Pub-Sub Profile for Authentication and Authorization for Constrained Environments (ACE)

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Table of Contents

1. Introduction

The publisher-subscriber setting allows for devices with limited reachability to communicate via a broker that enables store-and-forward messaging between the devices. The pub-sub scenario using the Constrained Application Protocol (CoAP) is specified in [I-D.ietf-core-coap-pubsub], while the one using MQTT is specified in REF MQTT. This document defines a way to authorize nodes in a CoAP pub-sub type of setting, using the ACE framework [I-D.ietf-ace-oauth-authz], and to provide the keys for protecting the communication between these nodes. This document gives detailed specifications for MQTT and CoAP pub-sub, but can easily be adapted for other transport protocol as well.¶

2. Application Profile Overview

The objective of this document is to specify how to authorize nodes, provide keys, and protect a pub-sub communication, using [I-D.ietf-ace-key-groupcomm], which itself expands the Ace framework ([I-D.ietf-ace-oauth-authz]), and transport profiles ([I-D.ietf-ace-dtls-authorize], [I-D.ietf-ace-oscore-profile], REF MQTT profile). The pub-sub communication protocol can be based on CoAP, as described in [I-D.ietf-core-coap-pubsub], MQTT (see REF MQTT comm), or other transport.¶

The architecture of the scenario is shown in Figure 1.¶

             +----------------+   +----------------+
             |                |   |                |
             | Authorization  |   | Authorization  |
             |    Server 1    |   |    Server 2    |
             |                |   |                |
             +----------------+   +----------------+
                      ^                  ^  ^
                      |                  |  |
     +---------(A)----+                  |  +-----(D)------+
     |   +--------------------(B)--------+                 |
     v   v                                                 v
+------------+             +------------+              +------------+
|            |             |            |              |            |
| Publisher  | ----(E)---> |   Broker   |              | Subscriber |
|            |             |            | <----(F)---- |            |
|            |             |            | -----(G)---> |            |
+------------+             +------------+              +------------+

The RS is the broker, which contains the topic. This node corresponds to the Dispatcher, in [I-D.ietf-ace-key-groupcomm]. The AS1 hosts the policies about the Broker: what endpoints are allowed to Publish on the Broker. The Clients access this node to get write access to the Broker. The AS2 hosts the policies about the topic: what endpoints are allowed to access what topic. This node represents both the AS and Key Distribution Center roles from [I-D.ietf-ace-key-groupcomm].¶

There are four phases, the first three can be done in parallel.¶

1. The Publisher requests publishing access to the Broker at the AS1, and communicates with the Broker to set up security.¶
2. The Publisher requests access to a specific topic at the AS2¶
3. The Subscriber requests access to a specific topic at the AS2.¶
4. The Publisher and the Subscriber securely post to and get publications from the Broker.¶

This exchange aims at setting up 2 different security associations: on the one hand, the Publisher has a security association with the Broker, to protect the communication and securely authorize the Publisher to publish on a topic (Security Association 1). On the other hand, the Publisher has a security association with the Subscriber, to protect the publication content itself (Security Association 2). The Security Association 1 is set up using AS1 and a transport profile of [I-D.ietf-ace-oauth-authz], the Security Association 2 is set up using AS2 and [I-D.ietf-ace-key-groupcomm].¶

Note that, analogously to the Publisher, the Subscriber can also set up an additional security association with the Broker, using an AS, in the same way the Publisher does with AS1. In this case, only authorized Subscribers would be able to get notifications from the Broker. The overhead would be that each Subscriber should access the AS and get all the information to start a secure exchange with the Broker.¶

+------------+             +------------+              +------------+
|            |             |            |              |            |
| Publisher  |             |   Broker   |              | Subscriber |
|            |             |            |              |            |
|            |             |            |              |            |
+------------+             +------------+              +------------+
      :   :                       :                           :
      :   '------ Security -------'                           :
      :         Association 1                                 :
      '------------------------------- Security --------------'
                                     Association 2

