On Nov 16, 2003, at 12:24 PM, lrk wrote:
"Stupid crypto", probably. Unless I'm missing something, this only works
if A(A(M)) = M. Symetric crypto, not just symetric keys.
NEVER willingly give the cryptanalyst the same message encrypted with the same system using two different keys.
For the simple case, suppose F(X) = X ^ S (exclusive or with a string generated from the key).
Then M = A(M) ^ B(M) ^ B(A(M)), right?
Probably something similar for other symetric systems.
This is Shamir's Three-Pass protocol and it doesn't require a symmetric system, it requires a commutative system. See Schneier p 516 (section 22.3) or [1] for details.
so A(A(M)) != M
Unless I'm mistaken, this commutative system does not leak information in the same way as XOR does.
- Jeremiah
[1] http://www.afn.org/~afn21533/keyexchg.htm
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