Who created the Caesar cipher? History of shift ciphers

As everybody knows, it was Gaius Julius Caesar, the great Roman general and dictator, who created the Caesar cipher to protect the privacy of letters sent to his friends and family members, as well as battlefield orders. That's it, we're done, right?

Well, not so fast. The origin of the Caesar cipher is much more fascinating and obscured by the passage of time.

All that we know for sure is the following passage from a Roman historian, Suetonius:

Which translates in the language of Shakespeare as follows:

To summarize:

• Suetonius gives the earliest known description of somebody using a shift cipher, and this person was Caesar himself.
• The idea of cyclic letter substitution is probably much, much older, and nobody really knows who created the Caesar cipher.
• While the origin of the Caesar cipher is lost in history, Divus Iulius did help have it named after him, as he certainly contributed to popularizing the Caesar shift cipher.

Before we focus on explaining what a shift cipher is, we must answer one extremely important question. Suetonius explains how to decode the Caesar cipher, but if I have an extremely important secret message which cannot fall into the hands of an enemy, how can I encode it with the Caesar cipher?

Let us reread the passage of Suetonius. To decode a Caesar's message, one has to substitute each letter with the letter that stands three places to the left in the alphabet, so:

• D with A;
• E with B, and
• F with C, etc.

It is easy to guess what Caesar had to do to encode the message: he substituted each letter with the letter that stands three places to the right in the alphabet, so A got replaced with D, etc. The end of the alphabet requires some thinking. What would he do with letters like X? Well, since the letters at the very beginning of the alphabet are still very much available, the most natural thing to do is to wrap the alphabet around, so X gets replaced with A, etc.

As you can see, it is not at all hard to encode with the Caesar cipher. Its ease may be one reason why the pragmatic Roman general employed this method. This leads us to the more general notion of shift ciphers.

While we do not know if Caesar ever wondered what a shift cipher is, we can be sure he understood the principle very well!

Modern cryptographers refer to the encryption method described by Suetonius as the Caesar cipher or Caesar shift cipher. It consists of replacing each letter of the message with the letter that in the alphabet stands a given number of fields to the right or to the left, with the end of the alphabet wrapped up. (In a more sophisticated language, scientists refer to this wrapping up as the periodic boundary condition.)

Caesar's cipher is a particular instance of a shift cipher, where the shifting parameter $L=3$ corresponds to Caesar looking three places to the right to replace each letter of his message.

We can express the principle of shifting to the left by using negative values of the integer parameter. For instance, the shift cipher with $L=-2$ consists of replacing each letter with the letter that stands two places to the left, with the end of the alphabet wrapped up. That is, we replace:

• D with B;
• C with A, and
• B with Z, etc.

It is totally possible to use a shift code without wrapping up the ends as long as one comes up with some extra symbols for the letters in the final portion of the alphabet. In fact, we have historical reports about it! And guess who was using it? Octavian August, a nephew and adopted son of our dear Julius Caesar! We can only wonder if he came up with it himself or if someone explained the trick to him! 😂

If you crave historical sources, here's the relevant passage. We can see that Octavian was not very creative: the special symbol he invented was just a double A:

Which translates into...

Although the shift cipher follows incredibly simple rules, in cryptography it laid the foundations for far more complex methods, for instance, the Vigenère cipher invented in the Renaissance.

The surviving Caesar’s letters to Cicero and other friends do not contain, unfortunately, any passages encoded by him.

With Omni Calculator, you can, however, impersonate the great conqueror of Gallia! Suppose Gaius Julius wants to secretly tell Cicero that the die has been cast. He would then perform the following encryption:

ALEA IACTA EST → DOHD LDFWD HVW

He could get the reply that arms should yield to the toga:

FHGDQW DUPD WRJDH → CEDANT ARMA TOGAE

Don't hesitate to use Omni's Caesar cipher calculator to verify whether both Caesar and Cicero got it right!

As you can see, even though we do not know who created the Caesar cipher, this does not prevent us from sending encrypted orders in the name of Caesar!