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Versions: 00 01 draft-ietf-ace-oauth-authz

Network Working Group                                      H. Tschofenig
Internet-Draft                                               ARM Limited
Intended status: Informational                             March 8, 2015
Expires: September 9, 2015


   The OAuth 2.0 Bearer Token Usage over the Constrained Application
                            Protocol (CoAP)
                  draft-tschofenig-ace-oauth-bt-01.txt

Abstract

   This specification describes how to use OAuth 2.0 bearer tokens to
   access protected resources using the Constrained Application Protocol
   (CoAP).  Any party in possession of a bearer token (a "bearer") can
   use it to get access to the associated resources (without
   demonstrating possession of a cryptographic key).  To prevent misuse,
   bearer tokens need to be protected from disclosure in storage and in
   transport.

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 http://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 9, 2015.

Copyright Notice

   Copyright (c) 2015 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
   (http://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



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   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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Requests  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Responses . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   OAuth enables clients to access protected resources by obtaining an
   access token, which is defined in "The OAuth 2.0 Authorization
   Framework" [4] as "a string representing an access authorization
   issued to the client", rather than using the resource owner's
   credentials directly.

   Tokens are issued to clients by an authorization server and the
   client uses the access token to access the protected resources hosted
   by the resource server.  This specification describes how to make
   protected resource requests when the access token is a bearer token
   and conveyed from the client to the resource server using the
   Constrained Application Protocol (CoAP) [3].  To secure the
   communication exchange the Datagram Transport Layer Security Version
   1.2 [1] is mandatory to use.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in "Key words for use in
   RFCs to Indicate Requirement Levels" [5].

   This document also re-uses terminology from RFC 6749 [4] and RFC 6750
   [2].








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3.  Introduction

   The abstract OAuth 2.0 flow illustrated in Figure 1 describes the
   interaction between the client, resource owner, authorization server,
   and resource server (described in [[4]).  The following two steps are
   specified within this document:

      (E) The client requests the protected resource from the resource
      server and authenticates by presenting the access token.

      (F) The resource server validates the access token, and if valid,
      serves the request.

   This document also imposes semantic requirements upon the access
   token returned in step (D).


        +--------+                               +---------------+
        |        |--(A)- Authorization Request ->|   Resource    |
        |        |                               |     Owner     |
        |        |<-(B)-- Authorization Grant ---|               |
        |        |                               +---------------+
        |        |
        |        |                               +---------------+
        |        |--(C)-- Authorization Grant -->| Authorization |
        | Client |                               |     Server    |
        |        |<-(D)----- Access Token -------|               |
        |        |                               +---------------+
        |        |
        |        |                               +---------------+
        |        |--(E)----- Access Token ------>|    Resource   |
        |        |                               |     Server    |
        |        |<-(F)--- Protected Resource ---|               |
        +--------+                               +---------------+


                Figure 1: Abstract OAuth 2.0 Protocol Flow.

4.  Requests

   Access tokens are embedded in a CoAP message by the use of the
   "Bearer" option.  The definition is shown in Figure 2.









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   +-----+----+---+---+---+----------------+--------+--------+---------+
   | No. | C  | U | N | R | Name           | Format | Length | Default |
   +-----+----+---+---+---+----------------+--------+--------+---------+
   | TBD | x  |   |   |   | Bearer         | opaque | var    | (none)  |
   +-----+----+---+---+---+----------------+--------+--------+---------+

   Legend:
     C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable


                    Figure 2: Bearer Option Definition.

   Figure 3 shows an example request from the client to the resource
   server.


   Header: GET (T=CON, Code=0.01, MID=0x7d34,
               Uri-Path:"resource", Bearer: "mF_9.B5f-4.1JqM")


             Figure 3: Example CoAP Request with Bearer Token.

5.  Responses

   If the request does not contain an access token that enables access
   to the protected resource, the resource server MUST respond with a
   4.01 "Unauthorized" error message.

   QUESTION: Should the response also provide information about the
   scheme (e.g., 'Bearer Tokens' vs. 'Proof-of-Possession Tokens')?
   Should it contain a "realm" attribute as well?  Should a scope value
   be returned to provide some guidance about the available scopes at
   that resource server?

