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

Nico Williams <nico@cryptonector.com> Thu, 28 May 2015 16:40 UTC

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Date: Thu, 28 May 2015 11:40:02 -0500
From: Nico Williams <nico@cryptonector.com>
To: Simon Josefsson <simon@josefsson.org>
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Subject: Re: [kitten] draft-hansen-scram-sha256 and incorporating session hashing for channel binding
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On Thu, May 28, 2015 at 05:11:22PM +0200, Simon Josefsson wrote:
> > > 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

And RFC5929.

My vote is for (2).  I don't mind (1) in addition, but I want (2).

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

We disagree as to (3).  This is advice we can give to apps in the
security considerations section, not something we can force a SASL or
GSS mechanism to do because API-wise the mechanism doesn't get a choice.

> 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
> [...]

No, they don't.  Certainly not GSS ones, and the SASL implementations
I'm familiar with don't either.

E.g., GSS_Init_sec_context() doesn't get a handle to a channel to bind
to, it only gets the channel binding as already extracted by the app.
Worse, GSS_Init_sec_context() doesn't even get the channel binding type.

(3) is unworkable.

Nico
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