A History of Cryptography - Introduction to Cryptography with Java Applets - page 27

TABLE 1.22

ABCDE

A A B C D E

B F G H I J

C K L M N O

D P Q/Z R S T

E U V W X Y

Now, we do the second transformation. Form new pairs by associating each letter in the
upper half with the letter below it; this yields

DC EE AE AA CD AC AC EB CA CE AA EA DA ED CA EE ED EC.

Now, using the same matrix given previously, map these pairs back to their single letter
equivalents. This third transformation yields the final ciphertext (grouped into 5-letter
blocks):

RYEAN CCVKO AUPXK YXW

(If we encounter a DB pair, it doesn't matter if we map it back to a Q or a Z.) In order to
decipher, an individual simply maps the ciphertext letters to their letter pair equivalents,
perhaps writing them vertically to save time.

DE AA CA AE CC AE DE CE EE
CE EA DC CB AE AA AD AE DC

She then regards the pairs horizontally, and regains the plaintext

TAKEM ETOYO URLEA DER.

This cipher is similar to a German wartime cipher (called the ADFGVX cipher) that per-
plexed the allied cryptanalysts for quite some time. It was a surprisingly simple cipher, but
it was only cracked after great expenditure of time and effort.
This cipher, like the German ADFGVX cipher, uses a property called fractionation, which
means a permutation on parts of a unit rather than among the units. In this cipher, this hap-
pens when we split the letter pairs. A mapping from a single character initially formed these
pairs, so it is akin to moving “half ” of a plaintext character. This is what confounds attempts
at breaking such ciphers. Not all substitution/transposition ciphers use this property.

EXERCISES

1. Write a Java program to simulate one of the ciphers in this chapter. Prompt the user to

enter a message, and then return the corresponding ciphertext. Then decrypt the mes-