OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer
draft-fett-oauth-dpop-01

Web Authorization Protocol                                       D. Fett
Internet-Draft                                                   yes.com
Intended status: Standards Track                              J. Bradley
Expires: October 4, 2019                                          Yubico
                                                             B. Campbell
                                                           Ping Identity
                                                          T. Lodderstedt
                                                                 yes.com
                                                                M. Jones
                                                               Microsoft
                                                           April 2, 2019

OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer
                        draft-fett-oauth-dpop-01

Abstract

   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 to detect 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 October 4, 2019.

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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Main Objective  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Concept . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  DPoP JWT Syntax . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Token Request (Binding Tokens to a Public Key)  . . . . . . .   6
   6.  Resource Access (Proof of Possession for Access Tokens) . . .   8
   7.  Refresh Token Usage (Proof of Possession for Refresh Tokens)    8
   8.  Public Key Confirmation . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  OAuth Access Token Type Registration . . . . . . . . . .   9
     10.2.  JWT Confirmation Methods Registration  . . . . . . . . .  10
     10.3.  JSON Web Signature and Encryption Type Values
            Registration . . . . . . . . . . . . . . . . . . . . . .  10
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  10
     11.1.  Token Replay at the Same Authorization Server  . . . . .  10
     11.2.  Token Replay at the Same Resource Server Endpoint  . . .  11
     11.3.  Signed JWT Swapping  . . . . . . . . . . . . . . . . . .  11
     11.4.  Comparison to mTLS and OAuth Token Binding . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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.

   Due to a sub-par user experience of TLS client authentication in user
   agents and a lack of support for HTTP token binding, neither

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   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 tokens and refresh tokens that can be used if neither mTLS nor
   OAuth Token Binding are available.  It uses proof-of-possession based
   on a public/private key pair and application-level signing.

   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",
   "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.

   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 tries to ensure that
   token replay at a different endpoint is prevented.

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

   Secondary objectives are discussed in Section 11.

3.  Concept

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

   Figure 1: Basic DPoP Flow

   The new elements introduced by this specification are shown in
   Figure 1:

   o  (A) In the Token Request, the client sends an authorization grant,
      e.g., an authorization code or a refresh token, to the
      authorization server in order to obtain an access token (and
      potentially a refresh token).  The client proves the possession of
      a private key belonging to some public key by sending a request
      header containing a JWT that was signed using this private key.
      The corresponding public key is contained in the same request.

   o  (B) The AS binds (sender-constrains) the access token to the
      public key claimed by the client; 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
      "Bearer+DPoP".  If a refresh token is issued to the client, it is
      sender-constrained in the same way if the client is a public
      client.  Note: refresh tokens are automatically bound to the
      "client_id" of a confidential client, 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 adding a header to the request
      that, again, contains a JWT signed with this private key.  The JWT
      contains the endpoint URL and the request method.  The resource
      server needs to receive information about which public key to
      check against.  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
      (request not shown).

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   o  (D) The resource server refuses to serve the request if the
      signature check fails or the data in the JWT do not match, e.g.,
      the request URI does not match the URI claim in the JWT.

   o  Steps (A) and (B) can be repeated using a refresh token to obtain
      fresh access tokens.  In this case, the client sends a DPoP proof
      JWT as in step (C) above.  The client can optionally proof the
      possession of a new private/public key pair to which the new
      tokens are then bound by the authorization server.  Otherwise, the
      authorization server binds the new tokens to the previously used
      public key.

   The mechanism presented herein is not a client authentication method.
   In fact, a primary use case are 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.

   Note: DPoP does not directly ensure message integrity but relies on
   the TLS layer for that purpose.

4.  DPoP JWT Syntax

   DPoP uses so-called DPoP JWTs for binding public keys (DPoP Binding
   JWT) and proving knowledge about private keys (DPoP Proof JWT).

