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Versions: (draft-sengul-ace-mqtt-tls-profile) 00 01 02 03 04

ACE Working Group                                              C. Sengul
Internet-Draft                                                   Nominet
Intended status: Standards Track                                A. Kirby
Expires: April 7, 2020                                          Oxbotica
                                                            P. Fremantle
                                                University of Portsmouth
                                                         October 5, 2019


                        MQTT-TLS profile of ACE
                   draft-ietf-ace-mqtt-tls-profile-01

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 MQTT 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 April 7, 2020.

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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  ACE-Related Terminology . . . . . . . . . . . . . . . . .   4
     1.3.  MQTT-Related Terminology  . . . . . . . . . . . . . . . .   4
   2.  Protocol Interactions . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Authorizing Connection Establishment  . . . . . . . . . .   7
       2.1.1.  Client Token Request to the Authorization Server (AS)   8
       2.1.2.  Client Connection Request to the Broker (C) . . . . .   8
         2.1.2.1.  Proof-of-Possession over Predefined Field . . . .  10
         2.1.2.2.  Proof-of-Possession via challenge/response  . . .  11
         2.1.2.3.  Unauthorised Request: Authorisation Server
                   Discovery . . . . . . . . . . . . . . . . . . . .  12
       2.1.3.  Token Validation  . . . . . . . . . . . . . . . . . .  12
       2.1.4.  The Broker's Response to Client Connection Request  .  13
     2.2.  Authorizing PUBLISH Messages  . . . . . . . . . . . . . .  13
       2.2.1.  PUBLISH Messages from the Publisher Client to the
               Broker  . . . . . . . . . . . . . . . . . . . . . . .  13
       2.2.2.  PUBLISH Messages from the Broker to the Subscriber
               Clients . . . . . . . . . . . . . . . . . . . . . . .  14
     2.3.  Authorizing SUBSCRIBE Messages  . . . . . . . . . . . . .  14
     2.4.  Token Expiration and Reauthentication . . . . . . . . . .  15
     2.5.  Handling Disconnections and Retained Messages . . . . . .  15
   3.  Reduced Protocol Interactions for MQTT v3.1.1 . . . . . . . .  16
     3.1.  Token Transport . . . . . . . . . . . . . . . . . . . . .  16
     3.2.  Handling Authorization Errors . . . . . . . . . . . . . .  17
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  19
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Checklist for profile requirements . . . . . . . . .  21
   Appendix B.  The Authorization Information Endpoint . . . . . . .  21
   Appendix C.  Document Updates . . . . . . . . . . . . . . . . . .  22
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23








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1.  Introduction

   This document specifies a profile for the ACE framework
   [I-D.ietf-ace-oauth-authz].  In this profile, Clients and a Broker
   use MQTT to exchange Application messages.  The protocol relies on
   TLS for communication security between entities.  The MQTT protocol
   interactions are described based on the MQTT v5.0 - the OASIS
   Standard [MQTT-OASIS-Standard-v5].  It is expected that MQTT
   deployments will retain backward compatibility for MQTT v3.1.1
   clients, and therefore, this document describes a reduced set of
   protocol interactions suited to MQTT v3.1.1 - the OASIS Standard
   [MQTT-OASIS-Standard].  However, it is RECOMMENDED to use MQTT v5.0
   as it works more naturally with ACE-style authentication and
   authorization.

   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.
   The MQTT Broker is responsible for distributing messages published by
   the publishers to the appropriate subscribers.  Each publish message
   contains a Topic Name, 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 (RS).  In the rest of the document
   RS and Broker are used interchangeably.  To provide communication
   confidentiality and Resource Server authentication, TLS is used.
   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 Clients and the Broker.

   o  Connection establishment between the Clients and the Broker

   o  Publish messages from the Clients to the Broker, and from the
      Broker to the Clients

   o  Subscribe messages from the Clients to the Broker

   While the Client-Broker exchanges are over MQTT, the required Client-
   AS and RS-AS interactions are described for HTTPS-based
   communication, using 'application/ace+json' content type, and unless



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   otherwise specified, using JSON encoding.  The token may be a
   reference, or JWT.  For JWT tokens, this document follows RFC 7800
   [RFC7800] for PoP semantics for JWTs.  The Client-AS and RS-AS may
   also be based on CoAP.  It is also possible to use 'application/
   ace+cbor' content type, and CBOR encoding, and CWT and associated PoP
   semantics to reduce the protocol memory and bandwidth requirements.
   For more information on Proof of Possession semantics for CWTs, see
   Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)
   [I-D.ietf-ace-cwt-proof-of-possession].

