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Versions: (draft-sengul-ace-mqtt-tls-profile) 00 01 02

ACE Working Group                                              C. Sengul
Internet-Draft                                                   Nominet
Intended status: Standards Track                                A. Kirby
Expires: November 8, 2019                                       Oxbotica
                                                            P. Fremantle
                                                University of Portsmouth
                                                             May 7, 2019


                        MQTT-TLS profile of ACE
                   draft-ietf-ace-mqtt-tls-profile-00

Abstract

   This document specifies a profile for the ACE (Authentication and
   Authorization for Constrained Environments) to enable authorization
   in an MQTT-based publish-subscribe messaging system.  Proof-of-
   possession keys, bound to OAuth2.0 access tokens, are used to
   authenticate and authorize publisher and subscriber clients.  The
   protocol relies on TLS for confidentiality and server authentication.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 8, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  ACE-Related Terminology . . . . . . . . . . . . . . . . .   4
     1.3.  MQTT-Related Terminology  . . . . . . . . . . . . . . . .   4
   2.  Basic Protocol Interactions . . . . . . . . . . . . . . . . .   5
     2.1.  Authorizing Connection Establishment  . . . . . . . . . .   6
       2.1.1.  Client Authorization Server (CAS) and Authorization
               Server (AS) Interaction . . . . . . . . . . . . . . .   7
       2.1.2.  Client Connection Request to the Broker . . . . . . .   8
       2.1.3.  Token Validation  . . . . . . . . . . . . . . . . . .  10
       2.1.4.  The Broker's Response to Client Connection Request  .  11
     2.2.  Authorizing PUBLISH Messages  . . . . . . . . . . . . . .  12
       2.2.1.  PUBLISH Messages from the Publisher Client to the
               Broker  . . . . . . . . . . . . . . . . . . . . . . .  12
       2.2.2.  PUBLISH Messages from the Broker to the Subscriber
               Clients . . . . . . . . . . . . . . . . . . . . . . .  12
     2.3.  Authorizing SUBSCRIBE Messages  . . . . . . . . . . . . .  13
     2.4.  Token Expiration  . . . . . . . . . . . . . . . . . . . .  13
     2.5.  Handling Disconnections and Retained Messages . . . . . .  13
   3.  Improved Protocol Interactions with MQTT v5 . . . . . . . . .  14
     3.1.  Token Transport via Authentication Exchange (AUTH)  . . .  14
     3.2.  Authorization Errors and Client Re-authentication . . . .  16
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  18
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  Checklist for profile requirements . . . . . . . . .  20
   Appendix B.  The Authorization Information Endpoint . . . . . . .  21
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   This document specifies a profile for the ACE framework
   [I-D.ietf-ace-oauth-authz].  In this profile, clients and a resource
   server use MQTT to communicate.  The protocol relies on TLS for
   communication security between entities.  The basic protocol
   interactions follow MQTT v3.1.1 - the OASIS Standard
   [MQTT-OASIS-Standard].  In addition, this document describes



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   improvements to the basic protocol with the new MQTT v5.0 - the OASIS
   Standard [MQTT-OASIS-Standard-v5] (e.g., improved authentication
   exchange and error reporting).  Both versions are expected to be
   supported in practice, and therefore, covered in this document.

   MQTT is a publish-subscribe protocol and supports two main types of
   client operation: publish and subscribe.  Once connected, a client
   can publish to multiple topics, and subscribe to multiple topics;
   however, for this document, these actions are described separately.
   The MQTT broker is responsible for distributing messages published by
   the publishers to the appropriate subscribers.  Each publish message
   contains a topic, which is used by the broker to filter the
   subscribers for the message.  Subscribers must subscribe to the
   topics to receive the corresponding messages.

   In this document, message topics are treated as resources.  Clients
   use an access token, bound to a key (the proof-of-possession key) to
   authorize with the MQTT broker their connection and publish/subscribe
   permissions to topics.  In the context of this ACE profile, the MQTT
   broker acts as the resource server.  To provide communication
   confidentiality and resource server authentication, TLS is used.

   Clients use client authorization servers [I-D.ietf-ace-actors] to
   obtain tokens from the authorization server.  The communication
   protocol between the client authorization server and the
   authorization server is assumed to be HTTPS.  Also, if the broker
   supports token introspection, it is assumed to use HTTPS to
   communicate with the authorization server.  These interfaces MAY be
   implemented using other protocols, e.g., CoAP or MQTT.  This document
   makes the same assumptions as the Section 4 of the ACE framework
   [I-D.ietf-ace-oauth-authz] regarding client and RS registration with
   the AS and establishing of keying material.

