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Versions: 00 01

ACE Working Group                                              M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                                L. Seitz
Expires: September 10, 2020                                    Combitech
                                                            F. Palombini
                                                             Ericsson AB
                                                           S. Echeverria
                                                                G. Lewis
                                                                 CMU SEI
                                                          March 09, 2020


    Notification of Revoked Access Tokens in the Authentication and
       Authorization for Constrained Environments (ACE) Framework
             draft-tiloca-ace-revoked-token-notification-01

Abstract

   This document specifies a method of the Authentication and
   Authorization for Constrained Environments (ACE) framework, which
   allows an Authorization Server to notify Clients and Resource Servers
   (i.e., registered devices) about revoked Access Tokens.  The method
   relies on resource observation for the Constrained Application
   Protocol (CoAP), with Clients and Resource Servers observing a Token
   Revocation List on the Authorization Server.  Resulting unsolicited
   notifications of revoked Access Tokens complement alternative
   approaches such as token introspection, while not requiring
   additional endpoints on Clients and Resource Servers.

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 September 10, 2020.






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Copyright Notice

   Copyright (c) 2020 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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Token Hash  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  The TRL Resource  . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Update of the TRL Resource  . . . . . . . . . . . . . . .   7
   5.  The TRL Endpoint  . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Full Query of the TRL . . . . . . . . . . . . . . . . . .   8
     5.2.  Diff Query of the TRL . . . . . . . . . . . . . . . . . .   9
   6.  Upon Registration . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Notification of Revoked Tokens  . . . . . . . . . . . . . . .  11
   8.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Authentication and Authorization for Constrained Environments (ACE)
   [I-D.ietf-ace-oauth-authz] is a framework that enforces access
   control on IoT devices acting as Resource Servers.  In order to use
   ACE, both Clients and Resource Servers have to register with an
   Authorization Server and become a registered device.  Once
   registered, a Client can send a request to the Authorization Server
   for an Access Token for a Resource Server.  For a Client to access
   the Resource Server, the Client must present the issued Access Token
   at the Resource Server, which then validates and stores it.



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   Even though Access Tokens have expiration times, there are
   circumstances by which an Access Token may need to be revoked before
   its expiration time, such as: (1) a registered device has been
   compromised, or is suspected of being compromised; (2) a registered
   device is decommissioned; (3) there has been a change in access
   policies for a registered device; and (4) there has been a change in
   the ACE profile for a registered device.

   As discussed in Section 6.1 of [I-D.ietf-ace-oauth-authz], only
   client-initiated revocation is currently specified [RFC7009] for
   OAuth 2.0, based on the assumption that Access Tokens in OAuth are
   issued with a relatively short lifetime.  However, this may not be
   the case for constrained, intermittently connected devices, that need
   Access Tokens with relatively long lifetimes.

   This document specifies a method for allowing registered devices to
   access and observe a Token Revocation List (TRL) resource on the
   Authorization Server, in order to get an updated list of revoked, but
   yet not expired, pertaining Access Tokens.  In particular, registered
   devices rely on resource observation for the Constrained Application
   Protocol (CoAP) [RFC7641].  The benefits of this method are that it
   complements token introspection and does not require any additional
   endpoints on the registered devices.

1.1.  Terminology

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

   Readers are expected to be familiar with the terms and concepts
   described in the ACE framework for Authentication and Authorization
   [I-D.ietf-ace-oauth-authz], as well as with terms and concepts
   related to CBOR Web Tokens (CWTs) [RFC8392], and JSON Web Tokens
   (JWTs) [RFC7519].  The terminology for entities in the considered
   architecture is defined in OAuth 2.0 [RFC6749].  In particular, this
   includes Client, Resource Server, and Authorization Server.

   Readers are also expected to be familiar with the terms and concepts
   related to CBOR [RFC7049], JSON [RFC8259], the CoAP protocol
   [RFC7252], CoAP Observe [RFC7641], and the use of hash functions to
   name objects as defined in [RFC6920].

