC number of that document and delete this paragraph. Discussion Venues 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 communicate via a Broker, which enables store-and-forward messaging between these devices. This effectively enables a form of group communication, where all the Publishers and Subscribers participating in the same pub/sub topic are considered members of the same group associated with that topic. With a focus on the pub/sub architecture defined in for the Constrained Application Protocol (CoAP) , this document defines an application profile of the Authentication and Authorization for Constrained Environments (ACE) framework , which enables pub/sub communication where a group of Publishers and Subscribers securely communicate through a Broker using CoAP. Building on the message formats and processing defined in , this document specifies the provisioning and enforcement of authorization information for Clients to act as Publishers and/or Subscribers at the Broker, as well as the provisioning of keying material and security parameters that Clients use for protecting end-to-end their communications via the Broker. In order to protect the pub/sub operations at the Broker as well as the provisioning of keying material and security parameters, this profile relies on protocol-specific transport profiles of ACE (e.g., , , or ) to achieve communication security, server authentication, and proof-of-possession for a key owned by the Client and bound to an OAuth 2.0 Access Token. The content of published messages that are circulated by the Broker is protected end-to-end between the corresponding Publisher and the intended Subscribers. To this end, this profile relies on COSE and on keying material provided to the Publishers and Subscribers participating in the same pub/sub topic. In particular, source authentication of published content is achieved by means of the corresponding Publisher signing such content with its own private key. While this profile focuses on the pub/sub architecture for CoAP, this document also describes how it can be applicable to MQTT . Similar adaptations can also extend to further transport protocols and pub/sub architectures.

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 , and the Authorization Information Format (AIF) to express authorization information. In particular, analogously to , terminology for entities in the architecture such as Client (C), Resource Server (RS), and Authorization Server (AS) is defined in OAuth 2.0 .
• 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 . Unless otherwise indicated, the term "endpoint" is used here following its OAuth definition, aimed at denoting resources such as /token and /introspect at the AS, and /authz-info at the RS. This document does not use the CoAP definition of "endpoint", which is "An entity participating in the CoAP protocol".
• The terms and concepts of pub/sub group communication with CoAP, as described in .

A party interested in participating in group communication as well as already participating as a group member is interchangeably denoted as "Client", "pub/sub client", or "node".

• Group: a set of nodes that share common keying material and security parameters to protect their communications with one another. That is, the term refers to a "security group". This is not to be confused with an "application group", which has relevance at the application level and whose members are in this case the Clients acting as Publishers and/or Subscribers for a topic.

Examples throughout this document are expressed in CBOR diagnostic notation without the tag and value abbreviations.

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 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 and obtain 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 pub/sub communication protocol for their interaction with the Broker. When using the profile defined in this document, this protocol MUST be CoAP, which is used as described in . What is specified in this document can apply to other pub/sub protocols such as MQTT , or to further transport protocols. All Publishers and Subscribers MUST use CoAP when communicating with the KDC. Furthermore, both Publishers and Subscribers MUST use the same transport profile of ACE (e.g., for DTLS; or 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. All communications between the involved entities MUST be secured. The Client and the Broker MUST have a secure communication association, which they establish with the help of the AS and using a transport profile of ACE. This is shown by the interactions A and C in . During this process, the Client obtains an Access Token from the AS and uploads it to the Broker, thus providing an evidence of the topics that it is authorized to participate in, and with which permissions. The Client and the KDC MUST have a secure communication association, which they also establish with the help of the AS and using a transport profile of ACE. This is shown by the interactions A and B in . During this process, the Client obtains an Access Token from the AS and uploads it to the KDC, thus providing an evidence of the security groups that it can join, as associated with the topics of interest at the Broker. Based on the permissions specified in the Access Token, the Client can request the KDC to join a security group, after which the Client obtains from the KDC the keying material to use for communicating with the other group members. This builds on the process for joining security groups with ACE defined in 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 the ACE RS, verifies that the Client is authorized to perform data operations (i.e., publish, subscribe, read, delete) on a certain set of topics (Security Association 1). As discussed above, this security association is set up with the help of the AS and using a transport profile of ACE, when the Client obtains the Access Token to upload to the Broker. On the other hand, separately for each topic, all the Publisher and Subscribers for that topic have a common, group security association, through which the published content sent through the Broker is protected end-to-end (Security Association 2). As discussed above, this security association is set up and maintained as the different Clients request the KDC to join the security group, upon which they obtain from the KDC the corresponding group keying material to use for protecting end-to-end and verifying the content of their pub/sub group communication.

Security Associations between Publisher, Broker, and Subscriber.

In summary, this profile specifies the following functionalities.

1. A Client obtains the authorization to participate in a pub/sub topic at the Broker with certain permissions. This pertains operations defined in for taking part in pub/sub communication with CoAP.
2. A Client obtains the authorization to join a security group with certain permissions. This allows the Client to obtain from the KDC the group keying material for communicating with other group members, i.e., to protect end-to-end and verify the content published at the Broker on topics associated with the security group.
3. A Client obtains from the KDC the authentication credentials of other group members, and provides or updates the KDC with its authentication credential.
4. A Client leaves the group or is removed from the group by the KDC.
5. The KDC renews and redistributes the group keying material (rekeying), e.g., due to a membership change in the group.

lists the specifications on this application profile of ACE, based on the requirements defined in .

