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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: May 5, 2020                                            Oxbotica
                                                            P. Fremantle
                                                University of Portsmouth
                                                        November 2, 2019


                        MQTT-TLS profile of ACE
                   draft-ietf-ace-mqtt-tls-profile-02

Abstract

   This document specifies a profile for the ACE (Authentication and
   Authorization for Constrained Environments) framework 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 May 5, 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.  Authorizing Connection Requests . . . . . . . . . . . . . . .   7
     2.1.  Client Token Request to the Authorization Server (AS) . .   8
     2.2.  Client Connection Request to the Broker (C) . . . . . . .   8
       2.2.1.  Client-Server Authentication over TLS and MQTT  . . .   8
       2.2.2.  authz-info: The Authorization Information Topic . . .   9
       2.2.3.  Transporting Access Token Inside the MQTT CONNECT . .   9
       2.2.4.  Authentication Using AUTH Property  . . . . . . . . .  11
         2.2.4.1.  Proof-of-Possession Using a Challenge from the
                   TLS session . . . . . . . . . . . . . . . . . . .  11
         2.2.4.2.  Proof-of-Possession via Broker-generated
                   Challenge/Response  . . . . . . . . . . . . . . .  12
         2.2.4.3.  Unauthorised Request: Authorisation Server
                   Discovery . . . . . . . . . . . . . . . . . . . .  12
       2.2.5.  Token Validation  . . . . . . . . . . . . . . . . . .  13
       2.2.6.  The Broker's Response to Client Connection Request  .  13
   3.  Authorizing PUBLISH Messages  . . . . . . . . . . . . . . . .  14
     3.1.  PUBLISH Messages from the Publisher Client to the Broker   14
     3.2.  PUBLISH Messages from the Broker to the Subscriber
           Clients . . . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Authorizing SUBSCRIBE Messages  . . . . . . . . . . . . . . .  15
   5.  Token Expiration and Reauthentication . . . . . . . . . . . .  15
   6.  Handling Disconnections and Retained Messages . . . . . . . .  16
   7.  Reduced Protocol Interactions for MQTT v3.1.1 . . . . . . . .  16
     7.1.  Token Transport . . . . . . . . . . . . . . . . . . . . .  16
     7.2.  Handling Authorization Errors . . . . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     11.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Checklist for profile requirements . . . . . . . . .  22
   Appendix B.  Document Updates . . . . . . . . . . . . . . . . . .  22
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  23
   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 also 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 after connecting to the MQTT
   Broker, a Client can publish and subscribe to multiple topics.  The
   MQTT Broker is responsible for distributing messages published by the
   publishers to the appropriate subscribers.  Publisher messages
   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.  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.

   Clients use an access token, bound to a proof-of-possession (PoP) 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.  This document
   describes the following exchanges between Clients and the Broker.

   o  Authorizing connection requests from the Clients to 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

   To provide communication confidentiality and Resource Server
   authentication, TLS is used, and TLS 1.3 is RECOMMENDED.  This
   document makes the same assumptions as the Section 4 of the ACE
   framework [I-D.ietf-ace-oauth-authz] regarding Client and RS



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   registration with the Authorization Server (AS) and setting up keying
   material.  While the Client-Broker exchanges are only over MQTT, the
   required Client-AS and RS-AS interactions are described for HTTPS-
   based communication, using 'application/ace+json' content type, and
   unless otherwise specified, using JSON encoding.  The token may be a
   reference, or JSON Web Token (JWT).  For JWT tokens, this document
   follows RFC 7800 [RFC7800] for PoP semantics for JWTs.  The Client-AS
   and RS-AS may also be other than HTTPS e.g., CoAP or MQTT.  It may
   also be possible to use 'application/ace+cbor' content type, and CBOR
   encoding, and CBOR Web Token (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].



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

   Broker
           The Server in MQTT.  It acts as an intermediary between
           Clients that publishes 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.

   QoS level
           The level of assurance for the delivery of an Application
           Message.  The QoS level can be 0-2, where "0" indicates "At
           most once delivery", "1" "At least once delivery", and "2"
           "Exactly once delivery".

   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.




