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
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Internet-Drafts are draft documents valid for a maximum of six months
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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
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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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