Getting Authorisation to Join a Pub/sub security group (A) 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 sec. group -->]| | | | | | | | | | | |------- Authorisation Request (Audience: KDC) ------->| | |<------ Authorisation Response (Audience: KDC) -------| | | | | | | | | | |--------- Upload of authorisation information ------------->| |<-------- Establishment of secure association ------------->| | | | | | | | | |----- Request to join the security group for the topic ---->| |<-------- Keying material for the security group -----------| | | | | | | | | |--------------- Resource Request --------------->| | | | (Publication/Subscription to the topic) | | | | | | | ]]>

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, using the conversion method specified in Sections and of . In case JSON is used, the Content-Format of the message has to be changed accordingly. Exact definitions of these exchanges are out of scope for this document.

Topic Discovery at the Broker (Optional) The discovery of a topic at the Broker can be performed by discovering the corresponding topic resources hosted at the Broker. For example, the Client can send a lookup request to /.well-known/core at the Broker, specifying as lookup criterion the resource type "core.ps.conf" (see ). 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 Unauthorized Resource Request (see ). An example using 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 authorized to operate. In particular the Client is authorized to retrieve the representation of a topic resource, from which the Client can retrieve information related to the topic in question, as specified in . This profile extends the set of CoAP Pub/sub Parameters that is possible to specify within the representation of a topic resource, as originally defined in . In particular, this profile defines the following two parameters that the Broker can specify in a response from a topic resource (see ). Note that, when these parameters are transported in their respective fields of the message payload, the Content-Format application/core-pubsub+cbor defined in MUST be used.

• 'kdc_uri', with value the URI of the group membership resource at KDC, where Clients can send a request to join the security group associated with the topic in question. The URI is encoded as a CBOR text string. Clients will have to obtain an Access Token from the AS to upload to the KDC, before starting the process to join the security group at the KDC.
• 'sec_gp', specifying the name of the security group associated with the topic in question, as a stable and invariant identifier. The name of the security group is encoded as a CBOR text string.

Furthermore, the Resource Type (rt=) Link Target Attribute value "core.ps.gm" is registered in (REQ10), and can be used to describe group-membership resources at KDC, e.g., by using a 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 name of the topics that the Client wishes to access, together with the corresponding permissions. If the audience is the KDC, the scope specifies the name of the security groups that the Client wishes to join, together with the corresponding permissions. This parameter is encoded as a CBOR byte string, whose value is the binary encoding of a CBOR array. The format MUST follow the data model AIF-PUBSUB-GROUPCOMM defined in .
• '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 item specifying the name of the groups pertaining to the scope entry. The permission set ("Tperm") is specialized as a CBOR unsigned integer with value R, specifying the permissions that the Client wishes to have in the groups indicated by "Toid". More specifically, the following applies when, as defined in this document, a scope entry includes as set of permissions for user-related operations performed by a pubsub Client.

• The object identifier ("Toid") is a CBOR text string, specifying the name of one application group (topic) or of the corresponding security group to which the scope entry pertains.
• The permission set ("Tperm") is a CBOR unsigned integer, whose value R specifies the operations that the Client wishes to or has been authorized to perform on the resources at the Broker associated with the application group (topic) indicated by "Toid", or on the resources at the KDC associated with the security group indicated by "Toid" (REQ1). The value R is computed as follows.
  • Each operation (i.e., permission detail) in the permission set is converted into the corresponding numeric identifier X taken from the following set.
    • Admin (0): This operation is reserved for scope entries that express permissions for Administrators of pub/sub groups.
    • AppGroup (1): This operation is signaled as wished/authorized when "Toid" specifies the name of an application group (topic), while it is signaled as not wished/authorized when Toid specifies the name of a security group.
    • 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.
  • The set of N numeric identifiers is converted into the single value R, by taking two to the power of each numeric identifier X_1, X_2, ..., X_N, and then computing the inclusive OR of the binary representations of all the power values.
  Since this application profile considers user-related operations, the "Admin" operation is signaled as not wished/authorized. That is, the scope entries MUST have the least significant bit of "Tperm" set to 0.

If the "Toid" of a scope entry in an access token specifies the name of an application group (i.e., the "AppGroup" operation is signaled as authorized), the Client has also the permission to retrieve the configuration of the application group (topic) whose name is indicated by "Toid", by sending a GET or FETCH request to the corresponding topic resource at the Broker. The specific interactions between the Client and the Broker are defined in . 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] ]]>

Authorisation response The AS responds with an Authorization Response to each request, containing claims, as defined in and with the following additions:

• The AS MUST include the 'expires_in' parameter. Other means for the AS to specify the lifetime of Access Tokens are out of the scope of this document.
• The AS MUST include the 'scope' parameter, when the value included in the Access Token differs from the one specified by the Client in the Authorization Request. In such a case, the second element of each scope entry specifies the set of interactions that the Client is authorized for that scope entry, encoded as specified in .

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 KDC The Client transfers its access token to the KDC using one of the methods defined in the Section 3.3 of . 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 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. 'kdcchallange' is a challenge N_S generated by the KDC, and is RECOMMENDED to be an 8-byte long random nonce. Later when joining the group, the Publisher can use the 'kdcchallenge' 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'. If 'sign_info' is included in the Token Transfer Request, the KDC SHOULD include the 'sign_info' parameter in the Token Transfer Response. Note that the joining node may have obtained such information by alternative means e.g., the 'sign_info' may have been pre-configured (OPT3). The following applies for each element 'sign_info_entry'.

• 'sign_alg' MUST take its value from the "Value" column of one of the recommended algorithms in the "COSE Algorithms" registry (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.

