OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens
draft-ietf-oauth-mtls-17

OAuth Working Group                                          B. Campbell
Internet-Draft                                             Ping Identity
Intended status: Standards Track                              J. Bradley
Expires: August 2, 2018                                           Yubico
                                                             N. Sakimura
                                               Nomura Research Institute
                                                          T. Lodderstedt
                                                           YES Europe AG
                                                        January 29, 2018


OAuth 2.0 Mutual TLS Client Authentication and Certificate Bound Access
                                 Tokens
                        draft-ietf-oauth-mtls-07

Abstract

   This document describes Transport Layer Security (TLS) mutual
   authentication using X.509 certificates as a mechanism for OAuth
   client authentication to the authorization sever as well as for
   certificate bound sender constrained access tokens as a method for a
   protected resource to ensure that an access token presented to it by
   a given client was issued to that client by the authorization server.

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 https://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 August 2, 2018.

Copyright Notice

   Copyright (c) 2018 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



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   (https://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
   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 . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Mutual TLS for OAuth Client Authentication  . . . . . . . . .   4
     2.1.  PKI Mutual TLS OAuth Client Authentication Method . . . .   5
       2.1.1.  PKI Authentication Method Metadata Value  . . . . . .   5
       2.1.2.  Client Registration Metadata  . . . . . . . . . . . .   5
     2.2.  Self-Signed Certificate Mutual TLS OAuth Client
           Authentication Method . . . . . . . . . . . . . . . . . .   6
       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  . . . . . . .   7
     3.1.  X.509 Certificate 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  . . . . . . . . . . . . . .   9
   4.  Implementation Considerations . . . . . . . . . . . . . . . .  10
     4.1.  Authorization Server  . . . . . . . . . . . . . . . . . .  10
     4.2.  Resource Server . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Sender Constrained Access Tokens Without Client
           Authentication  . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Certificate Bound Access Tokens . . . . . . . . . . . . .  11
     4.5.  Implicit Grant Unsupported  . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     5.1.  TLS Versions and Best Practices . . . . . . . . . . . . .  11
     5.2.  X.509 Certificate Spoofing  . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  JWT Confirmation Methods Registration . . . . . . . . . .  12
     6.2.  OAuth Authorization Server Metadata Registration  . . . .  12
     6.3.  Token Endpoint Authentication Method Registration . . . .  12
     6.4.  OAuth Token Introspection Response Registration . . . . .  13
     6.5.  OAuth Dynamic Client Registration Metadata Registration .  13
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Relationship to Token Binding  . . . . . . . . . . .  16
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  16



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   Appendix C.  Document(s) History  . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   This document describes Transport Layer Security (TLS) mutual
   authentication using X.509 certificates as a mechanism for OAuth
   client authentication to the 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.  This
   document describes an additional mechanism of client authentication
   utilizing mutual TLS [RFC5246] certificate-based authentication,
   which provides better security characteristics than shared secrets.
   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, 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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.







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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 presents its
   X.509 certificate and proves possession of the corresponding private
   key to 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

   This section defines, as an extension of OAuth 2.0, Section 2.3
   [RFC6749], two distinct methods of using mutual TLS X.509 client
   certificates as client credentials.  The requirement of mutual TLS
   for client authentication 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).

   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 requests to the 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.  The authorization server can locate the client
   configuration 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.  Sections Section 2.1 and
   Section 2.2 below define two ways of binding a certificate to a
   client as two distinct client authentication methods.








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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 expected DN configured or registered for 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 by
   obtaining a new certificate with the same subject DN from a trusted
   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 indicate 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.





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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 an X.509 certificate or a trusted
   source for its X.509 certificates (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
   certificate matches the subject public key info of one of the
   certificates configured or registered for that particular client.
   The 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 "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 the authorization
   server.

2.2.1.  Self-Signed Certificate 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 indicate 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
   Self-Signed Certificate method of binding a certificate to a client
   using mutual TLS client authentication, this specification defines
   and registers the following authentication method metadata value.

   self_signed_tls_client_auth
      Indicates that client authentication to the authorization server
      will occur using mutual TLS with the client utilizing a self-
      signed certificate.

2.2.2.  Client Registration Metadata

   For the Self-Signed Certificate method of binding a certificate to a
   client using mutual TLS client authentication, the existing
   "jwks_uri" or "jwks" metadata parameters from [RFC7591] are used to
   convey the client's certificates and public keys, where the X.509
   certificates are represented using the JSON Web Key (JWK) [RFC7517]



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   "x5c" parameter (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 by the client on the connection to 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 Thumbprint Confirmation Method for JWT

   When access tokens are represented as 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 known as certificate fingerprints).




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   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



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   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:




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   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.

   If the authorization server supports the 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.

   The authorization server may also consider hosting the token
   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 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 manner as for the
   Self-Signed Certificate method such that it does not verify that the
   certificate presented by the client during the handshake is signed by



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   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.

4.5.  Implicit Grant Unsupported

   This document describes binding an access token to the client
   certificate presented on the TLS connection from the client to the
   authorization server's token endpoint, however, certificate binding
   of access tokens issued directly from the authorization endpoint via
   the implicit grant flow is explicitly out of scope.  End users
   interact directly with the authorization endpoint using a web browser
   and the use of client certificates in user's browsers bring
   operational and usability issues, which make it undesirable to
   support certificate bound access tokens issued in the implicit grant
   flow.  Implementations wanting to employ certificate bound sender
   constrained access tokens should utilize grant types that involve the
   client making an access token request directly to the token endpoint
   (e.g. the authorization code and refresh token grant types).

