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Network Working Group                                    A. Backman, Ed.
Internet-Draft                                                    Amazon
Intended status: Standards Track                       November 19, 2019
Expires: May 22, 2020


     Proof-of-Possession Tokens for OAuth Using JWS HTTP Signatures
               draft-richanna-oauth-http-signature-pop-00

Abstract

   This document describes a method of generating and validating proof-
   of-possession tokens for use with OAuth 2.0.  The required proof is
   provided via a JSON Web Signature (JWS) representing a signature of a
   minimal subset of elements from the HTTP request.

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
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   This Internet-Draft will expire on May 22, 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Generating the Signature  . . . . . . . . . . . . . . . . . .   3
   4.  Sending the signed object . . . . . . . . . . . . . . . . . .   3
     4.1.  HTTP Authorization header . . . . . . . . . . . . . . . .   3
     4.2.  HTTP Form body  . . . . . . . . . . . . . . . . . . . . .   4
     4.3.  HTTP Query parameter  . . . . . . . . . . . . . . . . . .   4
   5.  Validating the request  . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  The 'pop' OAuth Access Token Type . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
     7.1.  Denial of Service . . . . . . . . . . . . . . . . . . . .   5
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   5
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   In order to prove possession of an access token and its associated
   key, an OAuth 2.0 client needs to compute some cryptographic function
   and present the results to the protected resource as a signature.
   The protected resource then needs to verify the signature and compare
   that to the expected keys associated with the access token.  This is
   in addition to the normal token protections provided by a bearer
   token [RFC6750] and transport layer security (TLS).

   Furthermore, it is desirable to bind the signature to the HTTP
   request.  Ideally, this should be done without replicating the
   information already present in the HTTP request more than required.
   However, many HTTP application frameworks insert extra headers, query
   parameters, and otherwise manipulate the HTTP request on its way from
   the web server into the application code itself.  It is the goal of
   this draft to have a signature protection mechanism that is
   sufficiently robust against such deployment constraints while still
   providing sufficient security benefits.

   The key required for this signature calculation is distributed via
   mechanisms described in companion documents (see
   [I-D.ietf-oauth-pop-key-distribution] and
   [I-D.ietf-oauth-pop-architecture]).  The JSON Web Signature (JWS)
   specification [RFC7515] is used for computing a digital signature
   (which uses asymmetric cryptography) or a keyed message digest (in
   case of symmetric cryptography).





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   The mechanism described in this document assumes that a client is in
   possession of an access token and asociated key.  That client then
   creates a signature of elements of the HTTP request as a JWS, as
   described in [[ draft-richanna-http-request-signing-jws-00 ]],
   including the access token as an additional top-level member of the
   signature's payload JSON object, and issues a request to a resource
   server for access to a protected resource using the signature as its
   authorization.  The protected resource validates the signature and
   parses the JSON object to obtain token information.

2.  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 RFC
   2119 [RFC2119].

   Other terms such as "client", "authorization server", "access token",
   and "protected resource" are inherited from OAuth 2.0 [RFC6749].

   We use the term 'sign' (or 'signature') to denote both a keyed
   message digest and a digital signature operation.

3.  Generating the Signature

   This client creates a JSON Web Signature (JWS) over elements of the
   HTTP request, as described in [[ draft-richanna-http-jwt-signature-00
   ]].  The client includes the following additional top-level members
   within the JSON object payload of the JWS:

   at REQUIRED.  The access token value.  This string is assumed to have
      no particular format or structure and remains opaque to the
      client.

   The JWS is signed using the algorithm appropriate to the associated
   access token key, usually communicated as part of key distribution
   [I-D.ietf-oauth-pop-key-distribution].

4.  Sending the signed object

   In order to send the signed object to the protected resource, the
   client includes it in one of the following three places.

4.1.  HTTP Authorization header

   The client SHOULD send the signed object to the protected resource in
   the _Authorization_ header.  The value of the signed object in JWS
   compact form is appended to the _Authorization_ header as a PoP



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   value.  This is the preferred method.  Note that if this method is
   used, the _Authorization_ header MUST NOT be included in the
   protected elements of the signed object.

   GET /resource/foo
   Authorization: PoP eyJ....omitted for brevity...