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 . Each resource is marked as REQUIRED or OPTIONAL to be hosted at the KDC. 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). PUT (Publishers).
/ace-group/GROUPNAME/nodes/NODENAME/cred	REQUIRED. Authentication credential for NODENAME in the group GROUPNAME.	POST (Publishers)
/ace-group/GROUPNAME/kdc-cred	REQUIRED if a group re-keying mechanism is used. 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. 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 .

Joining a Security Group This section describes the interactions between a Client and the KDC to join a security group. Source authentication of a message sent within the group is ensured by means of a digital signature embedded in the message. Subscribers must be able to retrieve Publishers' authentication credentials from a trusted repository, to verify source authentication of received messages. Hence, on joining a security group, a Publisher is expected to provide its own authentication credential to the KDC. On a successful join, the Clients receive from the KDC the symmetric COSE Key used as shared group key to protect the payload of a published topic data. The message exchange between the joining node and the KDC follows what is defined in and only additions or modifications to that specification are defined in this document.

Join Flow | | | |<-----Join Response (CoAP) ------| ]]>

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 specify the group that the Client is attempting to join, i.e., the group name, and the permissions it wishes to have in the group. This value corresponds to one scope entry, as defined in .
• 'get_creds': It MAY be present if the Client wishes to join as a Subcriber and wants to retrieve the public keys of all the Publishers upon joining. Otherwise, this parameter MUST NOT be present. If the parameter is present, the parameter MUST encode the CBOR simple value null (0xf6). Note that the parameter 'role_filter' is not necessary, as the KDC returns the authentication credentials of Publishers by default.
• 'client_cred': The use of this parameter is detailed in .
• 'cnonce': It specifies a dedicated nonce N_C generated by the Client. It is RECOMMENDED to use use an 8-byte long random nonce. Join Requests MUST include a new 'cnonce' at each join attempt.
• 'client_cred_verify': The use of this parameter is detailed in .

As a Publisher Client has its own authentication credential to use in a group, it MUST support the client_cred', 'cnonce', and 'client_cred_verify' parameters.

Client Credentials in 'client_cred' One of the following cases can occur when a new Client attempts to join a security group.

• The joining node is not a Publisher, i.e., it is not going to send data to the application group. In this case, the joining node is not required to provide its own authentication credential to the KDC. In case the joining node still provides an authentication credential in the 'client_cred' parameter of the Join Request (see ), the KDC silently ignores that parameter, as well as the related parameter 'client_cred_verify'.
• The joining node wishes to join as a Publisher, and:
  • The KDC already acquired the authentication credential of the joining node either during a past group joining process, or when establishing a secure communication association using asymmetric proof-of-possession keys. If the joining node's proof-of-possession key is compatible with the signature algorithm used in the security group and with possible associated parameters, then the corresponding authentication credential can be used in the group. In this case, the joining node MAY choose not to provide again its authentication credential to the KDC in order to limit the size of the Join Request.
  • The KDC has not acquired an authentication credential. Then, the joining node MUST provide a compatible authentication credential in the 'client_cred' parameter of the Join Request (see ).

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

Proof-of-Possession The 'client_cred_verify' parameter contains the proof-of-possession evidence, and is computed as defined below (REQ14). The Publisher signs the scope, concatenated with N_S and concatenated with N_C, using the private key corresponding to the public key in the 'client_cred' parameter. The N_S may be either:

• The challenge received from the KDC in the 'kdcchallenge' parameter of the 2.01 (Created) response to the Token Transfer Request (see ).
• 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-group-pubsub-app" defined in of this document.
• 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-group-pubsub-app" defined in of this document.
• If the Join Request is a retry in response to an error response from the KDC, which included a new 'kdcchallenge' parameter, then N_S MUST be the new value from this parameter.

It is up to applications to define how N_S is computed in further alternative settings.

Join Response On receiving the Join Request, the KDC processes the request as defined in , and returns a success or error response. If the 'client_cred' parameter is present, the KDC verifies the signature in the 'client_cred_verify' parameter. As PoP input, the KDC uses the value of the 'scope' parameter from the Join Request as a CBOR byte string, concatenated with N_S encoded as a CBOR byte string, concatenated with N_C encoded as a CBOR byte string. As public key of the joining node, the KDC uses either the one included in the authentication credential retrieved from the 'client_cred' parameter of the Join Request or the one from the already stored authentication credential from previous interactions with the joining node. The KDC verifies the PoP evidence, which is a signature, by using the public key of the joining node, as well as the signature algorithm used in the group and possible corresponding parameters. In the case of any join request error, the KDC and the Client attempting to join follow the procedure defined in . In case of success, the KDC responds with a Join Response, whose payload formatted as a CBOR map MUST contain the following fields as per :