Note that AS1 and AS2 might either be co-resident or be 2 separate physical entities, in which case access control policies must be exchanged between AS1 and AS2, so that they agree on rights for joining nodes about specific topics. How the policies are exchanged is out of scope for this specification.¶

3. PubSub Application Profiles

Each profile defined in this document uses [I-D.ietf-ace-key-groupcomm], which expands the ACE framework. This section defines which exact parameters from [I-D.ietf-ace-key-groupcomm] have to be used, and the values for each parameter. Since [I-D.ietf-ace-oauth-authz] recommends the use of CoAP anc CBOR, this document describes the exchanges assuming CoAP and CBOR are used. However, using HTTP instead of CoAP is possible, using the corresponding parameters and methods. Analogously, JSON [RFC8259] can be used instead of CBOR, using the conversion method specified in Sections 4.1 and 4.2 of [RFC7049]. In case JSON is used, the Content Format or Media Type of the message has to be changed accordingly.¶

The Publisher and the Subscriber map to the Client in [I-D.ietf-ace-key-groupcomm], the AS2 maps to the AS and to the KDC, the Broker maps to the Dispatcher.¶

Note that both publishers and subscribers use the same profile.¶

   Client                            Broker             AS2
      | [----- Resource Request ---->] |                 |
      |                                |                 |
      | [<-- AS1, AS2 Information ---] |                 |
      |                                                  |
      | [------ Pub Key Format Negociation Request --->] |
      |                                                  |
      | [<---- Pub Key Format Negociation Response ----] |
      |                                                  |
      | -- Authorization + Key Distribution Request ---> |
      |                                                  |
      | <-- Authorization + Key Distribution Response -- |
      |                                                  |

    4.01 Unauthorized
    Content-Format: application/ace+cbor
    {"AS": "coaps://as1.example.com/token,
    coaps://as2.example.com/pubsubkey"}

4. Publisher

In this section, it is specified how the Publisher requests, obtains and communicates to the Broker the access token, as well as the retrieval of the keying material to protect the publication.¶

             +----------------+   +----------------+
             |                |   |                |
             | Authorization  |   | Authorization  |
             |    Server 1    |   |    Server 2    |
             |                |   |                |
             +----------------+   +----------------+
                      ^                  ^
                      |                  |
     +---------(A)----+                  |
     |   +--------------------(B)--------+
     v   v
+------------+             +------------+
|            | ----(C)---> |            |
| Publisher  |             |   Broker   |
|            |             |            |
|            |             |            |
+------------+             +------------+

This is a combination of two independent phases:¶

• one is the establishment of a secure connection between Publisher and Broker, using an ACE transport profile such as DTLS [I-D.ietf-ace-dtls-authorize], OSCORE [I-D.ietf-ace-oscore-profile] or REF MQTT Profile. (A)(C)¶
• the other is the Publisher's retrieval of keying material to protect the publication. (B)¶

In detail:¶

(A) corresponds to the Access Token Request and Response between Publisher and Authorization Server to retrieve the Access Token and RS (Broker) Information. As specified, the Publisher has the role of a CoAP client, the Broker has the role of the CoAP server.¶

(C) corresponds to the exchange between Publisher and Broker, where the Publisher sends its access token to the Broker and establishes a secure connection with the Broker. Depending on the Information received in (A), this can be for example DTLS handshake, or other protocols. Depending on the application, there may not be the need for this set up phase: for example, if OSCORE is used directly. Note that, in line with what defined in the ACE transport profile used, the access token includes the scope (i.e. pubsub topics on the Broker) the Publisher is allowed to publish to. For implementation semplicity, it is RECOMMENDED that the ACE transport profile used and this specification use the same format of "scope".¶

(A) and (C) details are specified in the profile used.¶

(B) corresponds to the retrieval of the keying material to protect the publication end-to-end with the subscribers (see Section 6.1), and uses [I-D.ietf-ace-key-groupcomm]. The details are defined in Section 3.1.¶