   This document describes the authorization of the following exchanges
   between publisher and subscriber clients, and the broker.

   o  Connection establishment between the clients and the broker

   o  Publish messages from the publishers to the broker, and from the
      broker to the subscribers

   o  Subscribe messages from the subscribers to the broker

   In Section 2, these exchanges are described based on the MQTT v3.1.1
   - the OASIS Standard [MQTT-OASIS-Standard].  These exchanges are also
   supported by the new MQTT v5 - the OASIS Standard
   [MQTT-OASIS-Standard-v5].  Section 3 describes how they may be
   improved by the new MQTT v5.



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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174], when, and only when, they appear in all
   capitals, as shown here.

1.2.  ACE-Related Terminology

   The terminology for entities in the architecture is defined in OAuth
   2.0 RFC 6749 [RFC6749] and ACE actors [I-D.ietf-ace-actors], such as
   "Client" (C), "Resource Server" (RS) and "Authorization Server" (AS).

   The term "endpoint" is used following its OAuth definition, to denote
   resources such as /token and /introspect at the AS.

   The term "Resource" is used to refer to an MQTT "topic name," which
   is defined in Section 1.3.  Hence, the "Resource Owner" is any entity
   that can authoritatively speak for the "topic".

   Certain security-related terms such as "authentication",
   "authorization", "confidentiality", "(data) integrity", "message
   authentication code", and "verify" are taken from RFC 4949 [RFC4949].

1.3.  MQTT-Related Terminology

   The document describes message exchanges as MQTT protocol
   interactions.  For additional information, please refer to the MQTT
   v3.1.1 - the OASIS Standard [MQTT-OASIS-Standard] or the MQTT v5 -
   the OASIS Standard [MQTT-OASIS-Standard-v5].

   Topic name
           The label attached to an application message, which is
           matched to a subscription.

   Topic filter
           An expression that indicates interest in one or more topic
           names.  Topic filters may include wildcards.

   Subscription
           A subscription comprises a Topic filter and a maximum quality
           of service (QoS).

   Application Message
           The data carried by the MQTT protocol.  The data has an
           associated QoS level and a Topic name.




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   MQTT sends various control messages across a network connection.  The
   following is not an exhaustive list and the control packets that are
   not relevant for authorization are not explained.  These include, for
   instance, the PUBREL and PUBCOMP packets used in the 4-step handshake
   required for the QoS level 2.

   CONNECT
           Client request to connect to the broker.  After a network
           connection is established, this is the first packet sent by a
           client.

   CONNACK
           The broker connection acknowledgment.  The first packet sent
           from the broker to a client is a CONNACK packet.  CONNACK
           packets contain return codes indicating either a success or
           an error state to a client.

   PUBLISH
           Publish packet that can be sent from a client to the broker,
           or from the broker to a client.

   PUBACK
           Response to PUBLISH packet with QoS level 1.  PUBACK can be
           sent from the broker to a client or a client to the broker.

   PUBREC
           Response to PUBLISH packet with QoS level 2.  PUBREC can be
           sent from the broker to a client or a client to the broker.

   SUBSCRIBE
           The client subscribe request.

   SUBACK
           Subscribe acknowledgment.

   PINGREQ A ping request sent from a client to the broker.  It signals
           to the broker that the client is alive, and is used to
           confirm that the broker is still alive.

2.  Basic Protocol Interactions

   This section describes the following exchanges between publisher and
   subscriber clients, the broker, and the authorization server
   according to the MQTT v3.1.1 - the OASIS Standard
   [MQTT-OASIS-Standard].  These exchanges are compatible also with the
   new MQTT v5 - the OASIS Standard [MQTT-OASIS-Standard-v5].  In
   addition, Section 3 describes how these exchanges may be improved
   with the MQTT v5.



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   o  Authorizing connection establishment between the clients and the
      broker

   o  Authorizing publish messages from the publishers to the broker,
      and from the broker to the subscribers

   o  Authorizing subscribe messages from the subscribers to the broker

   Message topics are treated as resources.  The publisher and
   subscriber clients are assumed to have identified the topics of
   interest out-of-band (topic discovery is not a feature of the MQTT
   protocol).

   A connection request carries a token specifying the permissions that
   the client has (e.g., publish permission to a given topic).  A
   resource owner can pre-configure policies at the AS that give clients
   publish or subscribe permissions to different topics.

2.1.  Authorizing Connection Establishment

   This section specifies how publishers and subscribers establish an
   authorized connection to an MQTT broker.  The token request and
   response use the /token endpoint of the authorization server, as
   specified in Section 5 of the ACE framework
   [I-D.ietf-ace-oauth-authz].

   Figure 1 shows the basic protocol flow during connection
   establishment.  The step (C), client onboarding, is out of the scope
   of this document.  Steps (E) and (F) are optional.






