4.2.  HTTP Form body

   If the client is sending the request as a form-encoded HTTP message
   with parameters in the body, the client MAY send the signed object as
   part of that form body.  The value of the signed object in JWS
   compact form is sent as the form parameter _pop_access_token_.  Note
   that if this method is used, the body hash cannot be included in the
   protected elements of the signed object.

   POST /resource
   Content-type: application/www-form-encoded

   pop_access_token=eyJ....omitted for brevity...

4.3.  HTTP Query parameter

   If neither the _Authorization_ header nor the form-encoded body
   parameter are available to the client, the client MAY send the signed
   object as a query parameter.  The value of the signed object in JWS
   compact form is sent as the query parameter _pop_access_token_. Note
   that if this method is used, the _pop_access_token_ parameter MUST
   NOT be included in the protected elements of the signed object.

   GET /resource?pop_access_token=eyJ....

5.  Validating the request

   Just like with a bearer token [RFC6750], while the access token value
   included in the signed object is opaque to the client, it MUST be
   understood by the protected resource in order to fulfill the request.
   Also like a bearer token, the protected resource traditionally has
   several methods at its disposal for understanding the access token.
   It can look up the token locally (such as in a database), it can
   parse a structured token (such as JWT [RFC7519]), or it can use a
   service to look up token information (such as introspection
   [RFC7662]).  Whatever method is used to look up token information,
   the protected resource MUST have access to the key associated with
   the access token, as this key is required to validate the signature
   of the incoming request.  Validation of the signature is done
   according the rules defined in [[ draft-richanna-http-jwt-
   signature-00 ]].



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6.  IANA Considerations

6.1.  The 'pop' OAuth Access Token Type

   Section 11.1 of [RFC6749] defines the OAuth Access Token Type
   Registry and this document adds another token type to this registry.

   Type name:  pop

   Additional Token Endpoint Response Parameters:  (none)

   HTTP Authentication Scheme(s):  Proof-of-possession access token for
      use with OAuth 2.0

   Change controller:  IETF

   Specification document(s):  [[ this document ]]

7.  Security Considerations

7.1.  Denial of Service

   This specification includes a number of features which may make
   resource exhaustion attacks against resource servers possible.  For
   example, a resource server may need to process the incoming request,
   verify the access token, perform signature verification, and might
   (in certain circumstances) have to consult back-end databases or the
   authorization server before granting access to the protected
   resource.  Many of these actions are shared with bearer tokens, but
   the additional cryptographic overhead of validating the signed
   request needs to be taken into consideration with deployment of this
   specification.

   An attacker may exploit this to perform a denial of service attack by
   sending a large number of invalid requests to the server.  The
   computational overhead of verifying the keyed message digest alone is
   not likely sufficient to mount a denial of service attack.  To help
   combat this, it is RECOMMENDED that the protected resource validate
   the access token (contained in the "at" member of the signed
   structure) before performing any cryptographic verification
   calculations.

8.  Privacy Considerations

   This specification addresses machine to machine communications and
   raises no privacy considerations beyond existing OAuth transactions.





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9.  Acknowledgements

   The authors thank the OAuth Working Group for input into this work.

   In particular, the authors thank Justin Richer for his work on
   [I-D.ietf-oauth-signed-http-request], on which this specification is
   based.

10.  Normative References

   [I-D.ietf-oauth-pop-architecture]
              Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
              Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
              Architecture", draft-ietf-oauth-pop-architecture-08 (work
              in progress), July 2016.

   [I-D.ietf-oauth-pop-key-distribution]
              Bradley, J., Hunt, P., Jones, M., Tschofenig, H., and M.
              Meszaros, "OAuth 2.0 Proof-of-Possession: Authorization
              Server to Client Key Distribution", draft-ietf-oauth-pop-
              key-distribution-07 (work in progress), March 2019.

   [I-D.ietf-oauth-signed-http-request]
              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing HTTP Requests for OAuth", draft-ietf-oauth-signed-
              http-request-03 (work in progress), August 2016.

   [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>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [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>.



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   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

Author's Address

   Annabelle Backman (editor)
   Amazon

   Email: richanna@amazon.com









































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