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] 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.  The Clients are MQTT Clients, which connect to the
   Broker to publish and subscribe to Application Messages.  For
   additional information, please refer to the MQTT v5.0 - the OASIS
   Standard [MQTT-OASIS-Standard-v5] or the MQTT v3.1.1 - the OASIS
   Standard [MQTT-OASIS-Standard].

   MQTTS
           Secured transport profile of MQTT.  MQTTS runs over TLS.

   Broker



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           The Server in MQTT and acts as an intermediary between
           Clients that publish Application Messages, and the Clients
           that made Subscriptions.  The Broker acts as the Resource
           Server for the Clients.

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

   Topic Name
           The label attached to an Application Message, which is
           matched to a Subscription.

   Subscription
           A subscription comprises a Topic Filter and a maximum Quality
           of Service (QoS).

   Topic Filter
           An expression that indicates interest in one or more Topic
           Names.  Topic Filters may include wildcards.

   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




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           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.

   Will
           An application message published by the Server after the
           network connection is closed in cases where the network
           connection is not closed normally.  If the Will Flag is set,
           then the payload of the CONNECT message has information about
           the Will.  The Will consists of the Will Properties, Will
           Topic, and Will Payload fields in the CONNECT message.

2.  Protocol Interactions

   This section describes the following exchanges between Clients, the
   Broker, and the Authorization Server according to the MQTT v5.0.

   o  Authorizing connection establishment between the Clients and the
      Broker

   o  Authorizing publish messages from the Clients to the Broker, and
      from the Broker to the Clients

   o  Authorizing subscribe messages from Clients to the Broker

   Section 3 describes how these exchanges can also be supported using
   the MQTT v3.1.1.  MQTT v5.0 brokers MAY also only support the basic
   operation; however, this is NOT RECOMMENDED.

   In this profile document, message topics are treated as resources.
   The Clients are assumed to have identified the publish/subscribe
   topics of interest out-of-band (topic discovery is not a feature of
   the MQTT protocol).  A resource owner can pre-configure policies at
   the AS that give Clients publish or subscribe permissions to
   different topics.






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2.1.  Authorizing Connection Establishment

   This section specifies how Clients establish an authorized connection
   to an MQTT Broker.  Figure 1 shows the basic protocol flow during
   connection establishment.The token request and response use the
   /token endpoint of the authorization server, specified in the
   Section 5.6 of the ACE framework [I-D.ietf-ace-oauth-authz].  Steps
   (D) and (E) are optional, and use the introspection endpoint,
   specified in the Section 5.7 of the ACE framework.  The Client and
   Broker use HTTPS to communicate to AS via these endpoints.  If the
   Client is resource-constrained, a Client Authorisation Server may
   carry out the token request on behalf of the Client, and later,
   onboard the Client with the token.  Also, these interfaces may be
   implemented using other protocols, e.g., CoAP or MQTT.  The
   interactions between a Client and its Client Authorization Server for
   token onboarding, and the MQTTS support for token requests are out of
   scope of this document.

                             +---------------------+
                             | Client              |
                             |                     |
      +---(A) Token request--| Client -            |
      |                      | Authorization       |
      |   +-(B) Access token-> Server Interface    |
      |   |                  |       (HTTPS)       |
      |   |                  |_____________________|
      |   |                  |                     |
   +--v-------------+        |  Pub/Sub Interface  |
   |  Authorization |        |     (MQTTS)         |
   |  Server        |        +-----------^---------+
   |________________|            |       |
      |    ^             (C)Connection  (F)Connection
      |    |               request +    response
      |    |               access token  |
      |    |                     |       |
      |    |                 +---v--------------+
      |    |                 |   Broker (MQTTS) |
      |    |                 |__________________|
      |    +(D)Introspection-|                  |
      |   request (optional) | RS-AS interface  |
      |                      |     (HTTPS)      |
      +-(E)Introspection---->|__________________|
        response (optional)

                    Figure 1: Connection establishment






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2.1.1.  Client Token Request to the Authorization Server (AS)

   The first step in the protocol flow (Figure 1 (A)) is the token
   acquisition by the Client from the AS.  When requesting an access
   token from the AS, the Client MAY include parameters in its request
   as defined in Section 5.6.1 of the ACE framework
   [I-D.ietf-ace-oauth-authz].  The media format is 'application/
   ace+json'.  The profile parameter MUST be set to 'mqtt_tls'.  The
   OAuth 2.0 AS uses a JSON structure in the payload of its responses
   both to client and RS.

