A-B-a-b encryption

lrk crypto at ovillatx.sytes.net
Sun Nov 16 12:24:02 EST 2003

On Fri, Nov 14, 2003 at 09:19:25AM +0100, martin f krafft wrote:
> it came up lately in a discussion, and I couldn't put a name to it:
> a means to use symmetric crypto without exchanging keys:
>   - Alice encrypts M with key A and sends it to Bob
>   - Bob encrypts A(M) with key B and sends it to Alice
>   - Alice decrypts B(A(M)) with key A, leaving B(M), sends it to Bob
>   - Bob decrypts B(M) with key B leaving him with M.
> Are there algorithms for this already? What's the scheme called?

"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.

crypto at ovillatx.sytes.net

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