Mutual TLS Profile for OAuth 2.0
draft-ietf-oauth-mtls-03

OAuth Working Group                                          B. Campbell
Internet-Draft                                             Ping Identity
Intended status: Standards Track                              J. Bradley
Expires: January 28, 2018                                         Yubico
                                                             N. Sakimura
                                               Nomura Research Institute
                                                          T. Lodderstedt
                                                           YES Europe AG
                                                           July 27, 2017

                    Mutual TLS Profile for OAuth 2.0
                        draft-ietf-oauth-mtls-03

Abstract

   This document describes Transport Layer Security (TLS) mutual
   authentication using X.509 certificates as a mechanism for OAuth
   client authentication to the token endpoint as well as for
   certificate bound sender constrained access 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
   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 January 28, 2018.

Copyright Notice

   Copyright (c) 2017 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

Campbell, et al.        Expires January 28, 2018                [Page 1]
Internet-Draft              OAuth Mutual TLS                   July 2017

   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Mutual TLS for Client Authentication  . . . . . . . . . . . .   4
     2.1.  Mutual TLS Client Authentication to the Token Endpoint  .   4
     2.2.  Authorization Server Metadata . . . . . . . . . . . . . .   5
     2.3.  Dynamic Client Registration . . . . . . . . . . . . . . .   5
   3.  Mutual TLS Sender Constrained Resources Access  . . . . . . .   6
     3.1.  X.509 Certificate SHA-256 Thumbprint Confirmation Method
           for JWT . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Confirmation Method for Token Introspection . . . . . . .   8
   4.  Implementation Considerations . . . . . . . . . . . . . . . .   9
     4.1.  Authorization Server  . . . . . . . . . . . . . . . . . .   9
     4.2.  Resource Server . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Sender Constrained Access Tokens Without Client
           Authentication  . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Certificate Bound Access Tokens . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  JWT Confirmation Methods Registration . . . . . . . . . .  10
       5.1.1.  Registry Contents . . . . . . . . . . . . . . . . . .  10
     5.2.  OAuth Authorization Server Metadata Registration  . . . .  11
       5.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .  11
     5.3.  Token Endpoint Authentication Method Registration . . . .  11
       5.3.1.  Registry Contents . . . . . . . . . . . . . . . . . .  11
     5.4.  OAuth Token Introspection Response Registration . . . . .  11
       5.4.1.  Registry Contents . . . . . . . . . . . . . . . . . .  11
     5.5.  OAuth Dynamic Client Registration Metadata Registration .  12
       5.5.1.  Registry Contents . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     6.1.  TLS Versions and Best Practices . . . . . . . . . . . . .  12
     6.2.  X.509 Certificate Spoofing  . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  15
   Appendix B.  Document(s) History  . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

Campbell, et al.        Expires January 28, 2018                [Page 2]
Internet-Draft              OAuth Mutual TLS                   July 2017

1.  Introduction

   This document describes Transport Layer Security (TLS) mutual
   authentication using X.509 certificates as a mechanism for OAuth
   client authentication to the token endpoint as well as for sender
   constrained access to OAuth protected resources.

   The OAuth 2.0 Authorization Framework [RFC6749] defines a shared
   secret method of client authentication but also allows for the
   definition and use of additional client authentication mechanisms
   when interacting with the authorization server's token endpoint.
   This document describes an additional mechanism of client
   authentication utilizing mutual TLS [RFC5246] certificate-based
   authentication, which provides better security characteristics than
   shared secrets.

   Mutual TLS sender constrained access to protected resources ensures
   that only the party in possession of the private key corresponding to
   the certificate can utilize the access token to get access to the
   associated resources.  Such a constraint is unlike the case of the
   basic bearer token described in [RFC6750], where any party in
   possession of the access token can use it to access the associated
   resources.  Mutual TLS sender constrained access binds the access
   token to the client's certificate thus preventing the use of stolen
   access tokens or replay of access tokens by unauthorized parties.

