Web Authorization Protocol                                       D. Fett
Internet-Draft                                                   yes.com
Intended status: Standards Track                             B. Campbell
Expires: May 2, 2020                                       Ping Identity
                                                              J. Bradley
                                                          T. Lodderstedt
                                                                M. Jones
                                                                D. Waite
                                                           Ping Identity
                                                        October 30, 2019

OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer


   This document describes a mechanism for sender-constraining OAuth 2.0
   tokens via a proof-of-possession mechanism on the application level.
   This mechanism allows for the detection of replay attacks with access
   and refresh tokens.

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 2, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Main Objective  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Concept . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  DPoP Proof JWTs . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Checking DPoP Proofs  . . . . . . . . . . . . . . . . . .   6
   5.  Token Request (Binding Tokens to a Public Key)  . . . . . . .   7
   6.  Resource Access (Proof of Possession for Access Tokens) . . .   8
   7.  Public Key Confirmation . . . . . . . . . . . . . . . . . . .   9
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     9.1.  DPoP Proof Replay . . . . . . . . . . . . . . . . . . . .  10
     9.2.  Signed JWT Swapping . . . . . . . . . . . . . . . . . . .  11
     9.3.  Signature Algorithms  . . . . . . . . . . . . . . . . . .  11
     9.4.  Message Integrity . . . . . . . . . . . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  OAuth Access Token Type Registration . . . . . . . . . .  11
     10.2.  JSON Web Signature and Encryption Type Values
            Registration . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   [I-D.ietf-oauth-mtls] describes methods to bind (sender-constrain)
   access tokens using mutual Transport Layer Security (TLS)
   authentication with X.509 certificates.

   [I-D.ietf-oauth-token-binding] provides mechanisms to sender-
   constrain access tokens using HTTP token binding.

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   Due to a sub-par user experience of TLS client authentication in user
   agents and a lack of support for HTTP token binding, neither
   mechanism can be used if an OAuth client is a Single Page Application
   (SPA) running in a web browser.

   This document outlines an application-level sender-constraining for
   access and refresh tokens that can be used in cases where neither
   mTLS nor OAuth Token Binding are available.  It uses proof-of-
   possession based on a public/private key pair and application-level

   DPoP can be used with public clients and, in case of confidential
   clients, can be combined with any client authentication method.

1.1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   This specification uses the terms "access token", "refresh token",
   "authorization server", "resource server", "authorization endpoint",
   "authorization request", "authorization response", "token endpoint",
   "grant type", "access token request", "access token response", and
   "client" defined by The OAuth 2.0 Authorization Framework [RFC6749].

2.  Main Objective

   Under the attacker model defined in [I-D.ietf-oauth-security-topics],
   the mechanism defined by this specification aims to prevent token
   replay at a different endpoint.

   More precisely, if an adversary is able to get hold of an access
   token or refresh token because it set up a counterfeit authorization
   server or resource server, the adversary is not able to replay the
   respective token at another authorization or resource server.

   Secondary objectives are discussed in Section 9.

3.  Concept

   The main data structure introduced by this specification is a DPoP
   proof JWT, described in detail below.  A client uses a DPoP proof JWT
   to prove the possession of a private key belonging to a certain
   public key.  Roughly speaking, a DPoP proof is a signature over some
   data of the HTTP request to which it is attached to and a timestamp.

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   +--------+                                          +---------------+
   |        |--(A)-- Token Request ------------------->|               |
   | Client |        (DPoP Proof)                      | Authorization |
   |        |                                          |     Server    |
   |        |<-(B)-- DPoP-bound Access Token ----------|               |
   |        |        (token_type=DPoP)                 +---------------+
   |        |        PoP Refresh Token for public clients
   |        |
   |        |                                          +---------------+
   |        |--(C)-- DPoP-bound Access Token --------->|               |
   |        |        (DPoP Proof)                      |    Resource   |
   |        |                                          |     Server    |
   |        |<-(D)-- Protected Resource ---------------|               |
   |        |                                          +---------------+

   Figure 1: Basic DPoP Flow

   The basic steps of an OAuth flow with DPoP are shown in Figure 1:

   o  (A) In the Token Request, the client sends an authorization code
      to the authorization server in order to obtain an access token
      (and potentially a refresh token).  The client attaches a DPoP
      proof to the request in an HTTP header.

   o  (B) The AS binds (sender-constrains) the access token to the
      public key claimed by the client in the DPoP proof; that is, the
      access token cannot be used without proving possession of the
      respective private key.  This is signaled to the client by using
      the "token_type" value "DPoP".

