RE: SHA-1 broken
Michael,
But wouldn't it render a login-based hashing system resistant to the
current hashing problems if it is implemented something like:
--
result = hashFunc1( input + hashFunc1(input) + salt )
//
// instead of
//
result = hashFunc1( input + salt )
--
We can see that the input to the functions is the same, so although a
collision could be found within one or the other but it would not give
the correct result unless the hashFunc1( foo ) = hashFunc2( foo )
where foo is the magical input that gives the same result as "bar"
(the initial password).
-- Michael
> -----Original Message-----
> From: Scovetta, Michael V [mailto:Michael.Scovetta@xxxxxx]
> Sent: Friday, 18 February 2005 8:34 AM
> To: Kent Borg; Gadi Evron
> Cc: bugtraq@xxxxxxxxxxxxxxxxx
> Subject: RE: SHA-1 broken
>
> 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