[OAUTH-WG] rfc6749 question about the optional use of the client_id in the request body

Tom Van Oppens <Tom.Van.Oppens@be.ibm.com> Wed, 24 January 2018 10:19 UTC

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Subject: [OAUTH-WG] rfc6749 question about the optional use of the client_id in the request body
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Dear Oauth Mailing List

After some discussion i had i wanted to ask you for some guidance.

For the following request



We had some discussions whether or not this request is a valid request, to 
be more exact wether the clientid can be in the body.
Section 2.3.1 states 
A client MAY use the "client_id" request parameter to identify itself when 
sending requests to the token endpoint. 

But at the same time in the case of a client password (2.3.1)
The clientid and secret are carried in the basic auth header as a form of 
authentication as a preferred method ,
But the standard states that if you choose to use the body as a form of 
authentication that if you can ommit the clientsecret the clientsecret is 
an empty string, therefore passing only the client_id is the same as 
passing the client_id and an empty string clientsecret .

So the current request would be according to the spec interpreted as 
Authentication 1) basic auth cleintid:secret
Authentication 2) body auth  clientd and blank secret

You can choose to use the client_id in the body with public clients or in 
the confidential client (the Lloyds situation) if you choose to add the 
clientsecret there as well and are not using the basic auth header (this 
is due to spec section 2.3 which states 
The client MUST NOT use more than one authentication method in each 

In short there is no way in the spec that allows for the oauth provider to 
distinguish between your intention of sending in the client_id again for 
identification and a malformed request with double authentication.

So my stance is (for now) that you cannot send a clientid when you find 
yourself in the clientid with a corresponding password situation.
Is that a correct statement ?
and if it is not how would that work ?
and if it is, when can you send the clientid in the body but use something 
else for authentication  (something like mtls ?) ?

Kind Regards
Van Oppens Tom

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