   o  If a refresh token is issued to a public client, it is sender-
      constrained in the same way.  For confidential clients, refresh
      tokens are bound to the "client_id", which is more flexible than
      binding it to a particular public key.

   o  (C) If the client wants to use the access token, it has to prove
      possession of the private key by, again, adding a header to the
      request that carries the DPoP proof.  The resource server needs to
      receive information about the public key to which the access token
      is bound.  This information is either encoded directly into the
      access token (for JWT structured access tokens), or provided at
      the token introspection endpoint of the authorization server (not

   o  (D) The resource server refuses to serve the request if the
      signature check fails or the data in the DPoP proof is wrong,

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      e.g., the request URI does not match the URI claim in the DPoP
      proof JWT.

   o  When a refresh token that is sender-constrained using DPoP is used
      by the client, the client has to provide a DPoP proof just as in
      the case of a resource access.  The new access token will be bound
      to the same public key.

   The mechanism presented herein is not a client authentication method.
   In fact, a primary use case is public clients (single page
   applications) that do not use client authentication.  Nonetheless,
   DPoP is designed such that it is compatible with "private_key_jwt"
   and all other client authentication methods.

   DPoP does not directly ensure message integrity but relies on the TLS
   layer for that purpose.  See Section 9 for details.

4.  DPoP Proof JWTs

   DPoP uses so-called DPoP proof JWTs for binding public keys and
   proving knowledge about private keys.

4.1.  Syntax

   A DPoP proof is a JWT ([RFC7519]) that is signed (using JWS,
   [RFC7515]) using a private key chosen by the client (see below).  The
   header of a DPoP JWT contains at least the following parameters:

   o  "typ": type header, value "dpop+jwt" (REQUIRED).

   o  "alg": a digital signature algorithm identifier as per [RFC7518]
      (REQUIRED).  MUST NOT be "none" or an identifier for a symmetric
      algorithm (MAC).

   o  "jwk": representing the public key chosen by the client, in JWK
      format, as defined in [RFC7515] (REQUIRED)

   The body of a DPoP proof contains at least the following claims:

   o  "jti": Unique identifier for the DPoP proof JWT (REQUIRED).  The
      value MUST be assigned such that there is a negligible probability
      that the same value will be assigned to any other DPoP proof used
      in the same context during the time window of validity.  Such
      uniqueness can be accomplished by encoding (base64url or any other
      suitable encoding) at least 96 bits of pseudorandom data or by
      using a version 4 UUID string according to [RFC4122].  The "jti"
      SHOULD be used by the server for replay detection and prevention,
      see Section 9.1.

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   o  "htm": The HTTP method for the request to which the JWT is
      attached, as defined in [RFC7231] (REQUIRED).

   o  "htu": The HTTP URI used for the request, without query and
      fragment parts (REQUIRED).

   o  "iat": Time at which the JWT was created (REQUIRED).

   Figure 2 shows the JSON header and payload of a DPoP proof JWT.

     "jwk": {

   Figure 2: Example JWT content for "DPoP" proof header.

   Note: To keep DPoP simple to implement, only the HTTP method and URI
   are signed in DPoP proofs.  Nonetheless, DPoP proofs can be extended
   to contain other information of the HTTP request (see also
   Section 9.4).

4.2.  Checking DPoP Proofs

   To check if a string that was received as part of an HTTP Request is
   a valid DPoP proof, the receiving server MUST ensure that

   1.  the string value is a well-formed JWT,

   2.  all required claims are contained in the JWT,

   3.  the "typ" field in the header has the value "dpop+jwt",

   4.  the algorithm in the header of the JWT indicates an asymmetric
       digital signature algorithm, is not "none", is supported by the
       application, and is deemed secure,

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   5.  that the JWT is signed using the public key contained in the
       "jwk" header of the JWT,

   6.  the "htm" claim matches the HTTP method value of the HTTP request
       in which the JWT was received (case-insensitive),

   7.  the "htu" claims matches the HTTP URI value for the HTTP request
       in which the JWT was received, ignoring any query and fragment

   8.  the token was issued within an acceptable timeframe (see
       Section 9.1), and

   9.  that, within a reasonable consideration of accuracy and resource
       utilization, a JWT with the same "jti" value has not been
       received previously (see Section 9.1).

   Servers SHOULD employ Syntax-Based Normalization and Scheme-Based
   Normalization in accordance with Section 6.2.2. and Section 6.2.3. of
   [RFC3986] before comparing the "htu" claim.

5.  Token Request (Binding Tokens to a Public Key)

   To bind a token to a public key in the token request, the client MUST
   provide a valid DPoP proof JWT in a "DPoP" header.  The HTTPS request
   shown in Figure 3 illustrates the protocol for this (with extra line
   breaks for display purposes only).

