Mutual TLS Profiles for OAuth Clients
draft-ietf-oauth-mtls-00

Abstract

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

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on November 10, 2017.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Notation and Conventions
• 1.2. Terminology
• 2. Mutual TLS for Client Authentication
• 2.1. Mutual TLS Client Authentication to the Token Endpoint
• 2.2. Authorization Server Metadata
• 2.3. Dynamic Client Registration
• 3. Mutual TLS Sender Constrained Resources Access
• 3.1. X.509 Certificate SHA-256 Thumbprint Confirmation Method for JWT
• 4. IANA Considerations
• 4.1. JWT Confirmation Methods Registration
• 4.1.1. Registry Contents
• 4.2. Token Endpoint Authentication Method Registration
• 4.2.1. Registry Contents
• 4.3. OAuth Dynamic Client Registration Metadata Registration
• 4.3.1. Registry Contents
• 5. Security Considerations
• 5.1. TLS Versions and Best Practices
• 5.2. Client Identity Binding
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Acknowledgements
• Appendix B. Document(s) History
• Authors' Addresses

1. Introduction

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

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

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

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

1.1. Requirements Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Terminology

This specification uses the following phrases interchangeably: 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.

Transport Layer Security (TLS) Mutual Authentication

Mutual TLS

Mutual TLS X.509 client certificate authentication

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

2. Mutual TLS for Client Authentication

2.1. Mutual TLS Client Authentication to the Token Endpoint

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

For all access token requests to the token endpoint, regardless of the grant type used, the client MUST include the client_id parameter, described in OAuth 2.0, Section 2.2 [RFC6749]. The presence of the client_id parameter enables the authorization server to easily identify the client independently from the content of the certificate and allows for trust models to vary as appropriate for a given deployment. The authorization server can locate the client configuration by the client identifier and check the certificate presented in the TLS Handshake against the expected credentials for that client. As described in Section 5.2, the authorization server MUST enforce some method of binding a certificate to a client.

2.2. Authorization Server Metadata

tls_client_auth is used as a new value of the token_endpoint_auth_methods_supported metadata parameter to indicate server support for mutual TLS as a client authentication method in authorization server metadata such as [OpenID.Discovery] and [I-D.ietf-oauth-discovery].

2.3. Dynamic Client Registration

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

The value tls_client_auth is used to indicate the client's intention to use mutual TLS as an authentication method to the token endpoint for the token_endpoint_auth_method client metadata field.

For authorization servers that associate certificates with clients using subject information in the certificate, the following two new string metadata parameters can be used:

tls_client_auth_subject_dn

The expected subject distinguished name of the client certificate can be represented using tls_client_auth_subject_dn.

tls_client_auth_issuer_dn

The metadata parameter tls_client_auth_issuer_dn can optionally be used to constrain the expected distinguished name of the root issuer of the client certificate.

For authorization servers that use the key or full certificate to associate clients with certificate, the existing jwks_uri or jwks metadata parameters from [RFC7591] shall be used.

3. Mutual TLS Sender Constrained Resources Access

When mutual TLS X.509 client certificate authentication is used at the token endpoint, the authorization server is able to bind the issued access token to the client certificate. Such a binding is accomplished by associating a hash of 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 [RFC7662]. The specific method for associating the certificate with the access token is determined by the authorization server and the protected resource, and is beyond the scope for 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 to authenticate to the token endpoint.

The protected resource MUST obtain the client certificate used for TLS authentication and MUST verify that the hash of that certificate exactly matches the hash of the certificate associated with the access token. If the hash values do not match, the resource access attempt MUST be rejected with an error.

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

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

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

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

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