   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.  This document follows
   RFC 7800 [RFC7800] for PoP semantics for JWTs.  The PoP token
   includes a 'cnf' parameter with a symmetric or asymmetric PoP key.
   Note that the 'cnf' parameter in the web tokens are to be consumed by
   the resource server and not the Client.

   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 (C)

   This section describes how the Client transports 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].  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.

   After the token acquisition, the Client connects to the RS (Broker)
   using the CONNECT message of MQTT over TLS.  For server
   authentication, the client MAY either have the ability to receive and
   validate a certificate or a raw public key from the Broker.  The



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   client needs to use this raw public key in the TLS handshake together
   with an out-of-band validation technique (see RFC 7250 [RFC7250] for
   details).

   Figure 2 shows the structure of the MQTT CONNECT control message used
   in MQTT v5.0.  A CONNECT message is composed of a fixed header, a
   variable header and a payload.  The fixed header contains Control
   Packet Type (CPT), Reserved, and Remaining Length.  The Variable
   Header contains the Protocol Name, Protocol Level, Connect Flags,
   Keep Alive, and Properties.  The Connect Flags in the variable header
   specify the behavior of the MQTT connection.  It also indicates the
   presence or absence of fields in the Payload.  The payload contains
   one or more encoded fields, namely a unique Client identifier for the
   Client, a Will Topic, Will Payload, User Name and Password.  All but
   the Client identifier can be omitted depending on flags in the
   Variable Header.

          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      |
          +------------------------------------------------------+
          |           Payload: Client Identifier                 |
          +------------------------------------------------------+

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

   Connect Flags include Clean Start, Will, Will QoS, Will Retain,
   Password and Username flags.  Figure 6 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 |           |        |         |     |      |
   +-----------------------------------------------------------+
   | 0       | 0   |    X      |   X X  |   X     |  1   |  0  |
   +-----------------------------------------------------------+

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



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   To achieve a clean session (i.e., the session starts without an
   existing session), the Clean Start Flag MUST be set to 1.  In
   addition, if the Session Expiry Interval is present in the CONNECT
   message, it MUST be set to 0.

   The Will Flag indicates that a Will message needs to be sent if
   network connection is not closed normally.  The situations in which
   the Will message is published include disconnections due to I/O or
   network failures, and the server closing the network 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.  To avoid publishing Will Messages in the case of
   temporary network disconnections, the Client my specify a Will Delay
   Interval in Will Properties.  Section 2.5 explains how the Broker
   deals with the retained messages in further detail.

   For token transport, the RS SHOULD support AUTH (Authentication
   Exchange) method.  The RS MAY support token transport via username
   and password, which is described in Section 3 for MQTT v3.1.1.  The
   rest of this section describes the AUTH method, for which the
   username and password flags MUST be set to 0.

   To implement the AUTH (Authentication Exchange) method, the Client
   MUST set the Authentication Method as a property of a CONNECT packet
   by using the property identifier 21 (0x15).  This is followed by a
   UTF-8 Encoded String containing the name of the authentication
   method, which MUST be set to '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 22 (0x16) and is binary data.  Based
   on the Authentication Data, this profile allows:

   o  Proof-of-Possession over predefined field

   o  Proof-of-Possession via challenge/response

   o  Unauthorised request: Authorisation Server discovery

2.1.2.1.  Proof-of-Possession over Predefined Field

   For this option, the Authentication Data MUST contain the token and
   the keyed message digest (MAC) or the Client signature.  To calculate
   the keyed message digest (MAC) or the Client signature, the Client



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   SHOULD apply the PoP key to the CONNECT payload.  The CONNECT payload
   has at least a Client Identifier, and if the Will Flag is set to 1,
   may contain Will-related information.  The Client Identifier is a
   MUST be a UTF-8 Encoded String (i.e., is prefixed with a two-byte
   integer length field that gives the number of bytes in a UTF-8
   encoded string itself).  The Client Identifier may be 1-23 UTF-8
   encoded bytes, and contain only the characters
   "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".
   However, according to MQTTv5 standard, the Broker may except longer
   Client Identifiers, and characters not included in the list given
   above.  Clients MUST change their Client Identifier for each session,
   if the Client Identifier is the only source of randomness in the
   payload to defend against a replay attack.  If the Client reuses its
   Client Identifier across different sessions, the Authentication Data
   MUST also contain a nonce, and the keyed message digest (MAC) or the
   Client signature MUST be computed over this nonce.  Finally, the
   token is validated as described in Section 2.1.3 and the server
   responds with a CONNACK.