   A DPoP JWT 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 the following fields:

   o  "typ": type header, value "dpop_binding+jwt" for a DPoP Binding
      JWT or "dpop_proof+jwt" for a DPoP Proof 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).

   The body of a DPoP JWT contains the following fields:

   o  "jti": Unique identifier for this JWT chosen freshly when creating
      the JWT (REQUIRED).  SHOULD be used by the AS for replay detection
      and prevention.  See Security Considerations [1].

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

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

   o  "exp": Expiration time of the JWT (REQUIRED).  See Security
      Considerations [2].

   o  "cnf": Confirmation claim as per [RFC7800] containing a member
      "dpop+jwk", representing the public key chosen by the client in
      JWK format (REQUIRED for DPoP Binding JWTs, OPTIONAL for DPoP
      Proof JWTs).

   An example DPoP JWT is shown in Figure 2.

   {
       "typ": "dpop_binding+jwt",
       "alg": "ES512",
   }.{
       "jti": "HK2PmfnHKwXP",
       "http_method": "POST",
       "http_uri": "https://server.example.com/token",
       "exp": "..."
       "cnf":{
           "dpop+jwk": {
                "kty" : "EC",
                "kid" : "11",
                "crv" : "P-256",
                "x" : "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
                "y" : "3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4"
           }
       }
   }

   Figure 2: Example JWT contents for "DPoP-Binding" header.

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 public key and prove the possession of the corresponding
   private key.  The HTTPS request shown in Figure 3 illustrates the
   protocol for this (with extra line breaks for display purposes only).

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   POST /token HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded;charset=UTF-8
   DPoP-Binding: eyJhbGciOiJSU0ExXzUi ...

   grant_type=authorization_code
   &code=SplxlOBeZQQYbYS6WxSbIA
   &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
   (remainder of JWK omitted for brevity)

   Figure 3: Token Request for a DPoP bound token.

   The HTTP header "DPoP-Binding" MUST contain a DPoP Binding JWT signed
   using the private key chosen by the client.

   It is RECOMMENDED that clients reuse the same JWT if possible to
   improve the performance of the client, the data transfer (caching),
   and the authorization server.

   If the authorization server receives a "DPoP-Binding" header in a
   token request, the authorization server MUST check that:

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

   2.  all required claims are contained in the JWT,

   3.  the "typ" field in the header has the correct value,

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

   5.  the JWT is signed using the public key contained in the "cnf"
       claim of the JWT,

   6.  the "http_method" and "http_uri" claims match the respective
       values for the HTTP request in which the header was received,

   7.  the token has not expired, and

   8.  if replay protection is desired, that a JWT with the same "jti"
       value has not been received previously.

   If these checks are successful, the authorization server MUST
   associate the access token with the public key.  It then sets
   "token_type" to "Bearer+DPoP" in the token response.  The client MAY
   use the value of the "token_type" parameter to determine whether the
   server supports the mechanisms specified in this document.

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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.  More precisely, the client MUST create a DPoP Proof JWT
   and sign it using the previously chosen private key.  The signed JWT
   MUST then be sent in the "DPoP-Proof" request header.

   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 8) it MUST check that:

   1.  a header "DPoP-Proof" was received in the HTTP request,

   2.  the header's value is a well-formed DPoP Proof JWT,

   3.  all required claims are contained in the JWT,

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

   5.  the JWT is signed using the public key to which the access token
       was bound,

   6.  the "typ" field in the header has the correct value,

   7.  the "http_method" and "http_uri" claims match the respective
       values for the HTTP request in which the header was received,

   8.  the token has not expired, and

   9.  if replay protection is desired, that a JWT with the same "jti"
       value has not been received previously.

   If any of these checks fails, the resource server MUST NOT grant
   access to the resource.

7.  Refresh Token Usage (Proof of Possession for Refresh Tokens)

   At the token endpoint, public clients using a refresh token MUST
   provide a proof of possession in the same way as for access tokens.

8.  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.

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   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 "dpop+jwk" of the "cnf" claim, as shown in Figure 4.

