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Is a Playfair really excluded ?
#1
Recently I went into thinking about the K4 "masking technique" could be (sort of) a Playfair, after all.

Yes, I know, Playfair suppose 25 or 36 alphabet, but Jim stated clearly that K4 had all the letters of the alphabet used.

I went into deep thoughts, and resurfaced with a novelty.  Who told us that the alphabet must be a square ?  It is only a convention...

My "alphacross" solution

Let's take 4 alphabets (like in four squares) : BLUE, RED, GREEN, YELLOW.  Reverse Green and Yellow, just to add a bit of spice.

To encipher : HELLOWORLD

BLUEACDFGHIJKMNOPQRSTVWXYZ REDABCFGHIJKLMNOPQSTUVWXYZ
ZYXWVUTSQPOMLKJIHFDCBANERG ZXVUTSRQPNMKJIHGFDCBAWOLEY

We take the rectangle formed by the two first letters (HE) from the alphabet in the top left to the alphabet in the bottom right corner, then take the opposite letters to get YP.

The result is : YPXYVIFIQY

Note that it covers the whole alphabet, being the length you want and yet simple as a-b-c.

Disadvantage

If you compare to the plain vanilla four square thing, the drawback here is that a given letter will always be enciphered with the same letter, making the whole thing more or less a dual-monoalphabetic transposition.  Still, used in combination with a transposition, it could have been hard to break, because to break it, you have to figure how it works, first.
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