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Re: Security team successfully cracks SSL using 200 PS3's and MD5
From: "Steven M. Bellovin" <smb () cs columbia edu>
Date: Sat, 3 Jan 2009 10:49:04 -0500

On Sat, 03 Jan 2009 09:35:06 -0500
William Warren <hescominsoon () emmanuelcomputerconsulting com> wrote:

Everyone seems to be stampeding to SHA-1..yet it was broken in 2005.
So we trade MD5 for SHA-1?  This makes no sense.

(a) SHA-1 was not broken as badly.  The best attack is, as I recall,
2^63, which is computationally infeasible without special-purpose
hardware.

(b) Per a paper Eric Rescorla and I wrote, there's no usable
alternative, since too many protocols (including TLS) don't negotiate
hash functions before presenting certificates.  In particular, this
means that a web site can't use SHA-256 because (1) most clients won't
support it; and (2) it can't tell which ones do.  (Note that this
argument applies just as much to combinations of hash functions --
anything that *the large majority of today's* browsers don't implement
isn't usable.)

These two points lead us to (c): security is a matter of economics, not
algorithms.  Switching now to something else loses more in connectivity
or customers than you would lose from such an expensive attack.

                --Steve Bellovin, http://www.cs.columbia.edu/~smb


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