Re: SHA-1 broken
Sadly, there is no magic bullet for the SHA-1 problem. Let me say, in
classic Bugtraq style, that I believe the "temporary workaround for this
vulnerability" is to move to SHA-512 as quickly as possible.
NIST was going to recommend SHA-256 and SHA-512 by 2010, but for the
security-conscious the time is now.
The "computer security response" should not be to re-jigger the hashes,
bet on crypto tricks that haven't seen any review, and guess at the
computational complexity of the result.
The only fix will be informed analysis of the new paper from the Chinese
team (which hasn't even been released yet) and the informed development
of a solid cryptographic response.
Anatole
On Fri, Feb 18, 2005 at 05:06:42PM +1100, Michael Silk wrote:
> Michael,
>
> But with such functions the point is that "input" isn't a function,
> it's a string - and it can only be the inverse of one, not both; i.e.
> the result of "invHashFunc1( foo )" _wont_ equal "invHashFunc2( foo
> )".
>
> So if the user is attempting to break a login screen with his
> invHashFunc's, and the hash of the users password is implemented as
> described, they can't possibly provide the right inversions for _both_
> functions in one string; unless they happen to be the same.
>
> No?
>
> -- Michael
>
>
> On Fri, 18 Feb 2005 00:45:24 -0500, Scovetta, Michael V
> <Michael.Scovetta@xxxxxx> wrote:
> > Michael,
> > I'm not sure that it would help significantly. If the end-result of
> > this research on breaking hash algorithms is to create "inverse-MD5" and
> > "inverse-SHA" functions, then:
> > input = invHashFunc2( substring(invHashFunc1(result)) )
> >
> > By our assumptions, invHashFunc1 and invHashFunc2 are both tractable, the
> > substring function would simply add a polynomial factor to the calculation
> > to guess it right.
> >
> > You could create arbitrarily complex functions, like:
> > MD5(SHA(input+salt)+MD5(input+salt)+salt)
> > But in the end, if invHashFunc1 and invHashFunc2 are both tractable, then
> > nothing you do could help it (beyond a polynomial factor). And keeping the
> > actual algorithm-composition secret wouldn't help much either.
> >
> > -Mike
> >
> > -----Original Message-----
> > From: Michael Silk [mailto:michaelsilk@xxxxxxxxx]
> > Sent: Thu 2/17/2005 10:30 PM
> > To: Scovetta, Michael V; bugtraq@xxxxxxxxxxxxxxxxx
> > Cc:
> > Subject: 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
> >
> >
> >