bugtraq - Re: SHA-1 broken

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Date: Sat, 19 Feb 2005 14:37:57 +1100
From: Michael Cordover <michael.cordover@...il.com>
To: Bugtraq <bugtraq@...urityfocus.com>
Subject: Re: SHA-1 broken

On this topic, I might actually say that a concatenation of two hashes
is very secure if the two hashes are sufficiently different.

Although MD5 and SHA1 are reasonably similar, let's suppose for a
moment that they use entirely different mechanisms.  If this were so
and the crack time for MD5 was 2**50, for SHA1 2**65, then the crack
time for CONCAT(MD5(string), SHA1(string)) would be about 2**115. 
This is because you need to find something that colides for both. 
This is *far* more difficult and therefore far more collision
resistant.  Unfortunately this requires 288 bits - far more than each
of the old hashes.  Still, that much memory doesn't tend to be a
problem ;).

Regards,

Michael Cordover

-- 
http://mine.mjec.net/