OAuth Working Group                                          B. Campbell
Internet-Draft                                             Ping Identity
Intended status: Standards Track                              J. Bradley
Expires: April 21, August 29, 2019                                          Yubico
                                                             N. Sakimura
                                               Nomura Research Institute
                                                          T. Lodderstedt
                                                              YES.com AG
                                                        October 18, 2018
                                                       February 25, 2019

    OAuth 2.0 Mutual TLS Client Authentication and Certificate Bound Certificate-Bound
                             Access Tokens
                        draft-ietf-oauth-mtls-12
                        draft-ietf-oauth-mtls-13

Abstract

   This document describes OAuth client authentication and certificate certificate-
   bound access tokens using mutual Transport Layer Security (TLS)
   authentication with X.509 certificates.  OAuth clients are provided a
   mechanism for authentication to the authorization server using mutual
   TLS, based on either self-signed certificates or public key
   infrastructure (PKI).  OAuth authorization servers are provided a
   mechanism for binding access tokens to a client's mutual TLS
   certificate, and OAuth protected resources are provided a method for
   ensuring that such an access token presented to it was issued to the
   client presenting the token.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on April 21, August 29, 2019.

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   Copyright (c) 2018 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   4   5
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4   5
   2.  Mutual TLS for OAuth Client Authentication  . . . . . . . . .   4   5
     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 . . . . . . . .   6
       2.1.1.  PKI Method Metadata Value . . . . . . .   5
     2.2.  Self-Signed Certificate Mutual TLS OAuth Client
           Authentication Method . . . . . . .   6
       2.1.2.  Client Registration Metadata  . . . . . . . . . . . .   6
       2.2.1.
     2.2.  Self-Signed Certificate Authentication Mutual TLS Method
               Metadata Value  . . . . . . . . .   7
       2.2.1.  Self-Signed Method Metadata Value . . . . . . . . . .   6   8
       2.2.2.  Client Registration Metadata  . . . . . . . . . . . .   6   8
   3.  Mutual TLS Client Certificate Bound Access Tokens . . . . . .   7   8
     3.1.  X.509  JWT Certificate Thumbprint Confirmation Method for JWT    7  . . . . .   9
     3.2.  Confirmation Method for Token Introspection . . . . . . .   8  10
     3.3.  Authorization Server Metadata . . . . . . . . . . . . . .   9  11
     3.4.  Client Registration Metadata  . . . . . . . . . . . . . .  10  11
   4.  Implementation Considerations . .  Public Clients and Certificate Bound Tokens . . . . . . . . .  11
   5.  Metadata for Mutual TLS Endpoint Aliases  . . . . .  10
     4.1.  Authorization Server . . . . .  12
   6.  Implementation Considerations . . . . . . . . . . . . .  10
     4.2.  Resource Server . . .  13
     6.1.  Authorization Server  . . . . . . . . . . . . . . . . . .  10
     4.3.  Certificate Bound Access Tokens Without Client
           Authentication  14
     6.2.  Resource Server . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  14
     6.3.  Certificate Expiration and Bound Access Tokens  . . . . .  15
     6.4.  Implicit Grant Unsupported  . . . . . . . . .  11
     4.5.  Implicit Grant Unsupported . . . . . .  15
     6.5.  TLS Termination . . . . . . . . .  11
     4.6.  TLS Termination . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . .  12
   5.  Security Considerations . . . . . . . . .  15
     7.1.  Certificate Thumbprint Binding  . . . . . . . . . .  12
     5.1. . . .  15
     7.2.  TLS Versions and Best Practices . . . . . . . . . . . . .  12
     5.2.  16
     7.3.  X.509 Certificate Spoofing  . . . . . . . . . . . . . . .  12
     5.3.  16
     7.4.  X.509 Certificate Parsing and Validation Complexity . . .  12
   6.  16
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  17
     9.1.  JWT Confirmation Methods Registration . . . . . . . . . .  13
     6.2.  OAuth  17
     9.2.  Authorization Server Metadata Registration  . . . .  13
     6.3. . . .  17
     9.3.  Token Endpoint Authentication Method Registration . . . .  13
     6.4.  OAuth  18
     9.4.  Token Introspection Response Registration . . . . .  14
     6.5.  OAuth . . .  18
     9.5.  Dynamic Client Registration Metadata Registration .  14
   7. . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     7.2.  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16  21
   Appendix A.  Example Certificate, JSON Web Key, "cnf" Claim, Certificate and Confirmation
                Method JWK . . . . . .  22
   Appendix B.  Relationship to Token Binding  . . . . . . . . . . .  23
   Appendix C.  Acknowledgements . . . . . .  17
   Appendix B.  Relationship to Token Binding  . . . . . . . . . . .  18
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  19  24
   Appendix D.  Document(s) History  . . . . . . . . . . . . . . . .  19  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23  29

1.  Introduction

   This document describes

   The OAuth 2.0 Authorization Framework [RFC6749] enables third-party
   client authentication and certificate
   bound applications to obtain delegated access tokens using mutual TLS [RFC5246] authentication with
   X.509 certificates.  OAuth clients are provided mechanisms for
   authentication to protected
   resources.  In the authorization server using mutual TLS. prototypical abstract OAuth
   authorization servers are provided a mechanism for binding flow, illustrated in
   Figure 1, the client obtains an access
   tokens to a client's mutual TLS certificate, token from an entity known as
   an authorization server and OAuth protected
   resources are provided a method for ensuring then uses that token when accessing
   protected resources, such as HTTPS APIs.

