The following is proposed language for a new section, 2.2, in the spec. This is
part of the spec that deals with client authentication. This proposal is
completely orthogonal to anything in section 4 such as grant types. At Eran's
suggestion I added in an example into the text to help readers understand both
what the feature is for and how it is used.
Eran also suggested that we should probably introduce a client_cred_type
parameter (since we are explicitly support multiple types of client parameters)
just to be consistent with grant_type. I don't have strong feelings on the
subject and am happy to go with whatever the group wants.
Thoughts?
Thanks,
Yaron
2.2 Assertion Client Credentials
In scenarios where security is at a premium one wants to avoid sending secrets
across the Internet, even on encrypted connections. Instead one wants to send
values derived from the secret that prove to the receiver that the sender is in
possession of the secret without actually sending the secret. Typically the way
derived values are created is by generating an assertion that is then either
HMAC'd or digitally signed using an agreed upon secret. By validating the HMAC
or digital signature on the assertion the receiver of the assertion can prove
to themselves that the entity that generated the assertion was in possession of
the secret without actually communicating the secret directly.
So, for example, a client can establish a secret using an out of band mechanism
with a resource server. As part of this out of band communication the client
and resource server agree that the client will authenticate itself using a SAML
assertion with agreed upon parameters that will be signed by the provisioned
secret.
Later on the client interacts with an end-user and uses the end-user
authorization process with the resource server to get an authorization code.
The client then sends an access token request to the token endpoint for the
resource server and includes, along with the authorization code, a SAML
assertion it signed with the previously agreed key and parameters. The SAML
assertion proves to the token endpoint the identity of the client and the
authorization code provides the link to the end-user authorization. If the
access token request comes back with a refresh token then in the future the
client can get new access tokens by submitting an access token request
containing both a properly signed and formatted SAML assertion as well as the
refresh token.
The following defines how to send an assertion as client credentials. If the
assertion client credentials are used then the client MUST include the
credentials using the following request parameters:
client_id
REQUIRED. The client identifier.
client_assertion_type
REQUIRED. The format of the assertion as defined by the
authorization server. The value MUST be an absolute URI.
client_assertion
REQUIRED. The client assertion.
For example (line breaks are for display purposes only):
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
grant_type=refresh_token&client_id=s6BhdRkqt3&
client_assertion=PHNhbWxwOl...[omitted for brevity]...ZT4%3D&
client_assertion_type=urn%3Aoasis%3Anames%sAtc%3ASAML%3A2.0%3Aassertion&
refresh_token=n4E90119d
The client MUST NOT include the client credentials using more than one
mechanism. If more than one mechanism is used regardless whether the
credentials are identical or valid, the server MUST reply with an HTTP 400
status code (Bad Request) and include the "multiple_credentials" error code.
Token endpoints can differentiate between client assertion credentials and
other client credential types by looking for the presence of the
client_assertion and client_assertion_type attributes which will only be
present with client assertion credentials.
From: [email protected] [mailto:[email protected]] On Behalf Of Yaron
Goland
Sent: Friday, June 25, 2010 3:33 PM
To: Eran Hammer-Lahav; [email protected]
Subject: [OAUTH-WG] Proposal for text for section 2
Motivating Scenario: A client makes a request to a token endpoint. It uses an
authorization token to authenticate itself and a refresh token to authenticate
it's delegated right. This approach to authenticating clients is used in
enterprises all the time because it's good security practice to send values
derived from a secret (e.g. an HMAC, digital signature, etc.) rather than the
secret itself across the wire, even if the wire is encrypted.
Proposed Text:
2.2 Assertion Client Credentials
The assertion client credentials include a client identifier, assertion and
assertion type. If the assertion client credentials are used then the client
MUST include the credentials using the following request parameters:
client_id
REQUIRED. The client identifier.
client_assertion_type
REQUIRED. The format of the assertion as defined by the
authorization server. The value MUST be an absolute URI.
client_assertion
REQUIRED. The client assertion.
For example (line breaks are for display purposes only):
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
grant_type=refresh_token&client_id=s6BhdRkqt3&
client_assertion=PHNhbWxwOl...[omitted for brevity]...ZT4%3D&
client_assertion_type=urn%3Aoasis%3Anames%sAtc%3ASAML%3A2.0%3Aassertion&
refresh_token=n4E90119d
The client MUST NOT include the client credentials using more than one
mechanism. If more than one mechanism is used regardless whether the
credentials are identical or valid, the server MUST reply with an HTTP 400
status code (Bad Request) and include the "multiple_credentials" error code.
From: Eran Hammer-Lahav [mailto:[email protected]]
Sent: Friday, June 25, 2010 11:31 AM
To: Yaron Goland; [email protected]
Subject: RE: Clients authenticating with assertions
We never had support for two assertions in one request.
The client authenticates itself and can include an assertion (or use type
'none'). The client credentials are the "client assertion" and the assertion is
about the resource owner.
Also, you can define an assertion type that's a composite assertion (of one
more more).
EHL
From: [email protected] [mailto:[email protected]] On Behalf Of Yaron
Goland
Sent: Friday, June 25, 2010 11:26 AM
To: [email protected]
Subject: [OAUTH-WG] Clients authenticating with assertions
If a client wants to authenticate itself to a token endpoint to get an access
token using an assertion how should it do it?
Grant_Type = assertion doesn't seem right because that assertion should be from
the resource owner who delegated the permission, not from the client, right? In
other words one can end up with an access token request with two assertions,
one from the client and one from the resource owner. How is this done?
Thanks,
Yaron
P.S. I looked for something like client_assertion and client_assertion_type in
section 2 of -08 but didn't see it. Sorry if I missed it.
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