[Openid-specs-ab] Questions about multiple audiences for ID Tokens using MAC algorithms

Mike Jones Michael.Jones at microsoft.com
Thu Oct 24 18:26:12 UTC 2013

http://openid.bitbucket.org/openid-connect-core-1_0.html#IDTokenValidation contains this text that George asked about in his review:
7.  If the alg parameter of the JWT header is a MAC based algorithm such as HS256, HS384, or HS512, the octets of the UTF-8 representation of the client_secret corresponding to the client_id contained in the aud (audience) Claim are used as the key to validate the signature. Multiple audiences are not supported for MAC based algorithms.

George wrote:
"Why not? Wouldn't the secret associated with the azp work for the client to validate the id_token?  If we want interoperability across the use of audience and azp we are going to need to describe how it works in an extension document. It is not clear from this spec how it is to work and I was on most of the calls:)"

These questions arise:

1.      Does anyone remember the history behind the highlighted sentence?  I'm pretty sure that this was written before we had an "azp" claim.

2.      If there's an "azp" claim and an "aud" claim and the values are different, which Client Secret would be the right one to use as the key value?  (George seems to be suggesting that it's the one associated with the Client ID in the "azp" value.)

3.      If we did want to relax the restriction prohibiting multiple audiences, which value would be used for the key?  And would all the parties that need to valid the ID Token signature actually have access to this value?

4.      Or should we leave the text above as-is for now, and deal with this case as an extension later, if a need for it ever comes up?

5.      If we're not defining how multi-valued audiences would work with MAC signatures for now, should we also tighten this be requiring that any "azp" value that is include have the same value as the single-valued audience value?

                                                            -- Mike

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