   Mutual TLS sender constrained access tokens and mutual TLS client
   authentication are distinct mechanisms that don't necessarily need to
   be deployed together.

1.1.  Requirements Notation and Conventions

   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].

1.2.  Terminology

   This specification uses the following phrases interchangeably:

      Transport Layer Security (TLS) Mutual Authentication

      Mutual TLS

   These phrases all refer to the process whereby a client uses it's
   X.509 certificate to authenticate itself with a server when
   negotiating a TLS session.  In TLS 1.2 [RFC5246] this requires the

Campbell, et al.        Expires January 28, 2018                [Page 3]
Internet-Draft              OAuth Mutual TLS                   July 2017

   client to send Client Certificate and Certificate Verify messages
   during the TLS handshake and for the server to verify these messages.

2.  Mutual TLS for Client Authentication

2.1.  Mutual TLS Client Authentication to the Token Endpoint

   The following section defines, as an extension of OAuth 2.0,
   Section 2.3 [RFC6749], the use of mutual TLS X.509 client
   certificates as client credentials.  The requirement of mutual TLS
   for client authentications is determined by the authorization server
   based on policy or configuration for the given client (regardless of
   whether the client was dynamically registered or statically
   configured or otherwise established).  OAuth 2.0 requires that access
   token requests by the client to the token endpoint use TLS.  In order
   to utilize TLS for client authentication, the TLS connection MUST
   have been established or reestablished with mutual X.509 certificate
   authentication (i.e. the Client Certificate and Certificate Verify
   messages are sent during the TLS Handshake [RFC5246]).

   For all access token requests to the token endpoint, regardless of
   the grant type used, the client MUST include the "client_id"
   parameter, described in OAuth 2.0, Section 2.2 [RFC6749].  The
   presence of the "client_id" parameter enables the authorization
   server to easily identify the client independently from the content
   of the certificate and allows for trust models to vary as appropriate
   for a given deployment.  The authorization server can locate the
   client configuration by the client identifier and check the
   certificate presented in the TLS Handshake against the expected
   credentials for that client.  The authorization server MUST enforce
   some method of binding a certificate to a client.  The following two
   binding methods are defined:

   PKI  The PKI method uses a distinguished name (DN) to identify the
      client.  The TLS handshake is utilized to validate the client's
      possession of the private key corresponding to the public key in
      the certificate and to validate the corresponding certificate
      chain.  The client is successfully authenticated if the subject
      information in the certificate matches the configured DN.  The
      client may prescribe the DN of the issuer of its certificates.
      The authorization server will enforce this restriction after the
      TLS handshake took place.  Setting the issuer to a certain CA
      securely scopes the DN of the client to this CA and shall prevent
      an attacker from impersonating a client by using a certificate for
      the client's DN obtained from a different CA.  The PKI method
      facilitates the way X.509 certificates are traditionally being
      used for authentication.  It also allows the client to rotate its

Campbell, et al.        Expires January 28, 2018                [Page 4]
Internet-Draft              OAuth Mutual TLS                   July 2017

      X.509 certificates without the need to modify its respective
      authentication data at the authorization server.

   Public Key  The Public Key method uses public keys to identify
      clients.  As pre-requisite, the client registers a X.509
      certificate or a trusted source for its X.509 certificates (jwks
      uri as defined in [RFC7591]) with the authorization server.
      During authentication, TLS is utilized to validate the client's
      possession of the private key corresponding to the public key
      presented in the respective TLS handshake.  In contrast to the PKI
      method, the certificate chain is not validated in this case.  The
      client is successfully authenticated, if the subject public key
      info of the validated certificate matches the subject public key
      info of one the certificates configured for that particular
      client.  The Public Key method allows to use mutual TLS to
      authenticate clients without the need to maintain a PKI.  When
      used in conjunction with a trusted X.509 certificate source, it
      also allows the client to rotate its X.509 certificates without
      the need to change its respective authentication data at the
      authorization server.