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   AUTH
           Authentication Exchange.  An AUTH packet is sent from the
           Client to the Broker or to the Broker to the Client as part
           of an extended authentication exchange.  AUTH Properties
           include Authentication Method and Authentication Data.  The
           Authentication Method is set in the CONNECT packet, and
           consequent AUTH packets follow the same Authentication
           Method.  The contents of the Authentication Data are defined
           by the Authentication Method.

   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.  The "Keep Alive"
           period is set in the CONNECT message.

   PINGRESP
           Response sent by the Broker to the Client in response to
           PINGREQ.  It indicates the Broker is alive.

   Will
           If the network connection is not closed normally, the Server
           sends a last Will message for the Client, if the Client
           provided one in its CONNECT message.  If the Will Flag is
           set, then the payload of the CONNECT message includes
           information about the Will.  The information consists of the
           Will Properties, Will Topic, and Will Payload fields.






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2.  Authorizing Connection Requests

   This section specifies how Client connections can be authorized by an
   MQTT Broker.Figure 1 shows the basic protocol flow during connection
   set-up.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.  The Client and Broker use
   only MQTT to communicate between them.

   If the Client is resource-constrained, the Client's Authorisation
   Server may carry out the token request on behalf of the Client, and
   later, onboard the Client with the token.  Also, the C-AS and Broker-
   AS interfaces may be implemented using protocols other than HTTPS,
   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 set-up




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2.1.  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 follows the token request as is
   described in Section 5.6.1 of the ACE framework
   [I-D.ietf-ace-oauth-authz], howevever, it MUST set the profile
   parameter to 'mqtt_tls'.  The media format is 'application/ace+json'.
   The AS uses a JSON structure in the payload of its responses both to
   the Client and the RS.

   If the AS successfully verifies the access token request and
   authorizes the Client for the indicated audience (i.e., RS) and
   scopes (i.e., 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.
   The 'cnf' parameter in the web tokens are to be consumed by the
   resource server and not the Client.  The PoP token may include a
   'rs_cnf' parameter containing the information about the public key
   used by the RS to authenticate as described in
   [I-D.ietf-ace-oauth-params].

   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].

2.2.  Client Connection Request to the Broker (C)

2.2.1.  Client-Server Authentication over TLS and MQTT

   The Client and the Broker MUST perform mutual authentication.  The
   Client MAY authenticate to the Broker over MQTT or TLS.  For MQTT,
   the options are "None" and "ace".  For TLS, the options are "Anon"
   for anonynous client, and "Known(RPK/PSK)" for Raw Public Keys (RPK)
   and Pre-Shared Keys (PSK), respectively.  Combined, the Client
   authentication takes the following options:

   o  "TLS:Anon-MQTT:None": This option is used only for the topics that
      do not require authorization, including the "authz-info" topic.
      Publishing to the "authz-info" topic is described in
      Section 2.2.2.

   o  "TLS:Anon-MQTT:ace": The token is transported inside the CONNECT
      message, and MUST be validated using one of the methods described



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      in Section 2.2.2.  This also supports a tokenless connection
      request for AS discovery.

   o  "TLS:Known(RPK/PSK)-MQTT:none": For the RPK, the token MUST have
      been published to the "authz-info" topic.  For the PSK, the token
      MAY have be provided in the "psk_identity".  The TLS session set-
      up is as described in DTLS profile for ACE
      [I-D.ietf-ace-dtls-authorize].

   o  "TLS:Known(RPK/PSK)-MQTT:ace": This option SHOULD NOT be chosen.
      In any case, the token transported in the CONNECT overwrites any
      permissions passed during the TLS authentication.

   It is RECOMMENDED that the Client follows TLS:Anon-MQTT:ace.

   The Broker MUST be authenticated during TLS handshake.  If the Client
   authentication included TLS:Known(RPK/PSK), then the Broker is
   authenticated using the respective method.  For the other Client
   Authentication cases, to authenticate the Broker, the client MAY
   either have the ability to receive and validate a server-side
   certificate or an RPK from the Broker against the 'rs_cnf' parameter
   in the token.

