The Caesar cipher: the three-letter shift described by Suetonius

Few ciphers are as well known as the one that bears the name of Gaius Julius Caesar. Its fame is due above all to a short passage in Suetonius's Lives of the Caesars, written at the beginning of the second century AD. After recalling the dictator's works and letters, the historian reports that when Caesar had to send confidential news he wrote in such a way that the letters did not form recognisable words.

The testimony of Suetonius

Suetonius also explains the rule, which was surprisingly simple. Each letter was replaced by the one three places further along in the alphabet: D in place of A, E in place of B, and so on. Anyone wishing to decipher had to perform the reverse operation, substituting, in the historian's words, “quartam elementorum litteram, id est D pro A”. It is a monoalphabetic substitution cipher, in which each letter always corresponds to the same one.

Aulus Gellius, in the second century, confirms that there was a commentary by the grammarian Probus devoted to the hidden meaning of the letters in Caesar's epistles. He mentions in particular the correspondence with his associates Gaius Oppius and Cornelius Balbus, who looked after his affairs in Rome while he was engaged in his military campaigns.

Augustus and the variants

Suetonius himself, in his life of Augustus, relates that Caesar's successor used an even simpler system: he shifted each letter by a single place, writing B for A and C for B. For X, the last letter of his alphabet, he did not go back to the beginning but wrote a double A. These details show that in Rome encryption was a personal habit of the powerful rather than an organised system of the state.

Caesar himself, in his Commentaries on the Gallic War, describes another expedient. During the siege of Quintus Cicero's camp by the Nervii in 54 BC, he sent a letter written in Greek characters, so that the enemy could not read it if it were intercepted. The letter, tied to a javelin and thrown into the camp, stuck in a tower and was noticed only on the third day.

Why it worked and why it is not enough today

A Caesar cipher has only twenty-five possible shifts in a twenty-six-letter alphabet, and anyone who suspects the method can try them all in a few minutes. In antiquity, however, security depended above all on the fact that few people could read and almost no one imagined that an incomprehensible text concealed a rule. Only with frequency analysis, described by Arab scholars in the ninth century, did simple substitution lose all protection.

The principle of shifting has survived in different forms. Leon Battista Alberti's cipher disk in the fifteenth century and Trithemius's table are nothing more than ways of changing the shift during the message. In the computer age ROT13, which shifts by thirteen places, has been used in online forums to hide puzzle solutions or plot spoilers, with no pretension to secrecy.

The Caesar cipher remains the starting point of almost every history of cryptography. Not because of its strength, which is minimal, but because it combines a precise ancient testimony, an illustrious name and an idea anyone can grasp in an instant: an alphabet sliding over itself which, with small modifications, gave rise to far more sophisticated tools.