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

Peter Gutmann <pgut001@cs.auckland.ac.nz> Wed, 11 July 2018 04:58 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Hubert Kario <hkario@redhat.com>
CC: David Benjamin <davidben@chromium.org>, Eric Rescorla <ekr@rtfm.com>, "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: [TLS] raising ceiling vs. floor (was: New Version Notification for draft-moriarty-tls-oldversions-diediedie-00.txt)
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Date: Wed, 11 Jul 2018 04:57:59 +0000
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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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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.  In fact the sole (significant) surviving member of the MD5/SHA-1 era,
RIPEMD-160, remains unbroken and uses dual hash chains within the same
function, not even as two independent functions.

So I'm not losing any sleep over this.

>not after quantum computers 

 ... or captured alien technology, or magic, or anything involving pyramids or the
  Aztecs...

>come into play

Yeah, not losing any sleep over those either.

Peter.

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