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

Hubert Kario <hkario@redhat.com> Tue, 10 July 2018 18:11 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: Tue, 10 Jul 2018 20:11:14 +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 Tuesday, 10 July 2018 18:38:22 CEST Peter Gutmann wrote:
> David Benjamin <davidben@chromium.org> writes:
> >EMS does not fix the ServerKeyExchange signature payload. It's still just
> >the randoms and not the full transcript.
> 
> Maybe we're talking about different things here, EMS hashes the full
> transcript, for 1.0 and 1.1 with the dual SHA-1 and MD5 hash, for 1.2 with
> whatever's negotiated, hopefully SHA-2 (even if SHA-1 is used, you've now
> got two hashes you need to defeat simultaneously, not one).

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)

read the SLOTH paper for details and references

> So while the
> ServerKeyExchange signature may not detect an attacker able to compromise
> SHA-1 in real time (and that statement alone should tell you how feasible
> the attack actually is)

yes, _now_
not for "foreseeable future" and not after quantum computers come into play
-- 
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