     +--------+                                 +---------------+
     |        |                                 |               |
     |        |<--(A)-- Get an access token presented to it was issued to --->| Authorization |
     |        |                                 |     Server    |
     |        |                                 |               |
     |        |                                 +---------------+
     |        |                                         ^
     |        |                                         |
     |        |
     |        |                               (C)       |
     | Client |                           Validate the client presenting
     |        |                           access token  |
     |        |
     |        |                                         |
     |        |                                         v
     |        |                                 +---------------+
     |        |                                 |      (C)      |
     |        |                                 |               |
     |        |<--(B)-- Use the token.

   The access token -->|   Protected   |
     |        |                                 |    Resource   |
     |        |                                 |               |
     +--------+                                 +---------------+

                Figure 1: Abstract OAuth 2.0 Authorization Framework [RFC6749] defines a shared
   secret method of Protocol Flow

   The flow illustrated in Figure 1 includes the following steps:

   (A)  The client authentication but also allows for definition makes an HTTPS "POST" request to the authorization
        server and use presents a credential representing the authorization
        grant.  For certain types of clients (those that have been
        issued or otherwise established a set of additional client authentication mechanisms when
   interacting directly with credentials) the
        request must be authenticated.  In the response, the
        authorization server.  This document
   describes server issues an additional mechanism of client authentication utilizing
   mutual TLS certificate-based authentication, which provides better
   security characteristics than shared secrets.  While [RFC6749]
   documents access token to the client.

   (B)  The client authentication for requests includes the access token when making a request to
        access a protected resource.

   (C)  The protected resource validates the access token endpoint,
   extensions in order to OAuth 2.0 (such
        authorize the request.  In some cases, such as Introspection [RFC7662] when the token is
        self-contained and cryptographically secured, the validation can
        be done locally by the protected resource.  While other cases
        require that the protected resource call out to the
        authorization server to determine the state of the token and
        obtain meta-information about it.

   Layering on the abstract flow above, this document standardizes
   enhanced security options for OAuth 2.0 utilizing client certificate
   based mutual TLS.  Section 2 provides options for authenticating the
   request in step (A).  While step (C) is supported with semantics to
   express the binding of the token to the client certificate for both
   local and remote processing in Section 3.1 and Section 3.2
   respectively.  This ensures that, as described in Section 3,
   protected resource access in step (B) is only possible by the
   legitimate client bearing the access token and holding the private
   key corresponding to the certificate.

   OAuth 2.0 defines a shared secret method of client authentication but
   also allows for 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 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], Revocation [RFC7009]) [RFC7009], and the Backchannel
   Authentication Endpoint in [OpenID.CIBA]) define endpoints that also
   utilize client authentication and the mutual TLS methods defined
   herein are applicable to those endpoints as well.

   Mutual TLS certificate bound certificate-bound access tokens ensure that only the party
   in possession of the private key corresponding to the certificate can
   utilize the token to access the associated resources.  Such a
   constraint is sometimes referred to as key confirmation, proof-of-
   possession, or holder-of-key and is unlike the case of the bearer
   token described in [RFC6750], where any party in possession of the
   access token can use it to access the associated resources.  Binding
   an access token to the client's certificate prevents the use of
   stolen access tokens or replay of access tokens by unauthorized
   parties.

   Mutual TLS certificate bound certificate-bound access tokens and mutual TLS client
   authentication are distinct mechanisms, which are complementary but
   don't necessarily need to be deployed or used 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.

1.2.  Terminology

   Mutual TLS

   Throughout this document the term "mutual TLS" refers 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 contemporary versions of TLS 1.2 [RFC8446] [RFC5246] this
   requires that the client to send Client the Certificate and Certificate Verify CertificateVerify
   messages during the TLS handshake and for the server to verify these the
   CertificateVerify and Finished 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, 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 TLS X.509 certificate
   authentication (i.e. the Client Certificate and Certificate Verify
   messages are sent during the TLS Handshake [RFC5246]). Handshake).

   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 the binding of a
   certificate to a specific
   between client and certificate as described in either Section 2.1 or
   Section 2.2 below.

