| OAuth Working Group | N. Sakimura |
| Internet-Draft | Nomura Research Institute |
| Intended status: Standards Track | K. Li |
| Expires: September 28, 2017 | Alibaba Group |
| J. Bradley | |
| Ping Identity | |
| March 27, 2017 |
The OAuth 2.0 Authorization Framework: JWT Pop Token Usage
draft-sakimura-oauth-jpop-04
This specification describes how to use JWT POP (Jpop) tokens that were obtained through [POPKD] in HTTP requests to access OAuth 2.0 protected resources. Only the party in possession of the corresponding cryptographic key for the Jpop token can use it to get access to the associated resources unlike in the case of the bearer token described in [RFC6750] where any party in posession of the access token can access the resource.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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This document specifies the method for the client to use a proof-of-possestion token against a protected resource. The format of such token is defined in section 3 of [RFC7800].
The same methods and JWT schema elements can be used with opaque tokens and OAuth 2.0 Token Introspection. [RFC7662]
[POPKD] can be used for a client to dynamically specify a key, or the Authorization Server can use information provided by the client at registration to provide the confirmation element.
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].
Unless otherwise noted, all the protocol parameter names and values are case sensitive.
For the purpose of this document, the terms defined in [RFC6749] and [RFC7800] are used.
JWT PoP token is a JWS signed JWT whose payload is a JWT Claims Set. The JWT claims set MUST include the following:
Their semantics are defined in [RFC7519] and [RFC7800].
Following is an example of such.
{
"iss": "https://server.example.com",
"aud": "https://resource.example.org",
"iat": "1360189224",
"exp": "1361398868",
"cnf":{...}
}
Figure 1: Example of JWT PoP Token.
There are several varieties of sender constrained token. Namely:
DN constrained token is typically used when X.509 client certificate authentication is used at the token endpoint. In this case, the constraint is expressed by including the following member at the top level of cnf claim.
The authorization server finds the relevant DN from the X.509 client certificate authentication that is performed at the token endpoint.
{
"iss": "https://server.example.com",
"sub": "joe@example.com",
"aud": "https://resource.example.org",
"exp": "1361398824",
"nbf": "1360189224",
"cnf":{
"dn": "cn=John Doe LLC,dc=client,dc=example,dc=com"
}
}
Figure 2: Example of DN Constrained JWT.
The constraint in the Client ID constrained token is expressed by including the following member at the top level of cnf claim.
The authorization server finds the client ID from the client ID used in the client authentication at the token endpoint.
Methods to express key constraints are extensively described in the section 3 of [RFC7800]. Such cnf claim is used in the access token described in section 3 to form a key constrained token. [RFC7800] defines 4 confirmation methods.
The client provides the corresponding keys or the pointers to the authorization server as a part of the client configuration. It can be done through out-of-band methods (e.g., developper portal) or through some form of dynamic registration, etc.
Following is an example of a JWT payload containing a JWK with a raw key.
{
"iss": "https://server.example.com",
"sub": "joe@example.com",
"aud": "https://resource.example.org",
"exp": "1361398824",
"nbf": "1360189224",
"cnf":{
"jwk":{
"kty": "EC",
"use": "sig",
"crv": "P-256",
"x": "18wHLeIgW9wVN6VD1Txgpqy2LszYkMf6J8njVAibvhM",
"y": "-V4dS4UaLMgP_4fY4j8ir7cl1TXlFdAgcx55o7TkcSA"
}
}
}
Figure 3: Example of a JWK Key Constrained JWT.
Following is an example of a JWT payload containing a jku URI.
{
"iss": "https://server.example.com",
"sub": "joe@example.com",
"aud": "https://resource.example.org",
"exp": "1361398824",
"nbf": "1360189224",
"cnf":{
"jku": "https://client.example.com/keys/client123-jwks"
}
}
Figure 4: Example of a jku Constrained JWT.
Following is an example of a JWT payload containing a x5t#s256 Certificate Thumbprint of a x509 certificate. .
