Re: [TLS] WGLC for "Deprecating TLSv1.0 and TLSv1.1"

mrex@sap.com (Martin Rex) Fri, 03 May 2019 14:56 UTC

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To: Hubert Kario <hkario@redhat.com>
Date: Fri, 3 May 2019 16:56:54 +0200 (CEST)
CC: tls@ietf.org, mrex@sap.com, Martin Thomson <mt@lowentropy.net>
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] WGLC for "Deprecating TLSv1.0 and TLSv1.1"
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Hubert Kario <hkario@redhat.com> wrote:
> 
> We've been over this Martin, the theoretical research shows that for Merkle-
> Damgård functions, combining them doesn't increase their security 
> significantly.

You are completely misunderstanding the results.

The security is greatly increased!

Nobody is afraid of the exhaustive search preimage attacks.

What folks with a little crypto clue are afraid of is
significantly-faster-than-exhaustive-search real-time preimage attacks.
And this is where

  TLSv1.0 + TLSv1.1 (rsa,SHA1+MD5)

is *significantly* stronger than

  TLSv1.2 (rsa,MD5) *cough* -- which a depressingly high number of clueless
          implementers actually implemented, see SLOTH
  TLSv1.2 (rsa,SHA1)


That is also trivially formally provable.

Assume that a real-time preimage attack for *one* of the functions is
discovered, and compare the resulting efforts.

 
-Martin