Re: SSH2 Encryption
On Thu, Jun 13, 2002 at 12:38:15PM +0200, Sergio Rodr?guez de Guzm?n Mart?nez wrote:
> peter@llama.nslug.ns.ca (2002-06-13 at 0330.28 -0300):
> >
> > Yes. MD5 has had some very minor breaks. It is easier to find
> > hash collisions than it should be. This means that it is possible to find
> > two messages that hash to the same value. You need to choose _both_
> > messages, so this doesn't help you e.g. make trojan binaries that have the
> > same MD5 hash as the originals.
> >
>
> MD5 generates a hash of 128 bits so the probability of finding a
> collision is 1/(2^128). So it seems that it is not so easy to find such
> a collision.
It is possible to do better than those odds when dealing with MD5, if you
get to choose both messages. That's why I said MD5 has had some breaks
against it; Cryptanalysis has resulted in an improvement over random
selection of message pairs for generating collisions.
>
> Of course SHA is more secure, it generates a hash of 160 bits so the
> probability of finding a collision is lower than MD5.
We were talking about MD5-96 and SHA1-96 message authentication codes for
the SSH protocol. I'm assuming that MD5-96 means that 96 bits of the MD5
hash are used, not all 128. Same goes for SHA-1. Since the same number of
bits is used, the strength against brute force collision-generation attempts
is equal. MD5 should be considered less secure than SHA-1 because of the
breaks found in it so far. AFAIK, there is no way to speed up finding a
collision for a given message, but it is reasonable to assume that the
likelihood of one being found is greater than for SHA-1.
BTW, you shouldn't say "of course". Producing a longer hash is not all
there is to making a secure hash function.
--
#define X(x,y) x##y
Peter Cordes ; e-mail: X(peter@llama.nslug. , ns.ca)
"The gods confound the man who first found out how to distinguish the hours!
Confound him, too, who in this place set up a sundial, to cut and hack
my day so wretchedly into small pieces!" -- Plautus, 200 BCE
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