5.  Security Considerations

5.1.  TLS Versions and Best Practices

   TLS 1.2 [RFC5246] is cited in this document because, at the time of
   writing, it is the 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].




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5.2.  X.509 Certificate Spoofing

   If the PKI method of client authentication is used, an attacker could
   try to impersonate a client using a certificate with the same subject
   DN but issued by a different CA, which the authorization server
   trusts.  To cope with that threat, the authorization server should
   only accept as trust anchors a limited number of CAs whose
   certificate issuance policy meets its security requirements.  There
   is an assumption then that the client and server agree on the set of
   trust anchors that the server uses to create and validate the
   certificate chain.  Without this assumption the use of a Subject DN
   to identify the client certificate would open the server up to
   certificate spoofing attacks.

6.  IANA Considerations

6.1.  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].

   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 ]]

6.2.  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].

   o  Metadata Name: "mutual_tls_sender_constrained_access_tokens"
   o  Metadata Description: Indicates authorization server support for
      mutual TLS sender constrained access tokens.
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.3 of [[ this specification ]]

6.3.  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].

   o  Token Endpoint Authentication Method Name: "tls_client_auth"
   o  Change Controller: IESG



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   o  Specification Document(s): Section 2.1.1 of [[ this specification
      ]]

   o  Token Endpoint Authentication Method Name:
      "self_signed_tls_client_auth"
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2.1 of [[ this specification
      ]]

6.4.  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].

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

6.5.  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]:

   o  Client Metadata Name:
      "mutual_tls_sender_constrained_access_tokens"
   o  Client Metadata Description: Indicates the client's intention to
      use mutual TLS sender constrained access tokens.
   o  Change Controller: IESG
   o  Specification Document(s): Section 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 2.1.2 of [[ this specification
      ]]

7.  References

7.1.  Normative References








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   [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,
              <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,
              <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,
              <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,
              <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,
              <https://www.rfc-editor.org/info/rfc6749>.

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

   [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>.

   [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>.






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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-08 (work in progress), November 2017.

   [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-05 (work in progress), October 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", 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,
              <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,
              <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,
              <https://www.rfc-editor.org/info/rfc7591>.

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





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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Appendix A.  Relationship to Token Binding

   OAuth 2.0 Token Binding [I-D.ietf-oauth-token-binding] enables the
   application of Token Binding to the various artifacts and tokens
   employed throughout OAuth.  That includes binding of an access token
   to a Token Binding key, which bears some similarities in motivation
   and design to the mutual TLS sender constrained resources access
   defined in this document.  Both documents define what is often called
   a proof-of-possession security mechanism for access tokens, whereby a
   client must demonstrate possession of cryptographic keying material
   when accessing a protected resource.  The details differ somewhat
   between the two documents but both have the authorization server bind
   the access token it issues to an asymmetric key pair on the client.
   The client then proves possession of the private key from that pair
   on the TLS connection over which the protected resource is accessed.

   The two documents then are effectively competing specifications, at
   least with respect to the binding of access tokens.  Token Binding
   uses bare keys that are generated on the client, which avoids many of
   the difficulties of creating, distributing, and managing certificates
   and has the potential to see wider scale adoption and deployment.
   However, at the time of writing, Token Binding is fairly new and
   there is relatively little support for it in available application
   development platforms and tooling.  Until better support for the
   underlying core Token Binding specifications exists, practical
   implementations of OAuth 2.0 Token Binding are infeasible.  Despite
   its name, Token Binding doesn't have a monopoly on the binding of
   tokens.  Mutual TLS, on the other hand, has been around for some time
   and enjoys widespread support in web servers and development
   platforms.  Mutual TLS for OAuth 2.0 can be built and deployed now
   using existing platforms and tools.  There are emerging and immediate
   scenarios, such as OAuth enabled financial transactions motivated by
   regulatory requirements in some cases, which demand the additional
   security protections of proof-of-possession access tokens.  This
   document aspires to provide standardized and expeditious solution for
   those scenarios.

Appendix B.  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.




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

Appendix C.  Document(s) History

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

   draft-ietf-oauth-mtls-07

   o  Update to use the boilerplate from RFC 8174

   draft-ietf-oauth-mtls-06

   o  Add an appendix section describing the relationship of this
      document to OAuth Token Binding as requested during the the
      Singapore meeting https://datatracker.ietf.org/doc/minutes-
      100-oauth/
   o  Add an explicit note that the implicit flow is not supported for
      obtaining certificate bound access tokens as discussed at the
      Singapore meeting https://datatracker.ietf.org/doc/minutes-
      100-oauth/
   o  Add/incorporate text to the Security Considerations on Certificate
      Spoofing as suggested https://mailarchive.ietf.org/arch/msg/oauth/
      V26070X-6OtbVSeUz_7W2k94vCo
   o  Changed the title to be more descriptive
   o  Move the Security Considerations section to before the IANA
      Considerations
   o  Elaborated on certificate bound access tokens a bit more in the
      Abstract
   o  Update draft-ietf-oauth-discovery reference to -08

   draft-ietf-oauth-mtls-05

   o  Editorial fixes

   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



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   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".





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   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






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   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

































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