"Approximate" hashes

[Marcel Popescu]

From: "Marcel Popescu" <Marcel_Popescu at 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"?

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. Does
anyone know of a mail transformation which could upset it? Can anyone see a
way to "attack" this by letting a significantly different message collide on
the same hash? (I'm ignoring the recent discoveries - they're not that
practical, I'm only trying to fight spam, not the government.)

Thanks,
Marcel

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