   POST /token HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded;charset=UTF-8
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik

   Figure 3: Token Request for a DPoP sender-constrained token.

   The HTTP header "DPoP" MUST contain a valid DPoP proof.

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   The authorization server, after checking the validity of the DPoP
   proof, MUST associate the access token issued at the token endpoint
   with the public key.  It then sets "token_type" to "DPoP" in the
   token response.

   A client typically cannot know whether a certain AS supports DPoP.
   It therefore SHOULD use the value of the "token_type" parameter
   returned from the AS to determine support for DPoP: If the token type
   returned is "Bearer" or another value, the AS does not support DPoP.
   If it is "DPoP", DPoP is supported.  Only then, the client needs to
   send the "DPoP" header in subsequent requests and use the token type
   "DPoP" in the "Authorization" header as described below.

   If a refresh token is issued to a public client at the token endpoint
   and a valid DPoP proof is presented, the refresh token MUST be bound
   to the public key contained in the header of the DPoP proof JWT.

   If a DPoP-bound refresh token is to be used at the token endpoint by
   a public client, the AS MUST ensure that the DPoP proof contains the
   same public key as the one the refresh token is bound to.  The access
   token issued MUST be bound to the public key contained in the DPoP

6.  Resource Access (Proof of Possession for Access Tokens)

   To make use of an access token that is token-bound to a public key
   using DPoP, a client MUST prove the possession of the corresponding
   private key by providing a DPoP proof in the "DPoP" request header.

   The DPoP-bound access token must be sent in the "Authorization"
   header with the prefix "DPoP".

   If a resource server detects that an access token that is to be used
   for resource access is bound to a public key using DPoP (via the
   methods described in Section 7) it MUST check that a header "DPoP"
   was received in the HTTP request, and check the header's contents
   according to the rules in Section 4.2.

   The resource server MUST NOT grant access to the resource unless all
   checks are successful.

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   GET /protectedresource HTTP/1.1
   Host: resource.example.org
   Authorization: DPoP eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWI
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik

   Figure 4: Protected Resource Request with a DPoP sender-constrained
   access token.

7.  Public Key Confirmation

   It MUST be ensured that resource servers can reliably identify
   whether a token is bound using DPoP and learn the public key to which
   the token is bound.

   Access tokens that are represented as JSON Web Tokens (JWT) [RFC7519]
   MUST contain information about the DPoP public key (in JWK format) in
   the member "jkt" of the "cnf" claim, as shown in Figure 5.

   The value in "jkt" MUST be the base64url encoding [RFC7515] of the
   JWK SHA-256 Thumbprint (according to [RFC7638]) of the public key to
   which the access token is bound.


   Figure 5: Example access token body with "cnf" claim.

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   When access token introspection is used, the same "cnf" claim as
   above MUST be contained in the introspection response.

   Resource servers MUST ensure that the fingerprint of the public key
   in the DPoP proof JWT equals the value in the "jkt" claim in the
   access token or introspection response.

8.  Acknowledgements

   We would like to thank David Waite, Filip Skokan, Mike Engan, and
   Justin Richer for their valuable input and feedback.

   This document resulted from discussions at the 4th OAuth Security
   Workshop in Stuttgart, Germany.  We thank the organizers of this
   workshop (Ralf Kuesters, Guido Schmitz).

9.  Security Considerations

   In DPoP, the prevention of token replay at a different endpoint (see
   Section 2) is achieved through the binding of the DPoP proof to a
   certain URI and HTTP method.  DPoP does not, however, achieve the
   same level of protection as TLS-based methods such as OAuth Mutual
   TLS [I-D.ietf-oauth-mtls] or OAuth Token Binding
   [I-D.ietf-oauth-token-binding] (see also Section 9.1 and
   Section 9.4).  TLS-based mechanisms can leverage a tight integration
   between the TLS layer and the application layer to achieve a very
   high level of message integrity and replay protection.  Therefore, it
   is RECOMMENDED to prefer TLS-based methods over DPoP if such methods
   are suitable for the scenario at hand.

9.1.  DPoP Proof Replay

   If an adversary is able to get hold of a DPoP proof JWT, the
   adversary could replay that token at the same endpoint (the HTTP
   endpoint and method are enforced via the respective claims in the
   JWTs).  To prevent this, servers MUST only accept DPoP proofs for a
   limited time window after their "iat" time, preferably only for a
   relatively brief period.  Servers SHOULD store the "jti" value of
   each DPoP proof for the time window in which the respective DPoP
   proof JWT would be accepted and decline HTTP requests for which the
   "jti" value has been seen before.  In order to guard against memory
   exhaustion attacks a server SHOULD reject DPoP proof JWTs with
   unnecessarily large "jti" values or store only a hash thereof.

