Why Historical Cryptologists Need to Be 'Part Mozart, Part Accountant'

Letters written by Mary, Queen of Scots were finally decoded after more than 500 years of mystery. Gustavo Tomsich/CORBIS/Corbis via Getty Images

From the pharaohs of ancient Egypt to the warring kingdoms of medieval Europe, humans have had plenty of good reasons to send secret messages. If you wanted to keep military plans from the enemy, or keep an illicit affair from your husband, you wrote it down as a "cipher," a coded message that could only be unlocked by someone possessing the key.

Before the computer age, if an historian came across a coded message in an archive of material, they would usually skip over it, because it just wasn't worth the trouble. Even ancient ciphers can be nearly impossible to crack with just a pencil and paper. But what juicy historical secrets were trapped in those ancient puzzles?

"If you're looking through a diary and you come across a passage that's enciphered, you know that's the best part," says Craig P. Bauer, a math professor at York College of Pennsylvania and editor-in-chief of the journal Cryptologia. "When a message can't be read, the sky's the limit. It can be absolutely anything. It can resolve a historic mystery or shed new light on the personality of a historic figure. Maybe history itself is rewritten."

Bauer should know. His journal, Cryptologia, made history in February 2023 by publishing the work of three amateur code breakers who decrypted a trove of secret letters written by Mary, Queen of Scots. The encoded letters dated to the 16th century, during Mary's 19-year imprisonment by Queen Elizabeth I and show how Mary remained a shrewd political operative even while locked away in a castle.

The team that cracked the Mary, Queen of Scots letters harnessed powerful code-breaking technology that wasn't available even a decade ago, and those new tools have ushered in what Bauer calls "the second golden age of decipherment." The question is: What secrets will these historical code breakers uncover next?

What is Historical Cryptology?

The "Zodiac Killer" sent two letters and a cryptogram to the San Francisco Chronicle boasting of his exploits in 1969. The code was broken 51 years later.
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Cryptology is the study of both writing and breaking codes. When we think of cryptologists, we might picture someone like the British mathematician Alan Turing, who cracked Nazi Germany's infamous Enigma machine during WWII.

But cryptologists don't just work for intelligence agencies like the NSA. There are also amateur code breakers who are fascinated with solving historical puzzles. Bauer is one of them. He wrote a book called "Unsolved!: The History and Mystery of the World's Greatest Ciphers from Ancient Egypt to Online Secret Societies," which catalogs some of the biggest ciphers that historical cryptologists are itching to crack.

The Zodiac cipher used to be one of them. For 51 years, no one could solve the riddle of the 340-character coded message written by the infamous Zodiac Killer in 1969. And then a team of amateur cryptologists led by David Oranchak cracked the Zodiac cipher in 2021 using supercomputers and custom decryption software. (The killer's identity remains a mystery.)

In recent years, historical cryptologists have broken ciphers created by the KKK, Marie Antoinette and the Masons. They even figured out how to decrypt the infamous Rohonc Codex, an ancient illustrated manuscript, discovered in the 1830s, written in an unknown nonsense language.

Bauer is quick to point out that it takes more than just raw computing power to break an ancient code.

"A good historical cryptologist has to be part accountant — able to very carefully keep track of numbers and statistics — but they also need to have the spirit of Mozart," says Bauer. "You have to be very creative and almost a psychologist to guess the key words or phrases to unlock the cipher. It requires a combination of creativity, powerful computers and persistence. If you lose one leg of that stool, you won't find a solution."

The team that deciphered Queen Mary's code used a process called hillclimbing, where a computer randomly assigns the symbols in the cipher to letters of the alphabet, decrypts the whole message, scores it based on readability and then repeats the process, only keeping the changes that increase the score of the translation. After the code was cracked, the cryptologists still needed to decipher the coded letters one word at a time and edit the transcriptions, a process that took one year.

How Historical Ciphers Work

Every historical cipher is created by some kind of algorithm (or formula), and Bauer says that encryption algorithms can be extremely complex — like the unbreakable end-to-end encryption that safeguards data traveling across the internet — or extremely simple.

"Julius Caesar used a very simple algorithm," says Bauer. "He would shift every letter over three spaces in the alphabet. Any child could shift it back and read the message."

In cryptology terminology, Caesar's method is called "transposition," any system that moves the letters of the alphabet around or scrambles their order. There's even a transposition cipher in the Hebrew Bible called atbash, in which the first letter of the Hebrew alphabet is swapped with the last letter, the second letter of the alphabet is swapped with the second-to-last letter, and so on.

A far more difficult cipher to solve is one that employs "substitution," in which the letters in the original message are replaced by other random letters, numbers or symbols. Substitution ciphers are created by using a key or "cipher alphabet." Mary, Queen of Scots used a cipher alphabet that replaced each word with a unique symbol. Such a cipher could only be decoded by someone possessing the same key.

To make a cipher even harder to crack, two different cipher keys could be employed simultaneously (called "polyalphabetic"), maybe one for the first word of a sentence and a different key for the second word. "Null" symbols might be added that have no meaning, just to throw off the code breakers. Another trick is to assign several different symbols to the most common words, a tactic called "homophonic substitution."

Teamwork Is Key in Historical Cryptology

The three men who solved the Queen Mary mystery all have day jobs: George Lasry is a computer scientist living in Israel, Norbert Biermann is a music professor in Germany and Satoshi Tomokiyo is an astrophysicist in Japan. They each had a passion for historical ciphers, but it's unlikely any one of them would have solved the Mary, Queen of Scots riddle on their own.

The three men found each other as part of the DECRYPT Project, an international effort to bring historians and cryptologists together and give them the computational tools to decipher ancient encrypted texts. Baer says that in the past, code breakers and historians mostly worked on stubborn problems in isolation, but that's changing.

"A lot of the great work in cryptology is done in teams of two or more people," says Baer. "George Lasry has established himself as the best code breaker outside of the government. He and his teammates are tearing things up."