• 'gkty': the key type "Group_PubSub_Keying_Material" (REQ18) registered in for the 'key' parameter.
• 'key': The keying material for group communication includes the following parameters (REQ17):
  • 'cred_fmt', specifying the format of authentication credentials used in the group. This parameter takes its value from the "Label" column of the "COSE Header Parameters" registry . 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', specifying the Signature Algorithm used to sign messages in the group. This parameter takes values from the "Value" column of the "COSE Algorithms" registry .
  • 'sign_params', specifying the parameters of the Signature Algorithm. This parameter is a CBOR array, which includes 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 .
  • 'group_key', specifying 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.
    • '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.
  • 'group_SenderId', specifying the Client's Sender ID encoded as a CBOR byte string. This field MUST be included if the Client is joining the security group as a Publisher, and MUST NOT be included otherwise. A Publisher Client MUST support the 'group_SenderId' parameter (REQ29). The Sender ID MUST be unique within the security group. The KDC MUST only assign an available Sender ID that has not been used in the security group since the last time when the current Gid value was assigned to the group (i.e., since the latest group rekeying, see ). The KDC MUST NOT assign a Sender ID to the joining node if the node is not joining the group as a Publisher. The Sender ID can be short in length. Its maximum length in bytes is the length in bytes of the AEAD nonce for the AEAD algorithm, minus 6. This means that, when using AES-CCM-16-64-128 as AEAD algorithm in the security group, the maximum length of Sender IDs is 7 bytes.
• 'num', specifying the version number of the keying material specified in the 'key' field. The initial value of the version number MUST be set to 0 upon creating the group (REQ16).
• 'exi', which MUST be present.
• 'ace-groupcomm-profile', which MUST be present and has value "coap_group_pubsub_app" (PROFILE_TBD), which is registered in (REQ19).
• 'creds', which MUST be present if the 'get_creds' parameter was present. Otherwise, it MUST NOT be present. The KDC provides the authentication credentials of all the Publishers in the security group.
• 'peer_roles', which MAY be omitted even if 'creds' is present, since each authentication credential conveyed in the 'creds' parameter: i) is associated with a Client authorized to be Publisher in the group; and ii) plays no role if such Client was also authorized to be Subscriber in the group. If 'creds' is not present, 'peer_roles' MUST NOT be present.
• 'peer_identifiers', which MUST be present if 'creds' is also present. Otherwise, it MUST NOT be present. The identifiers are the Publisher Sender IDs whose authentication credential is specified in the 'creds' parameter (REQ25).
• 'kdc_cred', which MUST be present if group re-keying is used, and is encoded as a CBOR byte string, with value the original binary representation of the KDC's authentication credential (REQ8).
• 'kdc_nonce', which MUST be present if 'kdc_cred' is present, and is encoded as a CBOR byte string, and including a dedicated nonce N_KDC generated by the KDC. For N_KDC, it is RECOMMENDED to use an 8-byte long random nonce.
• 'kdc_cred_verify', which MUST be present if 'kdc_cred' is present, and is encoded as a CBOR byte string. The KDC MUST compute the specified PoP evidence as a signature by using the signature algorithm used in the group, as well as its own private key associated with the authentication credential specified in the 'kdc_cred' parameter (REQ21).
• 'group_rekeying', which MAY be omitted if the KDC uses the "Point-to-Point" group rekeying scheme registered in as the default rekeying scheme in the group (OPT9). In any other case, the 'group_rekeying' parameter MUST be included.

After sending a successful Join Response, the KDC adds the Client to the list of current members of the security group, if that Client is not already a group member. Also, the Client is assigned a name NODENAME and a sub-resource /ace-group/GROUPNAME/nodes/NODENAME. Furthermore, the KDC associates NODENAME with the Client's access token and with the secure communication association that the KDC has with the Client. If the Client is a Publisher, its authentication credential is also associated with the tuple containing NODENAME, GROUPNAME, the current Gid, the newly assigned Publisher's Sender ID, and the Client's access token. The KDC MUST keep this association updated over time. Note that, as long as the secure communication association between the Client and the KDC persists, the same Client re-joining the group is recognized by the KDC by virtue of such a secure communication association. As a consequence, the re-joining Client keeps the same NODENAME and the associated subresource /ace-group/GROUPNAME/nodes/NODENAME. Also, if the Client is a Publisher, it receives a new Sender ID according to the same criteria defined above. If the application requires backward security, the KDC MUST generate updated security parameters and group keying material, and provide it to the current group members upon the new node's joining (see ). In such a case, the joining node is not able to access secure communication in the pub/sub group prior its joining. Upon receiving the Join Response, the joining node retrieves the KDC's authentication credential from the 'kdc_cred' parameter. The joining node MUST verify the proof-of-possession (PoP) evidence, which is a signature, specified in the 'kdc_cred_verify' parameter of the Join Response (REQ21).

Join Error Handling The KDC MUST reply with a 4.00 (Bad Request) error response (OPT4) to the Join Request in the following cases:

• The 'client_cred' parameter is present in the Join Request and its value is not an eligible authentication credential (e.g., it is not of the format accepted in the group) (OPT8).
• The 'client_cred' parameter is present but does not include the 'cnonce' and 'client_cred_verify' parameters.
• The 'client_cred' parameter is not present while the joining node is not going to join the group exclusively as a Subscriber, and any of the following conditions holds:
  • The KDC does not store an eligible authentication credential (e.g., of the format accepted in the group) for the joining node.
  • The KDC stores multiple eligible authentication credentials (e.g., of the format accepted in the group) for the joining node.
• The 'scope' parameter is not present in the Join Request, or it is present and specifies any set of permissions not included in the list defined in .

A 4.00 (Bad Request) error response from the KDC to the joining node MAY have content format application/ace-groupcomm+cbor and contain a CBOR map as payload. The CBOR map MAY include the 'kdcchallenge' parameter. If present, this parameter is a CBOR byte string, which encodes a newly generated 'kdcchallenge' value that the Client can use when preparing a new Join Request. In such a case, the KDC MUST store the newly generated value as the 'kdcchallenge' value associated with the joining node, which replaces the currently stored value (if any). Upon receiving the Join Response, if 'kdc_cred' is present but the Client cannot verify the PoP evidence, the Client MUST stop processing the Join Response and MAY send a new Join Request to the KDC. The KDC MUST return a 5.03 (Service Unavailable) response to a Client that sends a Join Request to join the security group as Publisher, in case there are currently no Sender IDs available to assign.

