Re: [OAUTH-WG] Pete Resnick's No Objection on draft-ietf-oauth-assertions-17: (with COMMENT)

[Barry Leiba <[e-mail]>]

>
>
>>    Assertions used in the protocol exchanges defined by this
>>    specification MUST always be protected against tampering using a
>>    digital signature or a keyed message digest applied by the issuer.
>>
>> Why is that? Aren't you using assertions over a protected channel (as
>> required by the spec) and therefore not need to sign the assertions?
>> Indeed, why would a self-issued Bearer Assertion need to be signed at
>> all? Does that even make sense?
>>
>>
> Yes, assertions are sent over a protected channel, which does provide
> integrity protection for the transport form client to AS and also gives
> server authentication. But it doesn't provide client authentication, which
> is kind of the point of the Client Authentication part of this draft. And
> for authorization the signing or MACing is what authenticates the issuer of
> the assertion - sometimes the issuer is the client but often the issuer
> will be a 3rd party system.
>
> I do agree with you in one specific case that, if the client is trusted to
> be the assertion issuer and the client is properly authenticated, then an
> unsigned assertion could be reasonably used as an authorization grant. But
> it's a fairly rare and very specific case. And one that can be accommodated
> in other ways. So it's not worth introducing the complexity and potential
> security problems that having the signature be option would bring.
>

In other words, the assertion must be protected against tampering *by the
party that presents the assertion*.  That is a significant point, and you
should say it.

Barry