2.1.  PKI Mutual TLS OAuth Client Authentication Method

   The PKI (public key infrastructure) method of mutual TLS OAuth client
   authentication uses adheres to the way in which X.509 certificates are
   traditionally used for authentication.  It relies on a subject
   distinguished name (DN) or a subject alternative name (SAN) and
   validated certificate chain [RFC5280] to identify authenticate 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 subject configured or registered for
   that particular client (note that a predictable treatment of DN
   values, such as the distinguishedNameMatch rule from [RFC4517], is
   needed in comparing the certificate's subject DN to the client's
   registered DN).  If  Revocation checking is possible with the PKI method
   but if and how to check a certificate's revocation status is a
   deployment decision at the discretion of the authorization server.  The PKI
   method facilitates the way X.509 certificates are traditionally being
   used for authentication.  It also allows the client to
   Clients can rotate its their X.509 certificates without the need to
   modify its the 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

   For the PKI method of mutual TLS client authentication, this
   specification defines and registers the following authentication
   method metadata value into the "OAuth Token Endpoint Authentication
   Methods" registry [IANA.OAuth.Parameters].

   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

   In order to convey the expected subject of the certificate, the
   following metadata parameter is parameters are introduced for the OAuth 2.0
   Dynamic Client Registration Protocol [RFC7591] in support of the PKI
   method of binding a mutual TLS client authentication.  A client using the
   "tls_client_auth" authentication method MUST use exactly one of the
   below metadata parameters to indicate the certificate subject value
   that the authorization server is to a client: expect when authenticating the
   respective client.

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

   tls_client_auth_san_dns
      A string containing the value of an expected dNSName SAN entry in
      the certificate, which the OAuth client will use in mutual TLS
      authentication.

   tls_client_auth_san_uri
      A string containing the value of an expected
      uniformResourceIdentifier SAN entry in the certificate, which the
      OAuth client will use in mutual TLS authentication.

   tls_client_auth_san_ip
      A string representation of an IP address in either dotted decimal
      notation (for IPv4) or colon-delimited hexadecimal (for IPv6, as
      defined in [RFC4291] section 2.2) that is expected to be present
      as an iPAddress SAN entry in the certificate, which the OAuth
      client will use in mutual TLS authentication.

   tls_client_auth_san_email
      A string containing the value of an expected rfc822Name SAN entry
      in the certificate, which 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 an its X.509 certificate certificates (using
   "jwks" defined in [RFC7591]) or a trusted source for its X.509
   certificates (such as the (using "jwks_uri" defined in
   [RFC7591] that references a JSON Web Key [RFC7517] Set containing the
   client's certificates and public keys) from [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 client's certificate chain is not
   validated by the server in this case.  The client is successfully
   authenticated if the subject public key info of the certificate
   matches that it presented during the subject public key info of
   handshake matches one of the certificates configured or registered
   for that particular client.  The Self-Signed Certificate method
   allows to the use of 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

   For the Self-Signed Certificate method of mutual TLS client
   authentication, this specification defines and registers the
   following authentication method metadata value into the "OAuth Token
   Endpoint Authentication Methods" registry [IANA.OAuth.Parameters].

   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 with
   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 via JSON Web Key (JWK) [RFC7517]
   "x5c" parameter.  Note that Sec 4.7 of [RFC7517] requires that the
   key in a JWK Set
   (JWKS) [RFC7517].  The "jwks" metadata parameter is a JWK Set
   containing the first certificate of the "x5c" parameter must match the client's public key keys as an array of JWKs while the
   "jwks_uri" parameter is a URL that references a client's JWK Set.  A
   certificate is represented by other with the "x5c" parameter of an individual
   JWK within the set.  Note that the members of the JWK representing
   the public key (e.g. "n" and "e" for RSA, "x" and "y" for EC). EC) are
   required parameters per [RFC7518] so will be present even though they
   are not utilized in this context.  Also note that that sec 4.7 of
   [RFC7517] requires that the key in the first certificate of the "x5c"
   parameter match the public key represented by those other members of
   the JWK.

3.  Mutual TLS Client Certificate Bound Access Tokens

   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.  Binding the access token to the client certificate in
   that fashion has the benefit of decoupling that binding from the
   client's authentication with the authorization server, which enables
   mutual TLS during protected resource access to serve purely as a
   proof-of-possession mechanism.  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 obtain, from its TLS implementation
   layer, 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
   client certificate bound certificate-bound access tokens is defined in Section 3.3 and
   Section 3.4 respectively.

3.1.  X.509  JWT 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 [RFC7800] member "x5t#S256"
   for the X.509 Certificate SHA-256 Thumbprint.  The value of the
   "x5t#S256" member is a base64url-encoded [RFC4648] SHA-256 [SHS] hash
   (a.k.a. thumbprint, fingerprint or digest) of the DER encoding of the
   X.509 certificate [RFC5280].  The base64url-encoded value MUST omit
   all trailing pad '=' characters and MUST NOT include any line breaks,
   whitespace, or other additional characters.