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                               +----------------+
      +---(A) Token request----| Client         |
      |                        | Authorization  |
      |   +-(B) Access token-->| Server         |
      |   |                    |________________|
      |   |                            |
      |   |                (C) Client On-boarding
      |   |                            |
      |   |                  +---------v-----+
   +--v-------------+        | Publisher or  |
   |                |        | Subscriber    |
   |  Authorization |        |_______________|
   |  Server        |            |       ^
   |________________|            |       |
      |    ^             (D)Connection  (G)Connection
      |    |               request +    response
      |    |               access token  |
      |    |                     |       |
      |    |                 +---v--------------+
      |    |                 |   Broker         |
      |    +(E)Introspection-| Resource Server  |
      |   request (optional) |                  |
      +-(F)Introspection---->|__________________|
        response (optional)

                    Figure 1: Connection establishment

2.1.1.  Client Authorization Server (CAS) and Authorization Server (AS)
        Interaction

   The first step in the protocol flow (Figure 1 (A)) is the token
   acquisition by the client authorization server (CAS) from the AS.  If
   a client has enough resources and can support HTTPS, or optionally
   the AS supports MQTTS, these steps can instead be carried out by a
   client directly.

   When requesting an access token from the AS, the CAS MAY include
   parameters in its request as defined in Section 5.6.1 of the ACE
   framework [I-D.ietf-ace-oauth-authz].  The content type is set to
   "application/json".  The profile parameter is set to 'mqtt_tls'.

   If the AS successfully verifies the access token request and
   authorizes the client for the indicated audience (e.g., RS) and
   scopes (e.g., publish/subscribe permissions over topics), the AS
   issues an access token (Figure 1 (B)).  The response includes the
   parameters described in Section 5.6.2 of the ACE framework
   [I-D.ietf-ace-oauth-authz].  The included token is assumed to be
   Proof-of-Possession (PoP) token by default.  Hence, a 'cnf' parameter



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   with a symmetric or asymmetric PoP key is returned.  The token may be
   a reference, or a CBOR or JWT web token.  Note that the 'cnf'
   parameter in the web tokens are to be consumed by the resource server
   and not the client.  For more information on Proof of Possession
   semantics in JWTs see RFC 7800 [RFC7800] and for CWTs, see Proof-of-
   Possession Key Semantics for CBOR Web Tokens (CWTs)
   [I-D.ietf-ace-cwt-proof-of-possession].

   In the case of an error, the AS returns error responses for HTTP-
   based interactions as ASCII codes in JSON content, as defined in
   Section 5.2 of RFC 6749 [RFC6749].

2.1.2.  Client Connection Request to the Broker

   Once the client acquires the token, it can use it to request an MQTT
   connection to the broker over a TLS session with server
   authentication (Figure 1 (D)).  This section describes the client
   transporting the token to the broker (RS) via the CONNECT control
   message after the TLS handshake.  This is similar to an earlier
   proposal by Fremantle et al. [fremantle14].  An improvement to this
   is presented in Section 3 for the MQTT v5 - the OASIS Standard
   [MQTT-OASIS-Standard-v5].  Alternatively, the token may be used for
   the TLS session establishment as described in the DTLS profile for
   ACE [I-D.gerdes-ace-dtls-authorize].  In this case, both the TLS PSK
   and RPK handshakes MAY be supported.  This may additionally require
   that the client transports the token to the broker before the
   connection establishment.  To this end, the broker MAY support
   /authz-info endpoint via the "authz-info" topic.  Then, to transport
   the token, clients publish to "authz-info" topic unauthorized.  The
   topic "authz-info" MUST be publish-only for clients (i.e., the
   clients are not allowed to subscribe to it).  This option is
   described in more detail in Appendix B.

   When the client wishes to connect to the broker, it uses the CONNECT
   message of MQTT.  Figure 2 shows the structure of the MQTT CONNECT
   control message.















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          0            8            16            24            32
          +------------------------------------------------------+
          |CPT=1 | Rsvd.|Remaining len.| Protocol  name len. = 4 |
          +------------------------------------------------------+
          |                      'M' 'Q' 'T' 'T'                 |
          +------------------------------------------------------+
          | Proto.level=4|Connect flags|          Keep alive     |
          +------------------------------------------------------+
          | Payload                                              |
          |     Username as access token (UTF-8)                 |
          |     Password length (2 Bytes)                        |
          |     Password data as signature/MAC (binary)          |
          |                           ...                        |
          +------------------------------------------------------+

    Figure 2: MQTT CONNECT control message.  (CPT=Control Packet Type,
               Rsvd=Reserved, len.=length, Proto.=Protocol)

   To communicate the necessary connection parameters, the Client uses
   the appropriate flags of the CONNECT message.  Figure 3 shows how the
   MQTT connect flags MUST be set to initiate a connection with the
   broker.

