bugtraq - Re: Combining Hashes

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Message-ID: <200502190055.05271.aaron@synacksecurity.com>
Date: Sat, 19 Feb 2005 00:54:56 -0400
From: Aaron Mizrachi (unmanarc) <aaron@...acksecurity.com>
To: bugtraq@...urityfocus.com
Subject: Re: Combining Hashes

El Vie 18 Feb 2005 11:24, Kent Borg escribió:
> Concatenating two different hashes, for example SHA-1 and MD5,
> apparently does not add as much security as one might hope.
>
> What about more complicated compositions?  For example, a reader
> comment posted on Bruce Schneier's blog
> (http://www.schneier.com/blog/archives/2005/02/sha1_broken.html)
> suggests the following:
>
> d1=SHA-1(data)
> d2=MD5(data)
> d3=SHA-1(d1+data+d2)
>
> The final digest would be d1+d2+d3
>
> (where "+" is concatenation)
>
>
> I admit I don't know why this might be significantly better than
> d1+d2, I was hoping someone here would.
>
>
> -kb

Having d1+d2 may leave some useful information in order to obtain a collition 
more fastest because we can intersect these functions... SHA-1(d1+data+d2) is 
relative better than d1+d2. I dont think that is really secure... d2 or d1 
may leave some useful information. we need to study and probe that. 

I dont recomend something as: HASH(HASH(data)+data) until a research of 
propietries of that where investigated and mathematical proved. The better 
method (i think) is: HASH(HASH(data)), because adds two layer... and have the 
same or more security than HASH(data). it's simple... if you use HASH(data), 
you can obtain HASH(HASH(data)), and crack from HASH(HASH(data)) (if 2-ble 
round hash is more weakness).

A simple probe of a very basic crypto-system that isn't good idea have two 
rounds are: XOR, the second round leave the original text. With one way 
functions may happen something similar.

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