   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: 2: Example JWT Claims Set with an X.509 Certificate Thumbprint
                            Confirmation Method

   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 client certificate bound certificate-bound 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 "cnf" with "x5t#S256"
   member 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

   The following is an example 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 the more
   specific certificate thumbprint confirmation method.

   The following is an example of an 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: 3: Example Introspection Response for a Certificate Bound
                               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:

   tls_client_certificate_bound_access_tokens
      OPTIONAL.  Boolean value indicating server support for mutual TLS
      client certificate bound certificate-bound 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:

   tls_client_certificate_bound_access_tokens
      OPTIONAL.  Boolean value used to indicate the client's intention
      to use mutual TLS client certificate bound certificate-bound access tokens.  If
      omitted, the default value is "false".

4.  Implementation Considerations

4.1.  Authorization Server

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

   If the authorization server wants to support mutual  Public Clients and Certificate Bound Tokens

   Mutual TLS OAuth client authentication and other certificate-bound access
   tokens can be used independently of each other.  Use of certificate-
   bound access tokens without mutual TLS OAuth client authentication methods authentication,
   for example, is possible in parallel,
   it should make mutual support of binding access tokens to a TLS optional.

   If
   client certificate for public clients (those without authentication
   credentials associated with the "client_id").  The authorization
   server supports the Self-Signed Certificate
   method, it should would configure the TLS stack in a way the same manner as for the
   Self-Signed Certificate method such that it does not verify whether that 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 CA.  Individual instances of a client authentication, on would create a
   separate host name or port in order to prevent unintended impact on
   the self-
   signed certificate for mutual TLS behavior of its other endpoints, e.g. with both the authorization
   endpoint.

4.2.  Resource Server

   Since the resource server relies on the
   and resource server.  The authorization server would not use the
   mutual TLS certificate to
   perform client authentication, there is no need authenticate the client at the OAuth layer
   but would bind the issued access token to that certificate, for which
   the client has proven possession of the corresponding private key.
   The access token is then bound to the certificate and can only be
   used by the client possessing the certificate and corresponding
   private key and utilizing them to negotiate mutual TLS on connections
   to the resource server.  When the authorization server issues a
   refresh token to validate such a client, it SHOULD also bind the trust chain refresh token
   to the respective certificate.  And check the binding when the
   refresh token is presented to get new access tokens.  The
   implementation details of the client's certificate in any binding the refresh token are at the
   discretion of the methods defined in this document. authorization server.

5.  Metadata for Mutual TLS is used only as
   a proof-of-possession mechanism during protected resource access. Endpoint Aliases

   The resource process of negotiating client certificate-based mutual TLS
   involves a TLS server should therefore configure requesting a certificate from the TLS stack in client
   (the client does not provide one unsolicited).  Although a way server can
   be configured such that it client certificates are optional, meaning
   that the connection is allowed to continue when the client does not verify whether
   provide a certificate, the act of a server requesting a certificate presented by
   can result in undesirable behavior from some clients.  This is
   particularly true of web browsers as TLS clients, which will
   typically present the
   client during end-user with an intrusive certificate
   selection interface when the handshake is signed by server requests a trusted CA certificate.

4.3.  Certificate Bound Access Tokens Without Client Authentication

   Mutual

   Authorization servers supporting both clients using mutual TLS OAuth client authentication and
   conventional clients MAY chose to isolate the server side mutual TLS client
   certificate bound access tokens can be used independently of each
   other.  Use
   behaviour to only clients intending to do mutual TLS, thus avoiding
   any undesirable effects it might have on conventional clients.  The
   following authorization server metadata parameter is introduced to
   facilitate such separation:

   mtls_endpoint_aliases
      OPTIONAL.  A JSON object containing alternative authorization
      server endpoints that, when present, an OAuth client intending to
      do mutual TLS uses in preference to the conventional endpoints.
      The parameter value itself consists of certificate bound access tokens without one or more endpoint
      parameters, such as "token_endpoint", "revocation_endpoint",
      "introspection_endpoint", etc., conventionally defined for the
      top-level of authorization server metadata.  An OAuth client
      intending to do mutual TLS (for OAuth client authentication, for example, authentication and/or
      to acquire or use certificate-bound tokens) when making a request
      directly to the authorization server MUST use the alias URL of the
      endpoint within the "mtls_endpoint_aliases", when present, in
      preference to the endpoint URL of the same name at top-level of
      metadata.  When an endpoint is possible not present in support
      "mtls_endpoint_aliases", then the client uses the conventional
      endpoint URL defined at the top-level of
   binding access tokens the authorization server
      metadata.  Metadata parameters within "mtls_endpoint_aliases" that
      do not define endpoints to a TLS which an OAuth client certificate makes a direct
      request have no meaning and SHOULD be ignored.

