An international group of codebreakers has released a new document describing the extensive effort, teamwork, and computer programming needed to decode a famous serial killer’s fifty-year-old secret message. Despite one codebreaker posting a video explanation of their process on YouTube in 2020, the team’s new document further demonstrates the amount of work involved in their accomplishment.
Between 1968-69, an individual who called themselves the Zodiac killed at least five people in Northern California. During this time and in the years following, the killer sent a series of letters to local newspapers with a total of four ciphers. To this day, no one has been formally identified as the Zodiac Killer, and only two of his codes have been solved.
One of these codes, however, was long considered the most challenging to decipher. Published in newspapers on November 12, 1969, the 340-character code (referred to as “Z340”) puzzled both amateur and professional codebreakers for years. However, in December 2020, an international team announced their belief that they had finally decoded the Zodiac’s message. A subsequent review by the FBI supported the solution proposed by David Oranchak, Sam Blake, and Jarl Van Eycke, solving a 51-year-old mystery.
[Related: Codebreakers have finally unlocked the secret messages of Mary, Queen of Scots.]
“I HOPE YOU ARE HAVING LOTS OF FUN IN TRYING TO CATCH ME,” the Zodiac Killer’s Z340 message starts, and he also denies making 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
Discovered this week by 404 Media, the 39-page document (accompanied by 23 pages of source materials) presents the intriguing and complicated history of 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 numerous unsuccessful attempts, the team believed that Z340 involved both homophonic substitution (one letter replaced by one or more symbols) and transposition (letters rearranged according to a systematic logic). However, this did not significantly narrow down the possibilities. As Discover Magazine explained in a 2021 article, the codebreakers then faced hundreds of thousands of potential approaches to deciphering Z340.
To address all these possibilities, the team used AZDecrypt, a decryption program focused on homophonic decoding created by Van Eycke. The complex mathematics behind AZDecrypt are extensive—but for reference, the codebreakers state that their program can solve up to 200 homophonic substitution codes per second with 99 percent accuracy. After enhancing the software to include transposition options, AZDecrypt got to work and soon made progress. Eventually, the team successfully decoded Z340.
[Related: This ancient language puzzle was unsolvable—until a PhD student broke the code.]
Interestingly, the authors suggest that it's possible the Zodiac Killer didn't intend Z340 to be so difficult to decode. Speaking in 2021Oranchak believes that the computational power required to break Z340 was not available in 1969. The Zodiac's first cipher, Z408, was decoded shortly after being published, so it's probable that he intended to make Z340 more challenging, but accidentally made it too difficult as a random unintended outcome of the encoding process.
But as they clarify in their whitepaper, it wasn't only 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.
