A History of Cryptography - Introduction to Cryptography with Java Applets - page 21

E XAMPLE .

We use the following square

L O V E I/J

S A M N Y

P D R T H

G B C F K

Q U W X Z

to encrypt the message

AMBASSADOR SHOT.

First, group the letters in pairs

AM BA SS AD OR SH OT.

Now look for any doubled letter pairs and insert an X between them. Regroup the plain-
text.

AM BA SX SA DO RS HO TX

If there are not enough letters to make the final pair, add another X at the end, as done
here. If one follows the rules outlined previously, one should obtain the following cipher-
text:

MN UD QN AM BA MP ID FE

The rules for decryption should be easy to figure out; the same Playfair square is used.
(Of course—the square is the key.) The ciphertext pair of letters c, and d, are mapped to the
plaintext letters p and q in the following way.

1. If c and d are in both different columns and different rows, they define the corners of a

square. The other 2 corners are p and q; p is the letter in the same column as c.

2. If c and d are in the same row, p is the letter to the left of c, and q is the letter to the left

of d (wrapping around if necessary).

3. If c and d are in the same column, p is the letter above c, and q is the letter above d (with

wrap-around).

Because of the way enciphering was done, doubled letter ciphertext pairs will not occur.
The recipient must remove from the recovered plaintext any letter X's which do not make
sense. They must also determine, since I and J are equated, whether a recovered plaintext
I/J is an I or a J.