   +-----------------------------------------------------------+
   |User name|Pass.|Will retain|Will QoS|Will Flag|Clean| Rsvd.|
   | flag    |flag |           |        |         |     |      |
   +-----------------------------------------------------------+
   | 1       | 1   |    X      |   X X  |   X     |  1   |  0  |
   +-----------------------------------------------------------+

              Figure 3: MQTT CONNECT flags.  (Rsvd=Reserved)

   To ensure that the client and the broker discard any previous session
   and start a new session, the Clean Session Flag MUST be set to 1.

   The Will flag indicates that a Will message needs to be sent when a
   client disconnection occurs.  The situations in which the Will
   message is published include disconnections due to I/O or network
   failures, and the server closing the networking connection due to a
   protocol error.  The client may set the Will flag as desired (marked
   as 'X' in Figure 3).  If the Will flag is set to 1 and the broker
   accepts the connection request, the broker must store the Will
   message, and publish it when the network connection is closed
   according to Will QoS and Will retain parameters, and MQTT Will
   management rules.  Section 2.5 explains how the broker deals with the
   retained messages in further detail.





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   Finally, Username and Password flags MUST be set to 1 to ensure that
   the Payload of the CONNECT message includes both Username and
   Password fields.

   The CONNECT message defaults to ACE for authentication and
   authorization.  For the basic operation described in this section,
   the Username field MUST be set to the access token.  The Password
   field MUST be set to the keyed message digest (MAC) or signature
   associated with the access token for proof-of-possession.  The client
   MAY apply the PoP key either to the entire request by computing a
   keyed message digest (for symmetric key) or a digital signature (for
   asymmetric key).  The CONNECT message is assumed to have enough
   randomness in the payload, and inside a TLS session (excluding the
   0-RTT case) will not be exposed to a replay attack.  When either
   cannot be guaranteed, the Password MAY also contain a nonce.

   Section 3.1.3 of MQTT v3.1.1 - the OASIS Standard
   [MQTT-OASIS-Standard] defines the MQTT Username as a UTF-8 encoded
   string, which is prefixed by a 2-byte length field followed by UTF-8
   encoded character data up to 65535 bytes.  Therefore an access token
   that is not a valid UTF-8 MUST be Base64 [RFC4648] encoded.  (The
   MQTT Password allows binary data up to 65535 bytes, and so, does not
   require encoding.)

2.1.3.  Token Validation

   RS MUST verify the validity of the token.  This validation MAY be
   done locally (e.g., in the case of a self-contained token) or the RS
   MAY send an introspection request to the AS.  If introspection is
   used, this section follows similar steps to those described in
   Sections 5.7 of the ACE framework [I-D.ietf-ace-oauth-authz].  The
   communication between AS and RS MAY be HTTPS, but it, in every case,
   MUST be confidential, mutually authenticated and integrity protected.

   The broker MUST check if the token is active either using 'exp' claim
   of the token or 'active' parameter of the introspection response.

   The access token is constructed by the AS such that RS can associate
   the access token with the client key.  This document assumes that the
   Access Token is a PoP token as described in
   [I-D.ietf-ace-oauth-authz].  Therefore, the necessary information is
   contained in the 'cnf' claim of the access token and may use either
   public or shared key approaches.  The client uses the signature or
   the MAC in the password field to prove the possession of the key.
   The resource server validates the signature or the MAC over the
   contents of the packet, authenticating the client.





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   The broker uses the scope field in the token (or in the introspection
   result) to determine the publish and subscribe permissions for the
   client.  If the Will flag is set, then the broker MUST check that the
   token allows the publication of the Will message too.

   If the token is not self-contained and the broker uses token
   introspection, it MAY cache the validation result to decide whether
   to accept subsequent PUBLISH and SUBSCRIBE messages as these
   messages, which are sent after a connection set-up, do not contain
   access tokens.  If the introspection result is not cached, then the
   RS needs to introspect the saved token for each request.

   Scope strings SHOULD be encoded as a permission, followed by an
   underscore, followed by a topic filter.  Two permissions apply to
   topics: 'publish' and 'subscribe'.  An example scope field may
   contain multiple such strings, space delimited, e.g., 'publish_topic1
   subscribe_topic2/#'.  Hence, this access token would give 'publish'
   permission to the 'topic1', 'subscribe' permission to all the
   subtopics of 'topic2'.

   Also, if present in the access token, RS must check that the 'iss'
   corresponds to AS, the 'aud' field (if not used to define topics)
   corresponds to RS.  It also has to check whether 'nbf' and 'iat'
   claims are present and valid.

2.1.4.  The Broker's Response to Client Connection Request

   Based on the validation result (obtained either via local inspection
   or using the /introspection interface of the AS), the broker MUST
   send a CONNACK message to the client.

   The broker responses may follow either the MQTT v3.1.1 - the OASIS
   Standard [MQTT-OASIS-Standard] or the MQTT v5 - the OASIS Standard
   [MQTT-OASIS-Standard-v5], depending on which version(s) the broker
   supports.

