Re: [Ace] [EXTERNAL] Re: Martin Duke's No Objection on draft-ietf-ace-oauth-params-13: (with COMMENT)

[Seitz Ludwig <[e-mail]>]

Hi Martin, Ben,

If I were to change the offending sentence like so:
"It is RECOMMENDED that an AS reject a request containing a symmetric key value ... (Note: this does not apply to key identifiers referencing a symmetric key)"

(the "Note..." part being the new clarification), would that help making the intention distinction more visible?

/Ludwig


> -----Original Message-----
> From: Benjamin Kaduk <[e-mail]>
> Sent: den 21 mars 2021 03:17
> To: Martin Duke <[e-mail]>
> Cc: The IESG <[e-mail]>; [e-mail]; [e-mail]; draft-ietf-
> [e-mail]
> Subject: [EXTERNAL] Re: [Ace] Martin Duke's No Objection on draft-ietf-ace-
> oauth-params-13: (with COMMENT)
> 
> Hi Martin,
> 
> On Thu, Mar 18, 2021 at 11:44:53AM -0700, Martin Duke via Datatracker
> wrote:
> >
> >
> > ----------------------------------------------------------------------
> > COMMENT:
> > ----------------------------------------------------------------------
> >
> > In sec 3.1 it says the AS SHOULD reject req_cnf if the key is
> > symmetric. But in Sec 5 it presents a totally reasonable use case
> > where the C and RS hold a previously established (symmetric?) key.
> > These observations are somewhat contradictory. Should 3.1 include a
> > qualifier. Would the AS know about this key a priori so that it can
> > ignore the recommendation? If not, how can this be done safely?
> 
> I think there is a subtle distinction between the two cases, if I am
> remembering correctly.  In particular, in Section 3.1 it says that "[i]t is
> RECOMMENDED that an AS reject a request containing a symmetric key
> value", and the last word ("value") is important!  This is saying, if the client
> tries to propose to the AS the actual symmetric key to be (encapsulated in
> the token and) used to secure C/RS communications, the AS typically should
> reject it, since a constrained client is likely to have a much worse RNG than
> the AS.  If, on the other hand, some out-of-band management system has
> provisioned a symmetric key shared by C and RS, that key is presumed to be
> strong, but in the scenario depicted in Section 5 it is "the key-identifier of a
> previously established key between C and RS" that "req_cnf" conveys.
> Note that this scenario is only the identifier, not the key value itself.
> 
> This is clearly a pretty subtle distinction to make, and if you have any
> suggestions for how to word things to make it more obvious, we'd love to
> have them.
> 
> Thanks,
> 
> Ben