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178 CHAPTER 9. DIGRAPHIC CIPHERS
Examples:
(1) Using keyword square, encipher rectangle
The arrangement of the alphabet is
S Q U A R
E B C D F
G H I K L
M N O P T
V W X Y Z
Then we follow the directions. re forms corners of a rectangle with SF.
Since S is on r’s row, and F is on e’s, re becomes SF. ct forms a rectangle
with FO, and an with QP. gl are on the same row, so moving each one to
the right g becomes H and (circling around) l becomes G. Finally, perhaps
adding an x as a null, ex becomes CV. So the final ciphertext is SFFOQ
PHGCV.
(2) Use the same keyword to decipher RKNKQ DFM.
Each of the pairs RK, NK,QD, and FM are two of the four corners of a rectangle.
Replace each letter by the corner of the rectangle with the same
height.
(3) Encipher foreign affairs using the keyword playfair.
(4) Use the same keyword to decipher QMIGH PSZQF BKKN. 9
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄
Of course, naming this cipher for Playfair is already unfair to Wheatstone.
But further, Wheatstone recommended using the cipher with rectangular matrices,
rather than only square ones, and mixing the alphabet as in the Keyword
Mixed ciphers rather as in the simpler Keyword ciphers. Unfortunately, these
improvements tended to be forgotten.
The Playfair cipher is quick to set-up and easy to use. It is harder to break
than a monoalphabetic cipher, but only somewhat so. To see why, notice that
the only possible substitutes for e are the letters in its row and the one letter
directly beneath it. (In the keyword SQUARE example earlier, these are the
letters BCDFG.) So the letters in e’s row will be used a lot. This gives some hope
of setting up an entire and row in an unknown Playfair cipher. Additionally,
bigrams such as re and er will always be encoded by a reversal, for example as
SF and FS. From here, with some guided guesswork and some training, anyone
can be taught to break Playfair ciphers accurately and fairly quickly.
9 (2) alphabet, (3 ) LTIGR EQPYZ PY, (4) three attaches.