   Below is an example of an authorization server metatdata document
   with the "mtls_endpoint_aliases" parameter, which indicates aliases
   for public clients the token, revocation, and introspection endpoints that an OAuth
   client intending to do mutual TLS would in preference to the
   conventional token, revocation, and introspection endpoints.  Note
   that the endpoints in "mtls_endpoint_aliases" use a different host
   than their conventional counterparts, which allows the authorization
   server (via SNI or clients utilizing other actual distinct hosts) to differentiate its TLS
   behavior as appropriate.

   {
     "issuer": "https://server.example.com",
     "authorization_endpoint": "https://server.example.com/authz",
     "token_endpoint": "https://server.example.com/token",
     "introspection_endpoint": "https://server.example.com/introspect",
     "revocation_endpoint": "https://server.example.com/revo",
     "jwks_uri": "https://server.example.com/jwks",
     "response_types_supported": ["code"],
     "response_modes_supported": ["fragment","query","form_post"],
     "grant_types_supported": ["authorization_code", "refresh_token"],
     "token_endpoint_auth_methods_supported":
                     ["tls_client_auth","client_secret_basic","none"],
     "tls_client_certificate_bound_access_tokens": true
     "mtls_endpoint_aliases": {
       "token_endpoint": "https://mtls.example.com/token",
       "revocation_endpoint": "https://mtls.example.com/revo",
       "introspection_endpoint": "https://mtls.example.com/introspect"
     }
   }

      Figure 4: Example Authorization Server Metadata with Mutual TLS
                             Endpoint Aliases

6.  Implementation Considerations
6.1.  Authorization Server

   The authorization server needs to set up its TLS configuration
   appropriately for the OAuth client authentication methods of it
   supports.

   An authorization server that supports mutual TLS client
   authentication and other client authentication methods or public
   clients in parallel would make mutual TLS optional (i.e. allowing a
   handshake to continue after the server requests a client certificate
   but the client does not send one).

   In order to support the Self-Signed Certificate method, the
   authorization server.  The
   authorization server would configure the TLS stack in the same manner as for the Self-Signed Certificate
   method such a way that
   it does not verify that whether the certificate presented by the client
   during the handshake is signed by a trusted CA.
   Individual instances CA certificate.

   As described in Section 3, the authorization server binds the issued
   access token to the TLS client certificate, which means that it will
   only issue certificate-bound tokens for a certificate which the
   client has proven possession of the corresponding private key.

   The authorization server may also consider hosting the token
   endpoint, and other endpoints requiring client authentication, on a
   separate host name or port in order to prevent unintended impact on
   the TLS behavior of its other endpoints, e.g. the authorization
   endpoint.  As described in Section 5, it may further isolate any
   potential impact of the server requesting client would create certificates by
   offering a self-signed
   certificate distinct set of endpoints on a separate host or port,
   which are aliases for the originals that a client intending to do
   mutual TLS with both will use in preference to the conventional endpoints.

6.2.  Resource Server

   OAuth divides the roles and responsibilities such that the resource
   server relies on the authorization server to perform client
   authentication and obtain resource server. owner (end-user) authorization.
   The authorization resource server would makes authorization decisions based on the access
   token presented by the client but does not use directly authenticate the mutual
   TLS certificate
   client per se.  The the manner in which an access token is bound to authenticate
   the client at certificate decouples it from the OAuth layer but
   would bind specific method that the issued
   client used to authenticate with the authorization server.  Mutual
   TLS during protected resource access token can therefore serve purely as a
   proof-of-possession mechanism.  As such, it is not necessary for the
   resource server to that certificate, which validate the
   client has proven possession trust chain of the corresponding private key. client's
   certificate in any of the methods defined in this document.  The
   access token is then bound to
   resource server would therefore configure the TLS stack in a way that
   it does not verify whether the certificate and can only be used presented by the client possessing the certificate and corresponding private key
   and utilizing them to negotiate mutual TLS on connections to
   during the
   resource server.

4.4. handshake is signed by a trusted CA certificate.

6.3.  Certificate Expiration and 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.

6.4.  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, such 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 certificate-
   bound access tokens issued in the implicit grant flow.
   Implementations wanting to employ certificate bound certificate-bound 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).

4.6.

6.5.  TLS Termination

   An authorization server or resource server MAY choose to terminate
   TLS connections at a load balancer, reverse proxy, or other network
   intermediary.  How the client certificate metadata is securely
   communicated between the intermediary and the application server in
   this case is out of scope of this specification.

5.

7.  Security Considerations

5.1.