Other Group Operations through the KDC

Querying for Group Information

• '/ace-group': All Clients can send a FETCH request to retrieve a set of group names associated with their group identifiers specified in the request payload. Each element of the CBOR array 'gid' is a CBOR byte string (REQ13), which encodes the Gid of the group (see ) for which the group name and the URI to the group-membership resource are provided.
• '/ace-group/GROUPNAME': All Clients can use GET requests to retrieve the symmetric group keying material of the group with the name GROUPNAME. The value of the GROUPNAME URI path and the group name in the access token scope ('gname') MUST coincide.
• '/ace-group/GROUPNAME/creds': The KDC acts as a repository of authentication credentials for the Publishers that are member of the security group with name GROUPNAME. The members of the group that are Subscribers can send GET/FETCH requests to this resource in order to retrieve the authentication credentials of all or a subset of the group members that are Publishers. The KDC silently ignores the Sender IDs included in the 'get_creds' parameter of the request that are not associated with any current Publisher group member (REQ26). The response from the KDC MAY omit the parameter 'peer_roles', since each authentication credential conveyed in the 'creds' parameter: i) is associated with a Client authorized to be Publisher in the group; and ii) plays no role if such Client was also authorized to be Subscriber in the group. If 'creds' is not present, 'peer_roles' MUST NOT be present.
• '/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.

Updating Authentication Credentials A Publisher with node name NODENAME can contact the KDC to upload a new authentication credential to use in the security group with name GROUPNAME, and replace the currently stored one. To this end, it sends a CoAP POST request to its associated sub-resource /ace-group/GROUPNAME/nodes/NODENAME/cred. The KDC replaces the stored authentication credential of this Client for the group GROUPNAME with the one specified in the request.

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

Rekeying a Group The KDC MUST trigger a group rekeying as described in due to a change in the group membership or the current group keying material approaching its expiration time. The KDC MAY trigger regularly scheduled update of the group keying material. Upon generating the new group keying material and before starting its distribution, the KDC MUST increment the version number of the group keying material. The KDC MUST preserve the current value of the Sender ID of each member in that group. The KDC MUST also generate a new Group Identifier (Gid) for the group, and use it as identifier of the new group key provided through the group rekeying and hereafter to Clients (re-)joining the security group (see ). The default rekeying scheme is Point-to-Point (see ), where the KDC individually targets each node to rekey, using the pairwise secure communication association with that node. If the group rekeying is performed due to one or multiple Clients joining the group as Publishers, then a rekeying message includes the Sender IDs and authentication credentials that those clients are going to use in the group. This information is specified by means of the parameters 'creds' and 'peer_identifiers', like done in the Join Response message. In the same spirit, the 'peer_roles' parameter MAY be omitted.

PubSub Protected Communication (C)

Secure communication between Publisher and Subscriber | Broker | | Subscriber | | | | | <----(E)---- | | | | | | -----(F)---> | | +------------+ +------------+ +------------+ ]]>

(D) corresponds to the publication of a topic on the Broker, using a CoAP PUT. The publication (the resource representation) is protected with COSE () by the Publisher. The (E) message is the subscription of the Subscriber, and uses a CoAP GET with the Observe option set to 0 (zero) , as per . The (F) message is the response from the Broker, where the publication is protected with COSE by the Publisher. (ToDo: Add Delete to the flow?)

Example of protected communication for CoAP | | | protected with COSE | | | | <--- GET /topic ----- | | | Observe:0 | | | | | | ---- Response ------> | | | protected with COSE | ]]>

Using COSE Objects To Protect The Resource Representation The Publisher uses the symmetric COSE Key received from the KDC to protect the payload of the Publish operation (see ). Specifically, the Publisher creates a COSE_Encrypt0 object by means of the COSE Key currently used as group key. The encryption algorithm and Base IV to use are specified by the 'alg' and 'Base IV' parameters of the COSE Key, together with its key identifier in the 'kid' parameter. Also, the Publisher uses its private key corresponding to the public key sent to the KDC for countersigning the COSE_Encrypt0 object as specified in . The countersignature is specified in the 'Countersignature version 2' parameter, within the 'unprotected' field of the COSE_Encrypt0 object. Finally, the Publisher sends the COSE_Encrypt0 object conveying the countersignature to the Broker, as payload of the PUT request sent to the topic-data of the topic targeted by the Publish operation. Upon receiving a response from the topic-data resource at the Broker, the Subscriber uses the 'kid' parameter from the 'Countersignature version 2' parameter within the 'unprotected' field of the COSE_Encrypt0 object, in order to retrieve the Publisher's public key from the Broker or from its local storage. Then, the Subscriber uses that public key to verify the countersignature. In case of successful verification, the Subscriber uses the 'kid' parameter from the 'unprotected' field of the COSE_Encrypt0 object, in order to retrieve the COSE Key used as current group key from its local storage. Then, the Subscriber uses that group key to verify and decrypt the COSE_Encrypt0 object. In case of successful verification, the Subscriber delivers the received topic data to the application. The COSE_Encrypt0 object is constructed as follows. The 'protected' field MUST include:

• The 'alg' parameter, with value the identifier of the AEAD algorithm specified in the 'alg' parameter of the COSE Key used as current group key.

