I was 12 when I first became obsessed with the Zodiac Killer’s ciphers. While other kids in my class were trading Pokémon cards, I was tracing cryptic symbols onto graph paper, convinced I’d be the one to crack the code that had stumped the FBI. Spoiler alert: I didn’t. But two decades later, I’m still trying—and now you can too.
The Zodiac Killer terrorized Northern California in the late 1960s, murdering at least five people (though he claimed to have killed 37). But what made him truly notorious wasn’t just his killing spree—it was his bizarre communication campaign.
Between 1969 and 1970, he sent four encrypted messages to local newspapers. These weren’t your average “catch me if you can” taunts. These were methodically crafted puzzles designed to both showcase his supposed brilliance and mock authorities who couldn’t decode them. (As subtle as a bloodstain on white carpet, this guy’s ego.)
The Z408: The One That Cracked First
The first cipher—known as Z408—arrived in three pieces sent to different newspapers in July 1969. At 408 characters long, it looked like an impenetrable wall of symbols, numbers, and zodiac signs.
But here’s where the story gets interesting: it wasn’t FBI agents or cryptography experts who solved it. It was a high school teacher and her husband.
Donald and Bettye Harden cracked it in just one week, revealing a disturbing message from the killer about collecting slaves for his afterlife. The message was rambling, disturbing, and completely devoid of any identifying information. Classic serial killer move—all tease, no payoff.
The Z340: The 51-Year Mystery
The second cipher—the Z340—proved much harder to crack. For 51 years, it remained one of criminology’s most frustrating puzzles. I spent countless nights in college staring at printouts of this thing, convinced the solution was just one algorithm away. (Ryan, my husband, still brings this up whenever I get too deep into a new case: “Remember when you thought you were smarter than the entire FBI cryptography department?”)
Finally, in December 2020, an international team of code-breakers solved the Z340 using specialized software. The message? More rambling, more taunting, more disappointment for anyone hoping for a confession or identity reveal.
What’s fascinating is how the Zodiac deliberately made this cipher harder than the first. He knew people had solved his previous puzzle, so he added layers of complexity—transposition, multiple symbols for the same letter, and deliberate errors to throw off pattern recognition.
Would have survived this crime? Absolutely not. I can barely remember my Netflix password.
The Unsolved Mysteries: Z13 and Z32
Two of the Zodiac’s ciphers remain unsolved to this day: the Z13 and Z32.
The Z13 is particularly tantalizing because it supposedly contains the killer’s name. Just 13 characters that might solve one of America’s most infamous cold cases. The Z32, meanwhile, was included in a letter describing a bomb plot that never materialized.
Every few years, someone claims to have solved these remaining ciphers, but none of these solutions have been widely accepted by cryptography experts. The problem? Short ciphers are actually harder to crack than longer ones because there’s less text to identify patterns.
Why We Can’t Stop Trying
The allure of these ciphers isn’t just about solving a puzzle—it’s about potentially closing a case that has haunted investigators for over 50 years.
Modern technology offers new hope. Advanced pattern recognition algorithms and retrieval-augmented generation systems are being applied to these ciphers, running millions of possible solutions in seconds.
What makes the Zodiac’s ciphers so compelling is that they exist in this perfect intersection of true crime, puzzle-solving, and technological challenge. They’re the ultimate brain teaser with the highest possible stakes.
Want to Try Your Hand?
If you’re feeling brave (or just need something to do during your next insomnia episode), there are several online communities dedicated to cracking these remaining ciphers.
Start with the basics of cryptography—understanding substitution ciphers, transposition methods, and how to identify patterns. Then graduate to specialized software designed for breaking historical codes.
Just remember: the Zodiac deliberately created these puzzles to be difficult. He wanted the attention, the newspaper headlines, the knowledge that people would be obsessing over his words decades later.
And here we are, still giving him exactly what he wanted.
(But between us? I still think I might crack it someday. Just don’t tell Ryan—he’s already worried about the serial killer books on my nightstand.)