   In MQTT v3.1.1 - the OASIS Standard [MQTT-OASIS-Standard], it is not
   possible to support AS discovery via sending a tokenless CONNECT
   message to the broker.  This is because a CONNACK packet does not
   include a means to provide additional information to the client.
   Therefore, AS discovery needs to take place out-of-band.  This is
   remedied in the MQTT v5 - the OASIS Standard [MQTT-OASIS-Standard-v5]
   and a solution is described in Section 3.

   If the RS accepts the connection, it MUST store the token.






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2.2.  Authorizing PUBLISH Messages

2.2.1.  PUBLISH Messages from the Publisher Client to the Broker

   On receiving the PUBLISH message, the broker MUST use the type of
   message (i.e., PUBLISH) and the topic name in the message header to
   compare against the cached token or its introspection result.

   If the client is allowed to publish to the topic, the RS must publish
   the message to all valid subscribers of the topic.  The broker may
   also return an acknowledgment message if the QoS level is greater
   than or equal to 1.

   In case of a failure, it is not possible to return an error in MQTT
   v3.1.1 - the OASIS Standard [MQTT-OASIS-Standard].  Acknowledgement
   messages only indicate success.  In the case of an authorization
   error, the broker SHOULD disconnect the client.  Otherwise, it MUST
   ignore the PUBLISH message.  Also, DISCONNECT messages are only sent
   from a client to the broker.  So, server disconnection needs to take
   place below the application layer.  However, in MQTT v5 - the OASIS
   Standard [MQTT-OASIS-Standard-v5], it is possible to indicate failure
   and provide a reason code.  Section 3 describes in more detail how
   MQTT v5 handles PUBLISH authorization errors.

2.2.2.  PUBLISH Messages from the Broker to the Subscriber Clients

   To forward PUBLISH messages to the subscribing clients, the broker
   identifies all the subscribers that have valid matching topic
   subscriptions (i.e., the tokens are valid, and token scopes allow a
   subscription to the particular topic name).  The broker sends a
   PUBLISH message with the topic name and the topic message to all the
   valid subscribers.

   In MQTT, after connection establishment, there is no way to inform a
   client that an authorization error has occurred for previously
   subscribed topics, e.g., token expiry.  In the case of an
   authorization error, the broker disconnects the client.  In the MQTT
   v3.1.1 - the OASIS Standard [MQTT-OASIS-Standard], the MQTT
   DISCONNECT messages are only sent from a client to the broker.
   Therefore, the server disconnection needs to take place below the
   application layer.  In MQTT v5 - the OASIS Standard
   [MQTT-OASIS-Standard-v5], a server-side DISCONNECT message is
   possible and described in Section 3.








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2.3.  Authorizing SUBSCRIBE Messages

   In MQTT, a SUBSCRIBE message is sent from a client to the broker to
   create one or more subscriptions to one or more topics.  The
   SUBSCRIBE message may contain multiple topic filters.  The topic
   filters may include wildcard characters.

   On receiving the SUBSCRIBE message, the broker MUST use the type of
   message (i.e., SUBSCRIBE) and the topic filter in the message header
   to compare against the stored token or introspection result.

   As a response to the SUBSCRIBE message, the broker issues a SUBACK
   message.  For each topic filter, the SUBACK packet includes a return
   code matching the QoS level for the corresponding topic filter.  In
   the case of failure, the return code, in MQTT v3.1.1, must be 0x80
   indicating 'Failure'.  In MQTT v5, the appropriate return code is
   0x87, indicating that the client is 'Not authorized'.  Note that, in
   both MQTT versions, a reason code is returned for each topic filter.
   Therefore, the client may receive success codes for a subset of its
   topic filters, while being unauthorized for the rest.

2.4.  Token Expiration

   The broker MUST check for token expiration whenever a CONNECT,
   PUBLISH or SUBSCRIBE message is received or sent.  The broker SHOULD
   check for token expiration on receiving a PINGREQUEST message.  This
   may allow for early detection of a token expiry.

   The token expiration is checked by checking the 'exp' claim of a CWT/
   JWT or via performing an introspection request with the Authorization
   server as described in Section 5.7 of the ACE framework
   [I-D.ietf-ace-oauth-authz].  In the basic operation, token
   expirations MAY lead to disconnecting the associated client.
   However, in MQTT v5 - the OASIS Standard [MQTT-OASIS-Standard-v5],
   better error handling and re-authentication are possible.  This is
   explained in more detail in Section 3.

2.5.  Handling Disconnections and Retained Messages

   According to MQTT v3.1.1 - the OASIS Standard [MQTT-OASIS-Standard],
   only Client DISCONNECT messages are allowed.  In MQTT v5 - the OASIS
   Standard [MQTT-OASIS-Standard-v5], server-side DISCONNECT messages
   are possible, allowing to return '0x87 Not Authorized' return code to
   the client.