2.2.  Authorization Server Metadata

   In authorization server metadata, such as [OpenID.Discovery] and
   [I-D.ietf-oauth-discovery], the
   "token_endpoint_auth_methods_supported" parameter indicates client
   authentication methods to the token endpoint supported by the
   authorization server.  This document introduces the value
   "tls_client_auth" for use in "token_endpoint_auth_methods_supported"
   to indicate server support for mutual TLS client authentication
   utilizing the PKI method.  And for the support of mutual TLS client
   authentication utilizing the Public Key method, the value
   "pub_key_tls_client_auth" is used in
   "token_endpoint_auth_methods_supported".

   This document also introduces a new authorization server metadata
   parameter:

   mutual_tls_sender_constrained_access_tokens
      OPTIONAL.  Boolean value indicating server support for mutual TLS
      sender constrained access tokens.  If omitted, the default value
      is "false".

2.3.  Dynamic Client Registration

   This document adds the following values and metadata parameters to
   OAuth 2.0 Dynamic Client Registration [RFC7591].

Campbell, et al.        Expires January 28, 2018                [Page 5]
Internet-Draft              OAuth Mutual TLS                   July 2017

   The client metadata parameter
   "mutual_tls_sender_constrained_access_tokens" is a Boolean value used
   to indicate the client's intention to use mutual TLS sender
   constrained access tokens.  If omitted, the default value is "false".

   For the PKI method of binding a certificate to a client, the value
   "tls_client_auth" is used to indicate the client's intention to use
   mutual TLS as an authentication method to the token endpoint for the
   "token_endpoint_auth_method" client metadata parameter.  And the
   following two metadata parameters are introduced in support of the
   PKI method of binding a certificate to a client:

   tls_client_auth_subject_dn
      An [RFC4514] string representation of the expected subject
      distinguished name of the certificate the OAuth client will use in
      mutual TLS authentication.

   tls_client_auth_root_dn
      OPTIONAL.  An [RFC4514] string representation of a distinguished
      name that can optionally be used to constrain, for the given
      client, the expected distinguished name of the root issuer of the
      client certificate.

   With the Public Key method of binding a certificate to a client, the
   value "pub_key_tls_client_auth" is used for the
   "token_endpoint_auth_method" client metadata parameter to indicate
   the client's intention to use mutual TLS with a self-signed
   certificate as an authentication method.  For the Public Key method,
   the existing "jwks_uri" or "jwks" metadata parameters from [RFC7591]
   are used to convey client's public keys, where the X.509 certificates
   are represented using the "x5c" parameter from [RFC7517].

3.  Mutual TLS Sender Constrained Resources Access

   When mutual TLS is used at the token endpoint, the authorization
   server is able to bind the issued access token to the client
   certificate.  Such a binding is accomplished by associating the
   certificate with the token in a way that can be accessed by the
   protected resource, such as embedding the certificate hash in the
   issued access token directly, using the syntax described in
   Section 3.1, or through token introspection as described in
   Section 3.2.  Other methods of associating a certificate with an
   access token are possible, per agreement by the authorization server
   and the protected resource, but are beyond the scope of this
   specification.

   The client makes protected resource requests as described in
   [RFC6750], however, those requests MUST be made over a mutually

Campbell, et al.        Expires January 28, 2018                [Page 6]
Internet-Draft              OAuth Mutual TLS                   July 2017

   authenticated TLS connection using the same certificate that was used
   for mutual TLS at the token endpoint.

   The protected resource MUST obtain the client certificate used for
   mutual TLS authentication and MUST verify that the certificate
   matches the certificate associated with the access token.  If they do
   not match, the resource access attempt MUST be rejected with an error
   per [RFC6750] using an HTTP 401 status code and the "invalid_token"
   error code.

3.1.  X.509 Certificate SHA-256 Thumbprint Confirmation Method for JWT

   When access tokens are represented as a JSON Web Tokens
   (JWT)[RFC7519], the certificate hash information SHOULD be
   represented using the "x5t#S256" confirmation method member defined
   herein.