{
"iss": "https://server.example.com",
"sub": "joe@example.com",
"aud": "https://resource.example.org",
"exp": "1361398824",
"nbf": "1360189224",
"cnf":{
"x5t#s256": "w5cK0ebwmCZUYDB2Y5SlESsXE8o9yZg05O89jdNidgI"
}
}
Figure 5: Example of a x5t#s256 Certificate Thumbprint Constrained JWT.
The resource server that supports this specification MUST authenticate the Client by having it demonstrate that it is the holder of the key associated with the access token being used. The confirmation method can be broadly categorized in two forms.
For this, the following steps are taken:
To obtain it, first create a JSON with a name "nonce" and the value being what was received in the previous step. The JWS MUST contain a kid header element if the client has more than one signing key published via JWKS URI e.g.,
{
"nonce":"dcd98b7102dd2f0e8b11d0f600bfb0c093",
"nc":"00000001",
"cnonce":"0a4f113b"
}
Then, jws-on-nonce is obtained by creating a compact serialization of JWS on this JSON.
credentials = "Jpop" jpop-response jpop-response = at-response "," s-response at-response = "at" "=" access-token (* As specified by [POPKD] *) s-response = "s" "=" jws-on-nonce (* Created in the STEP3. *) access-token = quoted-string jws-on-nonce = quoted-string
In the following example, the access token and the jws-on-nonce are represented as access.token.jwt and jws.of.nonce for the sake of brevity.
GET /resource/1234 HTTP/1.0 Host: server.example.com Authorization: Jpop at="access.token.jwt", s="jws.of.nonce"
Figure 6: Example resouce request
If the client requests the resource without the proper authoization header, the resource server returns a HTTP 401 response with WWW-Authenticate header as defined in section 4.1 of [RFC7235] with the challenge as follows:
challenge = "Jpop" jpop-challenge
jpop-challenge = "nonce" "=" nonce-value
nonce-value = quoted-string
Following example depicts what the response would look like.
HTTP/1.0 401 Unauthorized Server: HTTPd/0.9 Date: Wed, 14 March 2017 09:26:53 GMT WWW-Authenticate: Jpop nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093"
Figure 7: Example error response.
A new scheme has been registered in the HTTP Authentication Scheme Registry as follows:
Authentication Scheme Name: Jpop
Reference: Section 3 of this specification
Notes (optional): The Named Authentication scheme is intended to be used only with OAuth Resource Access, and thus does not support proxy authentication.
The "dn" JWT confirmation method relies its security property on the X.509 client certificate authentication. In particular, the validity of the certificate needs to be verified properly. It involves the traversal of all the certificate chain and the certificate validation (e.g., with OCSP).
The client's secret key must be kept securely. Otherwise, the notion of PoP breaks down.
It should be noted that JWE confirmation method is significantly weaker form of the PoP, as the resource server and the authorization server can masquerade as the client.
When using the signature method the client must specify to the AS the aud it intends to send the token to, so that it can be included in the AT.
A malicious RS could receive a AT with no aud or a logical audience and then replay the AT and jws-on-nonce to the actual server.
NOTE another approach would be to include the resource in the jws-on-nonce
When a AS uses dynamic client registration it may accept software statements supplied by a federation operator. Those software statements can contain a JWKS-URI that is hosted by the federation operator or protected by a certificate provisioned from a trusted root. These methods would allow the federation operator to administratively revoke the keys at the JWKS-URI without requiring the JWKS to contain x5c elements with CA issued certificates and having to have the RS perform full certificate validation for each request.
The authors thank the following people for providing valuable feedback to this document. Nov Matake (YAuth).
| [POPA] | Hunt, P., "OAuth 2.0 Proof-of-Possession (PoP) Security Architecture", March 2015. |
| [RFC7636] | Sakimura, N., Bradley, J. and N. Agarwal, "Proof Key for Code Exchange by OAuth Public Clients", RFC 7636, DOI 10.17487/RFC7636, September 2015. |
| [TINTRO] | Richer, J., "OAuth 2.0 Token Introspection", July 2015. |