   In the case of a DISCONNECT, due to the Clean Session flag, the
   broker deletes all session state but MUST keep the retained messages.
   By setting a RETAIN flag in a PUBLISH message, the publisher



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   indicates to the broker that it should store the most recent message
   for the associated topic.  Hence, the new subscribers can receive the
   last sent message from the publisher for that particular topic
   without waiting for the next PUBLISH message.  In the case of a
   disconnection, the broker MUST continue publishing the retained
   messages as long as the associated tokens are valid.

   In case of disconnections due to network errors or server
   disconnection due to a protocol error (which includes authorization
   errors), the Will message must be sent if the client supplied a Will
   in the CONNECT request message.  The token provided in the CONNECT
   request must cover the Will topic.  The Will message MUST be
   published to the Will topic when the network connection is closed
   regardless of whether the corresponding token has expired.

3.  Improved Protocol Interactions with MQTT v5

   In the new MQTT v5 - the OASIS Standard [MQTT-OASIS-Standard-v5],
   several new capabilities are introduced, which enable better
   integration with ACE.  The newly enhanced authentication and re-
   authentication methods support a wider range of authentication flows
   beyond username and password.  With the MQTT v5, there is a clearly
   defined approach for using token-based authorization.  Also, it is
   possible for a client to request a re-authentication avoiding
   disconnection.  Finally, MQTT v5 generally improves error reporting,
   enabling better response to authorization failures during publishing
   messages to the subscribers.

3.1.  Token Transport via Authentication Exchange (AUTH)

   To initiate the authentication and authorization flow, as before, the
   CAS initiates the token request as in Section 2.1.  When the client
   wishes to connect to the RS (broker), it uses the CONNECT message of
   MQTT.  Figure 4 shows the structure of the MQTT CONNECT control
   message used in MQTT v5.
















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          0            8            16            24            32
          +------------------------------------------------------+
          |CPT=1 | Rsvd.|Remaining len.| Protocol  name len. = 4 |
          +------------------------------------------------------+
          |                      'M' 'Q' 'T' 'T'                 |
          +------------------------------------------------------+
          | Proto.level=5|Connect flags|          Keep alive     |
          +------------------------------------------------------+
          |                 Property length                      |
          |          Auth. Method (0x15) | 'ace'                 |
          |          Auth. Data (0x16)   | empty or token or     |
          |                                token + PoP data      |
          +------------------------------------------------------+

    Figure 4: MQTT CONNECT control message.  (CPT=Control Packet Type,
               Rsvd=Reserved, len.=length, Proto.=Protocol)

   To communicate the necessary connection parameters, the client uses
   the appropriate flags of the CONNECT message.  To achieve a clean
   session (i.e., the session should start without an existing session),
   the new MQTT v5 session flags MUST be set appropriately: the Clean
   Start Flag MUST be set to 1 and Session Expiry Interval MUST be set
   to 0.

   With the enhanced authentication capabilities, it is not necessary to
   overload the username and password fields in the CONNECT message for
   ACE authentication.  Nevertheless, the RS MUST support both methods
   for supporting the token: (1) Token transport via username and
   password and (2) using the new AUTH (Authentication Exchange) method.
   The token transport via username and password is as described in
   Section 2.1.2.  The rest of this section describes the AUTH method.

   To use the AUTH method, the username flag MUST be set to 0, and the
   password flag MUST be set to 0.  The client can set the
   Authentication Method as a property of a CONNECT packet by setting
   Auth Properties (with the property identifier 0x15).  The client must
   MUST set the UTF-8 encoded string containing the name of the
   authentication method as 'ace'.  If the RS does not support this
   profile, it sends a CONNACK with a Reason Code of '0x8C (Bad
   authentication method)'

   The Authentication Method is followed by the Authentication Data,
   which has a property identifier 0x16.  Authentication data is binary
   data and is defined by the authentication method.  The RS MAY support
   different implementations for transporting the authentication data.
   The first option is that Authentication data contains both the token
   and the keyed message digest (MAC) or signature as described in
   Section 2.1.2.  The encoding of this field MAY use CBOR and COSE.  In



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   this case, the token validation proceeds as described in
   Section 2.1.3 and the server responds with a CONNACK.  The reason
   code of the CONNACK is '0x00 (Success)' if the authentication is
   successful.  In case of an invalid PoP token, the CONNACK reason code
   is '0x87 (Not Authorized)'.