   For example, in response to a protected resource request without a
   needed bearer token the error response shown in Figure 4 is sent.


   Header: ACK (T=ACK, Code=4.01, MID=0x7d34)


           Figure 4: Failed Request due to Missing Access Token.

   To provide information back to the client about the failure of the
   request the following error codes are defined.  These error codes are
   conveyed within the 'Error' option, which is defined in Figure 5.





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   +-----+----+---+---+---+----------------+--------+--------+---------+
   | No. | C  | U | N | R | Name           | Format | Length | Default |
   +-----+----+---+---+---+----------------+--------+--------+---------+
   | TBD |    |   |   |   | Error          | uint   |  0-2   | (none)  |
   +-----+----+---+---+---+----------------+--------+--------+---------+

   Legend:
     C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable


                    Figure 5: Error Option Definition.

   This specification defines the following error codes that are used
   with the 'Error' option:

   invalid_request (0)

      The request is missing a required parameter, includes an
      unsupported parameter or parameter value, repeats the same
      parameter, uses more than one method for including an access
      token, or is otherwise malformed.  The resource server MUST
      respond with the 4.00 (Bad Request) status code.

   invalid_token (1)

      The access token provided is expired, revoked, malformed, or
      invalid for other reasons.  The resource MUST respond with the
      4.01 (Unauthorized) status code.  The client MAY request a new
      access token and retry the protected resource request.

   insufficient_scope (2)

      The request requires higher privileges than provided by the access
      token.  The resource server MUST respond with the 4.03 (Forbidden)
      status code.

   QUESTION: Is the granularity of the error messages useful enough for
   client implementations to take actions?

   As an example, in response to a request using an expired access token
   the following error is returned.


   Header: ACK (T=ACK, Code=4.01, MID=0x7d34,
                Error="1")


            Figure 6: Failed Request due Expired Access Token.



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6.  Security Considerations

   The security threats of this specification are identical to those
   discussed in RFC 6750 since the encoding of the request does not
   change the security threats.

   It is nevertheless worthwhile to replicate the security
   recommendation here for readers who do not want to consult another
   document.

   Safeguard bearer tokens:  Client implementations MUST ensure that
      bearer tokens are not leaked to unintended parties, as they will
      be able to use them to gain access to protected resources.  This
      is the primary security consideration when using bearer tokens and
      underlies all the more specific recommendations that follow.

   Validate TLS certificate chains:  The client MUST validate the
      certificate chain, if the resource server is authenticated using a
      certificate-based ciphersuite in DTLS, when making requests to
      protected resources.  Failing to do so may enable man-in-the-
      middle attacks.

   Always use DTLS (coaps):  Clients MUST always use DTLS [1] when
      making requests with bearer tokens.  Failing to do so exposes the
      token to third parties and could consequently give attackers
      unintended access.

   Issue short-lived bearer tokens:  Authorization servers SHOULD issue
      short-lived bearer tokens.  Using short-lived bearer tokens
      reduces the impact of them being leaked.

   Issue scoped bearer tokens:  Authorization servers MUST issue bearer
      tokens that contain an audience restriction, scoping their use to
      the intended relying party or set of relying parties.

7.  IANA Considerations

   This specification requests IANA to allocate two values, as shown
   below, in the 0..255 range of the CoAP option number registry
   established with RFC 7252.











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    +--------+-----------+------------+
    | Number | Name      | Reference  |
    +--------+-----------+------------+
    | TBD-61 | Bearer    | [this RFC] |
    | TBD-62 | Error     | [this RFC] |
    +--------+-----------+------------+


   TBD: Add a registry for error codes.

8.  Acknowledgements

   The author would like to thank Mike Jones and Dick Hardt for their
   work on RFC 6750.  This document is heavily inspired by their work.

9.  Normative References

   [1]        Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [2]        Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, October 2012.

   [3]        Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, June 2014.

   [4]        Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
              6749, October 2012.

   [5]        Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

Author's Address

   Hannes Tschofenig
   ARM Limited
   Austria

   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at











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