   Note that, unless otherwise indicated, the term "endpoint" is used
   here following its OAuth definition, aimed at denoting resources such
   as /token and /introspect at the Authorization Server, and /authz-



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   info at the Resource Server.  This document does not use the CoAP
   definition of "endpoint", which is "An entity participating in the
   CoAP protocol."

   This specification also refers to the following terminology.

   o  Token hash: identifier of an Access Token, in binary format
      encoding.  The token hash has no relation to other possibly used
      token identifiers, such as the "cti" (CWT ID) claim of CBOR Web
      Tokens (CWTs) [RFC8392].

   o  Token Revocation List (TRL): a collection of token hashes, in
      which the corresponding Access Tokens have been revoked but are
      not expired yet.

   o  TRL resource: a resource on the Authorization Server, with a TRL
      as its representation.

   o  TRL endpoint: an endpoint at the Authorization Server associated
      to the TRL resource.  The default name of the TRL endpoint in a
      url-path is '/revoke/trl'.  Implementations are not required to
      use this name, and can define their own instead.

   o  Registered device: a device registered at the Authorization
      Server, as a Client, a Resource Server, or both.  A registered
      device acts as caller of the TRL endpoint.

   o  Administrator: entity authorized to get full access to the TRL at
      the Authorization Server, and acting as caller of the TRL
      endpoint.  An administrator is not necessarily a registered device
      as defined above, i.e. a Client requesting Access Tokens or a
      Resource Server consuming Access Tokens.  How the administrator
      authorization is established and verified is out of the scope of
      this specification.

   o  Pertaining Access Token:

      *  With reference to an administrator, an Access Token issued by
         the Authorization Server.

      *  With reference to a registered device, an Access Token intended
         to be owned by that device.  An Access Token pertains to a
         Client if the Authorization Server has issued the Access Token
         and provided it to that Client.  An Access Token pertains to a
         Resource Server if the Authorization Server has issued the
         Access Token to be consumed by that Resource Server.





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2.  Protocol Overview

   This protocol defines how a CoAP-based Authorization Server informs
   Clients and Resource Servers, i.e. registered devices, about revoked
   Access Tokens.  How the relationship between the registered device
   and the Authorization Server is established is out of the scope of
   this specification.

   At a high level, the steps of this protocol are as follows.

   o  Upon startup, the Authorization Server creates a TRL resource.  At
      any point in time, the TRL resource represents the list of all
      revoked Access Tokens issued by the Authorization Server that are
      yet not expired.

   o  When a device is registered at the Authorization Server, it
      receives the url-path to the TRL resource.  After the registration
      procedure is finished, the registered device sends an Observation
      Request to that TRL resource as described in [RFC7641], i.e. a GET
      request with an Observe option set to 0 (register).  Upon
      receiving the request, the Authorization Server adds the
      registered device to the list of observers of the TRL resource.
      At any time, the registered device can send a GET request to the
      TRL endpoint, in order to get the current list of pertaining
      revoked Access Tokens.

   o  When an Access Token is revoked, the Authorization Server adds the
      corresponding token hash to the TRL.  Also, when a revoked Access
      Token eventually expires, the Authorization Server removes the
      corresponding token hash from the TRL.  In either case, after
      updating the TRL, the Authorization Server sends Observe
      Notifications as described in [RFC7641].  That is, one Observe
      Notification is sent to each registered device the Access Token
      pertains to, and specifies the current updated list of token
      hashes in the portion of the TRL pertaining to that device.

   o  An administrator can observe and access the TRL like a registered
      device, while getting the full updated representation of the TRL.

   Figure 1 provides a high-level overview of the service provided by
   this protocol.  Each dotted line associated to a pair of registered
   devices indicates the Access Token that they both own.  In
   particular, Figure 1 shows the Observe Notifications sent by the
   Authorization Server to four registered devices and one
   administrator, upon revocation of the issued Access Tokens t1, t2 and
   t3, with token hash th1, th2 and th3, respectively.





