From Sixteenth-Century Cryptography to the New Millennium — The Last 500 Years - Codes: The Guide to Secrecy from Ancient to Modern Times - page 68

consisting of 26 letters. He put two clock-style hands over them, one long and
one short, which were employed in unison to choose plaintext and ciphertext
equivalents. However, given that there is 27 26 = 1 unit difference between

plaintext and ciphertext in his device, while Wadsworth had 33

−

24 = 9 units
difference meant that Wheatstone's apparatus was far less secure than that of
Wadsworth. In another, perhaps fitting, misattribution, a cipher that Wheat-
stone did create and which was far superior to the device for which he is known,
has been attributed to another. In 1854, Wheatstone invented the first literal
digraphic cipher in history. (The attentive reader will recall that on page 54 we
mentioned that Porta created the first digraphic cipher, but with signs , rather
than letters.) However, Wheatstone's friend Lord Lyon Playfair, who sponsored
it at the British Foreign OQce, has his name attached to it. This is how it works.
Consider the Table 2.1 (where the letters WX are considered as a single
symbol):

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The Playfair Cipher

A Z I WX D
E U T G Y

O N K Q M

Table 2.1

H F J L S

V R P B C

Pairs of letters are enciphered according to the following rules.

(a) If two letters are in the same row, then their ciphertext equivalents are
immediately to their right. For instance, VC in plaintext is RV in cipher-
text. (This means that if one is at the right or bottom edge of the table,
then one “wraps around” as indicated in the example.)

(b) If two letters are in the same column, then their cipher equivalents are the
letters immediately below them. For example, ZF in plaintext is UR in
ciphertext, and XB in plaintext is GW in ciphertext.

(c) If two letters are on the corners of a diagonal of a rectangle formed by
them, then their cipher equivalents are the letters in the opposite corners
and the same row as the plaintext letter. For instance, UL in plaintext
becomes GF in ciphertext and SZ in plaintext is FD in ciphertext.

(d) If the same letter occurs as a pair in plaintext, then we agree by convention
to put a Z between them and encipher.

(e) If a single letter remains at the end of the plaintext, then a Z is added to
it to complete the digraph.

One merely reverses the rules to decipher.

Example 2.1 Suppose that we know the following was enciphered using the
Playfair cipher.