A-B-a-b encryption

[Jeremiah Rogers]

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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