bug#13949: Aw: Re: bug#13949: fill-paragraph is buggy, but using MD5 is even more buggy

[Andreas Schwab <schwab@linux-m68k.org>]

"Petros Travioli" <travioli@gmx.de> writes:

>> > But that's exactly what happens when you are using hash functions to
>> > verify buffer equality, just with a more complicated mathematical
>> > formulation and at a slightly different scale.
>> >
>> > So don't use hash functions to a two-sided correct answer to test buffer
>> > equality. For a one-sided answer (if hash(x) != hash(y) then x != y), you
>> > are fine.
>> 
>> There is a difference between a hash function and a cryptographic hash
>> function.  An inportant property of a cryptographic hash function is the
>> avalanche effect, that means a small change in the plaintext will result
>> in a big change in the hash value.  That makes such a hash function
>> suitable for the reverse condition x != y => hash(x) != hash(y), with a
>> very high probability of being true.
>> 
> So far most old crypto functions have been broken. There is no doubt this will happen to any newer one sooner or later. If any single person would lose his work because of a random collision, this is an argument agains crypto hash functions.

This is irrelevant.  See avalanche effect.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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