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From: Mok-Kong Shen on 2 Oct 2009 09:48
Hi,

A recent post of mosherubin on the distinguished Chaocipher reminds
me of the methodologies of classical crypto in general and as a
consequence also of a conversation I had long time ago with a friend.
We babbled one day over the application of polyalphabetic substituion
ciphers and had as a topic in our chat the following variation of the
use of substitution tables, which, though we were convinced must with
very high probability had already been tried out in theory or in
practice (and possibly even subsequently discarded due to
disadvantages), we hadn't known of a mention in the meagre collection
of books then available to us.

Let there be chosen two 26x26 substitution tables T1 and T2, whose
columns are random permutations of the alphabet. The encipherment
with table T of the plaintext character P with the key character
K will be denoted by C = T( K, P ), resulting in the ciphertext
character C.

The encryption of the plaintext character sequence P_i (i=1 ...) is
then, with K_1 being supplied at the start:

K_i = T1( C_(i-1), P_(i-1) )

C_i = T2( K_i, P_i )

A special case is where T1 and T2 are identical.

My friend and I hadn't pondered too much over the scheme (our coffee
cups were soon empty), let alone actually tried it out. I myself
have almost forgotten it and have only slowly retrieved it from
the 2nd order storage of my poor brain. I am taking the liberty to
post it here, because I suppose it could eventually serve as a stuff
for recreation for those experts interested in classical cryptology.

Thanks,

M. K. Shen
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Was sich ueberhaupt sagen laesst, laesst sich klar sagen;
und wovon man nicht sprechen kann, darueber muss man schweigen.

L. Wittgenstein
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