Re: [kitten] draft-hansen-scram-sha256 and incorporating session hashing for channel binding

Simon Josefsson <simon@josefsson.org> Thu, 28 May 2015 15:11 UTC

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Date: Thu, 28 May 2015 17:11:22 +0200
From: Simon Josefsson <simon@josefsson.org>
To: Nico Williams <nico@cryptonector.com>
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Subject: Re: [kitten] draft-hansen-scram-sha256 and incorporating session hashing for channel binding
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> > You then go on to say: "Personally, I would prefer to change to
> > another mandatory channel binding that is secure for all TLS
> > versions."
> 
> This is not really appropriate here because it's the applications that
> need to do this, and we can't say anything here about this that will
> force them to.

> A reference to TLS-SESSION-HASH of the same level (i.e., normative
> or informative) as RFCs 5246 and 5929 would be nice.

I believe that what is required is

  1) scram-sha256 has a normative reference to tls-session-hash; or

  2) tls-session-hash uses an Update: that makes it applicapable to all
  TLS versions, and that it is clarified (if not already the case) that
  tls-session-hash must be used; or

  3) scram-sha256 uses a new channel binding that is secure with or
  without tls-session-hash.

I believe 1) and 2) would be worse than 3) for the next ~5 years or
so, and things being equal after that.  SASL libraries/applications
rarely have any influence over TLS internals, but they directly
influence the channel binding used.  Using another channel binding for
hansen-scram-sha256 should not be difficult, as far as I can tell --
SCRAM implementations needs to be changed to support the other stuff
for SCRAM-SHA256 so they could just as well be modified to support a
new channel binding too.  But I would not object to 1) or 2), if people
prefer that.

/Simon