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

From: "Pete Resnick" <[email-masked]>
Date: Tue, 14 Oct 2014 13:42:13 -0700

Pete Resnick has entered the following ballot position for
draft-ietf-oauth-assertions-17: No Objection

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COMMENT:
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3 -

   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?

4.1 -

   grant_type
      REQUIRED.  The format of the assertion as defined by the
      authorization server.  The value MUST be an absolute URI.

That MUST is unnecessary. It's just definitional from 6749, 4.5 (which,
as it happens, doesn't use 2119 language for this). s/MUST/will

   assertion
      REQUIRED.  The assertion being used as an authorization grant.
      Specific serialization of the assertion is defined by profile
      documents.  The serialization MUST be encoded for transport within
      HTTP forms.  It is RECOMMENDED that base64url be used.

The MUST seems weird here. Are you saying that the profile could not
possibly have a serialization for an assertion that did not require
further encoding? But the RECOMMENDED seems downright wrong: Either an
implementer *needs* to know the encoding independently of the profile,
and therefore this needs to be a MUST, or the profile is going to
describe the encoding, which could be base64url or could be something
else, and the implementation will do whatever the profile says. If you
really want to say something here, I suggest replacing the last two
sentences with:

      If the assertion is going to be transported within HTTP forms, the
      profile document needs to describe what (if any) encoding will be
      needed to do so. The base64url encoding is widely implemented and
      therefore suggested.
    
    scope
    [...]
                                                   As such, the
      requested scope MUST be equal or lesser than the scope originally
      granted to the authorized accessor.
      
s/MUST/will (unless you explain whether it's the server or the client
that's supposed to be obeying that MUST, and for what reason)
      
      If the scope parameter and/or
      value are omitted, the scope MUST be treated as equal to the scope
      originally granted to the authorized accessor.  The Authorization
      Server MUST limit the scope of the issued access token to be equal
      or lesser than the scope originally granted to the authorized
      accessor.

In the first sentence, is this MUST for the server? (That is, shouldn't
it be, "If the scope parameter and/or value are omitted, the server MUST
use the value of the scope originally granted to the authorized
accessor."?) And anyway, don't these two sentences contradict 6749, which
says:

   The authorization server MAY fully or partially ignore the scope
   requested by the client, based on the authorization server policy or
   the resource owner's instructions.
   [...]
   If the client omits the scope parameter when requesting
   authorization, the authorization server MUST either process the
   request using a pre-defined default value or fail the request
   indicating an invalid scope.

Here and throughout: s/non-normative example/example (As far as I know,
there are no other kinds in IETF documents.)

4.1.1 - s/MUST construct/constructs

4.2, client_assertion_type and client_assertion: See comments from 4.1
regarding grant_type and assertion.

4.2.1 - s/MUST construct/constructs

5.2 -

s/MUST identify/identifies

For clarification:
OLD
      Assertions that do
      not identify the Authorization Server as an intended audience MUST
      be rejected.
NEW
      The Authorization Server MUST reject any assertion that does not
      contain the its own identity as the intended audience.
END

---

From: Brian Campbell <[email-masked]>
Date: Wed, 15 Oct 2014 17:06:52 -0600

Thanks for your review and feedback, Pete. Replies are inline below...

> http://datatracker.ietf.org/doc/draft-ietf-oauth-assertions/
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.

> as it happens, doesn't use 2119 language for this). s/MUST/will

Makes sense.

>       needed to do so. The base64url encoding is widely implemented and
In reading it again, I agree with you that it's weird and not appropriate
here. In fact the JWT profile itself does not require any further encoding.

Barring any objection, I suggest that the last two sentences just be
removed. So it would just be,

  "assertion
      documents."

> that's supposed to be obeying that MUST, and for what reason)

They are both supposed to obey it - the client shouldn't ask for more and
the server will reject the request, if it does.

Is "will" more appropriate than "MUST" here? Or maybe a non 2119 "should"?

>    indicating an invalid scope.
Yes, I think you're correct that there is a contradiction with 6749. And,
honestly, I'm surprised at the text there as I read it again. I don't think
that was the intent.

I'd propose that the sentence, " If the scope parameter and/or
      value are omitted, the scope MUST be treated as equal to the scope
      originally granted to the authorized accessor." be removed.

> there are no other kinds in IETF documents.)

I thought I picked that language up from some other draft or RFC but I'm
now not sure where it came from and can't easily find other examples of the
same thing.  So I am happy to remove the "non-normative" throughout, if it
is already understood and/or not customary to say so.

> 4.1.1 - s/MUST construct/constructs

OK

> regarding grant_type and assertion.

Agree and same answer.

> 4.2.1 - s/MUST construct/constructs

OK

> s/MUST identify/identifies

OK

>       contain the its own identity as the intended audience.

OK, I agree that reads better.

---

From: Barry Leiba <[email-masked]>
Date: Wed, 15 Oct 2014 19:18:24 -0400

> 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

---

From: Brian Campbell <[email-masked]>
Date: Wed, 15 Oct 2014 17:35:19 -0600

Fair point. I'll add some text saying that in the next revision.

On Wed, Oct 15, 2014 at 5:18 PM, Barry Leiba <[email-masked]>
wrote:

>> 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.
>>
>

---

From: Pete Resnick <[email-masked]>
Date: Wed, 15 Oct 2014 21:30:26 -0500

> Thanks for your review and feedback, Pete. Replies are inline below...

Thanks for addressing the comments, including Barry's followup. Just on 
the questions:

> On Tue, Oct 14, 2014 at 2:42 PM, Pete Resnick 
> <[email-masked] <mailto:[email-masked]>> wrote:
>
>         scope
>         [...]
>                                                        As such, the
> They are both supposed to obey it - the client shouldn't ask for more 
> and the server will reject the request, if it does.
>

Ah, so you're saying that a client could conceivably (either purposely 
or accidentally) try to sneak through a larger scope than it should, and 
the client MUST NOT do that, and the server MUST reject if it gets one. 
OK, that makes sense. (I do tend to like active MUSTs -- the foo MUST do 
X or the bar MUST NOT do Y -- but this is probably OK as is.)

> I thought I picked that language up from some other draft or RFC but 
> I'm now not sure where it came from and can't easily find other 
> examples of the same thing.  So I am happy to remove the 
> "non-normative" throughout, if it is already understood and/or not 
> customary to say so.

Yeah, we've seen other RFCs with such language. I've whined about it in 
the past. Some authors roll their eyes at me. You are welcome to roll 
your eyes if you like, but I find such text silly. :-)

Thanks for the rest of the planned changes. Looks good.

pr

-- 
Pete Resnick<http://www.qualcomm.com/~presnick/>
Qualcomm Technologies, Inc. - +1 (858)651-4478