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                           +---------------+
                           |               |
                           | Authorization |
                           |    Server     |
                           |               |
                           +-------o-------+
                       revoke/trl  |  TRL: {th1,th2,th3}
                                   |
                                   |
    +-----------------+------------+------------+------------+
    |                 |            |            |            |
    |                 |            |            |            |
    | th1,th2,th3     | th1,th2    | th1        | th3        | th2,th3
    v                 v            v            v            v
   +---------------+ +----------+ +----------+ +----------+ +----------+
   |               | |          | |          | |          | |          |
   | Administrator | | Client 1 | | Resource | | Client 2 | | Resource |
   |               | |          | | Server 1 | |          | | Server 2 |
   +---------------+ +----------+ +----------+ +----------+ +----------+
                        :    :        :           :            :    :
                        :    :   t1   :           :     t3     :    :
                        :    :........:           :............:    :
                        :                                           :
                        :                    t2                     :
                        :...........................................:


                        Figure 1: Protocol Overview

   A more detailed example describing the protocol flow and message
   exchange between the Authorization Server and a registered device is
   provided in Section 8.

3.  Token Hash

   The token hash of an Access Token is generated as follows.

   1.  The Authorization Server defines ENCODED_TOKEN, as the value of
       the 'access_token' field from the Authorization Server response
       where that Access Token was included and returned from the /token
       endpoint to the requesting Client.

   2.  The Authorization Server defines HASH_INPUT as:

       *  If CBOR was used to transport the Access Token, the byte-
          string value of ENCODED_TOKEN.





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       *  If JSON was used to transport the Access Token, the binary
          representation of the String value of ENCODED_TOKEN, which
          would depend on the used charset.

   3.  The Authorization Server generates a hash value of HASH_INPUT as
       per Section 6 of [RFC6920].  The resulting output in binary
       format is used as the token hash.

   The specifically used hash-function MUST be collision-resistant on
   byte-strings, and MUST be selected from the "Named Information Hash
   Algorithm" Registry defined in Section 9.4 of [RFC6920].

4.  The TRL Resource

   Upon startup, the Authorization Server creates a single TRL resource.

   The initial content of the TRL resource representation MUST be an
   empty CBOR array, i.e. the TRL is initialized as empty.

   The order of the token hashes in the CBOR array is irrelevant, and
   the CBOR array MUST be treated as a set in which the order has no
   significant meaning.

4.1.  Update of the TRL Resource

   The Authorization Server updates the TRL in the following two cases.

   o  When a non-expired Access Token is revoked, the token hash of the
      Access Token is added to the TRL resource representation.  That
      is, the token hash is added to the CBOR array used as TRL resource
      representation.

   o  When a revoked Access Token expires, the token hash of the Access
      Token is removed from the TRL resource representation.  That is,
      the token hash is removed from the CBOR array used as TRL resource
      representation.

5.  The TRL Endpoint

   Consistently with Section 6.5 of [I-D.ietf-ace-oauth-authz], all
   communications between a caller of the TRL endpoint and the
   Authorization Server MUST be encrypted, integrity and replay
   protected.  Furthermore, responses from the Authorization Server to
   the caller MUST be bound to the caller's request.

   The Authorization Server MUST implement measures to prevent access to
   the TRL endpoint by entities other than registered devices and
   authorized administrators.



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   The TRL endpoint supports only the GET method, and provides two types
   of query of the TRL.

   o  Full query: the Authorization Server returns the token hashes of
      the revoked Access Tokens currently in the TRL and pertaining to
      the issuer of the GET request.  The processing of a full query and
      the related response format are defined in Section 5.1.

   o  Diff query: the Authorization Server returns a set of diff
      entries.  Each entry is related to one of the most recent updates,
      in the portion of the TRL pertaining to the issuer of the GET
      request.  In particular, the entry associated to one of such
      updates contains a list of token hashes, such that i) the
      corresponding revoked Access Tokens pertain to the issuer of the
      GET request; and ii) they were added to or removed from the TRL at
      that update.  The processing of a diff query and the related
      response format are defined in Section 5.2.

