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In the late 1960s, a serial killer identifying himself as “the Zodiac” killed at least five people in northern California and claimed to have murdered more. In November 1969, the Zodiac Killer sent a card to Chronicle of San Francisco containing a 340-character secret message that for over 50 years has gone unsolved by sleuths, crypto experts, amateur sleuths and other curious people.

Don’t wonder anymore, true crime aficionados.

After months of coding during the pandemic, three researchers on three different continents announced that they had finally decoded the message. Further reinforcing the claim, FBI experts verified the solution (and even tweeted about it). The encrypted message did not reveal the identity of the Zodiac, but it ended decades of speculation, conspiracy theories and guesses.

Crack the code

“It took a lot of computational effort and it was a real source of frustration for a lot of people,” says computer programmer David Oranchak in Roanoke, Va., Who has a background in cryptography and coordinated the effort. He’s spent years presenting misguided detectives’ theories about the meaning of the 340-character code and the identity of its author. “So many people conjure up coincidences out of thin air, and the more coincidences they generate, the stronger their evidence.”

“That figure has always had such a target on its back,” says Sam Blake, an applied mathematician at the University of Melbourne who worked with Oranchak.

Although the decryptors involved have each worked on encryption for years, the successful joint effort began in 2018 when Oranchak gave a keynote address on encryption at the American Cryptogram Association’s annual meeting in Asheville, North Carolina. . He posted the talk on YouTube where, as expected, it elicited hundreds of comments, many of which came from people who (falsely) claimed they had already resolved it.

But one person stood out: Blake. He responded to Oranchak’s speech with mathematical ideas on how to approach a code that includes both homophonic substitution – in which a letter can be exchanged for more than one symbol – and transposition – in which the letters are rearranged in a systematic way. Oranchak and Blake started to correspond and eventually generated hundreds of thousands of possible ways to read the code.

Phrase churn

To make sense of these, Oranchak enlisted the help of Jarl van Eycke, a Belgian warehouse. worker and decryptor who wrote AZdecrypt, software used to decode homophonic substitutions. Van Eycke used an updated version of his software to explore the possibilities. “Jarl just smashed it out of the park,” said Blake. At the end of November and beginning of December 2020, the trio restated their previous results, this time looking for words and phrases characteristic of other Zodiac writings.

And they found gold. Van Eycke’s software spat two sentences: trying to catch me and gas chamber. They corrected these sentences in the software, rerun it and other words like paradise [sic] and slaves began to appear. Finally, at the beginning of December, they had the message in total. Notably, the message included “it wasn’t me on the TV show,” referring to a telephone news program that aired just days before the show. the Chronicle received the figure in 1969.

Oranchak sent his solution to FBI contacts, and by the end of 2020 the FBI had verified the methodology and results. In March 2021, Blake wrote about how he had used Mathematica, a software for mathematics, for his part, and in January, van Eycke made headlines again when he deciphered an unresolved code old from 386 years old composed by a Dutch scientist.

Deciphering the 340-character cipher was so math-heavy, Oranchak says, that no one in 1969 probably could have decoded the Zodiac’s message – suggesting the killer didn’t know how difficult he had created a code. Additionally, he notes that today’s codes, like those that secure smartphone apps or transmit information online, would never fall into as brutal an effort of force as that used by Oranchak and his collaborators.

“They are just not receptive to this kind of attack,” he says. “The zodiac number was almost certainly constructed in pencil and paper, but it was complex enough to survive attacks for 51 years.”

Still not resolved

The Zodiac sent a total of four coded messages to the newspaper in 1969 and 1970. The first was 408 characters long and was deciphered within a week. The recently resolved 340-character cipher was the second. After these, the killer sent two very short digits, one with 13 characters and the other with only 32. In January 2021, an engineer in France claimed to have solved these, but Blake is dubious. “They are both too short to have a single solution,” he says.

The identity of the Zodiac Killer remains unknown.