Cracking the Zodiac Killer’s Cipher – How Mathematicians Finally Solved the Z340 Mystery

From Discover Magazine.com (2020):

Few cases have captured the public's imagination and baffled investigators quite like the Zodiac Killer. For decades, this unidentified serial murderer taunted law enforcement and the public alike with cryptic messages and ciphers, leaving a trail of terror in his wake.

One of the most infamous aspects of the Zodiac's sinister saga was the series of letters he sent to local newspapers, in which he claimed responsibility for his gruesome crimes and taunted authorities with cryptograms. Among these letters, one stood out as a tantalizing puzzle that would endure for years – the Zodiac Killer letter that finally got decoded.

Wonder no more, true-crime aficionados.

Who Was the Mysterious Zodiac Killer?

In the late 1960s, a serial killer self-identifying as “the Zodiac” killed at least five people in Northern California and claimed to have murdered more, according to Biography. In November 1969, the Zodiac Killer sent a card to the San Francisco Chronicle containing a 340-character secret message, also known as the Z340, that for more than 50 years went unsolved by detectives, cryptography experts, amateur sleuths, and curious others, according to a study in Arxiv.

After months of crunching code, three researchers on three different continents announced that they’d finally decoded the Z340 message. Further bolstering the claim, experts at the FBI verified the solution (and even tweeted about it).

……….

Who Cracked the Z340 Zodiac Killer Code?

Although the codebreakers involved had each been working on the cipher for years, the successful joint effort began in 2018 when Oranchak delivered a talk about the cipher at the annual meeting of the American Cryptogram Association in Asheville, North Carolina. He posted the talk on YouTube where, predictably, it elicited hundreds of comments, many of which came from people who (erroneously) claimed they’d already solved it.

But one person stood out: Blake. He responded to Oranchak’s talk with mathematical ideas about how to approach a code that includes both homophonic substitution — in which one letter might be swapped for more than one symbol — and transposition — in which letters are reordered in a systematic way. Oranchak and Blake began corresponding and eventually generated hundreds of thousands of possible ways to read the code.

To make sense of those, Oranchak brought in Jarl van Eycke, a Belgian warehouse worker and codebreaker who’d written AZdecrypt, software used for decoding homophonic substitutions. Van Eycke used an updated version of his software to churn through the possibilities. “Jarl just smashed it out of the park,” Blake says. In late November and early December 2020, the trio reprocessed their previous results, this time looking for words and phrasing characteristic of other writings by Zodiac.

And they struck gold. Van Eycke’s software spit out two phrases: trying to catch me and gas chamber. They fixed those phrases in the software, ran it again, and more words like paradice [sic] and slaves began to appear. Finally, in early December, they had the message in total. Notably, the message included “that wasn’t me on the TV show,” referencing a call-in news show that had aired just days before the Chronicle received the cipher in 1969. [read more]

Nice. Too bad the Zodiac Killer was never found.