"Approximate" hashes

Marcel Popescu Marcel_Popescu at microbilt.com
Wed Sep 1 10:55:59 EDT 2004


I am trying to build a Windows anti-spam thingy; it's supposed to "sit" in
between the mail client and the outer world, and indicate through mail
headers whether the incoming mail has a valid hashcash
http://www.hashcash.org/ "coin" (and, of course, to automatically add
hashcash to outgoing emails).

My problem is that I don't know what happens with the email in transit
(this, I believe, is an observation in the hashcash FAQ). I am worried that
some mail server might dislike ASCII characters with the high bit set, or
that a client uses some encoding which for some reason doesn't make it to
the destination unchanged.

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"?

I don't know if this is possible. But if it is, I though this would be a
good place to find out about it.

Thanks,
Marcel

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