   The TRL endpoint admits the following query parameters in a GET
   request.

   o  'diff': if included, its value MUST be set to "true" and indicates
      to perform a diff query of the TRL.

   o  'N': if included, its value MUST be a positive integer greater
      than 0.  This parameter is relevant when requesting to perform a
      diff query of the TRL, and indicates the maximum number of diff
      entries that a (notification) response should include.  This
      parameter MUST NOT be present if the 'diff' query parameter is not
      present.

5.1.  Full Query of the TRL

   In order to produce a (notification) response to a GET request asking
   for a full query of the TRL, the Authorization Server performs the
   following actions.

   1.  The Authorization Server builds from the current TRL resource
       representation a set HASHES of token hashes, such that:

       *  If the issuer of the GET request is a registered device,
          HASHES includes the token hashes of the Access Tokens
          pertaining to that registered device.  The Authorization
          Server can use the authenticated identity of the registered
          device to perform the necessary filtering on the TRL resource
          representation.





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       *  If the issuer of the GET request is an administrator, HASHES
          includes all the token hashes in the current TRL resource
          representation.

   2.  The Authorization Server prepares a 2.05 (Content) Response for
       the GET request issuer, with a CBOR Array as payload.  Each
       element of the array specifies one of the token hashes from the
       set HASHES.

       The order of the token hashes in the CBOR array is irrelevant,
       i.e. the CBOR array MUST be treated as a set in which the order
       has no significant meaning.

5.2.  Diff Query of the TRL

   In order to produce a (notification) response to a GET request asking
   for a diff query of the TRL, the Authorization Server performs the
   following actions.

   1.  The Authorization Server defines the positive integer SIZE.  If
       the GET request did not include the query parameter N, or N was
       greater than a pre-defined positive integer N_MAX, SIZE takes the
       value of N_MAX.  Otherwise, SIZE takes the value of N.

   2.  The Authorization Server prepares U <= SIZE diff entries.  The
       entries are related to the latest U updates that affect the
       portion of the TRL pertaining to the issuer of the GET request.
       In particular, the first entry refers to the most recent of such
       updates, the second entry refers to the second from last of such
       updates, and so on.  Each diff entry is a CBOR Map, which
       includes the following two elements.

       *  The first element has label "removed" and a CBOR Array as
          value.  Each element of the array is the token hash of an
          Access Token, that pertained to the issuer of the GET request
          and that was removed from the TRL during the update associated
          to the diff entry.

       *  The second element has label "added" and a CBOR Array as
          value.  Each element of the array is the token hash of an
          Access Token, that pertains to the issuer of the GET request
          and that was added to the TRL during the update associated to
          the diff entry.

   3.  The Authorization Server prepares a 2.05 (Content) Response for
       the issuer of the GET request, with a CBOR Array of U elements as
       payload.  Each element of the array specifies one of the CBOR
       Maps prepared at point 2 as diff entries.



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       Within the CBOR Array, the CBOR Maps are sorted to reflect the
       corresponding updates to the TRL in reverse chronological order.
       That is, the first CBOR Map relates to the most recent update to
       the portion of the TRL pertaining to the issuer of the GET
       request.

       However, in each of the CBOR Maps, the order of the token hashes
       in the CBOR arrays "removed" and "added" is irrelevant, i.e.
       those CBOR arrays MUST be treated as a set in which the order has
       no significant meaning.

   If the Authorization Server supports diff queries:

   o  The Authorization Server MUST keep track of N_MAX latest updates
      to the portion of the TRL that pertains each caller of the TRL
      endpoint.  The particular method to achieve this is
      implementation-specific.

   o  The Authorization Server SHOULD provide registered devices and
      administrators with the value of N_MAX, upon their registration
      (see Section 6).