2.2.2.  authz-info: The Authorization Information Topic

   In the cases when the Client MUST transport the token to the Broker
   before the TLS handshake, the Client connects to the Broker and
   publishes its token to the "authz-info" topic.  The "authz-info"
   topic MUST be publish-only for Clients (i.e., the Clients are not
   allowed to subscribe to it).  The Broker stores and indexes all
   tokens received to this topic in its key store similar to DTLS
   profile for ACE [I-D.ietf-ace-dtls-authorize].

   The Broker must verify the validity of the token (i.e., through local
   validation or introspection) as described in Section 2.2.5.  The
   Broker returns 'Not authorized' error to a PUBLISH request if the QoS
   level of this PUBLISH message is greater or equal to 1.  After
   publishing the token, the Client disconnects from the Broker and is
   expected to try reconnecting over TLS.

2.2.3.  Transporting Access Token Inside the MQTT CONNECT

   This section describes how the Client transports the token to the
   Broker (RS) inside the CONNECT message.  If this method is used, the
   Client TLS connection is expected to be anonymous, and the Broker is
   authenticated during the TLS connection set-up.  The approach
   described in this section is similar to an earlier proposal by
   Fremantle et al.  [fremantle14].



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   Figure 2 shows the structure of the MQTT CONNECT 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                          |
          +------------------------------------------------------+

    Figure 2: MQTT v5 CONNECT control message with ACE authentication.
                         (CPT=Control Packet Type)

   The CONNECT message flags include Username, Password, Will retain,
   Will QoS, Will Flag, Clean Start, and Reserved.  Figure 6 shows how
   the MQTT connect flags MUST be set to use AUTH packets for
   authentication and authorisation.  For AUTH, the username and
   password flags MUST be set to 0.  The RS MAY support token transport
   using username and password (the CONNECT message for that option is
   described in Section 7 for MQTT v3.1.1).

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

                     Figure 3: CONNECT flags for AUTH




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   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 may specify a Will Delay
   Interval in Will Properties.  Section 6 explains how the Broker deals
   with the retained messages in further detail.

   In MQTT v5, to achieve a clean session (i.e., the session starts
   without an existing session), the Client sets the Clean Start Flag to
   1 and, the Session Expiry Interval to 0 in the CONNECT message.
   However, in this profile, the Broker MUST always start with a clean
   session regardless of how these parameters are set.  The Broker MUST
   set the Session Present flag to 0 in the CONNACK packet as a
   response.

2.2.4.  Authentication Using AUTH Property

   To use AUTH, 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 using a challenge from the TLS session

   o  Proof-of-Possession via Broker generated challenge/response

   o  Unauthorised request: Authorisation Server discovery

2.2.4.1.  Proof-of-Possession Using a Challenge from the TLS session

   For this option, the Authentication Data MUST contain the token and
   the keyed message digest (MAC) or the Client signature.  The
   challenge that is used as part of the proof-of-possession, i.e., to
   calculate the keyed message digest (MAC) or the Client signature, is
   obtained using using a TLS exporter ([RFC5705] for TLS 1.2 and for



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   TLS 1.3, Section 7.5 of [RFC8446]).  The token is also validated as
   described in Section 2.2.5 and the server responds with a CONNACK
   with the appropriate response code.

2.2.4.2.  Proof-of-Possession via Broker-generated Challenge/Response

   For this option, the RS follows a Broker-generated 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 2.2.5.

                                Resource
                    Client      Server
                     |             |
                     |<===========>| TLS connection set-up
                     |             |
                     |             |
                     +------------>| 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

2.2.4.3.  Unauthorised Request: Authorisation Server Discovery

   Finally, this document allows the CONNECT message to have the
   Authentication Method set to 'ace' followed by an empty
   Authentication Data field.  This is the AS discovery option and the



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   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 defined in the Section 5.1.2 of the ACE framework
   [I-D.ietf-ace-oauth-authz].

2.2.5.  Token Validation

   The RS MUST verify the validity of the token either locally (e.g., in
   the case of a self-contained token) or the RS MAY send an
   introspection request to the AS.  RS MUST verify the claims according
   to the rules set in the Section 5.8.1.1 of the ACE framework
   [I-D.ietf-ace-oauth-authz].

