A-B-a-b encryption

[Ian Grigg]

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?
> I searched Schneier (non-extensively) but couldn't find a reference.

Perhaps you want to try looking at the various blinding
algorithms by Chaum (and Wagner, also Brands).  These use
schemes that are transitive (if that's the word).

iang

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