7.1.  Certificate Thumbprint Binding

   The binding between the certificate and access token specified in
   Section 3.1 uses a cryptographic hash of the certificate.  It relies
   on the hash function having sufficient preimage and second-preimage
   resistance so as to make it computationally infeasible to find or
   create another certificate that produces to the same hash output
   value.  The SHA-256 hash function was used because it meets the
   aforementioned requirement while being widely available.  If, in the
   future, certificate thumbprints need to be computed using hash
   function(s) other than SHA-256, it is suggested that additional
   related JWT confirmation methods members be defined for that purpose
   and registered in the the IANA "JWT Confirmation Methods" registry
   [IANA.JWT.Claims] for JWT "cnf" member values.

7.2.  TLS Versions and Best Practices

   In the abstract this document is applicable with any TLS version
   supporting certificate-based client authentication.  Both TLS 1.3
   [RFC8446] and TLS 1.2 [RFC5246] is are cited in this document herein because, at the time
   of writing, it 1.3 is the latest newest version that while 1.2 is the most widely
   deployed.  However,
   this document is applicable with other TLS versions supporting
   certificate-based client authentication, including the relatively
   recently published TLS 1.3 [RFC8446].  Implementation  General implementation and 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) can be found in [BCP195].

5.2.

7.3.  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
   (DN or SAN) but issued by a different CA, which the authorization
   server trusts.  To cope with that threat, the authorization server should
   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 subject to
   identify the client certificate would open the server up to
   certificate spoofing attacks.

5.3.

7.4.  X.509 Certificate Parsing and Validation Complexity

   Parsing and validation of X.509 certificates and certificate chains
   is complex and implementation mistakes have previously exposed
   security vulnerabilities.  Complexities of validation include (but
   are not limited to) [X509Pitfalls] [DangerousCode] [CX5P] [DCW] [RFC5280]:

   o  checking of Basic Constraints, basic and extended Key Usage
      constraints, validity periods, and critical extensions;

   o  handling of null-terminator bytes and non-canonical string
      representations in subject names;

   o  handling of wildcard patterns in subject names;

   o  recursive verification of certificate chains and checking
      certificate revocation.

   For these reasons, implementors SHOULD use an established and well-
   tested X.509 library (such as one used by an established TLS library)
   for validation of X.509 certificate chains and SHOULD NOT attempt to
   write their own X.509 certificate validation procedures.

6.

8.  Privacy Considerations

   In TLS versions prior to 1.3, the client's certificate is sent
   unencrypted in the initial handshake and can potentially be used by
   third parties to monitor, track, and correlate client activity.  This
   is likely of little concern for clients that act on behalf of a
   significant number of end-users because individual user activity will
   not be discernible amidst the client activity as a whole.  However,
   clients that act on behalf of a single end-user, such as a native
   application on a mobile device, should use TLS version 1.3 whenever
   possible or consider the potential privacy implications of using
   mutual TLS on earlier versions.

9.  IANA Considerations

6.1.

9.1.  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

9.2.  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 [RFC8414].

   o  Metadata Name: "tls_client_certificate_bound_access_tokens"
   o  Metadata Description: Indicates authorization server support for
      mutual TLS client certificate bound certificate-bound access tokens.
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.3 of [[ this specification ]]

6.3.

   o  Metadata Name: "mtls_endpoint_aliases"
   o  Metadata Description: JSON object containing alternative
      authorization server endpoints, which a client intending to do
      mutual TLS will use in preference to the conventional endpoints.

   o  Change Controller: IESG
   o  Specification Document(s): Section 5 of [[ this specification ]]

9.3.  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
   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
      ]]

9.4.  Token Introspection Response Registration

   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 (see
   Section 3.2), it needs to define and register the higher level "cnf"
   structure as an introspection response member in order to define and
   use the more specific certificate thumbprint confirmation method.

   As such, 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 [RFC7800] and [[ this specification ]]

6.5.  OAuth

9.5.  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: "tls_client_certificate_bound_access_tokens"
   o  Client Metadata Description: Indicates the client's intention to
      use mutual TLS client certificate bound certificate-bound 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 DN of the client certificate.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.1.2 of [[ this specification
      ]]

7.

   o  Client Metadata Name: "tls_client_auth_san_dns"
   o  Client Metadata Description: String value specifying the expected
      dNSName SAN entry in the client certificate.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.1.2 of [[ this specification
      ]]

   o  Client Metadata Name: "tls_client_auth_san_uri"
   o  Client Metadata Description: String value specifying the expected
      uniformResourceIdentifier SAN entry in the client certificate.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.1.2 of [[ this specification
      ]]

   o  Client Metadata Name: "tls_client_auth_san_ip"
   o  Client Metadata Description: String value specifying the expected
      iPAddress SAN entry in the client certificate.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.1.2 of [[ this specification
      ]]

   o  Client Metadata Name: "tls_client_auth_san_email"
   o  Client Metadata Description: String value specifying the expected
      rfc822Name SAN entry in the client certificate.
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.1.2 of [[ this specification
      ]]

10.  References

7.1.