   To authenticate the Client, the RS validates the signature or the
   MAC, depending on how the PoP protocol is implemented.  Validation of
   the signature or MAC MUST fail if the signature algorithm is set to
   "none", when the key used for the signature algorithm cannot be
   determined, or the computed and received signature/MAC do not match.
   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.

   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'.  If the Will Flag is set,then the Broker MUST
   check that the token allows the publication of the Will message
   (i.e., the scope is "publish_" followed by the Will Topic).

2.2.6.  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.  The Broker
   MUST also set Session Present to 0 in the CONNACK packet to signal a
   clean session to the Client.  In case of an invalid PoP token, the
   CONNACK reason code is '0x87 (Not Authorized)'.

   If the Broker accepts the connection, it MUST store the token until
   the end of connection.  On Client or Broker disconnection, the Client
   is expected to provide a token again inside the next CONNECT message.

   If the token is not self-contained and the Broker uses token
   introspection, it MAY cache the validation result to authorize the



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   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.

3.  Authorizing PUBLISH Messages

3.1.  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 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 5, and pass a new token
   following the same PoP methods as described in Figure 2.

3.2.  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.
   There is no way to inform the Clients with invalid tokens 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).







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4.  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.

5.  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.  The
   Broker MAY also check for token expiration periodically e.g., every
   hour.  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 AS 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'.  After sending a DISCONNECT message, the network
   connection is closed, and no more messages can be sent.  However, as
   a response to the PUBACK and SUBACK, 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 2.2.  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 i.e., before they receive a message
   with 'Not authorized' return code.




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6.  Handling Disconnections and Retained Messages

   In the case of a Client DISCONNECT, 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
   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.

7.  Reduced Protocol Interactions for MQTT v3.1.1

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

7.1.  Token Transport

   As in MQTT v5, The Token MAY either be transported before the TLS
   session publishing to the "authz-info" topic, or inside the CONNECT
   message.

   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.

   To transport the token to the Broker inside the CONNECT message, the
   Client uses the username and password fields of the CONNECT message.
   Figure 5 shows the structure of the MQTT CONNECT 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                                              |
          |     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     |  X   |  0  |
   +-----------------------------------------------------------+

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

   The Clean Session Flag is ignored, and the Broker always sets up a
   clean session.  On connection success, the Broker MUST set the
   Session Present flag to 0 in the CONNACK packet.

   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 as the header does not have a field to indicate the
   authentication method.  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 MUST apply the PoP key on the challenge
   derived from the TLS session as described in Section 2.2.4.1.





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   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.)

7.2.  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
      MUST 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.

8.  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.



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   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]

9.  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.

   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.  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 Section 2.2.2, 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.





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10.  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.

11.  References

11.1.  Normative References

   [I-D.ietf-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-ietf-ace-dtls-
              authorize-08 (work in progress), April 2019.

   [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-25
              (work in progress), October 2019.

   [I-D.ietf-ace-oauth-params]
              Seitz, L., "Additional OAuth Parameters for Authorization
              in Constrained Environments (ACE)", draft-ietf-ace-oauth-
              params-05 (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>.







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   [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>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/info/rfc5705>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

11.2.  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-11 (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 2.2.

   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: Several options are possible
      and descibed in Section 2.2.1.

   o  Content format: For the HTTPS interactions with AS, "application/
      ace+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: Client or its Client AS uses HTTPS /token interface
      of AS.

   o  /authz-info endpoint: It MAY be supported using the method
      described in Section 2.2.2, but is not protected.

   o  Token transport: Via "authz-info topic", or 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 2.2.

Appendix B.  Document Updates

   Version 01 to 02:

   o  Expanded Client connection authorization to capture different
      options for Client and Broker authentication over TLS and MQTT



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   o  Removed Payload (and specifically Client Identifier) from proof-
      of-possesion in favor of using tls-exporter for a TLS-session
      based challenge.

   o  Moved token transport via "authz-info" topic from the Appendix to
      the main text.

   o  Clarified Will scope.

   o  Added MQTT AUTH to terminology.

   o  Typo fixes, and simplification of figures.

   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 to explain AUTH packets exchang, 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.

Authors' Addresses








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   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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