Re: [TLS] Collisions (Re: Consensus Call: FNV vs SHA1)

Nicolas Williams <Nicolas.Williams@oracle.com> Mon, 10 May 2010 22:04 UTC

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Date: Mon, 10 May 2010 17:03:29 -0500
From: Nicolas Williams <Nicolas.Williams@oracle.com>
To: Simon Josefsson <simon@josefsson.org>
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Subject: Re: [TLS] Collisions (Re: Consensus Call: FNV vs SHA1)
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On Mon, May 10, 2010 at 04:56:52PM -0500, Nicolas Williams wrote:
> On Mon, May 10, 2010 at 11:48:09PM +0200, Simon Josefsson wrote:
> > Nicolas Williams <Nicolas.Williams@oracle.com> writes:
> > 
> > >  - Add a description of what happens if cached object checksums collide.
> > >
> > >    No, the current security considerations section doesn't deal with
> > >    this, and rightly so _if_ collisions are not a security problem, but
> > >    what happens when there are collisions?  Do hanshakes fail?
> > 
> > I agree that it is important to explain this.
> > 
> > If collisions happen, it appears that we do get slightly weaker
> > semantics of what it means for a handshake to succeed: we aren't
> > cryptographically certain (in the sense that there is cryptographic
> > reduction) that the client and server agree on the data used during the
> > handshake for cached items (CA cert list, server certificate) after the
> > handshake has concluded.
> 
> In the success case I think we are certain that we don't care about
> collisions, if there were any: the cryptographic properties of TLS
> ensure this.

Actually, maybe not!  I think on the server side we won't care about
collisions, but on the client side I think we care very much.  I think
the key is this: the client MUST NOT cache any objects from failed
handshakes.  Anyways, I've not done a careful analysis because I'm lazy
and I'd rather see what the authors have to say first :)

Nico
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