   Note: To accommodate for clock offsets, the server MAY accept DPoP
   proofs that carry an "iat" time in the near future (e.g., up to a few
   seconds in the future).

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9.2.  Signed JWT Swapping

   Servers accepting signed DPoP proof JWTs MUST check the "typ" field
   in the headers of the JWTs to ensure that adversaries cannot use JWTs
   created for other purposes in the DPoP headers.

9.3.  Signature Algorithms

   Implementers MUST ensure that only asymmetric digital signature
   algorithms that are deemed secure can be used for signing DPoP
   proofs.  In particular, the algorithm "none" MUST NOT be allowed.

9.4.  Message Integrity

   DPoP does not ensure the integrity of the payload or headers of
   requests.  The signature of DPoP proofs only contains the HTTP URI
   and method, but not, for example, the message body or other request

   This is an intentional design decision to keep DPoP simple to use,
   but as described, makes DPoP potentially susceptible to replay
   attacks where an attacker is able to modify message contents and
   headers.  In many setups, the message integrity and confidentiality
   provided by TLS is sufficient to provide a good level of protection.

   Implementers that have stronger requirements on the integrity of
   messages are encouraged to either use TLS-based mechanisms or signed
   requests.  TLS-based mechanisms are in particular OAuth Mutual TLS
   [I-D.ietf-oauth-mtls] and OAuth Token Binding

   Note: While signatures on (parts of) requests are out of the scope of
   this specification, signatures or information to be signed can be
   added into DPoP proofs.

10.  IANA Considerations

10.1.  OAuth Access Token Type Registration

   This specification registers the following access token type in the
   OAuth Access Token Types registry defined in [RFC6749].

   o  Type name: "DPoP"

   o  Additional Token Endpoint Response Parameters: (none)

   o  HTTP Authentication Scheme(s): Bearer

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   o  Change controller: IETF

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

10.2.  JSON Web Signature and Encryption Type Values Registration

   This specification registers the "dpop+jwt" type value in the IANA
   JSON Web Signature and Encryption Type Values registry [RFC7515]:

   o  "typ" Header Parameter Value: "dpop+jwt"

   o  Abbreviation for MIME Type: None

   o  Change Controller: IETF

   o  Specification Document(s): [[ this specification ]]

11.  References

11.1.  Normative References

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

   [RFC7638]  Jones, M. and N. Sakimura, "JSON Web Key (JWK)
              Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
              2015, <https://www.rfc-editor.org/info/rfc7638>.

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11.2.  Informative References

              Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", draft-ietf-oauth-
              mtls-17 (work in progress), August 2019.

              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", draft-ietf-
              oauth-security-topics-13 (work in progress), July 2019.

              Jones, M., Campbell, B., Bradley, J., and W. Denniss,
              "OAuth 2.0 Token Binding", draft-ietf-oauth-token-
              binding-08 (work in progress), October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,

   [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>.

   [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>.

Appendix A.  Document History

   [[ To be removed from the final specification ]]


   o  rework the text around uniqueness requirements on the jti claim in
      the DPoP proof JWT

   o  make tokens a bit smaller by using "htm", "htu", and "jkt" rather
      than "http_method", "http_uri", and "jkt#S256" respectively

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   o  more explicit recommendation to use mTLS if that is available

   o  added David Waite as co-author

   o  editorial updates


   o  added normalization rules for URIs

   o  removed distinction between proof and binding

   o  "jwk" header again used instead of "cnf" claim in DPoP proof

   o  renamed "Bearer-DPoP" token type to "DPoP"

   o  removed ability for key rotation

   o  added security considerations on request integrity

   o  explicit advice on extending DPoP proofs to sign other parts of
      the HTTP messages

   o  only use the jkt#S256 in ATs

   o  iat instead of exp in DPoP proof JWTs

   o  updated guidance on token_type evaluation


   o  fixed inconsistencies

   o  moved binding and proof messages to headers instead of parameters

   o  extracted and unified definition of DPoP JWTs

   o  improved description


   o  first draft

Authors' Addresses

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   Daniel Fett

   Email: mail@danielfett.de

   Brian Campbell
   Ping Identity

   Email: bcampbell@pingidentity.com

   John Bradley

   Email: ve7jtb@ve7jtb.com

   Torsten Lodderstedt

   Email: torsten@lodderstedt.net

   Michael Jones

   Email: mbj@microsoft.com

   David Waite
   Ping Identity

   Email: david@alkaline-solutions.com

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