OAuth Working Group                                          B. Campbell
Internet-Draft                                             Ping Identity
Intended status: Standards Track                              J. Bradley
Expires: January 28, April 14, 2018                                           Yubico
                                                             N. Sakimura
                                               Nomura Research Institute
                                                          T. Lodderstedt
                                                           YES Europe AG
                                                           July 27,
                                                        October 11, 2017

                    Mutual TLS Profile for OAuth 2.0
                        draft-ietf-oauth-mtls-03
                        draft-ietf-oauth-mtls-04

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 authorization sever as well as for
   certificate bound sender constrained access tokens.

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   This Internet-Draft will expire on January 28, April 14, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Mutual TLS for OAuth Client Authentication  . . . . . . . . . . . .   4
     2.1.  PKI Mutual TLS OAuth Client Authentication to the Token Endpoint Method . . . .   4
     2.2.  Authorization Server
       2.1.1.  PKI Authentication Method Metadata Value  . . . . . .   5
       2.1.2.  Client Registration Metadata  . . . . . . . . . . . .   5
     2.3.  Dynamic
     2.2.  Self-Signed Certificate Mutual TLS OAuth Client Registration
           Authentication Method . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Self-Signed Certificate Authentication Method
               Metadata Value  . . . . . . . . . . . . . . . . . . .   6
       2.2.2.  Client Registration Metadata  . . . . . . . . . . . .   6
   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
     3.3.  Authorization Server Metadata . . . . . . . . . . . . . .   9
     3.4.  Client Registration Metadata  . . . .   7
     3.2.  Confirmation Method for Token Introspection . . . . . . .   8 . . .   9
   4.  Implementation Considerations . . . . . . . . . . . . . . . .   9  10
     4.1.  Authorization Server  . . . . . . . . . . . . . . . . . .   9  10
     4.2.  Resource Server . . . . . . . . . . . . . . . . . . . . .   9  10
     4.3.  Sender Constrained Access Tokens Without Client
           Authentication  . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Certificate Bound Access Tokens . . . . . . . . . . . . .  10  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10  11
     5.1.  JWT Confirmation Methods Registration . . . . . . . . . .  10
       5.1.1.  Registry Contents . . . . . . . . . . . . . . . . . .  10  11
     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  12
     5.4.  OAuth Token Introspection Response Registration . . . . .  11
       5.4.1.  Registry Contents . . . . . . . . . . . . . . . . . .  11  12
     5.5.  OAuth Dynamic Client Registration Metadata Registration .  12
       5.5.1.  Registry Contents . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12  13
     6.1.  TLS Versions and Best Practices . . . . . . . . . . . . .  12  13
     6.2.  X.509 Certificate Spoofing  . . . . . . . . . . . . . . .  12  13
   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  17

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 authorization sever 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 directly with the authorization server's token endpoint. server.  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
   While [RFC6749] documents client authentication for requests to the
   token endpoint, extensions to OAuth 2.0 (such as Introspection
   [RFC7662] and Revocation [RFC7009]) define endpoints that also
   utilize client authentication and the mutual TLS methods defined
   herein are applicable to those endpoints as well.

   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 mechanisms, which are complementary but
   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 presents its
   X.509 certificate and proves possession of the corresponding private
   key to authenticate itself with a server when negotiating a TLS session.  In TLS 1.2 [RFC5246]
   this requires the 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 OAuth Client Authentication

2.1.  Mutual TLS Client Authentication to the Token Endpoint

   The following

   This section defines, as an extension of OAuth 2.0, Section 2.3
   [RFC6749], the use two distinct methods of using 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 OAuth client authentication, the TLS
   connection between the client and the authorization server 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, authorization server utilizing mutual TLS
   client authentication, 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. certificate.  The authorization server can locate the client
   configuration by using 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  Sections Section 2.1 and
   Section 2.2 below define two ways of binding methods are defined: a certificate to a
   client as two distinct client authentication methods.

2.1.  PKI Mutual TLS OAuth Client Authentication Method

   The PKI (public key infrastructure) method of mutual TLS OAuth client
   authentication uses a subject distinguished name (DN) and validated
   certificate chain 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 expected DN of the client to this CA and shall prevent
      an attacker from impersonating a client by using a certificate configured or registered for
      the client's DN obtained from a different CA. that particular client.
   The PKI method facilitates the way X.509 certificates are
   traditionally being used for authentication.  It also allows the
   client to rotate its 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 server by
   obtaining a X.509 new certificate or with the same subject DN from a trusted source for its
   certificate authority (CA).

2.1.1.  PKI Authentication Method Metadata Value

   The "OAuth Token Endpoint Authentication Methods" registry
   [IANA.OAuth.Parameters] contains values, each of which specify a
   method of authenticating a client to the authorization server.  The
   values are used to indicated supported and utilized client
   authentication methods in authorization server metadata, such as
   OpenID Connect Discovery [OpenID.Discovery] and OAuth 2.0
   Authorization Server Metadata [I-D.ietf-oauth-discovery], and in the
   OAuth 2.0 Dynamic Client Registration Protocol [RFC7591].  For the
   PKI method of mutual TLS client authentication, this specification
   defines and registers the following authentication method metadata
   value.

   tls_client_auth
      Indicates that client authentication to the authorization server
      will occur with mutual TLS utilizing the PKI method of associating
      a certificate to a client.

2.1.2.  Client Registration Metadata

   The following metadata parameter is introduced for the OAuth 2.0
   Dynamic Client Registration Protocol [RFC7591] 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.