   The second option that RS may accept is a challenge/response
   protocol.  If the Authentication Data only includes the token, the RS
   MUST respond with an AUTH packet, with the Authenticate Reason Code
   set to '0x18 (Continue Authentication)'.  This packet includes the
   Authentication Method, which MUST be set to 'ace' and Authentication
   Data.  The Authentication Data MUST NOT be empty and contains a
   challenge for the client.  The client responds to this with an AUTH
   packet, with a reason code '0x18 (Continue Authentication)'.
   Similarly, the client packet sets the Authentication Method to 'ace'.
   The Authentication Data in the client's response contains the
   signature or MAC computed over the RS's challenge.  To this, the
   server responds with a CONNACK and return code '0x00 (Success)' if
   the authentication is successful.  In case of an invalid PoP token,
   the CONNACK reason code is '0x87 (Not Authorized)'.

   Finally, this document allows the CONNECT message to have an empty
   Authentication Data field.  This is the AS discovery option and the
   RS responds with the CONNACK reason code '0x87 (Not Authorized)' and
   includes a User Property for the AS information.  AS Information
   contains the absolute URI of AS, and MAY also contain a cnonce as
   described in the Section 5.1 of the ACE framework
   [I-D.ietf-ace-oauth-authz].  This information MAY be CBOR encoded.

3.2.  Authorization Errors and Client Re-authentication

   MQTT v5 allows better error reporting.  To take advantage of this for
   PUBLISH messages, the QoS level should be set to greater than or
   equal to 1.  This guarantees that RS responds with either a PUBACK or
   PUBREC packet with reason code '0x87 (Not authorized)' in the case of
   an authorization error.  Similarly, for the SUBSCRIBE case, the
   SUBACK packet has a reason code set to '0x87 (Not authorized)' for
   the unauthorized topic(s).  When RS is forwarding PUBLISH messages to
   the subscribed clients, it may discover that some of the subscribers
   are no more authorized due to expired tokens.  In this case, the RS
   SHOULD send a DISCONNECT message with the reason code '0x87 (Not
   authorized)'.  Note that the server-side DISCONNECT is a new feature
   of MQTT v5 (in MQTT v3.1.1, the server needed to drop the
   connection).  RS MUST stop forwarding messages to the unauthorized
   subscribers.

   In the case of a PUBACK with '0x87 (Not authorized)', the client can
   update its token using the Re-authentication feature of MQTT v5.



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   Also, the clients can proactively update their tokens without waiting
   for such a PUBACK.  To re-authenticate, the client sends an AUTH
   packet with reason code '0x19 (Re-authentication)'.  The client MUST
   set the authentication method as 'ace' and transport the new token in
   the Authentication Data.  The client and the RS go through the same
   steps for proof of possession validation as described in the previous
   section.  If the re-authentication fails, the server MUST send a
   DISCONNECT with the reason code '0x87 (Not Authorized)'.

4.  IANA Considerations

   The following registrations are done for the ACE OAuth Profile
   Registry following the procedure specified in
   [I-D.ietf-ace-oauth-authz].

   Note to the RFC editor: Please replace all occurrences of "[RFC-
   XXXX]" with the RFC number of this specification and delete this
   paragraph.

   Profile name: mqtt_tls

   Profile description: Profile for delegating client authentication and
   authorization using MQTT as the application protocol and TLS For
   transport layer security.

   Profile ID:

   Change controller: IESG

   Reference: [RFC-XXXX]

5.  Security Considerations

   This document specifies a profile for the Authentication and
   Authorization for Constrained Environments (ACE) framework
   [I-D.ietf-ace-oauth-authz].  Therefore, the security considerations
   outlined in [I-D.ietf-ace-oauth-authz] apply to this work.

   In addition, the security considerations outlined in MQTT v3.1.1 -
   the OASIS Standard [MQTT-OASIS-Standard] and MQTT v5 - the OASIS
   Standard [MQTT-OASIS-Standard-v5] apply.  Mainly, this document
   provides an authorization solution for MQTT, the responsibility of
   which is left to the specific implementation in MQTT v5 - the OASIS
   Standard [MQTT-OASIS-Standard-v5].  In the following, we comment on a
   few relevant issues based on the current MQTT specifications.

   In this document, RS uses the PoP access token to authenticate the
   client.  If the client is able, TLS certificates sent from the client



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   can be used by the RS to authenticate the client.  The TLS
   certificate from the RS MUST be used by the client to authenticate
   the RS.

   To authorize a client's publish and subscribe requests in an ongoing
   session, the RS caches the access token after accepting the
   connection from the client.  However, if some permissions are revoked
   in the meantime, the RS may still grant publish/subscribe to revoked
   topics until the session ends or the token expires.  When permissions
   change dynamically, it is expected that AS follows a reasonable
   expiration strategy for the access tokens.

   The RS may monitor client behaviour to detect potential security
   problems, especially those affecting availability.  These include
   repeated token transfer attempts to the public "authz-info" topic,
   repeated connection attempts, abnormal terminations, and clients that
   connect but do not send any data.  If the RS supports the public
   "authz-info" topic, described in Appendix B, then this may be
   vulnerable to a DDoS attack, where many clients use the "authz-info"
   public topic to transport fictitious tokens, which RS may need to
   store indefinitely.

