After 51 years of silence, the Zodiac Killer’s infamous cipher was finally cracked—with the help of AI. Discover how a blend of human intuition and machine power unlocked one of true crime’s greatest mysteries.
Introduction: The Cipher That Whispered From the Shadows
In the chill of a Northern California autumn in 1969, a letter arrived at the San Francisco Chronicle. It wasn’t a reader submission—it was something darker. Handwritten in all caps, filled with threats, and boasting of brutal murders, the letter came from someone who called himself “Zodiac.” Enclosed was a 340-character cipher—an eerie grid of strange symbols and jumbled letters. He claimed it would reveal his identity. It did not.
For over 50 years, this cipher sat like a locked tomb in the annals of criminal history. The Zodiac reveled in theatrics. He dressed in a black executioner’s costume. He taunted police with phone calls and riddles. Then he vanished. But his unsolved cipher remained—a cold whisper from the past.
Until, in 2020, a new kind of detective stepped into the spotlight: Artificial Intelligence.
This is the story of how AI helped crack one of the most infamous codes in American criminal history. It’s a tale of obsession, innovation, and the blurry line between machine logic and human intuition. If you like puzzles, mystery, and a touch of the macabre—you’re in the right place.
Welcome to a Throwback Thursday you won’t forget.
The Zodiac Killer and the 340-Character Cipher
Between 1968 and 1974, the Zodiac Killer is believed to have murdered at least five people in Northern California. But beyond the violence, what defined him was his love of attention—and cryptic communication.
On November 8, 1969, he mailed a letter to the San Francisco Chronicle. Enclosed was a grid of symbols: 63 rows by 17 columns. It became known as the “Z340.”
This wasn’t just a basic code. It wasn’t even traditional cryptography. It had tricks: line-skipping, mirrored characters, diagonal messages. It mocked known decoding methods. And unlike his earlier cipher (Z408), which had been cracked within days, Z340 would baffle experts for more than half a century.
Some believed it was a hoax. Others swore it held a dark truth. Either way, the cipher became legend.
Cracking the Uncrackable: Decades of Dead Ends
FBI and Military Codebreakers
Soon after the cipher arrived, the FBI’s Cryptanalysis and Racketeering Records Unit got to work. They assumed it followed the logic of Z408—simple substitution. But it didn’t. After thousands of hours and no breakthrough, they quietly stepped back.
As cryptography historian David Kahn once said:
“What makes a codebreaker’s job impossible is uncertainty. If you don’t know the rules, you’re guessing in a field of infinite possibility.”
Others tried. The NSA. Private think tanks. Nothing worked.
Crowd-Sourced Obsession
By the 2000s, the internet took over. Reddit users, Zodiac forums, and independent researchers dove into the code. One of the most committed: David Oranchak, a software developer who’d been working on the Z340 since 2006.
He said:
“There were times when I thought maybe the cipher didn’t contain anything at all—just gibberish. But then I’d find a strange alignment or word fragment that kept me going. It was like the cipher was almost talking to you.”
Despite all the effort, it remained unsolved—until the right minds and the right machine came together.
The Breakthrough: Human Intuition Meets Artificial Intelligence
In December 2020, a trio of codebreakers finally cracked the cipher:
- David Oranchak (USA): Cryptography enthusiast and Zodiac expert
- Jarl Van Eycke (Belgium): Creator of the cipher-solving tool AZdecrypt
- Sam Blake (Australia): Mathematician and statistical analyst
Using AZdecrypt, they tested thousands of combinations, simulating how the Zodiac might have scrambled and substituted his message. The key insight? He used a diagonal transposition pattern. When the team rearranged the characters accordingly, words began to emerge:
“I HOPE YOU ARE HAVING LOTS OF FUN IN TRYING TO CATCH ME… I AM NOT AFRAID OF THE GAS CHAMBER… BECAUSE I NOW HAVE ENOUGH SLAVES TO WORK FOR ME IN PARADICE…”
The language, misspellings, and tone matched the Zodiac’s known writing. The FBI confirmed its authenticity.
Why AI Was the Game-Changer
Unlike brute-force methods, AZdecrypt used heuristics—techniques that prioritize likely solutions based on pattern recognition. It mimicked the “gut feeling” of a human puzzle-solver, but it could test thousands of possibilities per second.
