How cryptographers finally cracked one of the Zodiac Killer’s hardest codes
A new whitepaper offers a detailed look at how Z340 was decrypted after 51 years.
An international team of cryptographers has published a new whitepaper detailing the massive amounts of work, crowdsourcing, and computational programming that was required to translate a notorious serial killer’s half-century-old mystery message. Although one cryptographer uploaded a video rundown of their methodology to YouTube in 2020, the team’s new whitepaper further shows just how much work went into accomplishing their feat.
Between 1968-69, a man calling himself the Zodiac murdered at least five people in Northern California. During that time, as well as years after, the killer mailed a series of letters to local newspapers alongside a total of four ciphers. To this day, authorities have not formally named anyone as the Zodiac Killer, and only two of his cryptograms have been solved.
One of those, however, was long considered the most difficult to parse. First published in newspapers on November 12, 1969, the 340-character cipher (often referred to as “Z340”) baffled amateur and professional cryptographers alike for years. In December 2020, however, an international team announced they believed they finally cracked the Zodiac’s encoded message. A subsequent review by the FBI supported the solution offered by David Oranchak, Sam Blake, and Jarl Van Eycke, putting to rest a 51-year-old enigma.
[Related: Codebreakers have finally deciphered the lost letters of Mary, Queen of Scots.]
“I HOPE YOU ARE HAVING LOTS OF FUN IN TRYING TO CATCH ME,” the Zodiac Killer’s Z340 message begins, before clarifying he did not make the famous A.M. San Francisco television call-in on October 22, 1969.
THAT WASNT ME ON THE TV SHOW
WHICH BRINGS UP A POINT ABOUT ME
I AM NOT AFRAID OF THE GAS CHAMBER
BECAUSE IT WILL SEND ME TO PARADICE ALL THE SOONER
BECAUSE I NOW HAVE ENOUGH SLAVES TO WORK FOR ME
WHERE EVERYONE ELSE HAS NOTHING WHEN THEY REACH PARADICE
SO THEY ARE AFRAID OF DEATH
I AM NOT AFRAID BECAUSE I KNOW THAT MY NEW LIFE IS
LIFE WILL BE AN EASY ONE IN PARADICE DEATH
Zodiac’s Z340 message, typos included
First spotted this week by 404 Media, the 39-page paper (accompanied by 23 pages of source materials) offers the fascinating and complex history behind Z340. According to the three authors, arriving at their ultimate solution had been preceded by “many years of failed experiments, dead-end ideas, and efforts to summarize what was known about the [Zodiac Killer].”
After countless fruitless attempts, the team felt confident that Z340 included some combination of homophonic substitution (one letter swapped for one or more symbols) and transposition (letters reordered according to a certain systematic logic). Unfortunately, that didn’t exactly narrow down the possibilities. As Discover Magazine explained in a 2021 profile, the cryptographers then faced hundreds of thousands of possible approaches to reading Z340.
To tackle all those potentials, the team turned to AZDecrypt, a program dedicated to homophonic decryption built by Van Eycke. The mathematical intricacies behind AZDecrypt are intense—but just for reference, the codebreakers say their program can solve up to 200 homophonic substitution ciphers per second with a 99 percent accuracy rate. After augmenting the software a bit to incorporate transposition options, AZDecrypt got to work, and soon yielded its first breakthroughs. Before long, the team finally unraveled Z340.
[Related: This ancient language puzzle was impossible to solve—until a PhD student cracked the code.]
Interestingly, the writers theorize it’s entirely possible the Zodiac Killer didn’t intend Z340 to be this difficult to decode. Speaking in 2021, Oranchak believes the computational power needed to ultimately break Z340 wasn’t even available in 1969. The Zodiac’s very first cipher, Z408, was decoded just days after being published, so it’s likely he meant to make Z340’s enciphering methods harder—but accidentally went too far as “a random unintended result of the encipherment process.”
But as they make clear in their whitepaper, it wasn’t just computer software that solved one of the Zodiac Killer’s last mysteries. “The solution of this cipher was the result of a large, multi-decade group effort, and we ultimately stood on the shoulders of many others’ excellent cryptanalytic contributions,” they write.