{
  "scope" : ["Broker1/Temp", "publisher"],
  "client_id" : "publisher1",
  "client_cred" :
    { / COSE_Key /
      / type / 1 : 2, / EC2 /
      / kid / 2 : h'11',
      / alg / 3 : -7, / ECDSA with SHA-256 /
      / crv / -1 : 1 , / P-256 /
      / x / -2 : h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de1
      08de439c08551d',
      / y /-3 : h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e
      9eecd0084d19c',
  "cnonce" : h'd36b581d1eef9c7c,
  "client_cred_verify" : SIG
    }
}

{
  "profile" : "coap_pubsub_app",
  "kty" : "COSE_Key",
  "key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
          -1: h'02e2cc3a9b92855220f255fff1c615bc'}
}

5. Subscriber

In this section, it is specified how the Subscriber retrieves the keying material to protect the publication.¶

                                  +----------------+
                                  |                |
                                  | Authorization  |
                                  |    Server 2    |
                                  |                |
                                  +----------------+
                                            ^
                                            |
                                            +-----(D)------+
                                                           |
                                                           v
                                                       +------------+
                                                       |            |
                                                       | Subscriber |
                                                       |            |
                                                       |            |
                                                       +------------+

Step (D) between Subscriber and AS2 corresponds to the retrieval of the keying material to verify the publication end-to-end with the publishers (see Section 6.1). The details are defined in Section 3.1¶

This step is the same as (B) between Publisher and AS2 (Section 3.1), with the following differences:¶

• The Authorization + Joining Request MUST NOT contain the 'client_cred parameter', the role element in the 'scope' parameter MUST be set to "subscriber". The Subscriber MUST have access to the public keys of all the Publishers; this MAY be achieved in the Authorization + Joining Request by using the parameter 'get_pub_keys' set to empty array.¶
• The Authorization + Key Distribution Response MUST contain the 'pub_keys' parameter.¶

{
  "scope" : ["Broker1/Temp", "subscriber"],
  "get_pub_keys" : [ ]
}

{
  "profile" : "coap_pubsub_app",
  "scope" : ["Broker1/Temp", "subscriber"],
  "kty" : "COSE_Key"
  "key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
          -1: h'02e2cc3a9b92855220f255fff1c615bc'},
  "pub_keys" : [
   {
      1 : 2, / type EC2 /
      2 : h'11', / kid /
      3 : -7, / alg ECDSA with SHA-256 /
      -1 : 1 , / crv P-256 /
      -2 : h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de43
      9c08551d', / x /
      -3 : h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd
      0084d19c' / y /
    }
  ]
}

6. Pub-Sub Protected Communication

This section specifies the communication Publisher-Broker and Subscriber-Broker, after the previous phases have taken place. The operations of publishing and subscribing are defined in [I-D.ietf-core-coap-pubsub].¶

+------------+             +------------+              +------------+
|            |             |            |              |            |
| Publisher  | ----(E)---> |   Broker   |              | Subscriber |
|            |             |            | <----(F)---- |            |
|            |             |            | -----(G)---> |            |
+------------+             +------------+              +------------+

The (E) message corresponds to the publication of a topic on the Broker. The publication (the resource representation) is protected with COSE ([RFC8152]). The (F) message is the subscription of the Subscriber, which is unprotected, unless a profile of ACE [I-D.ietf-ace-oauth-authz] is used between Subscriber and Broker. The (G) message is the response from the Broker, where the publication is protected with COSE.¶

The flow graph is presented below.¶

  Publisher                Broker               Subscriber
      | --- PUT /topic ----> |                       |
      |  protected with COSE |                       |
      |                      | <--- GET /topic ----- |
      |                      |                       |
      |                      | ---- response ------> |
      |                      |  protected with COSE  |

16(
  [
    / protected / h'a2010c04421234' / {
        \ alg \ 1:12, \ AES-CCM-64-64-128 \
        \ kid \ 4: h'1234'
      } / ,
    / unprotected / {
      / iv / 5:h'89f52f65a1c580',
      / countersign / 7:[
        / protected / h'a10126' / {
          \ alg \ 1:-7
        } / ,
        / unprotected / {
          / kid / 4:h'11'
        },
        / signature / SIG / 64 bytes signature /
      ]
    },
    / ciphertext / h'8df0a3b62fccff37aa313c8020e971f8aC8d'
  ]
)