2.1.2.2.  Proof-of-Possession via challenge/response

   For this option, the RS follows a challenge/response protocol.  The
   success case is illustrated in Figure 4.  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.  Next, the token is validated as described
   in Section 2.1.3.

















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                                Resource
                    Client      Server
                     |             |
                     |<===========>| TLS connection establishment
                     |             |
                     |             |
                     +------------>| CONNECT with Authentication Data
                     |             | contains only token
                     |             |
                     <-------------+ AUTH '0x18 (Continue Authentication)'
                     |             | challenge
                     |             |
                     |------------>| AUTH '0x18 (Continue Authentication)'
                     |             | signature
                     |             |
                     |             |-----+ Token validation (may involve introspection)
                     |             |     |
                     |             |<----+
                     |             |
                     |<------------+ CONNACK '0x00 (Success)'

         Figure 4: PoP Challenge/Response Protocol Flow - Success

2.1.2.3.  Unauthorised Request: Authorisation Server Discovery

   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 (identified by 38 (0x26)) for the AS
   creation hints as dedined in the Section 5.1.2 of the ACE framework
   [I-D.ietf-ace-oauth-authz].

2.1.3.  Token Validation

   The 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 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.
   Also, if present in the access token, RS must check that the 'iss'
   corresponds to AS, the 'aud' field corresponds to RS.  It also has to
   check whether the 'nbf' and the 'iat' claims are present and valid.




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   To authenticate the Client, the RS validates the signature or the
   MAC, depending on how the PoP protocol is implemented.  To authorize
   the Client, the Broker uses the scope field in the token (or in the
   introspection result).  The scope field contains 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.

   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'.

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 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)'.

   If the RS accepts the connection, it MUST store the token until the
   end of connection.  On Client or RS disconnection, the token is
   discarded, and the Client MUST provide a token inside each CONNECT
   message.

   If the token is not self-contained and the Broker uses token
   introspection, it MAY cache the validation result to authorize the
   subsequent PUBLISH and SUBSCRIBE messages.  PUBLISH and SUBSCRIBE
   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.  The Broker
   SHOULD use a cache time out to introspect tokens regularly.

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



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   also return an acknowledgment message if the QoS level is greater
   than or equal to 1.

   In case of an authorization failure, an error MAY be returned to the
   Client.  For this the QoS level of the PUBLISH message, 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)'.

   On receiving a PUBACK with '0x87 (Not authorized)', the Client MAY
   reauthenticate as described in Section 2.4, and pass a new token
   following the same PoP methods as described in Figure 2.

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).  The Broker sends a PUBLISH
   message with the Topic name to all the valid subscribers.

   RS MUST stop forwarding messages to the unauthorized subscribers.
   For Clients with invalid tokens, there is no way to inform the Client
   that an authorization error has occurred other than sending a
   DISCONNECT message.  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 v5.0 (in MQTT v3.1.1, the server
   needs to drop the connection).

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 is 0x87, indicating that the
   Client is 'Not authorized'.  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.




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2.4.  Token Expiration and Reauthentication

   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 JWT
   or introspection response, 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].  Token expirations may trigger
   the RS to send PUBACK, SUBACK and DISCONNECT messages with return
   code set to 'Not authorised'.  As a response, the Client MAY re-
   authenticate by sending an AUTH packet with a Reason Code of 0x19
   (Re-authentication)

   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 Section 2.1.2.  If the re-
   authentication fails, the server MUST send a DISCONNECT with the
   reason code '0x87 (Not Authorized)'.  The Clients can also
   proactively update their tokens before they receive a message with
   'Not authorized' return code.

2.5.  Handling Disconnections and Retained Messages

   In the case of a Client 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 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 of that
   particular topic without waiting for the next PUBLISH message.  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 message.  The Client's token scopes MUST include 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.  In the case of a server-side
   DISCONNECT, the server returns the '0x87 Not Authorized' return code
   to the Client.




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3.  Reduced Protocol Interactions for MQTT v3.1.1

   This section describes a reduced set of protocol interactions for the
   MQTT v3.1.1 Client.

3.1.  Token Transport

   To transport the token to the Broker, the Clients use the username
   and password fields of the CONNECT control message after the TLS
   handshake.  Figure 5 shows the structure of the MQTT CONNECT message.

