A-B-a-b encryption

[martin f krafft]

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.

Thanks,

-- 
martin;              (greetings from the heart of the sun.)
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