Re: Added hashes.

"Ian Stirling" <root@mauve.demon.co.uk> wrote in message
news:439b6724$0$63085$ed2e19e4@ptn-nntp-reader04.plus.net...
> Given that assorted hashes (rc4,

rc4 is a cipher, not a hash.

> md5) seem to all have a finite lifespan,

md5 is not recommended for new applications, but there
should be no big rush to replace it in most current
applications.

> with others showing possible weaknesses.

Ditto for SHA1. So much for "Given...."

> Isn't it more secure to hash a file by hashing it twice
> with two seperate
> functions, and then xoring the result?

> Is this more secure than both, or only as secure as the
> weakest link?

What do you mean by "more secure?" Do you mean, "easier to
work backwards to get the plaintext?" That's not going to
happen if the plaintext is longer than the hash result. Do
you mean, "easier to concoct a plaintext that collides with
a given plaintext, is still meaningful, and is somehow
advantageous to the attacker?" Good luck doing that with
any of the standard hashes we have today (including MD4 or
SHA1).

>
> It seems naively that if the hashes are not closely
> related,
> then the problem of getting a hash collision is a
> completely new
> problem.

No. By hashing the file twice with two independent hashes
and then XORing the two results, you have created a third
hash for which the probability of collisions is just as
computable as it is for either hash alone. It is the nature
of a hash to have collisions.
Received on Sun Dec 11 14:26:59 2005