   If the Authorization Server does not support diff queries, it
   proceeds as when processing a full query (see Section 5.1).

6.  Upon Registration

   During the registration process at the Authorization Server, an
   administrator or a registered device receives the following
   information as part of the registration response.

   o  The url-path to the TRL endpoint at the Authorization Server.

   o  Optionally, a positive integer N_MAX, if the Authorization Server
      supports diff queries of the TRL resource (see Section 5.2).

   After the registration procedure is finished, the administrator or
   registered device performs a GET request to the TRL resource,
   including the CoAP Observe option set to 0 (register), in order to
   start an observation of the TRL resource at the Authorization Server,
   as per Section 3.1 of [RFC7641].  The GET request can express the
   wish for a full query (see Section 5.1) or a diff query (see
   Section 5.2).

   The Authorization Server MUST reply using the CoAP response code 2.05
   (Content) and the CoAP Observe option with value 1.  The response
   payload is formatted as defined in Section 5.1 or in Section 5.2, in




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   case the GET request was for a full query or a diff query of the TRL,
   respectively.

7.  Notification of Revoked Tokens

   In the case the TRL is updated (see Section 4.1), the Authorization
   Server sends Observe Notifications to every observer of the TRL
   resource.  Observe Notifications are sent as per Section 4.2 of
   [RFC7641].

   The content of each Observe Notification is formatted as defined in
   Section 5.1 or in Section 5.2, in case the original Observation
   Request was for a full query or a diff query of the TRL,
   respectively.

   Furthermore, an administrator or a registered device can send
   additional GET requests to the TRL endpoint at any time, in order to
   retrieve the token hashes of the pertaining revoked Access Tokens.
   When doing so, the caller of the TRL endpoint can perform a full
   query (see Section 5.1) or a diff query (see Section 5.2).

8.  Example

   Figure 2 shows an example interaction between a Resource Server RS
   and an Authorization Server AS, considering a CoAP observation and a
   full query of the TRL.

   The details of the registration process are omitted, but it is
   assumed that the Resource Server sends an unspecified payload to the
   Authorization Server, and then the Authorization Server replies with
   a 2.01 (Created) response.  In particular, the registration response
   contains a CBOR map, which includes a "trl" parameter, specifying the
   path of the TRL resource.

   The function 'h(x)' refers to the hash function used to compute the
   token hashes, as defined in Section 3 of this specifications and
   according to [RFC6920].  Assuming the usage of CWTs transported in
   CBOR, 'bstr.t1' and 'bstr.t2' denote the byte-string representations
   of the token hashes for the Access Tokens t1 and t2, respectively.

                 RS                                    AS
                 |                                      |
                 | Registration: POST                   |
                 +------------------------------------->|
                 |                                      |
                 |<-------------------------------------+
                 |           2.01 CREATED               |
                 |            Payload: {                |



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                 |                 ...                  |
                 |                 "trl" = "revoke/trl" |
                 |            }                         |
                 |                                      |
                 | GET Observe: 0                       |
                 |  coap://example.as.com/revoke/trl/   |
                 +------------------------------------->|
                 |                                      |
                 |<-------------------------------------+
                 |              2.05 CONTENT Observe: 1 |
                 |               Payload: []            |
                 |                   .                  |
                 |                   .                  |
                 |                   .                  |
                 |                                      |
                 |   (Access Tokens t1 and t2 issued    |
                 |  and successfully submitted to RS)   |
                 |                   .                  |
                 |                   .                  |
                 |                   .                  |
                 |                                      |
                 |     (Access Token t1 is revoked)     |
                 |                                      |
                 |<-------------------------------------+
                 |              2.05 CONTENT Observe: 2 |
                 |               Payload: [h(bstr.t1)]  |
                 |                    .                 |
                 |                    .                 |
                 |                    .                 |
                 |                                      |
                 |     (Access Token t2 is revoked)     |
                 |                                      |
                 |<-------------------------------------+
                 |              2.05 CONTENT Observe: 3 |
                 |               Payload: [h(bstr.t1),  |
                 |                         h(bstr.t2)]  |
                 |                    .                 |
                 |                    .                 |
                 |                    .                 |
                 |                                      |
                 |      (Access Token t1 expires)       |
                 |                                      |
                 |<-------------------------------------+
                 |              2.05 CONTENT Observe: 4 |
                 |               Payload:[h(bstr.t2)]   |
                 |                                      |

