The Cipher that Caesar Used

(From Chapter VII: The Caesar Cipher)

The Roman ruler Julius Caesar (100 B.C. - 44 B.C.) used a very simple cipher for secret communication. He substituted each letter of the alphabet with a letter three positions further along. Later, any cipher that used this "displacement" concept for the creation of a cipher alphabet, was referred to as a Caesar cipher. Of all the substitution type ciphers, this Caesar cipher is the simplest to solve, since there are only 25 possible combinations. Often this type of cipher is implemented on a wheel device. A disk or wheel has the alphabet printed on it and then a movable smaller disk or wheel with the same alphabet printed on it is mounted forming an inner wheel. The inner wheel then can be rotated so that any letter on one wheel can be aligned with any letter on the other wheel.

For example, if the inner wheel is rotated so that the letter M is placed under the letter A on the outer wheel, the Caesar cipher will have a displacement of 12. To encipher the letter P, locate it on the outer wheel and then write down the corresponding letter from the inner wheel, which in this case is B. The same can be accomplished by placing alphabets on two pieces of paper and sliding them back and forth to create a displacement.

The Caesar cipher computer program: The computer program that demonstrates the use of a Caesar substitution cipher displays alphabets on two lines that can be moved back and forth rather than a rotating circle.

Page 51 from "SECRET CODE BREAKER - A Cryptanalyst's Handbook "

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