A Roman rescue message tied to a javelin

In the winter of 54 BCE, a javelin landed inside a Roman camp under siege in Gaul. The javelin had a letter tied to it. The camp was commanded by Quintus Tullius Cicero, the younger brother of the famous orator Marcus Tullius Cicero, and enemy forces had surrounded it so tightly that ordinary messengers could not pass through. Caesar learned of the siege only after a messenger escaped and reached him. He sent a Gallic cavalryman back toward the camp with a reply. If the rider could not get close enough to deliver the message safely, Caesar told him to fasten the letter to a javelin and hurl it over the Roman fortifications.

The plan did work, although the javelin became stuck in a tower and was not noticed for two days. When a soldier finally spotted it and the letter reached Cicero, it carried reassuring news: Caesar was already marching to help with his legions. The message gave the trapped defenders an instant boost in morale. More importantly for the history of cryptography, Caesar had taken a precaution that made the letter unusually interesting. He had written the message in Greek characters instead of Latin, so an enemy who intercepted the letter would not easily read the Roman plans.

Greek letters were not a formal code. Educated Romans often knew Greek, and Caesar's choice to use Greek script was more about obscuring the message from casual eyes than creating an unbreakable cipher. Still, the episode reveals something important about Caesar's thinking: he cared about making sensitive military communications difficult for unintended readers to understand.

The Caesar cipher: a shift of three letters

The more famous method later associated with Caesar comes from Suetonius, who wrote The Twelve Caesars roughly 150 years after Caesar's death. According to Suetonius, Caesar protected confidential messages with a simple substitution cipher. He replaced each letter in his message with the letter that appeared three places farther along in the alphabet. For example, Suetonius noted that A was written as D. This small but systematic shift allowed Caesar to turn readable plaintext into a ciphertext that would not give up its meaning at a glance.

Caesar's Latin alphabet was not identical to the modern English alphabet, so when we describe the Caesar cipher today we often use the modern 26-letter version. In that version, A becomes D, B becomes E, C becomes F, and so on until the end of the alphabet, where X becomes A, Y becomes B, and Z becomes C. The shift is easy to apply and easy to reverse. Both sender and receiver only needed to remember the rule and follow it consistently. No equipment was necessary, no tables had to be carried, and no complex mathematics was involved.

ATTACK AT DAWN becomes DWWDFN DW GDZQ

A quick example shows how simple the system was. Take the phrase ATTACK AT DAWN. With a shift of three, each letter slides forward by three positions in the alphabet. A becomes D, T becomes W, C becomes F, and K becomes N. The encrypted result is DWWDFN DW GDZQ. In modern terminology, ATTACK AT DAWN is the plaintext, DWWDFN DW GDZQ is the ciphertext, and the agreed number of positions, in this case three, is the key.

Decryption works in reverse. Move every letter three places backward and the original phrase reappears. This symmetry made the scheme extraordinarily practical for field use. If a messenger was captured, the enemy might see the ciphertext, but without knowing the shift they would not immediately understand the message. That did not mean the system was secure against a determined opponent, but it did mean a quick glance would not reveal troop strengths, meeting points, or battle plans.

Why the Caesar cipher was easy to break

The weakness of the Caesar cipher is obvious to anyone who examines it. A modern 26-letter alphabet gives only 25 possible nontrivial shifts. An interceptor who suspects the use of a Caesar cipher can simply try every shift until recognizable words appear. DWWDFN DW GDZQ may look like random letters, but testing the shift of three brings back ATTACK AT DAWN immediately. Modern software can test all 25 possibilities in a fraction of a second. Even with paper and pencil, cracking such a message took little more than patience.

Frequency analysis is another method that can expose a Caesar shift. Certain letters appear far more often in a language than others. If a codebreaker knows how frequently letters occur in Latin or Greek, they can compare those patterns against the symbols in an intercepted message and infer which ciphertext letter stands for which plaintext letter. The Caesar cipher did not even require advanced cryptanalysis, but it set the stage for much more sophisticated substitution methods to come.

Augustus used a cipher of his own

Caesar was not the only Roman ruler to use letter substitution. According to Suetonius, Augustus used a similar cipher but with a shift of only one letter. In his system, A was written as B, B became C, and so on. Suetonius also noted a peculiar exception near the end of the alphabet: Augustus wrote AA in place of X. The detail may sound odd, but it reveals that Roman leaders were experimenting with ciphers and adapting them to their own preferences.

The fact that both Caesar and Augustus used substitution ciphers also demonstrates that the idea was not a one-off experiment. It was an accepted method for handling sensitive correspondence in a world where letters and messengers could easily be intercepted.

Al-Kindi and the birth of frequency analysis

It took many centuries for cryptanalysis to become a formal discipline. In the ninth century, an Arab polymath named Al-Kindi produced what is generally considered the earliest surviving description of frequency analysis. He recognized that language follows patterns and that some letters appear more often than others. A simple substitution cipher might hide the actual letters, but it cannot hide the statistical fingerprints of the original language.

Al-Kindi's work was important because it offered a systematic way to break substitution ciphers. Instead of guessing every possible shift, a codebreaker could examine repeated letters and common patterns in a ciphertext. By matching the most frequent symbols in the ciphertext to the most frequent letters in the underlying language, the codebreaker could uncover the message much more quickly. The idea changed cryptography from an art of hiding text into a contest between code makers and code breakers.

From simple shifts to stronger ciphers

After Al-Kindi, cryptographers looked for ways to make substitutions harder to analyze. One approach was to vary the substitution as a message progressed. Leon Battista Alberti experimented with multiple cipher alphabets in the fifteenth century. Johannes Trithemius published a square table of shifted alphabets, known as the tabula recta

Source:MakeUseOf News