   To represent the hash of a certificate in a JWT, this specification
   defines the new JWT Confirmation Method RFC 7800 [RFC7800] member
   "x5t#S256" for the X.509 Certificate SHA-256 Thumbprint.  The value
   of the "x5t#S256" member is a base64url-encoded SHA-256[SHS] hash
   (a.k.a. thumbprint or digest) of the DER encoding of the X.509
   certificate[RFC5280] (note that certificate thumbprints are also
   sometimes also known as certificate fingerprints).

   The following is an example of a JWT payload containing an "x5t#S256"
   certificate thumbprint confirmation method.

     {
       "iss": "https://server.example.com",
       "sub": "ty.webb@example.com",
       "exp": 1493726400,
       "nbf": 1493722800,
       "cnf":{
         "x5t#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
       }
     }

   Figure 1: Example claims of a Certificate Thumbprint Constrained JWT

   If, in the future, certificate thumbprints need to be computed using
   hash functions other than SHA-256, it is suggested that additional
   related JWT confirmation methods members be defined for that purpose.
   For example, a new "x5t#S512" (X.509 Certificate Thumbprint using
   SHA-512) confirmation method member could be defined by registering
   it in the the IANA "JWT Confirmation Methods" registry
   [IANA.JWT.Claims] for JWT "cnf" member values established by
   [RFC7800].

Campbell, et al.        Expires January 28, 2018                [Page 7]
Internet-Draft              OAuth Mutual TLS                   July 2017

3.2.  Confirmation Method for Token Introspection

   OAuth 2.0 Token Introspection [RFC7662] defines a method for a
   protected resource to query an authorization server about the active
   state of an access token as well as to determine meta-information
   about the token.

   For a mutual TLS sender constrained access token, the hash of the
   certificate to which the token is bound is conveyed to the protected
   resource as meta-information in a token introspection response.  The
   hash is conveyed using same structure as the certificate SHA-256
   thumbprint confirmation method, described in Section 3.1, as a top-
   level member of the introspection response JSON.  The protected
   resource compares that certificate hash to a hash of the client
   certificate used for mutual TLS authentication and rejects the
   request, if they do not match.

   Proof-of-Possession Key Semantics for JSON Web Tokens [RFC7800]
   defined the "cnf" (confirmation) claim, which enables confirmation
   key information to be carried in a JWT.  However, the same proof-of-
   possession semantics are also useful for introspected access tokens
   whereby the protected resource obtains the confirmation key data as
   meta-information of a token introspection response and uses that
   information in verifying proof-of-possession.  Therefore this
   specification defines and registers proof-of-possession semantics for
   OAuth 2.0 Token Introspection [RFC7662] using the "cnf" structure.
   When included as a top-level member of an OAuth token introspection
   response, "cnf" has the same semantics and format as the claim of the
   same name defined in [RFC7800].  While this specification only
   explicitly uses the "x5t#S256" confirmation method member, it needed
   to define and register the higher level "cnf" structure as an
   introspection response member in order to define and use its more
   specific "x5t#S256" confirmation method.

   The following is an example of an introspection response for an
   active token with an "x5t#S256" certificate thumbprint confirmation
   method.

Campbell, et al.        Expires January 28, 2018                [Page 8]
Internet-Draft              OAuth Mutual TLS                   July 2017

     HTTP/1.1 200 OK
     Content-Type: application/json

     {
       "active": true,
       "iss": "https://server.example.com",
       "sub": "ty.webb@example.com",
       "exp": 1493726400,
       "nbf": 1493722800,
       "cnf":{
         "x5t#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
       }
     }

        Figure 2: Example Introspection Response for a Certificate
                         Constrained Access Token

4.  Implementation Considerations

4.1.  Authorization Server

   The authorization server needs to setup its TLS configuration
   appropriately for the binding methods it supports.

   If the authorization server wants to support mutual TLS client
   authentication and other client authentication methods in parallel,
   it should make mutual TLS optional on the token endpoint.