Dr. Craig Bauer, editor of Cryptologia, noted:
“This wasn’t just a win for AI—it was a win for collaborative thinking. Humans framed the problem. The machine explored possibilities. Together, they found what neither could alone.”
Intelligence vs. Computation: Can AI Understand?
Beneath the technical triumph lies a deeper question: Can machines actually understand?
AI doesn’t feel fear. It doesn’t grasp mockery or death. It doesn’t know the cipher’s author was a killer. But it recognized patterns, extracted structure, and found coherence in chaos.
Dr. Shannon Vallor, a philosopher of AI, writes:
“Artificial intelligence is not artificial mind. It’s a mirror of our own cognitive structures—optimized and accelerated—but not embodied with our values or intuitions.”
So, no—AI didn’t understand the Zodiac. But it helped us hear him.
From Cold Cases to Ancient Codes: Where AI Goes Next
The success of cracking the Z340 has real-world implications:
Law Enforcement
AI now helps police:
- Reanalyze cold case files
- Compare linguistic styles
- Spot forensic patterns across time and cases
The FBI’s Sentinel system, for instance, uses AI to scan case documents rapidly for connections no human could detect alone.
Forensic Stylometry
Tools now detect linguistic “fingerprints”—repeating patterns in word choice and syntax. This helped attribute anonymous QAnon writings to a likely author (Orphanalytics, 2021).
Historical Cipher Solving
AI is also helping crack ancient texts. Projects at MIT and Google DeepMind are decoding damaged or lost languages using deep learning—something historians thought impossible just years ago.
Conclusion: The Cipher Spoke Back
For 51 years, the Zodiac’s cipher sat silent—just paper, symbols, and shadow. It became a ghost. But thanks to the tenacity of human minds and the power of machine logic, it finally spoke back.
Not with a name. Not with a confession. But with a chilling message—a reminder that mystery is still alive, and that we now have the tools to chase it.
There are more puzzles out there. In evidence lockers. In ancient ruins. In encrypted archives. And some of them are just waiting for the right partnership—of human insight and artificial intelligence—to be heard again.
Some secrets just need the right ears to listen.
References
- Fagan, K. (2020, December 11). Zodiac ‘340 Cipher’ cracked by code experts 51 years after it was sent to the S.F. Chronicle. San Francisco Chronicle. https://www.sfchronicle.com/crime/article/Zodiac-340-Cipher-cracked-by-code-experts-51-15793652.php
- Kübler, R. (2020, January 9). Where machine learning meets cryptography. Medium. https://medium.com/data-science/where-machine-learning-meets-cryptography-b4a23ef54c9e
- Federal Bureau of Investigation. (2020, December 11). Zodiac cipher solved. https://www.fbi.gov/news/stories/zodiac-cipher-solved-121120
- Oranchak, D. (2020, December 11). Let’s Crack Zodiac – Episode 5: The 340 is solved! [YouTube Video]. https://www.youtube.com/watch?v=Yj8cNq3bq8g
- Bauer, C. (2014). Unsolved!: The History and Mystery of the World’s Greatest Ciphers. Princeton University Press.
- Vallor, S. (2018). Technology and the Virtues: A Philosophical Guide to a Future Worth Wanting. Oxford University Press.
Additional Reading
- Schmeh, K. (2020). Codebreaking: A Practical Guide. Springer.
- Singh, S. (1999). The Code Book. Anchor Books.
- Kahn, D. (1996). The Codebreakers. Scribner.
- McDermott, Q. (2022, October). Cracking the QAnon Code. ABC Four Corners. https://www.abc.net.au/news/2022-10-17/cracking-the-qanon-code-using-ai/101535926
Additional Resources
- AZdecrypt Cipher Solver (Jarl Van Eycke): https://github.com/jarlvaneycke/azdecrypt
- Let’s Crack Zodiac (David Oranchak’s YouTube): https://www.youtube.com/c/Oranchak
- Project Gutenberg Cryptography Collection: https://www.gutenberg.org/
- FBI Vault: Zodiac Case Files: https://vault.fbi.gov/zodiac-killer
- Cryptologia Journal: https://www.tandfonline.com/loi/ucry20