Neal Koblitz critiques modern cryptography.
Steven M. Bellovin
smb at cs.columbia.edu
Tue Sep 4 19:11:07 EDT 2007
On Tue, 4 Sep 2007 16:46:58 -0400
Victor Duchovni <Victor.Duchovni at MorganStanley.com> wrote:
> The more specific scepticism of security
> proofs (I am reluctant to agree that these are actively harmful),
> seems to be a combination of the peer review issue above, and
> (often?) lack of tight bounds that make the proofs applicable to
> realistic parameter sizes.
I interpreted it a bit differently. In a nutshell, security is
different, for two reasons. One is, as you note, the parameter size
issue. There, the issue is twofold: first, that detailed analyses are
necessary, and not just worst-case bounds; second, that detailed
analyses -- though always of interest -- are a lot harder than simple
worst-case bounds. It's the job of referees to ensure that the right
analysis appears in security papers, and not just the easy, wrong one.
The other point, though, is more subtle. Any proof depends on your
axioms or system model. In security, though, an attacker can often
attack via a different model. Thus, we may have ciphers that are fine
at the 0s and 1s level but are vulnerable to things like differential
power analysis, cache timing, etc. Even at the 0s and 1s level, did
the proof of security account for things like related-key attacks?
Mathematicians have known since Euclid that axioms are important.
Security, though, is math embedded in the real world, and that
matters. Put another way, Euclidean geometry is completely valid as a
pure mathematical system. But that doesn't mean it applies in a
relativistic universe. Sure, we live far from any space-warping
masses, so we can pretend that the angles in our triangles add up to
180 degrees. In the security world, though, the attacker will toss a
black hole at us to warp the space around our provably-secure
triangular encryptor. Was that proof of security flawed? Ask Riemann
--Steve Bellovin, http://www.cs.columbia.edu/~smb
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