Bugtraq: RE: SHA-1 broken

["Scovetta, Michael V" <Michael.Scovetta () ca com>]

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 () borg org] 
Sent: Wednesday, February 16, 2005 6:27 PM
To: Gadi Evron
Cc: bugtraq () securityfocus com
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