The Caesar Cipher, named for Julius Caesar who used it in his military campaigns, is one of the simplest and easiest substitution ciphers in the world. While it is rarely used by itself because of the ease with which it is broken, it is still sometimes incorporated into more complicated ciphers.
A substitution cipher is a type of encryption that replaces each letter of a message with a different letter. The Caesar Cipher simply replaces each letter with the letter a set number of places from the original one. Thus, if you were to use a right shift of three letters, the text, “this is a test” would become “wklv lv d whvw.” A left shift of three would make the message, “qefp fp qbpq.” The key for such a cipher with a right shift would be…
A = D
B =E
C =F
D =G
E = H
F =I
G =J
H = K
I = L
J = M
K = N
L = O
M = P
N = Q
O = R
P = S
Q = T
R = U
S = V
T = W
U = X
V = Y
W = Z
X = A
Y = B
Z = C
According to the biographer Suetonius, Julius Caesar used this cipher with a left shift of three in his military campaigns. While there are a few older ciphers in the world, this was the first recorded use of any encryption technique. It is therefore of great historical significance. It not only helped Caesar in his military campaigns, but it left a lasting influence on cryptology. Among the public, the Caesar cipher is still the most popular method of encryption in use.
Given the ease with which it is broken, it is not known if the cipher was effective at keeping Caesar’s messages secret. We can speculate that it was successful, however, because most of his enemies could not speak Latin, so they would have found it especially difficult to crack his encoded messages without any specific techniques for doing so. Such techniques were not invented until the 9th century.
Two of these techniques are as simple as the cipher itself. The first involves frequency analysis. Given the fact that letters like “e” and “t” are more common in English writing than letters like “x” and “z”, one can break a substitution cipher by matching the letters that appear most frequently in the message with the letters that appear most frequently in the language was written.
After decoding some of the more common letters, it should be clear how the cipher has changed the text. This should make it easy to decode the less common words. Of course, this requires that one know in what language the message was written. Alternatively, one could simply try all possible results. Given that there are only 26 possible results for a standard Caesar cipher in English, it would not take very long to try them all.
Despite the ease with which it is broken, the cipher has been used by people for thousands of years in all kinds of settings. Sometimes, it has been broken, as it was when the Russian army tried to use it in World War I. Much of the time, however, it has worked perfectly, so it has remained one of the most popular encryption schemes for over two thousand years.
