compare-by-hash (was Re: sharing /etc/passwd)

[Giorgos Keramidas]

On 2004-09-27 10:27, Jason Stone <freebsd-security at dfmm.org> wrote:
> > Henson notes that since there's no absolutely guaranteed proof that
> > there are *no* collisions with a given hashing algorithm,
>
> true - quite the opposite, in fact - with a finite hash length and an
> infinite number of inputs, you are guaranteed an infinite number of
> collisions in any hashing algorithm - you're just going to have to spend
> longer than the lifetime of the universe to find them....

There is one difference between ``looking for collisions'' and being
bitten by undetected collisions though.

If the probability of a collision just happening with random user data
is 1/(2^128) we can't be sure that it will necessarily take the
transfer of an average number of 2^127 blocks before a collision
happens.  You might get one at the very first pair of blocks and then
no collisions ever after until the Sun burns out.

Using two different hashes for the same set of input data, which David
G. Andersen proposed, seems like a nice idea though.

- Giorgos