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

Viktor Dukhovni <ietf-dane@dukhovni.org> Mon, 06 May 2019 18:36 UTC

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From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Subject: Re: [TLS] WGLC for "Deprecating TLSv1.0 and TLSv1.1"
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> On May 6, 2019, at 7:18 AM, Hubert Kario <hkario@redhat.com> wrote:
> 
> ncatenation: To  strengthen  protocols  against  collisions  in  any  one  
> hash  function,  it  may  be  tempting  to use  a  combination  of  two  
> independent  hash  functions. For example, TLS versions up to 1.1 use a 
> concatenation of  MD5  and  SHA-1.  While  the  output  length  of  this 
> construction  is  288  bits,  it  does  not  offer  the  security of a 288-bit 
> hash function. In particular, Joux described a  multi-collision  attack  that  
> breaks  the  concatenation of  two  hash  functions  with  roughly  the  same  
> effort  as breaking the strongest one of the two [18].

With the proviso that the attack on the second hash function is *generic*
(i.e. 2^{n/2} brute-force).  Thus the attack on SHA-1 + MD5, has a cost
of 64 * cost of SHA-1 collision + O(2^64) MD5 operations (with the associated
memory costs of finding the collision).

Yes, it is far from as strong as a real 288-bit hash, and one would not
choose the construction in a new standard, but in *practice* it is likely
stronger than SHA-1, even if it is far from the strength one might naïvely
hope for.

The hypothetical Joux multi-collision attack on SHA1-MD5 would have to attack
the protocol in a way that can make use of collisions rather than the much
harder to find pre-images.  Are you aware of any attacks that reduce the
pre-image resistance of SHA-1+MD5 to that of SHA-1?

There's quite a gap between (rightly) concluding that concatenated hash
functions are not a good crypto building block, to concluding that a
particular construction is no stronger in practice in a particular
protocol.

Which is not to say that I have a concrete stength estimate for SHA-1+MD5 
vs. SHA-1 in TLS. Rather, the practical implication of the multi-collisions
attacks is more subtle than the basic theoretical conclusion that invalidates
their naïve strength estimate.

-- 
	Viktor.