   If the authorization server supports the Public Key method, it should
   configure the TLS stack in a way that it does not verify whether the
   certificate presented by the client during the handshake is signed by
   a trusted CA certificate.

   Please note: the Public Key method is intended to support client
   authentication using self-signed certificates.

   The authorization server may also consider hosting the token endpoint
   on a separate host name in order to prevent unintended impact on the
   TLS behavior of its other endpoints, e.g. authorization or
   registration.

4.2.  Resource Server

   From the perspective of the resource server, TLS client
   authentication is used as a proof of possession method only.  For the
   purpose of client authentication, the resource server may completely
   rely on the authorization server.  So there is no need to validate
   the trust chain of the client's certificate in any of the methods

Campbell, et al.        Expires January 28, 2018                [Page 9]
Internet-Draft              OAuth Mutual TLS                   July 2017

   defined in this document.  The resource server should therefore
   configure the TLS stack in a way that it does not verify whether the
   certificate presented by the client during the handshake is signed by
   a trusted CA certificate.

4.3.  Sender Constrained Access Tokens Without Client Authentication

   This document allows for the use of client authentication only or
   client authentication in combination with sender constraint access
   tokens.  Use of mutual TLS sender constrained access tokens without
   client authentication (e.g. to support binding access tokens to a TLS
   client certificate for public clients) is also possible.  The
   authorization server would configure the TLS stack in the same manor
   as for the Public Key method such that it does not verify that the
   certificate presented by the client during the handshake is signed by
   a trusted CA.  Individual instances of a public client would then
   create a self-signed certificate for mutual TLS with the
   authorization server and resource server.  The authorization server
   would not authenticate the client at the OAuth layer but would bind
   issued access tokens to the certificate, which the client has proven
   possession of the corresponding private key.  The access token is
   then mutual TLS sender constrained and can only be used by the client
   possessing the certificate and private key and utilizing them to
   negotiate mutual TLS on connections to the resource server.

4.4.  Certificate Bound Access Tokens

   As described in Section 3, an access token is bound to a specific
   client certificate, which means that the same certificate must be
   used for mutual TLS on protected resource access.  It also implies
   that access tokens are invalidated when a client updates the
   certificate, which can be handled similar to expired access tokens
   where the client requests a new access token (typically with a
   refresh token) and retries the protected resource request.

5.  IANA Considerations

5.1.  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].

5.1.1.  Registry Contents

   o  Confirmation Method Value: "x5t#S256"
   o  Confirmation Method Description: X.509 Certificate SHA-256
      Thumbprint

Campbell, et al.        Expires January 28, 2018               [Page 10]
Internet-Draft              OAuth Mutual TLS                   July 2017

   o  Change Controller: IESG
   o  Specification Document(s): Section 3.1 of [[ this specification ]]

5.2.  OAuth Authorization Server Metadata Registration

   This specification requests registration of the following value in
   the IANA "OAuth Authorization Server Metadata" registry
   [IANA.OAuth.Parameters] established by [I-D.ietf-oauth-discovery].

5.2.1.  Registry Contents

   o  Metadata Name: "mutual_tls_sender_constrained_access_tokens"
   o  Metadata Description: Indicates server support for mutual TLS
      sender constraint access tokens.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of [[ this specification ]]

5.3.  Token Endpoint Authentication Method Registration

   This specification requests registration of the following value in
   the IANA "OAuth Token Endpoint Authentication Methods" registry
   [IANA.OAuth.Parameters] established by [RFC7591].

5.3.1.  Registry Contents

   o  Token Endpoint Authentication Method Name: "tls_client_auth"
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of [[ this specification ]]

   o  Token Endpoint Authentication Method Name:
      "pub_key_tls_client_auth"
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of [[ this specification ]]

5.4.  OAuth Token Introspection Response Registration

   This specification requests registration of the following value in
   the IANA "OAuth Token Introspection Response" registry
   [IANA.OAuth.Parameters] established by [RFC7662].