                      Figure 2: Communication example



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9.  Security Considerations

   Security considerations are inherited from the ACE framework for
   Authentication and Authorization [I-D.ietf-ace-oauth-authz], from
   [RFC8392] as to the usage of CWTs, from [RFC7519] as to the usage of
   JWTs, from [RFC7641] as to the usage of CoAP Observe, and from
   [RFC6920] with regards to resource naming through hashes.  The
   following considerations also apply.

   The Authorization Server MUST ensure that each registered device can
   access and retrieve only its pertaining portion of the TRL.  To this
   end, the Authorization Server can perform the required filtering
   based on the authenticated identity of the registered device, i.e. a
   (non-public) identifier that the Authorization Server can securely
   relate to the registered device and the secure session they use to
   communicate.

   Disclosing any information about revoked Access Tokens to entities
   other than the intended registered devices may result in privacy
   concerns.  Therefore, the Authorization Server MUST ensure that,
   other than registered devices accessing their own pertaining portion
   of the TRL, only authorized and authenticated administrators can
   instead retrieve the full TRL.  To this end, the Authorization Server
   may rely on an access control list or similar.

   If a registered device has many non-expired Access Tokens associated
   to it that are revoked, the pertaining portion of the TRL could grow
   to a size bigger than what the registered device is prepared to
   handle upon reception, especially if relying on a full query of the
   TRL resource (see Section 5.1).  This could be exploited by attackers
   to negatively affect the behaviour of a registered device.  Short
   expiration times could help reduce the size of a TRL, but an
   Authorization Server SHOULD take measures to limit this size.

   Most of the communication about revoked Access Tokens presented in
   this specification relies on CoAP Observe Notifications sent from the
   Authorization Server to a registered device.  The suppression of
   those notifications by an external attacker that has access to the
   network would prevent registered devices from ever knowing that their
   pertaining Access Tokens have been revoked.  To avoid this, a
   registered device SHOULD NOT rely solely on the CoAP Observe
   notifications.  In particular, a registered device SHOULD also
   regularly poll the Authorization Server for the most current
   information about revoked Access Tokens, by sending GET requests to
   the TRL endpoint according to an application policy.






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10.  IANA Considerations

   This document has no actions for IANA.

11.  References

11.1.  Normative References

   [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-33
              (work in progress), February 2020.

   [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>.

   [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>.

   [RFC6920]  Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
              Keranen, A., and P. Hallam-Baker, "Naming Things with
              Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
              <https://www.rfc-editor.org/info/rfc6920>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.





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   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

11.2.  Informative References

   [RFC7009]  Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
              2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
              August 2013, <https://www.rfc-editor.org/info/rfc7009>.

Acknowledgments

   The authors sincerely thank Jim Schaad, Goeran Selander and Travis
   Spencer for their comments and feedback.

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC.

Authors' Addresses

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: marco.tiloca@ri.se


   Ludwig Seitz
   Combitech
   Djaeknegatan 31
   Malmoe  SE-21135 Malmoe
   Sweden

   Email: ludwig.seitz@combitech.se






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   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   Kista  SE-16440 Stockholm
   Sweden

   Email: francesca.palombini@ericsson.com


   Sebastian Echeverria
   CMU SEI
   4500 Fifth Avenue
   Pittsburgh, PA  15213-2612
   United States of America

   Email: secheverria@sei.cmu.edu


   Grace Lewis
   CMU SEI
   4500 Fifth Avenue
   Pittsburgh, PA  15213-2612
   United States of America

   Email: glewis@sei.cmu.edu


























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