Cryptography is nearly as old as human language. Many civilizations felt the need to conceal their messages using more or less sophisticated encryption methods, as demonstrated by some documentary examples that have survived in Egypt, China, Mesopotamia, India, Greece…

It is precisely in Egypt where the first example of cryptography has been found. The hieroglyphics of the inscriptions in the tomb of the official Khnumhotep II around 1900 B.C. were written with extra symbols, probably to confuse or hide their meaning from whoever read them.

Cryptography developed primarily in the military sphere. The Spartans, for example, used the scytale, which consisted of two rods of similar thickness, one for the sender and one for the receiver, on which a leather strip was wound where the message was written. Only by winding the strip on a rod of similar thickness could the message be read.

A model of the scytale
A model of the scytale. Credit: Luringen / Wikimedia Commons

This was a transposition cipher system, that is, one in which the letters are the same but the order changes. Many modern cryptography techniques are based on this.

Others are based on substitution encryption, of which a good example from antiquity is the Polybius Square, a method that consists of encoding the alphabet letters with the numerical coordinates of a table. Each letter is represented by two digits according to its position on the vertical and horizontal axes.

Polybius had not invented it, but it is called that because of the improvements that the Greek historian introduced into the system around 134 B.C., which consisted of using torches to transmit the encoded messages through that system.

Caesar's Cipher with shift 3
Caesar’s Cipher with shift 3. Credit: Don’t Know / Wikimedia Commons

But the first recorded use of a substitution cipher corresponds to none other than Julius Caesar, who used it in his private correspondence. Suetonius says in his biography:

Also, there are his letters to the Senate, which he seems to have first converted into pages and into the form of a commemorative little book, since previously the consuls and military leaders used to send only letters written on crossed paper. There are also letters addressed to Cicero, as well as to relatives about domestic matters, in which, if there was something that needed to be communicated in a more secret manner, he wrote it in notes, that is, structuring the order of the letters in such a way that no word could be formed; if someone wanted to decipher them, they would have to replace the fourth letter of the alphabet, that is, D by A and so on with the others.

Suetonius, Life of Caesar 56.6
Caesar Cipher Disk used by the Confederates in the American Civil War, now in the NSA Museum
Caesar Cipher Disk used by the Confederates in the American Civil War, now in the NSA Museum. Credit: austinmills / Wikimedia Commons

The system consisted, as Suetonius indicates, of replacing each letter with another at a fixed number of positions in the alphabet. Caesar used it with a shift of 3 positions, where D is replaced by A, E by B, and so on. The last letter of the Latin alphabet, which was X, he replaced with “AA”.

Aulus Gellius suggests that Caesar did not always use the same system, but sometimes used more complicated encryption. Also, that the grammarian Probus had written a treatise on the secret meaning of the letters in the composition of Caesar’s epistles, a work that unfortunately has not come down to us.

Traditionally, the invention of this encryption system is attributed to Julius Caesar, and that is why we know it as Caesar Cipher. Suetonius also says that Augustus used the same system, but with a shift of 1 instead of 3 as Caesar did. That is, B is replaced by A, C by B, etc.

Example of Caesar Cipher with Shift 3, as used by Julius Caesar
Example of Caesar Cipher with Shift 3, as used by Julius Caesar. Credit: Meilani.conley / Wikimedia Commons

An example of Caesar Cipher with a shift of 3, just as Julius Caesar used it.

It is unknown how effective this encryption could have been in Caesar’s time; possibly few of his enemies would have been able to decipher it, but currently it is one of the simplest to decode, with a little intuition and patience, of course.

A variant of the Caesar Cipher is ROT13, which was developed in Usenet forums since the early 1980s. ROT13 is nothing more than a Caesar Cipher with a shift of 13 (anyone who wants to try encoding and decoding with different shifts can use the rot13.com tool, which uses the English alphabet).

For example, using a shift of 3 like Julius Caesar, this website would be Od Eumxod Yhugh. But in ROT13, it changes to Ba Rzhkqba Ireqr.

This article was first published on our Spanish Edition on March 11, 2024. Puedes leer la versión en español en El sistema de cifrado que Julio César utilizaba en sus cartas para ocultar su contenido

Sources

A Brief History of Cryptography (Cypher Research Laboratories) | Reinke, E. C. (1962). Classical Cryptography. The Classical Journal, 58(3), 113–121. jstor.org/stable/3295135 | Reinhard Wobst, Cryptology Unlocked | Chris Savarese and Brian Hart, The Caesar Cipher | Wikipedia

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