How cryptographers finally cracked one of the Zodiac Killer’s hardest codes

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.