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

Torsten Lodderstedt torsten at lodderstedt.net
Fri Oct 25 05:32:45 UTC 2013


Hi all,

MAC as symmetric alg only makes sense (from a security perspective) for two parties. I consider sharing a symmetric key among more than two parties a bad design. So in my opinion this restriction makes sense. 

Wrt 5. Yes, we should tighten it.

Regards,
Torsten.



Mike Jones <Michael.Jones at microsoft.com> schrieb:
>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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