Re: [Ace] FW: WGLC comments on draft-ietf-ace-dtls-authorize

[Jim Schaad <[e-mail]>]

> -----Original Message-----
> From: Benjamin Kaduk <[e-mail]>
> Sent: Sunday, March 10, 2019 10:10 AM
> To: Göran Selander <[e-mail]>
> Cc: Jim Schaad <[e-mail]>; draft-ietf-ace-dtls-
> [e-mail]; [e-mail]
> Subject: Re: [Ace] FW: WGLC comments on draft-ietf-ace-dtls-authorize
> 
> On Fri, Mar 08, 2019 at 04:01:26PM +0000, Göran Selander wrote:
> >
> > On 2019-03-03, 02:44, "Jim Schaad" <[e-mail]> wrote:
> >
> >     I am responding to the review below in regards to the most recent
> version -06.
> >
> >     > -----Original Message-----
> >     >     > Section 3.3 - Figure 4 - Where is the 'alg' parameter defined at that
> level?
> >     >
> >     >     See next comment.
> >     >
> >     > [GS]  alg parameter included
> >     >
> >     >     > Section 3.3 - I am always bothered by the fact that PSK should really
> be
> >     > PSS
> >     >     > at this point.  The secret value is no longer a key and thus does not
> >     >     > necessarily have a length.  There is also a problem of trying to
> decide
> >     > what
> >     >     > the length of this value would be based on the algorithm.  If the
> client
> >     >     > offers TLS_PSK_WITH_AES_128_CCM_8 and
> >     > TLS_PSK_WITH_AES_256_CCM_8  (I may
> >     >     > have gotten these wrong but the intent should be understandable)
> then
> >     > what
> >     >     > length is the PSK supposed to be?
> >     >
> >     >     I think what you are saying is that for the shared secret (k) in the
> >     >     COSE_Key structure in Fig. 4, the AS needs to tell C what to do with
> >     >     that shared secret? This was the intention of the alg parameter
> (which
> >     >     has a not-so-useful value in this example).
> >
> >     Some of what is done here makes sense and some of it makes no sense
> at all.
> >
> >     Happy with the removal of the "alg" parameter in the root map.
> >
> >     Happy with the addition of the kid parameter in the COSE_Key object
> since this is required for doing DTLS w/o sending the token as the identifier.
> >
> >     I have no idea what the algorithm is doing here?  This is not currently a
> COSE algorithm, it is a TLS algorithm and thus would not make a great deal of
> sense.
> >
> > GS: I admit this does not make sense, neither here nor in Fig. 6.
> >
> > The terms of what the PSK length should be would be better covered by a
> statement along the lines of "When offering and/or accepting a TLS
> cryptographic suite, the length of the PSK should be at least as long as the
> symmetric encryption algorithms that are offered." This may already be
> pointed to in the TLS documents and thus can be referenced to rather than
> stated explicitly.
> 
> What would you do with a PSK that is longer than the input needed by the
> symmetric algorithm in use?

Ben, we are talking about TLS and this is the pre-shared secret.  It is an input to the KDF function and is not a symmetric algorithm key.

Jim

> 
> -Ben
> 
> > GS:
> > 1. If the PSK is not uniformly random, the security level is not given by the
> length. I note in the ACE framework: "The AS generates a random symmetric
> PoP key." Perhaps we should add 'uniform' to this text?
> >
> > 2. About the proposed text, how about making it into a consideration:
> "Note that the security level depends both of the length of PSK and the
> security of the TLS cipher suite and key exchange algorithm." I didn't find any
> text in TLS that I could reference.