The 'unprotected' field MUST include:

• The 'kid' parameter, with the same value specified in the 'kid' parameter of the COSE Key used as current group key. This value represents the current Group ID (Gid) of the security group associated with the application group (topic).
• The 'Partial IV' parameter, with value set to the current Sender Sequence Number of the Publisher. All leading bytes of value zero SHALL be removed when encoding the Partial IV, except in the case of Partial IV value 0, which is encoded to the byte string 0x00. The Publisher MUST initialize the Sender Sequence Number to 0 upon joining the security group, and MUST reset it to 0 upon receiving a new group key as result of a group rekeying (see ). The Publisher MUST increment its Sender Sequence Number value by 1, after having completed an encryption operation by means of the current group key.
• The 'Countersignature version 2' parameter, specifying the countersignature of the COSE_Encrypt0 object. In particular:
  • The 'protected' field includes the 'alg' parameter, with value the identifier of the Signature Algorithm used in the security group.
  • The 'unprotected' field includes the 'kid' parameter, with value the Publisher's Sender ID that the Publisher obtained from the KDC when joining the security group, as value of the 'group_SenderId' parameter of the Join Response (see ).
  • 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 .

1. Left-pad the Partial IV (PIV) with zeroes to exactly 5 bytes.
2. Left-pad the Sender ID of the Publisher that generated the Partial IV (ID_PIV) with zeroes to exactly the nonce length of the AEAD algorithm minus 6 bytes.
3. Concatenate the size of the ID_PIV (a single byte S) with the padded ID_PIV and the padded PIV.
4. XOR the result from the previous step with the Base IV.

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 or greater than 7 bytes. At the same time, it makes it easy to verify that the nonces will be unique when used with the the same group key, even though this is shared and used by all the Publishers in the security group. In fact:

• Since Publisher's Sender IDs are unique within the security group and they are not reassigned until a group rekeying occurs (see 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 nor any two Publishers use the same AEAD nonce with the same group key.

Replay Checks This section defines how a Subscriber Client checks whether the topic data conveyed in a received message from the Broker is a replay. TBD

Applicability to MQTT PubSub Profile The steps MQTT clients go through would be similar to the CoAP clients, and the payload of the MQTT PUBLISH messages will be protected using COSE. The MQTT clients needs to use CoAP to communicate to the KDC, to join security groups, and be part of the pairwise rekeying initiated by the KDC. Authorisation Server (AS) Discovery is defined in for MQTT v5 clients (and not supported for MQTT v3 clients). $SYS/ has been widely adopted as a prefix to topics that contain server-specific information or control APIs, and may be used for topic and KDC discovery. When the Client sends an authorisation request to the AS using the AIF-PUBSUB-GROUPCOMM data model in the authorisation response, the 'profile' claim is set to "mqtt_pubsub_app" as defined in . Both Publishers and Subscribers MUST authorise to the Broker with their respective tokens, as described in . A Publisher sends PUBLISH messages for a given topic and protects the payload with the corresponding key for the associated security group. A Subscriber may send SUBSCRIBE messages with one or multiple topic filters. A topic filter may correspond to multiple topics. The Broker forwards all PUBLISH messages to all authorised Subscribers, including the retained messages.

Security Considerations All the security considerations in apply. In the profile described above, the Publishers and Subscribers use asymmetric cryptography, which would make the message exchange quite heavy for small constrained devices. Moreover, all Subscribers must be able to access the public keys of all the Publishers to a specific topic to verify the publications. Even though access tokens have expiration times, an access token may need to be revoked before its expiration time (see for a list of possible circumstances). Clients can be excluded from future publications through re-keying for a certain topic. This could be set up to happen on a regular basis, for certain applications. How this could be done is out of scope for this work. The method described in MAY be used to allow an Authorization Server to notify the KDC about revoked access tokens. The Broker is only trusted with verifying that the Publisher is authorized to publish, but is not trusted with the publications itself, which it cannot read nor modify. With respect to the reusage of nonces for Proof-of-Possession input, the same considerations apply as in the . TODO: expand on security and privacy considerations

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 Registry IANA is asked to register the following entry in the "ACE Groupcomm Key Types" registry defined in .

• Name: Group_PubSub_Keying_Material
• Key Type Value: GROUPCOMM_KEY_TBD
• Profile: coap_group_pubsub_app, defined in of this document.
• Description: Encoded as described in the of this document.
• References: , , [RFC-XXXX]

ACE Groupcomm Profiles Registry IANA is asked to register the following entries in the "ACE Groupcomm Profiles" registry defined in .

• Name: coap_group_pubsub_app
• Description: Profile for delegating client authentication and authorization for publishers and subscribers in a CoAP pub/sub setting scenario in a constrained environment.
• CBOR Value: TBD
• Reference: [RFC-XXXX]

 

• Name: mqtt_pubsub_app
• Description: Profile for delegating client authentication and authorization for publishers and subscribers in a MQTT pub/sub setting scenario in a constrained environment.
• CBOR Value: TBD
• Reference: [RFC-XXXX]

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.

