A Brief History of Cryptography from Caesar to Bletchley Park

Abstract

In Julius Caesar’s Gallic Wars, he describes how he sent a message to the besieged Cicero. The messsage was encrypted by substituting Greek letters for Roman letters, then delivered in the most dramatic way imaginable. The messenger, unable to reach the camp, hurled a spear with the letter fastened to it with a thong. Although the spear lodged itself in a tower, nobody spotted it for two days. Eventually, it was taken down and delivered to Cicero, who read out the vital news to the entire camp, bringing enormous joy to his troops. This was the first documented use of a substitution cipher for military purposes. Substitution ciphers, as the name suggests, encrypt messages by replacing the original characters with different characters. This is in contrast to a transposition cipher, in which the characters remain the same, but they are transposed or rearranged to create an anagram. One of the most famous substitution ciphers is the so-called Caesar cipher, which simply replaces each letter in the message with the letter that is, say, three places further down the alphabet. Cryptographers often think in terms of the plain alphabet, the alphabet used to write the original message, and the cipher alphabet, the letters that are substituted in place of the plain letters, both of which are shown below. The plaintext is the technical name for the original message, while the ciphertext is the encrypted message. In this chapter, the plaintext is written in lower case and the ciphertext in upper case. Although this example involves a shift of 3, lesser or greater shifts are of course possible. The Caesar cipher can be generally stated as substituting each plain letter with the letter that is x places later in the alphabet, where x is between 1 and 25. This illustrates one of the basic principles of cryptography, namely the relationship between the algorithm and the key. In the Caesar cipher, the algorithm is the general idea of replacing the original letters with those that lie a fixed number of places further along the alphabet. The key, x, specifies the distance of the shift.