Re: manual cryptography

From: John Savard (jsavard_at_ecn.aSBLOKb.caNADA.invalid)
Date: 10/30/03 

Date: Thu, 30 Oct 2003 12:21:17 GMT

On Thu, 30 Oct 2003 06:31:31 -0500, "John E. Hadstate"
<jh113355@hotmail.com> wrote, in part:

>Hmmm. You're right. Suppose one were to generate the key material by
>autokeying the secret key and the alphabet (or any other pre-agreed text).
>Then just encrypt the plaintext by adding the resulting key material mod 26.
>It would take twice as much time, but would pretty well close the "probable
>word" loophole.

I'd be more inclined to follow the kind of thing Douglas Gwyn pointed
out - Playfair followed by a transposition.

But let's _really_ get fancy: use a straddling checkerboard, then
"Giant Playfair" using a 10 x 10 square of two-digit pairs, then
double transposition.

>Now, do you know of a way that is practical to compute manually to
>authenticate the ciphtertext?

With manual encryption, if it decrypts and makes sense, it's
authentic... as long as you _don't_ use something like the one-time
pad. Authentication is too prone to error to do manually, although I
suppose autokey might work as an intermediate layer.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html

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