10.1.  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,
              <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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [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.

10.2.  Informative References

   [DangerousCode]

   [CX5P]     Wong, D., "Common x509 certificate validation/creation
              pitfalls", September 2016,
              <https://www.cryptologie.net/article/374/
              common-x509-certificate-validationcreation-pitfalls>.

   [DCW]      Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
              D., and V. Shmatikov, "The Most Dangerous Code in the
              World: Validating SSL Certificates in Non-Browser
              Software",
              <http://www.cs.utexas.edu/~shmat/shmat_ccs12.pdf>.

   [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-06 (work in progress), March 2018.

   [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.CIBA]
              Fernandez, G., Walter, F., Nennker, A., Tonge, D., and B.
              Campbell, "OpenID Connect Client Initiated Backchannel
              Authentication Flow - Core 1.0", January 2019,
              <https://openid.net/specs/openid-client-initiated-
              backchannel-authentication-core-1_0.html>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4517]  Legg, S., Ed., "Lightweight Directory Access Protocol
              (LDAP): Syntaxes and Matching Rules", RFC 4517,
              DOI 10.17487/RFC4517, June 2006,
              <https://www.rfc-editor.org/info/rfc4517>.

   [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>.

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

   [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>.

   [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>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [X509Pitfalls]
              Wong, D., "Common x509 certificate validation/creation
              pitfalls", September 2016,
              <https://www.cryptologie.net/article/374/
              common-x509-certificate-validationcreation-pitfalls>.

Appendix A.  Example Certificate, JSON Web Key, "cnf" Claim, Certificate and Confirmation Method JWK

   For reference, an "x5t#S256" value and the X.509 Certificate from
   which it was calculated are provided in the following example
   figures.  A JWK representation of the certificate's public key along
   with the "x5c" member is also provided.

   "cnf":{"x5t#S256":"A4DtL2JmUMhAsvJj5tKyn64SqzmuXbMrJa0n761y5v0"}

                   Figure 3: 5: x5t#S256 Confirmation Claim

   -----BEGIN CERTIFICATE-----
   MIIBBjCBrAIBAjAKBggqhkjOPQQDAjAPMQ0wCwYDVQQDDARtdGxzMB4XDTE4MTAx
   ODEyMzcwOVoXDTIyMDUwMjEyMzcwOVowDzENMAsGA1UEAwwEbXRsczBZMBMGByqG
   SM49AgEGCCqGSM49AwEHA0IABNcnyxwqV6hY8QnhxxzFQ03C7HKW9OylMbnQZjjJ
   /Au08/coZwxS7LfA4vOLS9WuneIXhbGGWvsDSb0tH6IxLm8wCgYIKoZIzj0EAwID
   SQAwRgIhAP0RC1E+vwJD/D1AGHGzuri+hlV/PpQEKTWUVeORWz83AiEA5x2eXZOV
   bUlJSGQgjwD5vaUaKlLR50Q2DmFfQj1L+SY=
   -----END CERTIFICATE-----

               Figure 4: 6: PEM Encoded Self-Signed Certificate

   {
    "kty":"EC",
    "x":"1yfLHCpXqFjxCeHHHMVDTcLscpb07KUxudBmOMn8C7Q",
    "y":"8_coZwxS7LfA4vOLS9WuneIXhbGGWvsDSb0tH6IxLm8",
    "crv":"P-256",
    "x5c":[
     "MIIBBjCBrAIBAjAKBggqhkjOPQQDAjAPMQ0wCwYDVQQDDARtdGxzMB4XDTE4MTA
      xODEyMzcwOVoXDTIyMDUwMjEyMzcwOVowDzENMAsGA1UEAwwEbXRsczBZMBMGBy
      qGSM49AgEGCCqGSM49AwEHA0IABNcnyxwqV6hY8QnhxxzFQ03C7HKW9OylMbnQZ
      jjJ/Au08/coZwxS7LfA4vOLS9WuneIXhbGGWvsDSb0tH6IxLm8wCgYIKoZIzj0E
      AwIDSQAwRgIhAP0RC1E+vwJD/D1AGHGzuri+hlV/PpQEKTWUVeORWz83AiEA5x2
      eXZOVbUlJSGQgjwD5vaUaKlLR50Q2DmFfQj1L+SY="
      ]
    }

                          Figure 5: 7: JSON Web Key

Appendix B.  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 client certificate bound certificate-bound access tokens
   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 that it issues to an asymmetric key pair held by the
   client.  The client then proves possession of the private key from
   that pair with respect to the TLS connection over which the protected
   resource is accessed.

