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?
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.