2.2.  Self-Signed Certificate Mutual TLS OAuth Client Authentication
      Method

   This method of mutual TLS OAuth client authentication is intended to
   support client authentication using self-signed certificates.  As
   pre-requisite, the client registers a X.509 certificate or a trusted
   source for its X.509 certificates (jwks
      uri (such as the "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 within the certificate
   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 or registered for that particular client.
   The Public Key Self-Signed Certificate 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, "jwks_uri" for the client, it also allows the
   client to rotate its X.509 certificates without the need to change
   its respective authentication data directly with at the authorization
   server.

2.2.  Authorization Server

2.2.1.  Self-Signed Certificate Authentication Method Metadata

   In authorization server metadata, such as [OpenID.Discovery] and
   [I-D.ietf-oauth-discovery], the
   "token_endpoint_auth_methods_supported" parameter indicates Value

   The "OAuth Token Endpoint Authentication Methods" registry
   [IANA.OAuth.Parameters] contains values, each of which specify a
   method of authenticating a 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"  The
   values are used to indicate server support for mutual TLS client authentication
   utilizing the PKI method.  And for the support of mutual TLS indicated supported and utilized client
   authentication utilizing the Public Key method, the value
   "pub_key_tls_client_auth" is used methods 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 metadata, such as
   OpenID Connect Discovery [OpenID.Discovery] and metadata parameters to OAuth 2.0 Dynamic Client Registration [RFC7591].

   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,
   Authorization Server Metadata [I-D.ietf-oauth-discovery], and in the default value is "false".
   OAuth 2.0 Dynamic Client Registration Protocol [RFC7591].  For the PKI
   Self-Signed Certificate method of binding a certificate to a client, the value
   "tls_client_auth" is used to indicate the client's intention to use client
   using mutual TLS as an authentication method to the token endpoint for the
   "token_endpoint_auth_method" client metadata parameter.  And authentication, this specification defines
   and registers the following two metadata parameters are introduced in support of the
   PKI authentication method of binding a certificate metadata value.

   self_signed_tls_client_auth
      Indicates that client authentication to a client:

   tls_client_auth_subject_dn
      An [RFC4514] string representation of the expected subject
      distinguished name of the certificate the OAuth client authorization server
      will use in occur using 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 with the client utilizing a self-
      signed certificate.

   With

2.2.2.  Client Registration Metadata

   For the Public Key Self-Signed Certificate 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 using mutual TLS with a self-signed
   certificate as an authentication method.  For the Public Key method, client authentication, the existing
   "jwks_uri" or "jwks" metadata parameters from [RFC7591] are used to
   convey client's certificates and public keys, where the X.509
   certificates are represented using the JSON Web Key (JWK) [RFC7517]
   "x5c" parameter from [RFC7517]. (note that Sec 4.7 of RFC 7517 requires that the key
   in the first certificate of the "x5c" parameter must match the public
   key represented by other members of the JWK).

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
   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.

   Metadata to convey server and client capabilities for mutual TLS
   sender constrained access tokens is defined in Section 3.3 and
   Section 3.4 respectively.

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].

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 the 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.

     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

3.3.  Authorization Server Metadata

   This document introduces the following new authorization server
   metadata parameter to signal the server's capability to issue
   certificate bound access tokens:

   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".

3.4.  Client Registration Metadata

   The following new client metadata parameter is introduced to convey
   the client's intention to use certificate bound access tokens:

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

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. optional.

   If the authorization server supports the Public Key Self-Signed Certificate
   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
   endpoint, and other endpoints requiring client authentication, 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
   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 manner
   as for the Public Key Self-Signed Certificate 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
   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 authorization server support for
      mutual TLS sender constraint constrained access tokens.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 3.3 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 2.1.1 of [[ this specification
      ]]

   o  Token Endpoint Authentication Method Name:
      "pub_key_tls_client_auth"
      "self_signed_tls_client_auth"
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 2.2.1 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 ]]

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 constrained access tokens.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.3 3.4 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 2.1.2 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  To cope with that threat: 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
   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>.
              <https://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>.
              <https://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>.
              <https://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>.
              <https://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>.
              <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,
              <http://www.rfc-editor.org/info/rfc6750>.
              <https://www.rfc-editor.org/info/rfc6750>.

   [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>.
              <https://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
              discovery-07 (work in progress), August 2016. September 2017.

   [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. August 2015,
              <http://openid.net/specs/
              openid-connect-discovery-1_0.html>.

   [RFC7009]  Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
              2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
              August 2013, <https://www.rfc-editor.org/info/rfc7009>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <http://www.rfc-editor.org/info/rfc7517>.
              <https://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>.
              <https://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>.
              <https://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>.
              <https://www.rfc-editor.org/info/rfc7662>.

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-04

   o  Change the name of the 'Public Key method' to the more accurate
      'Self-Signed Certificate method' and also change the associated
      authentication method metadata value to
      "self_signed_tls_client_auth".
   o  Removed the "tls_client_auth_root_dn" client metadata field as
      discussed in https://mailarchive.ietf.org/arch/msg/oauth/
      swDV2y0be6o8czGKQi1eJV-g8qc
   o  Update draft-ietf-oauth-discovery reference to -07
   o  Clarify that MTLS client authentication isn't exclusive to the
      token endpoint and can be used with other endpoints, e.g.  RFC
      7009 revocation and 7662 introspection, that utilize client
      authentication as discussed in
      https://mailarchive.ietf.org/arch/msg/oauth/
      bZ6mft0G7D3ccebhOxnEYUv4puI
   o  Reorganize the document somewhat in an attempt to more clearly
      make a distinction between mTLS client authentication and
      certificate bound access tokens as well as a more clear
      delineation between the two (PKI/Public key) methods for client
      authentication
   o  Editorial fixes and clarifications

   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.
   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
   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