# Re: Hash question

• From: Tom St Denis <tom@xxxxxxx>
• Date: Tue, 20 Jul 2010 07:52:11 -0700 (PDT)

On Jul 20, 10:08 am, Peter Fairbrother <zenadsl6...@xxxxxxxxx> wrote:
The problem:

I have two unknown strings which differ by a known amount, and two
hashes. To make it easier, suppose the strings and the hashes are the
same length, 256 bits, and the strings differ in 32 bits.

The first hash is a hash of the first string, I want to know whether the
second hash is a hash of the second string, with good probability. How
hard is it?

Now resistance against that is not quite in the usual properties of a
hash - preimage resistance and so on - but I'm not very up on hashes, so
maybe someone has done some work on that problem. If so, can someone
give me a pointer to it please?

If not, any thoughts?

You're literally asking for a differential attack on the hash.
Cryptographic hashes are designed to not emit useful differentials.

Tom
.

## Relevant Pages

• Re: Is MD5 outdated ?
... >> produce identical hashes, never mind hashes differing by 1 bit? ... > bit will themselves differ in roughly half their bits, ... > in one crucial way and yet hash to the same digest is far easier said ... > desired way and yet produce the same hash. ...
(sci.crypt)
• Re: Is MD5 outdated ?
... > produce identical hashes, never mind hashes differing by 1 bit? ... I said that any two messages producing digests that differ in only one ... in one crucial way and yet hash to the same digest is far easier said ... desired way and yet produce the same hash. ...
(sci.crypt)
• Re: Hash question
... suppose the strings and the hashes are the ... The first hash is a hash of the first string, I want to know whether the ... "difference", including XOR, subtraction, and subtraction ...
(sci.crypt)
• Re: How to avoid rehashing?
... with strings. ... And keeping this in a hash. ... I use the words as keys since I want to find every occurence of the ... these hashes, which I need to do more than once. ...
(perl.beginners)
• Re: Is MD5 outdated ?
... when selecting any two hash values that differ by ... ] of strings of various lengths. ... if two hashes differ by one bit, then there exist two original strings ...
(sci.crypt)