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: Pub/sub topic name, corresponding to the security group
• Reference: [RFC-XXXX]

 

• Parameter: Tperm
• Name: pubsub-perm
• Description/Specification: Permissions corresponding to the topic data interactions in the pub/sub group
• Reference: [RFC-XXXX]

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: 294 (suggested)
• Reference: [RFC-XXXX]

 

• Content Type: application/aif+json;Toid="pubsub-topic",Tperm="pubsub-perm"
• Content Coding: -
• ID: 295 (suggested)
• Reference: [RFC-XXXX]

TLS Exporter Labels IANA is asked to register the following entry to the "TLS Exporter Labels" registry defined in and updated in .

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

References Normative References IANA IANA IANA 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. 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] 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] 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 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) 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Ericsson AB RISE AB This document defines how to use the Authentication and Authorization for Constrained Environments (ACE) framework to distribute keying material and configuration parameters for secure group communication. Candidate group members acting as Clients and authorized to join a group can do so by interacting with a Key Distribution Center (KDC) acting as Resource Server, from which they obtain the keying material to communicate with other group members. While defining general message formats as well as the interface and operations available at the KDC, this document supports different approaches and protocols for secure group communication. Therefore, details are delegated to separate application profiles of this document, as specialized instances that target a particular group communication approach and define how communications in the group are protected. Compliance requirements for such application profiles are also specified. Informative References 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. 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. 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. 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. This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework to enable authorization in a publish-subscribe messaging system based on Message Queuing Telemetry Transport (MQTT). Proof-of-Possession keys, bound to OAuth 2.0 access tokens, are used to authenticate and authorize MQTT Clients. The protocol relies on TLS for confidentiality and MQTT server (Broker) authentication. Ericsson AB Ericsson AB RISE AB RISE AB Nexus Group 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 all certificates compatible with the RFC 7925, IEEE 802.1AR (DevID), CNSA, RPKI, GSMA eUICC, and CA/Browser Forum Baseline Requirements profiles. When used to re-encode DER encoded X.509 certificates, the CBOR encoding can in many cases reduce the size of RFC 7925 profiled certificates with over 50% while also significantly reducing memory and code size compared to ASN.1. The CBOR encoded structure can alternatively be signed directly ("natively signed"), which does not require re- encoding for the signature to be verified. The document also specifies C509 Certificate Signing Requests, C509 COSE headers, a C509 TLS certificate type, and a C509 file format. Ericsson Ericsson RISE RISE This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Ephemeral Diffie-Hellman Over COSE (EDHOC) for achieving mutual authentication between an OAuth 2.0 Client and Resource Server, and it binds an authentication credential of the Client to an OAuth 2.0 access token. EDHOC also establishes an Object Security for Constrained RESTful Environments (OSCORE) Security Context, which is used to secure communications when accessing protected resources according to the authorization information indicated in the access token. This profile can be used to delegate management of authorization information from a resource-constrained server to a trusted host with less severe limitations regarding processing power and memory. RISE AB Ericsson AB CMU SEI CMU SEI 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 to access a Token Revocation List 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 Resource Servers. RISE AB Universitaet Duisburg-Essen Ericsson AB This document defines an application profile of the ACE framework for Authentication and Authorization, to request and provision keying material in group communication scenarios that are based on CoAP and are secured with Group Object Security for Constrained RESTful Environments (Group OSCORE). This application profile delegates the authentication and authorization of Clients, that 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 for a key owned by the Client and bound to an OAuth 2.0 Access Token.

Requirements on Application Profiles This section lists the specifications on this profile based on the requirements defined in .

