[14758] in cryptography@c2.net mail archive
Re: A-B-a-b encryption
daemon@ATHENA.MIT.EDU (lrk)
Sun Nov 16 17:37:56 2003
X-Original-To: cryptography@metzdowd.com
X-Original-To: cryptography@metzdowd.com
Date: Sun, 16 Nov 2003 11:24:02 -0600
From: lrk <crypto@ovillatx.sytes.net>
To: martin f krafft <madduck@madduck.net>
Cc: cryptography@metzdowd.com
In-Reply-To: <20031114081925.GA11764@piper.madduck.net>
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@ovillatx.sytes.net
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