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

[Andreas Schwab]

"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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