• REQ1: Specify the format and encoding of 'scope': see .
• REQ2: If the AIF format of 'scope' is used, register its specific instance of "Toid" and "Tperm" as Media Type parameters and a corresponding Content-Format, as per the guidelines in : see .
• REQ3: If used, specify the acceptable values for 'sign_alg': values from the "Value" column of the "COSE Algorithms" registry .
• REQ4: If used, specify the acceptable values for 'sign_parameters': format and values from the COSE algorithm capabilities as specified in the "COSE Algorithms" registry .
• REQ5: If used, specify the acceptable values for 'sign_key_parameters': its format and value are the same of the COSE capabilities array for the COSE key type of the keys used with the algorithm indicated in 'sign_alg', as specified for that key type in the "Capabilities" column of the "COSE Key Types" registry .
• REQ6: Specify the acceptable formats for authentication credentials and, if used, the acceptable values for 'cred_fmt': acceptable formats explicitly provide the public key as well as the comprehensive set of information related to the public key algorithm. Takes value from the "Label" column of the "COSE Header Parameters" registry .
• REQ7: If the value of the GROUPNAME URI path and the group name in the access token scope (gname) are not required to coincide, specify the mechanism to map the GROUPNAME value in the URI to the group name: not applicable; a perfect matching is required.
• REQ8: Define whether the KDC has an authentication credential and if this has to be provided through the 'kdc_cred' parameter: optional, see of this document.
• REQ9: Specify if any part of the KDC interface as defined in is not supported by the KDC: some are left optional, see of this document.
• REQ10: Register a Resource Type for the group-membership resource, which is used to discover the correct URL for sending a Join Request to the KDC: the Resource Type (rt=) Link Target Attribute value "core.ps.gm" is registered in Section .
• REQ11: Define what specific actions (e.g., CoAP methods) are allowed on each resource provided by the KDC interface, depending on whether the Client is a current group member; the roles that a Client is authorized to take as per the obtained access token; and the roles that the Client has as current group member: see of this document.
• REQ12: Categorize possible newly defined operations for Clients into primary operations expected to be minimally supported and secondary operations, and provide accompanying considerations: none added.
• REQ13: Specify the encoding of group identifier: CBOR byte string, with value used also to identify the current group key used in the security group (see ).
• REQ14: Specify the approaches used to compute and verify the PoP evidence to include in 'client_cred_verify', and which of those approaches is used in which case: see .
• REQ15: Specify how the nonce N_S is generated, if the token is not provided to the KDC through the Token Transfer Request to the authz-info endpoint (e.g., if it is used directly to validate TLS instead): see .
• REQ16: Define the initial value of the 'num' parameter: the initial value MUST be set to 0 (see ).
• REQ17: Specify the format of the 'key' parameter and register a corresponding entry in the "ACE Groupcomm Key Types" IANA registry: see and .
• REQ18: Specify the acceptable values of the 'gkty' parameter: Group_PubSub_Keying_Material, see .
• REQ19: Specify and register the application profile identifier: coap_group_pubsub_app, see and .
• REQ20: If used, specify the format and content of 'group_policies' and its entries. Specify the policies default values: ToDo.
• REQ21: Specify the approaches used to compute and verify the PoP evidence to include in 'kdc_cred_verify', and which of those approaches is used in which case. If external signature verifiers are supported, specify how those provide a nonce to the KDC to be used for computing the PoP evidence: see .
• REQ22: Specify the communication protocol that the members of the group must use: CoAP , and for pub/sub communication .
• REQ23: Specify the security protocol the group members must use to protect their communication. This must provide encryption, integrity and replay protection: a symmetric COSE Key is used to create a COSE_Encrypt0 object with an AEAD algorithm specified by the KDC.
• REQ24: Specify how the communication is secured between Client and KDC. Optionally, specify transport profile of ACE to use between Client and KDC: ACE transport profile such as for DTLS or OSCORE .
• REQ25: Specify the format of the identifiers of group members: the Sender ID defined in .
• REQ26: Specify policies at the KDC to handle ids that are not included in 'get_creds': see .
• REQ27: Specify the format of newly-generated individual keying material for group members, or of the information to derive it, and corresponding CBOR label: not applicable.
• REQ28: Specify which CBOR tag is used for identifying the semantics of binary scopes, or register a new CBOR tag if a suitable one does not exist already: see and of this document.
• 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', 'cnonce', and 'client_cred_verify' parameters; see . A Publisher Client that provides the token to the KDC, through the authz-info endpoint, MUST support the parameter 'kdcchallenge'; see .
• OPT1: Optionally, if the textual format of 'scope' is used, specify CBOR values to use for abbreviating the role identifiers in the group: no.
• OPT2: Optionally, specify the additional parameters used in the exchange of Token Transfer Request and Response: no.
• OPT3: Optionally, specify the negotiation of parameter values for signature algorithm and signature keys, if 'sign_info' is not used: see .
• 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': no.
• OPT6: Optionally, specify the encoding of 'creds_repo' if the default is not used: no.
• OPT7: Optionally, specify the functionalities implemented at the 'control_uri' resource hosted at the Client, including message exchange encoding and other details: no.
• OPT8: Optionally, specify the behavior of the handler in case of failure to retrieve an authentication credential for the specific node: The KDC MUST reply with a 4.00 (Bad Request) error response to the Join Request; see .
• 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 'control_group_uri' resource hosted at the Client, including message exchange encoding and other details: no.
• OPT11: Optionally, specify policies that instruct Clients to retain messages and for how long, if they are unsuccessfully decrypted: no.
• OPT12: Optionally, specify for the KDC to perform group rekeying (together or instead of renewing individual keying material) when receiving a Key Renewal Request: ToDo.
• OPT13: Optionally, specify how the identifier of a group member's authentication credential is included in requests sent to other group members: no.
• OPT14: Optionally, specify additional information to include in rekeying messages for the "Point-to-Point" group rekeying scheme: ToDo.
• OPT15: Optionally, specify if Clients must or should support any of the parameters defined as optional in : no.

Document Updates

Version -08 to -09

• Improved terminology section.
• Generalized scope format for future, admin-related extensions.
• Improved definition of permissions in the format of scope.
• Clarified alternative computing of N_S Challenge when DTLS is used.
• Use of the parameter 'exi' in the Join Response.
• Use of RFC 9290 instead of the custom format of error responses.
• Fixed construction of the COSE_Encrypt0 object.
• Fixed use of the resource type "core.ps.gm".
• Updated formulation of profile requirements.
• Clarification and editorial improvements.

Version -07 to -08

• Revised presentation of the scope format.
• Revised presentation of the Join Request-Response exchange.
• The 'cnonce' parameter must be present in the Join Request.
• The 'kid' of the group key is used as Group Identifier.
• Relaxed inclusion of the 'peer_roles' parameter.
• More detailed description of the encryption and signing operations.
• Defined construction of the AEAD nonce.
• Clarifications and editorial improvements.

Version -06 to -07

• Revised abstract and introduction.
• Clarified use of "application groups".
• Revised use of protocols and transport profiles with Broker and KDC.
• Revised overview of the profile and its security associations.
• Revised presentation of authorization flow.
• Subscribers cannot be anonymous anymore.
• Revised scope definition.
• Revised Join Response.
• Revised COSE countersignature, COSE encrypt objects.
• Further clarifications, fixes and editorial improvements.

Acknowledgments The authors wish to thank , , , , and for the useful discussion and reviews that helped shape this document. The work on this document has been partly supported by the H2020 project SIFIS-Home (Grant agreement 952652).