7. Security Considerations

In the profile described above, the Publisher and Subscriber use asymmetric crypto, which would make the message exchange quite heavy for small constrained devices. Moreover, all Subscribers must be able to access the public keys of all the Publishers to a specific topic to be able to verify the publications. Such a database could be set up and managed by the same entity having control of the topic, i.e. AS2.¶

An application where it is not critical that only authorized Publishers can publish on a topic may decide not to make use of the asymmetric crypto and only use symmetric encryption/MAC to confidentiality and integrity protect the publication, but this is not recommended since, as a result, any authorized Subscribers with access to the Broker may forge unauthorized publications without being detected. In this symmetric case the Subscribers would only need one symmetric key per topic, and would not need to know any information about the Publishers, that can be anonymous to it and the Broker.¶

Subscribers 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 Broker is only trusted with verifying that the Publisher is authorized to publish, but is not trusted with the publications itself, which it cannot read nor modify. In this setting, caching of publications on the Broker is still allowed.¶

TODO: expand on security and privacy considerations¶

8. IANA Considerations

Appendix A. Requirements on Application Profiles

This section lists the specifications on this profile based on the requirements defined in Appendix A of [I-D.ietf-ace-key-groupcomm]¶

• REQ1: Specify the encoding and value of the identifier of group or topic of 'scope': see Section 3.1).¶
• REQ2: Specify the encoding and value of roles of 'scope': see Section 3.1).¶
• REQ3: Optionally, specify the acceptable values for 'sign_alg': TODO¶
• REQ4: Optionally, specify the acceptable values for 'sign_parameters': TODO¶
• REQ5: Optionally, specify the acceptable values for 'sign_key_parameters': TODO¶
• REQ6: Optionally, specify the acceptable values for 'pub_key_enc': TODO¶
• REQ7: Specify the exact format of the 'key' value: COSE_Key, see Section 3.1.¶
• REQ8: Specify the acceptable values of 'kty' : "COSE_Key", see Section 3.1.¶
• REQ9: Specity the format of the identifiers of group members: TODO¶
• REQ10: Optionally, specify the format and content of 'group_policies' entries: not defined¶
• REQ11: Specify the communication protocol the members of the group must use: CoAP pub/sub.¶
• REQ12: Specify the security protocol the group members must use to protect their communication. This must provide encryption, integrity and replay protection: Object Security of Content using COSE, see Section 6.1.¶
• REQ13: Specify and register the application profile identifier : "coap_pubsub_app", see Section 8.1.¶
• REQ14: Optionally, specify the encoding of public keys, of 'client_cred', and of 'pub_keys' if COSE_Keys are not used: NA.¶
• REQ15: Specify policies at the KDC to handle id that are not included in get_pub_keys: TODO¶
• REQ16: Specify the format and content of 'group_policies': TODO¶
• REQ17: Specify the format of newly-generated individual keying material for group members, or of the information to derive it, and corresponding CBOR label : not defined¶
• REQ18: Specify how the communication is secured between Client and KDC. Optionally, specify tranport profile of ACE [I-D.ietf-ace-oauth-authz] to use between Client and KDC: pre-set, as KDC is AS.¶
• OPT1: Optionally, specify the encoding of public keys, of 'client_cred', and of 'pub_keys' if COSE_Keys are not used: NA¶
• OPT2: Optionally, specify the negotiation of parameter values for signature algorithm and signature keys, if 'sign_info' and 'pub_key_enc' are not used: NA¶
• OPT3: Optionally, specify the format and content of 'mgt_key_material': not defined¶
• OPT4: Optionally, specify policies that instruct clients to retain unsuccessfully decrypted messages and for how long, so that they can be decrypted after getting updated keying material: not defined¶

Acknowledgments

The author wishes to thank Ari Keraenen, John Mattsson, Ludwig Seitz, Goeran Selander, Cigdem Sengul, Jim Schaad and Marco Tiloca for the useful discussion and reviews that helped shape this document.¶

Authors' Addresses