5.4.1.  Registry Contents

   o  Claim Name: "cnf"
   o  Claim Description: Confirmation
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.2 of [[ this specification ]]

Campbell, et al.        Expires January 28, 2018               [Page 11]
Internet-Draft              OAuth Mutual TLS                   July 2017

5.5.  OAuth Dynamic Client Registration Metadata Registration

   This specification requests registration of the following client
   metadata definitions in the IANA "OAuth Dynamic Client Registration
   Metadata" registry [IANA.OAuth.Parameters] established by [RFC7591]:

5.5.1.  Registry Contents

   o  Client Metadata Name:
      "mutual_tls_sender_constrained_access_tokens"
   o  Client Metadata Description: Indicates the client's intention to
      use mutual TLS sender constraint access tokens.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.3 of [[ this specification ]]

   o  Client Metadata Name: "tls_client_auth_subject_dn"
   o  Client Metadata Description: String value specifying the expected
      subject distinguished name of the client certificate.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.3 of [[ this specification ]]

   o  Client Metadata Name: "tls_client_auth_root_dn"
   o  Client Metadata Description: String value specifying the expected
      distinguished name of the root issuer of the client certificate
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.3 of [[ this specification ]]

6.  Security Considerations

6.1.  TLS Versions and Best Practices

   TLS 1.2 [RFC5246] is cited in this document because, at the time of
   writing, it is latest version that is widely deployed.  However, this
   document is applicable with other TLS versions supporting
   certificate-based client authentication.  Implementation security
   considerations for TLS, including version recommendations, can be
   found in Recommendations for Secure Use of Transport Layer Security
   (TLS) and Datagram Transport Layer Security (DTLS) [BCP195].

6.2.  X.509 Certificate Spoofing

   If the PKI method is used, an attacker could try to impersonate a
   client using a certificate for the same DN issued by another CA,
   which the authorization server trusts.

   There are two ways to cope with that threat: the authorization server
   may decide to only accept a limited number of CAs whose certificate
   issuance policy meets its security requirements.  Alternatively or in

Campbell, et al.        Expires January 28, 2018               [Page 12]
Internet-Draft              OAuth Mutual TLS                   July 2017

   addition, the client may want to explicitly prescribe the CA it will
   use for obtaining its certificates.  The latter is supported by this
   document with the client registration parameter
   "tls_client_auth_root_dn".

7.  References

7.1.  Normative References

   [BCP195]   Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/bcp195>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4514]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol
              (LDAP): String Representation of Distinguished Names",
              RFC 4514, DOI 10.17487/RFC4514, June 2006,
              <http://www.rfc-editor.org/info/rfc4514>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6750>.

Campbell, et al.        Expires January 28, 2018               [Page 13]
Internet-Draft              OAuth Mutual TLS                   July 2017

   [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,
              <http://www.rfc-editor.org/info/rfc7800>.

   [SHS]      National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4, March 2012,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

7.2.  Informative References

   [I-D.ietf-oauth-discovery]
              Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", draft-ietf-oauth-
              discovery-04 (work in progress), August 2016.

   [IANA.JWT.Claims]
              IANA, "JSON Web Token Claims",
              <http://www.iana.org/assignments/jwt>.

   [IANA.OAuth.Parameters]
              IANA, "OAuth Parameters",
              <http://www.iana.org/assignments/oauth-parameters>.

   [OpenID.Discovery]
              Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
              Connect Discovery 1.0", February 2014.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <http://www.rfc-editor.org/info/rfc7517>.

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

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,
              <http://www.rfc-editor.org/info/rfc7591>.

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

Campbell, et al.        Expires January 28, 2018               [Page 14]
Internet-Draft              OAuth Mutual TLS                   July 2017

Appendix A.  Acknowledgements

   Scott "not Tomlinson" Tomilson and Matt Peterson were involved in
   design and development work on a mutual TLS OAuth client
   authentication implementation that informed some of the content of
   this document.

