Computer Security and Cryptography

3.2 CAESAR'S CIPHER

It is believed that Julius Caesar, in the period 58 BCE to 51 BCE, enciphered messages to his lawyer Marcus Tullius Cicero and other Roman senators using a monoalphabetic substitution. In the Caesar cipher, each plaintext letter was replaced by the letter standing three places to-the-right in the alphabet. If we neglect that the original Roman or Latin alphabet did not contain a J, U, or W, then, Julius' query in the present day Roman alphabet

               ANYONE  KNOW  WHERE  I  CAN  GET  DECENT  PIZZA?
               dqbrqh  nqrz  zkhuh  l  edq  jhw  ghfhqw  slccd?

would be enciphered as above.

For the alphabet of uppercase Latin letters {A, B,…, Z} identified with the integers in the Caesar shift substitution C_k is defined for each key by

Variations of the Caesar substitution with larger key spaces have been invented; one simple generalization, the affine Caesar substitution, is defined by the formula

where the key is a pair of integers j, k. A_{j, k} is a one-to-one transformation on the alphabet only ...