Re: Added hashes.

Juuso Hukkanen wrote:
>Paul Rubin wrote:
>>If neither is perfectly good, then the combination might be worse than
>>either one separately.
>
>2) Ok, (if) both are less than perfect... [...]
>could the combination break 2 other vice 'nice' hashes and make
>the third-hash 'bad'?

Asked and answered. To repeat what you've been told several times
already, the answer is "Yes, in principle it could happen".

Maybe you're having trouble imagining how this could happen in practice,
but in principle there is no good way to rule out the possibility.
See my note, or Paul Rubin's note, for justification. In practice it
seems like it would be very surprising if the combination was worse
than the two components, for any reasonable choice of two components,
but what I'm saying is that it is not guaranteed by any theorem --
there is no way to prove that bad things cannot happen.

Sure, if we somehow knew that the two hashes were "independent" (whatever
that means), then xor-ing them might be safe. But we don't know how
to verify with certainty that a pair of hashes is independent, any more
than we know how to verify with certainty that a single hash is secure.
Out of the frying pan, into the fire.
Received on Fri Dec 23 20:10:08 2005