   Token Binding uses bare keys that are generated on the client, which
   avoids many of the difficulties of creating, distributing, and
   managing certificates used in this specification.  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.  Mutual TLS, on the other hand, has
   been around for some time and enjoys widespread support in web
   servers and development platforms.  As a consequence, OAuth 2.0
   Mutual TLS Client Authentication and Certificate Bound Access Tokens
   can be built and deployed now using existing platforms and tools.  In
   the future, the two specifications are likely to be deployed in
   parallel for solving similar problems in different environments.
   Authorization servers may even support both specifications
   simultaneously using different proof-of-possession mechanisms for
   tokens issued to different clients.

Appendix C.  Acknowledgements

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

   This specification was developed within the OAuth Working Group under
   the chairmanship of Hannes Tschofenig and Rifaat Shekh-Yusef with
   Eric Rescorla and Benjamin Kaduk serving as Security Area Directors.
   Additionally, the following individuals contributed ideas, feedback,
   and wording that helped shape this specification: Sergey Beryozkin,
   Ralph Bragg, Sophie Bremer, Vladimir Dzhuvinov, Samuel Erdtman, Evan
   Gilman, Leif Johansson, Michael Jones, Phil Hunt, Benjamin Kaduk,
   Takahiko Kawasaki, Sean Leonard, Kepeng Li, Neil Madden, James
   Manger, Jim Manico, Nov Matake, Sascha Preibisch, Eric Rescorla,
   Justin Richer, Filip Skokan, Dave Tonge, and Hannes Tschofenig.

Appendix D.  Document(s) History

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

   draft-ietf-oauth-mtls-13

   o  Add an abstract protocol flow and diagram to serve as an overview
      of OAuth in general and baseline to describe the various ways in
      which the mechanisms defined herein are intended to be used.
   o  A little bit less of that German influence.

   o  Rework the TLS references a bit and, in the Terminology section,
      clean up the description of what messages are sent and verified in
      the handshake to do 'mutual TLS'.
   o  Move the explanation about "cnf" introspection registration into
      the IANA Considerations.
   o  Add CIBA as an informational reference and additional example of
      an OAuth extension that defines an endpoint that utilizes client
      authentication.
   o  Shorten a few of the section titles.
   o  Add new client metadata values to allow for the use of a SAN in
      the PKI MTLS client authentication method.
   o  Add privacy considerations attempting to discuss the implications
      of the client cert being sent in the clear in TLS 1.2.
   o  Changed the 'Certificate Bound Access Tokens Without Client
      Authentication' section to 'Public Clients and Certificate Bound
      Tokens' and moved it up to be a top level section while adding
      discussion of binding refresh tokens for public clients.
   o  Reword/restructure the main PKI method section somewhat to
      (hopefully) improve readability.
   o  Reword/restructure the Self-Signed method section a bit to
      (hopefully) make it more comprehensible.
   o  Reword the AS and RS Implementation Considerations somewhat to
      (hopefully) improve readability.
   o  Clarify that the protected protected resource obtains the client
      certificate used for mutual TLS from its TLS implementation layer.
   o  Add Security Considerations section about the certificate
      thumbprint binding that includes the hash algorithm agility
      recommendation.
   o  Add an "mtls_endpoint_aliases" AS metadata parameter that is a
      JSON object containing alternative authorization server endpoints,
      which a client intending to do mutual TLS will use in preference
      to the conventional endpoints.
   o  Minor editorial updates.

   draft-ietf-oauth-mtls-12

   o  Add an example certificate, JWK, and confirmation method claim.
   o  Minor editorial updates based on implementer feedback.
   o  Additional Acknowledgements.

   draft-ietf-oauth-mtls-11

   o  Editorial updates.
   o  Mention/reference TLS 1.3 RFC8446 in the TLS Versions and Best
      Practices section.

   draft-ietf-oauth-mtls-10
   o  Update draft-ietf-oauth-discovery reference to RFC8414

   draft-ietf-oauth-mtls-09

   o  Change "single certificates" to "self-signed certificates" in the
      Abstract

   draft-ietf-oauth-mtls-08

   o  Incorporate clarifications and editorial improvements from Justin
      Richer's WGLC review
   o  Drop the use of the "sender constrained" terminology per WGLC
      feedback from Neil Madden (including changing the metadata
      parameters from mutual_tls_sender_constrained_access_tokens to
      tls_client_certificate_bound_access_tokens)
   o  Add a new security considerations section on X.509 parsing and
      validation per WGLC feedback from Neil Madden and Benjamin Kaduk
   o  Note that a server can terminate TLS at a load balancer, reverse
      proxy, etc. but how the client certificate metadata is securely
      communicated to the backend is out of scope per WGLC feedback
   o  Note that revocation checking is at the discretion of the AS per
      WGLC feedback
   o  Editorial updates and clarifications
   o  Update draft-ietf-oauth-discovery reference to -10 and draft-ietf-
      oauth-token-binding to -06
   o  Add folks involved in WGLC feedback to the acknowledgements list

   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 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
   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
      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 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.com AG

   Email: torsten@lodderstedt.net