Caesar's shift cipher

The Roman writer Suetonius says Julius Caesar protected private messages by shifting each letter three places along the alphabet. The method is still called the Caesar cipher.

The date is approximate, and the account comes from a later writer.

Context

Suetonius, writing in the early second century CE, describes the method in his Life of Julius Caesar. If Caesar had something confidential to say, he wrote it in cipher, changing the order of the letters so that no word could be made out. To read it, one substitutes the fourth letter after each one, so D stands for A. Suetonius is the main surviving source, and he was writing roughly 150 years after Caesar's death.

Caesar reportedly used the system in letters to friends in Rome while he was campaigning in Gaul. Because a message passed through couriers and could be intercepted, a simple scramble was enough to stop casual readers. A grammarian named Valerius Probus is said to have written a treatise on Caesar's ciphers, but it is lost, so no detailed Roman description survives beyond Suetonius.

The cipher is weak by modern standards. It has only 25 useful shifts, and like any single-alphabet substitution it falls to frequency counting, because the commonest letters of a language still stand out after the shift. Its lasting value is as the simplest teaching example of a cipher, and shifted alphabets of the same type are still used in classroom exercises.