A-B-a-b encryption

Tim Dierks tim at dierks.org
Sun Nov 16 10:06:27 EST 2003

At 03:19 AM 11/14/2003, 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
>   - 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.

I don't know what it's called, but it's vulnerable to man-in-the-middle 
unless you've got some way to authenticate the parties (because Alice has 
no way to tell if she's decrypting Bob's B(A(m)) or Mallet's M(A(m)).

And if you've got some way to authenticate the parties (a shared secret or 
a public key or something), it's probably easier to leverage that into an 
encryption key.

  - Tim

PS - I'm looking for a full-time job. My resume is at 
http://www.dierks.org/tim/resume.html . Looking for architecture or 
technical management jobs; I'm in New York, NY, but I am willing to relocate.

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