6.  Privacy Considerations

   The privacy considerations outlined in [I-D.ietf-ace-oauth-authz]
   apply to this work.

   In MQTT, the RS is a central trusted party and may forward
   potentially sensitive information between clients.  Clients may
   choose to encrypt the payload of their messages.  However, this would
   not provide privacy for other properties of the message such as topic
   name.

7.  References

7.1.  Normative References

   [I-D.gerdes-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", draft-gerdes-ace-dtls-
              authorize-01 (work in progress), March 2017.








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   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-24
              (work in progress), March 2019.

   [MQTT-OASIS-Standard]
              Banks, A., Ed. and R. Gupta, Ed., "OASIS Standard MQTT
              Version 3.1.1 Plus Errata 01", 2015, <http://docs.oasis-
              open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html>.

   [MQTT-OASIS-Standard-v5]
              Banks, A., Ed., Briggs, E., Ed., Borgendale, K., Ed., and
              R. Gupta, Ed., "OASIS Standard MQTT Version 5.0", 2017,
              <http://docs.oasis-open.org/mqtt/mqtt/v5.0/os/
              mqtt-v5.0-os.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [fremantle14]
              Fremantle, P., Aziz, B., Kopecky, J., and P. Scott,
              "Federated Identity and Access Management for the Internet
              of Things", research International Workshop on Secure
              Internet of Things, September 2014,
              <http://dx.doi.org/10.1109/SIoT.2014.8>.

   [I-D.ietf-ace-actors]
              Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
              architecture for authorization in constrained
              environments", draft-ietf-ace-actors-07 (work in
              progress), October 2018.






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   [I-D.ietf-ace-cwt-proof-of-possession]
              Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
              possession-06 (work in progress), February 2019.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC7800]  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              RFC 7800, DOI 10.17487/RFC7800, April 2016,
              <https://www.rfc-editor.org/info/rfc7800>.

Appendix A.  Checklist for profile requirements

   o  AS discovery: For the basic protocol using either MQTT v3.1.1 or
      MQTT v5, the clients/client authorization servers need to be
      configured out-of-band.  RS does not provide any hints to help AS
      discovery.  AS discovery is possible with the MQTT v5 extensions
      described in Section 3.

   o  The communication protocol between the client and RS: MQTT

   o  The security protocol between the client and RS: TLS

   o  Client and RS mutual authentication: RS provides a server
      certificate during TLS handshake.  Client transports token and MAC
      via the MQTT CONNECT message.  Other methods for transporting the
      token with the MQTT v5 extensions described in Section 3.

   o  Content format: For the HTTPS interactions with AS, "application/
      json".  The MQTT payloads may be formatted JSON or CBOR.

   o  PoP protocols: Either symmetric or asymmetric keys can be
      supported.

   o  Unique profile identifier: mqtt_tls

   o  Token introspection: RS uses HTTPS /introspect interface of AS.

   o  Token request: CAS uses HTTPS /token interface of AS.




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   o  /authz-info endpoint: It MAY be supported using the method
      described in Appendix B, not protected.

   o  Token transport: In MQTT CONNECT message or using the AUTH
      extensions for MQTT v5 described in Section 3.

Appendix B.  The Authorization Information Endpoint

   The main document described a method for transporting tokens inside
   MQTT CONNECT messages.  In this section, we describe an alternative
   method to transport an access token.

   The method consists of the MQTT broker accepting PUBLISH messages to
   a public "authz-info" topic.  A client using this method MUST first
   connect to the broker, and publish the access token using the "authz-
   info" topic.  The broker must verify the validity of the token (i.e.,
   through local validation or introspection).  After publishing the
   token, the client disconnects from the broker and is expected to try
   reconnecting over TLS.

   In MQTT v3.1.1, after the client published to the "authz-info" topic,
   it is not possible for the broker to communicate the result of the
   token verification.  In MQTT v5, the broker can return 'Not
   authorized' error to a PUBLISH request for QoS greater or equal to 1.
   In any case, any token authorization failure affect the subsequent
   TLS handshake, which can prompt the client to obtain a valid token.

Acknowledgements

   The authors would like to thank Ludwig Seitz for his review and his
   input on the authorization information endpoint, presented in the
   appendix.

Authors' Addresses

   Cigdem Sengul
   Nominet
   2 Kingdom Street
   London  W2 6BD
   UK

   Email: Cigdem.Sengul@nominet.uk









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   Anthony Kirby
   Oxbotica
   1a Milford House, Mayfield Road, Summertown
   Oxford  OX2 7EL
   UK

   Email: anthony@anthony.org


   Paul Fremantle
   University of Portsmouth
   School of Computing, Buckingham House
   Portsmouth  PO1 3HE
   UK

   Email: paul.fremantle@port.ac.uk



































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