   Additionally, the authors would like to thank the following people
   for their input and contributions to the specification: Sergey
   Beryozkin, Vladimir Dzhuvinov, Samuel Erdtman, Phil Hunt, Sean
   Leonard, Kepeng Li, James Manger, Jim Manico, Nov Matake, Sascha
   Preibisch, Justin Richer, Dave Tonge, and Hannes Tschofenig.

Appendix B.  Document(s) History

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   draft-ietf-oauth-mtls-03

   o  Introduced metadata and client registration parameter to publish
      and request support for mutual TLS sender constrained access
      tokens
   o  Added description of two methods of binding the cert and client,
      PKI and Public Key.
   o  Indicated that the "tls_client_auth" authentication method is for
      the PKI method and introduced "pub_key_tls_client_auth" for the
      Public Key method
   o  Added implementation considerations, mainly regarding TLS stack
      configuration and trust chain validation, as well as how to to do
      binding of access tokens to a TLS client certificate for public
      clients, and considerations around certificate bound access tokens
   o  Added new section to security considerations on cert spoofing
   o  Add text suggesting that a new cnf member be defined in the
      future, if hash function(s) other than SHA-256 need to be used for
      certificate thumbprints

   draft-ietf-oauth-mtls-02

   o  Fixed editorial issue https://mailarchive.ietf.org/arch/msg/oauth/
      U46UMEh8XIOQnvXY9pHFq1MKPns
   o  Changed the title (hopefully "Mutual TLS Profile for OAuth 2.0" is
      better than "Mutual TLS Profiles for OAuth Clients").

   draft-ietf-oauth-mtls-01

   o  Added more explicit details of using RFC 7662 token introspection
      with mutual TLS sender constrained access tokens.

Campbell, et al.        Expires January 28, 2018               [Page 15]
Internet-Draft              OAuth Mutual TLS                   July 2017

   o  Added an IANA OAuth Token Introspection Response Registration
      request for "cnf".
   o  Specify that tls_client_auth_subject_dn and
      tls_client_auth_root_dn are RFC 4514 String Representation of
      Distinguished Names.
   o  Changed tls_client_auth_issuer_dn to tls_client_auth_root_dn.
   o  Changed the text in the Section 3 to not be specific about using a
      hash of the cert.
   o  Changed the abbreviated title to 'OAuth Mutual TLS' (previously
      was the acronym MTLSPOC).

   draft-ietf-oauth-mtls-00

   o  Created the initial working group version from draft-campbell-
      oauth-mtls

   draft-campbell-oauth-mtls-01

   o  Fix some typos.
   o  Add to the acknowledgements list.

   draft-campbell-oauth-mtls-00

   o  Add a Mutual TLS sender constrained protected resource access
      method and a x5t#S256 cnf method for JWT access tokens (concepts
      taken in part from draft-sakimura-oauth-jpop-04).
   o  Fixed "token_endpoint_auth_methods_supported" to
      "token_endpoint_auth_method" for client metadata.
   o  Add "tls_client_auth_subject_dn" and "tls_client_auth_issuer_dn"
      client metadata parameters and mention using "jwks_uri" or "jwks".
   o  Say that the authentication method is determined by client policy
      regardless of whether the client was dynamically registered or
      statically configured.
   o  Expand acknowledgements to those that participated in discussions
      around draft-campbell-oauth-tls-client-auth-00
   o  Add Nat Sakimura and Torsten Lodderstedt to the author list.

   draft-campbell-oauth-tls-client-auth-00

   o  Initial draft.

Authors' Addresses

   Brian Campbell
   Ping Identity

   Email: brian.d.campbell@gmail.com

Campbell, et al.        Expires January 28, 2018               [Page 16]
Internet-Draft              OAuth Mutual TLS                   July 2017

   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com
   URI:   http://www.thread-safe.com/

   Nat Sakimura
   Nomura Research Institute

   Email: n-sakimura@nri.co.jp
   URI:   https://nat.sakimura.org/

   Torsten Lodderstedt
   YES Europe AG

   Email: torsten@lodderstedt.net

Campbell, et al.        Expires January 28, 2018               [Page 17]