Re: [TLS] raising ceiling vs. floor (was: New Version Notification for draft-moriarty-tls-oldversions-diediedie-00.txt)

Hubert Kario <hkario@redhat.com> Wed, 11 July 2018 11:06 UTC

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From: Hubert Kario <hkario@redhat.com>
To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
Cc: David Benjamin <davidben@chromium.org>, Eric Rescorla <ekr@rtfm.com>, "<tls@ietf.org>" <tls@ietf.org>
Date: Wed, 11 Jul 2018 13:06:21 +0200
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Subject: Re: [TLS] raising ceiling vs. floor (was: New Version Notification for draft-moriarty-tls-oldversions-diediedie-00.txt)
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On Wednesday, 11 July 2018 06:57:59 CEST Peter Gutmann wrote:
> Hubert Kario <hkario@redhat.com>; writes:
> >defeating two hashes, when both use use the Merkle-Damgård construction, is
> >not much harder than breaking just one of them (increase of work factor
> >less than 2)
> 
> "In theory there is no difference between theory and practice.  In practice
> there is".
> 
> I'm aware of this long-standing theoretical weakness around multicollisions.
> I'm just as aware that in the fifteen-odd years since the Joux paper,
> no-one has ever managed to demonstrate an even remotely practical attack on
> dual hashes, despite the hugely tempting target of all of SSL/TLS being
> there as a reward.

2^77 is a rather high barrier of entry just to prove expected result – I'm not 
surprised about lack of practical attack at all.

Nobody has disproved the conclusion of that paper either, so we don't have the 
luxury of ignoring it.

-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic