Kent-- Compositions won't really help very much. Lets say (I'm sure the exact numbers are wrong here) that it takes brute-forcing MD5 takes 2**80, and brute-forcing SHA-1 takes 2**90. And due to recent discoveries, we can push those down to 2**50 and 2**55 respectively. Breaking a composition would still take on the order of 2**55 (the harder of the two)-- you're not going to make it exponentially harder to crack by composing. Doing something a little more slick like interweaving the bits of the two algorithms would make it geometrically harder, but not exponentially. You'd really have to get a new algorithm. Of course, this is assuming that the actual attack allows one to take some predefined input A, and compute some evil input A' such that Hash(A)=Hash(A'). If the attacks are simply to create colliding input data, then the underlying algorithm is still safe for most applications. Of course, I'm not a crypto-expert, so this may all be totally wrong. Michael Scovetta Computer Associates Senior Application Developer -----Original Message----- From: Kent Borg [mailto:kentborg@xxxxxxxx] Sent: Wednesday, February 16, 2005 6:27 PM To: Gadi Evron Cc: bugtraq@xxxxxxxxxxxxxxxxx Subject: Re: SHA-1 broken On Wed, Feb 16, 2005 at 02:56:27PM +0200, Gadi Evron wrote: > Now, we've all seen this coming for a while. > http://www.schneier.com/blog/archives/2005/02/sha1_broken.html > > Where do we go from here? I am feeling smug that in a project I am working on I earlier decided our integrity hashes would be a concatenation of MD5 and SHA-1, not that that's a fix, but it helps. I am also appreciating that hashes are used (this project included) for many different things, not all of which are directly affected by this break. Yes, this is a bad omen for the longevity of SHA-1 for other uses, so we will keep an eye on it. Something I am intrigued about is more sophiticated compositions of, say, SHA-1 and MD5. -kb