   {
       "iss": "https://server.example.com",
       "sub": "something@example.com",
       "exp": 1493726400,
       "nbf": 1493722800,
       "cnf":{
           "dpop+jwk": {
               "kty" : "EC",
               "kid" : "11",
               "crv" : "P-256",
               "x" : "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
               "y" : "3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4"
           }
       }
   }

   Figure 4: Example access token body with "cnf" claim.

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

9.  Acknowledgements

   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).

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: "Bearer+DPoP"

   o  Additional Token Endpoint Response Parameters: (none)

   o  HTTP Authentication Scheme(s): Bearer

   o  Change controller: IETF

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

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10.2.  JWT Confirmation Methods Registration

   This specification requests registration of the following value in
   the IANA "JWT Confirmation Methods" registry [IANA.JWT.Claims] for
   JWT "cnf" member values established by [RFC7800].

   o  Confirmation Method Value: "dpop+jwk"

   o  Confirmation Method Description: JWK encoded public key for dpop
      proof token

   o  Change Controller: IESG

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

10.3.  JSON Web Signature and Encryption Type Values Registration

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

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

   o  Abbreviation for MIME Type: None

   o  Change Controller: IETF

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

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

   o  Abbreviation for MIME Type: None

   o  Change Controller: IETF

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

11.  Security Considerations

   The Prevention of Token Replay at a Different Endpoint [3] is
   achieved through the binding of the DPoP JWT to a certain URI and
   HTTP method.

11.1.  Token Replay at the Same Authorization Server

   If an adversary is able to get hold of an DPoP-Binding JWT, it might
   replay it at the authorization server's token endpoint with the same
   or different payload.  The issued access token is useless as long as

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   the adversary does not get hold of a valid DPoP-Binding JWT for the
   corresponding resource server.

11.2.  Token Replay at the Same Resource Server Endpoint

   If an adversary is able to get hold of a DPoP-Proof JWT, the
   adversary could replay that token later at the same endpoint (the
   HTTP endpoint and method are enforced via the respective claims in
   the JWTs).  To prevent this, clients MUST limit the lifetime of the
   JWTs, preferably to a brief period.  Furthermore, the "jti" claim in
   each JWT MUST contain a unique (incrementing or randomly chosen)
   value, as proposed in [RFC7253].  Resource servers SHOULD store
   values at least for the lifetime of the respective JWT and decline
   HTTP requests by clients if a "jti" value has been seen before.

11.3.  Signed JWT Swapping

   Servers accepting signed DPoP 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.

11.4.  Comparison to mTLS and OAuth Token Binding

   o  mTLS stronger against intercepted connections

12.  References

12.1.  Normative References

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7253]  Krovetz, T. and P. Rogaway, "The OCB Authenticated-
              Encryption Algorithm", RFC 7253, DOI 10.17487/RFC7253, May
              2014, <https://www.rfc-editor.org/info/rfc7253>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <https://www.rfc-editor.org/info/rfc7518>.

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   [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>.

12.2.  Informative References

   [I-D.ietf-oauth-mtls]
              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-13 (work in progress), February 2019.

   [I-D.ietf-oauth-security-topics]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", draft-ietf-
              oauth-security-topics-12 (work in progress), March 2019.

   [I-D.ietf-oauth-token-binding]
              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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [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>.

   [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>.

12.3.  URIs

   [1] #Security

   [2] #Security

   [3] #Objective_Replay_Different_Endpoint

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Appendix A.  Document History

   [[ To be removed from the final specification ]]

   -00

   o  first draft

   -01

   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

Authors' Addresses

   Daniel Fett
   yes.com

   Email: mail@danielfett.de

   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com

   Brian Campbell
   Ping Identity

   Email: bcampbell@pingidentity.com

   Torsten Lodderstedt
   yes.com

   Email: torsten@lodderstedt.net

   Michael Jones
   Microsoft

   Email: mbj@microsoft.com

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