A History of Cryptography - Introduction to Cryptography with Java Applets - page 25

one-to-many mapping, but note that decryption is not, for none of the letter pairs in the pre-
vious table appear more than once. (You may wish to verify this.) Thus, decryption always
produces the correct message.
Those decrypting would probably have the inverse mappings organized according to the
ciphertext symbols, to aid in decryption. A listing of the inverse mappings of our sample
homophone is shown in Table 1.21.
Homophonic ciphers were very effective, and were used extensively in the past. Because
of their heavy dependence on the language being used, and because modern powerful block
ciphers are primarily independent of language, homophones are not commonly used today.

1.20

COMBINATION SUBSTITUTION/TRANSPOSITION CIPHERS
When substitution and transposition are used simultaneously, and especially when the respec-
tive block sizes are different, the result can be a very powerful cipher.

E XAMPLE . The following is a cipher that uses both substitution and transposition. Three
transformations will be involved. Suppose we use the ordinary alphabet, where substitu-
tions for plaintext are first made according to Table 1.22.
That is,

A maps to AA (row A, column A)
B maps to AB
C maps to AC
...
Q maps to DB
...
Y maps to EE
Z maps to DB.

(Note that the letters Q and Z map to the same pair. When decryption is done this should not
be a problem as Q and Z are very infrequent letters, and it should be easy to determine
which letter was intended). Suppose we wish to encipher the following message:

TAKE ME TO YOUR LEADER.

Now, convert each letter to its letter pair equivalent.

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

Take the second half of this text and place it under the first half.

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