A-B-a-b encryption

David Ross David.Ross at uq.net.au
Sun Nov 16 13:43:11 EST 2003

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

"Decrypting" B(A(M)) with key A will typically result in another 
encryption: a(B(A(M))), not B(M).  The operations are not associative,
commutative or distributive, so this will not work.

>   - Bob decrypts B(M) with key B leaving him with M.


- Dave.

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