[16077] in cryptography@c2.net mail archive
Re: "Approximate" hashes
daemon@ATHENA.MIT.EDU (David Honig)
Wed Sep 1 15:51:46 2004
X-Original-To: cryptography@metzdowd.com
X-Original-To: cryptography@metzdowd.com
Date: Wed, 01 Sep 2004 11:45:31 -0700
To: "Marcel Popescu" <Marcel_Popescu@microbilt.com>,
<cryptography@metzdowd.com>
From: David Honig <dahonig@cox.net>
In-Reply-To: <0c4c01c49034$bc1999b0$726e9cd9@mark>
At 06:02 PM 9/1/04 +0300, Marcel Popescu wrote:
>From: "Marcel Popescu" <Marcel_Popescu@microbilt.com>
>
>> Hence my question: is there some "approximate" hash function (which I
>could
>> use instead of SHA-1) which can verify that a text hashes "very close" to
>a
>> value? So that if I change, say, tabs into spaces, I won't get exactly the
>> same value, but I would get a "good enough"?
This is completely what secure hashes are supposed to prevent. *Any* change
in the input will flip half the hash-bits on average. Just like a block
cipher. There is no input "nearness" preservation. That's part of the point
of the algorithm.
>I just had an idea. Would this work?
>
>- let S be the input string, whose hash I want to verify
>- make S uppercase
>- remove everything but A-Z, 0-9, and common punctuation (!;:'",.?)
>- calculate the SHA1 hash of the result
>
>This should keep any insignificant changes out of the final result.
You can encode your message in some format which is not subject
to mangling, and use a hash of that encoding. You can then
decode the encoding back into unicode or whatever funky but
net-fragile character set you like. This is somewhat like
ascii-armoring of PGP-encrypted messages.
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