          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                                              |
          |     Client Identifier                                |
          |     Username as access token (UTF-8)                     |
          |     Password length (2 Bytes)                        |
          |     Password data as signature/MAC (binary)          |
          +------------------------------------------------------+

    Figure 5: MQTT CONNECT control message.  (CPT=Control Packet Type,
               Rsvd=Reserved, len.=length, Proto.=Protocol)

   Figure 6 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 6: MQTT CONNECT flags.  (Rsvd=Reserved)

   The Clean Session Flag MUST be set to 1.  The Client may set the Will
   Flag as desired (marked as 'X' in Figure 6).  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.  The Username field MUST be set to the access token.



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   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 MUST apply the PoP key to the payload as described in
   Section 2.1.2.1.

   In MQTT v3.1.1, the MQTT Username as a UTF-8 encoded string (i.e., is
   prefixed by a 2-byte length field followed by UTF-8 encoded character
   data) and may be 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.)

3.2.  Handling Authorization Errors

   Handling errors are more primitive in MQTT v3.1.1 due to not having
   appropriate error fields, error codes, and server-side DISCONNECTS.
   In the following, we list how errors are handled without such
   protocol support.

   o  CONNECT without a token: It is not possible to support AS
      discovery via sending a tokenless CONNECT message to the Broker.
      This is because a CONNACK packet in MQTT v3.1.1 does not include a
      means to provide additional information to the Client.  Therefore,
      AS discovery needs to take place out-of-band.  CONNECT attempt
      MUSY fail.

   o  Client-RS PUBLISH authorization failure: In case of a failure, it
      is not possible to return an error in MQTT v3.1.1.
      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.

   o  SUBSCRIBE authorization failure: In the SUBACK packet, the return
      code must be 0x80 indicating 'Failure' for the unauthorized
      topic(s).  Note that, in both MQTT versions, a reason code is
      returned for each Topic Filter.

   o  RS-Client PUBLISH authorization failure: 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.  These token expirations SHOULD lead to disconnecting the
      Client, rather than silently dropping messages.








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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 v5.0 - the
   OASIS Standard [MQTT-OASIS-Standard-v5] and MQTT v3.1.1 - the OASIS
   Standard [MQTT-OASIS-Standard] apply.  Mainly, this document provides
   an authorization solution for MQTT, the responsibility of which is
   left to the specific implementation in MQTT v5.0 standard.  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
   can be used by the RS to authenticate the Client.  The Client may
   authenticate the RS either using a server cerficate or the RPK
   method.  In the case of RPK, client needs to use this raw public key
   in the TLS handshake together with an out-of-band validation
   technique (see [RFC7250] for details).

   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



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   in the meantime, the RS may still grant publish/subscribe to revoked
   topics.  If the RS caches the token introspection responses, then the
   RS should use a reasonable cache timeout to introspect tokens
   regularly.  When permissions change dynamically, it is expected that
   AS also 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.

   [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.






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   [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>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [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-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-08 (work in progress), October 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>.



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   [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: AS discovery is possible with the MQTT v5.0
      described in Section 2.1.2.

   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 or RPK during TLS handshake.  Client transports token
      and MAC via the MQTT CONNECT message.

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

   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.

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

   o  Token transport: In MQTT CONNECT message for both versions of
      MQTT.  AUTH extensions also used for authentication and re-
      authentication for MQTT v5.0 as described in Section 2.1.2.

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.





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   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 v5.0, the Broker can return 'Not authorized' error to a
   PUBLISH request for QoS greater or equal to 1.  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 any case, any token authorization failure affect the subsequent
   TLS handshake, which can prompt the Client to obtain a valid token.

Appendix C.  Document Updates

   Version 00 to 01:

   o  Present the MQTTv5 as the RECOMMENDED version, and MQTT v3.1.1 for
      backward compatibility.

   o  Clarified Will message.

   o  Improved consistency in the use of terminology, and upper/lower
      case.

   o  Defined Broker and MQTTS.

   o  Clarified HTTPS use for C-AS and RS-AS communication.  Removed
      reference to actors document, and clarified the use of client
      authorization server.

   o  Clarified the Connect message payload and Client Identifier.

   o  Presented different methods for passing the token, and PoP.

   o  Added new figures for AUTH methods, updated CONNECT message
      figure.

Acknowledgements

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






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Authors' Addresses

   Cigdem Sengul
   Nominet
   4 Kingdom Street
   London  W2 6BD
   UK

   Email: Cigdem.Sengul@nominet.uk


   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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