Three Amateur Cryptographers Finally Decrypted the Zodiac Killer’s Letters: A Look Inside How They Solved a Half Century-Old Mystery

https://www.youtube.com/watch?v=-1oQLPRE21o

If we envision serial killers as figures who taunt law enforcement with cryptic messages sent to the media, we do so in large part because of the Zodiac Killer, who terrorized northern California in the late 1960s and early 70s. Though he seems to have stopped killing more than half a century ago, he remains an object of great fascination (and even became the subject of David Fincher’s acclaimed film Zodiac in 2007). As thoroughly as the case has been investigated, much remains unknown — not least what he actually said in some of his coded letters. But just this month, a team of three cryptography enthusiasts managed to break one of the Zodiac’s ciphers, finally revealing the contents of a 51-year old letter.

The Zodiac wrote this particular communiqué in a transposition cipher, which, as Ars Technica’s Dan Goodin writes, uses “rules to rearrange the characters or groups of characters in the message.” In the case of the 340, named for the number of symbols, the content “was probably rearranged by manipulating triangular sections cut from messages written into rectangles.” For the past half-century, nobody could successfully return the text to its original arrangement, but in 2020, there’s an app for that. Or rather, a software engineer named David Oranchak, a mathematician named Sam Blake, and a programmer named Jarl Van Eycke made an app for that. Goodin quotes Oranchak as saying the three had been “working on and off on solving the 340 since 2006.”

You can see Oranchak explain how he and his collaborators finally cracked the 340’s cipher in the video at the top of the post, the final episode of his five-part series Let’s Crack the Zodiac. This wasn’t a matter of simply whipping up the right piece of artificial intelligence and letting it rip: they had to generate hundreds of thousands of permutations of the message as well as attempts at decryptions of those messages. And even when recognizable words and phrases began to emerge in the results — “TRYING TO CATCH ME,” “THE GAS CHAMBER” — quite a bit of trial, error, and thought, remained to be done. It helped that Oranchak knew his Zodiac history, such as that someone claiming to be the killer mentioned not wanting to be sent to the gas chamber when he called in to a local television show on October 20, 1969, two weeks before the 340 was received.

Was it really him? The 340, when finally decoded — a process complicated by the mistakes the Zodiac made, not just in spelling but in executing his laborious, fully analog encryption process — seems to provide the answer:

I HOPE YOU ARE HAVING LOTS OF FUN IN TRYING TO CATCH ME
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

“The message doesn’t really say a whole lot,” admits Oranchak. “It’s more of the same attention-seeking junk from Zodiac. We were disappointed that he didn’t put any personally identifying information in the message, but we didn’t expect him to.” The Zodiac Killer remains unidentified, and indeed remains one of recent history’s more compelling villains, not just to those with an interest in true crime, but to those with an interest in cryptography as well. For two more messages still remain to be decoded, and in one of them he offers a short cipher that, he writes, contains his name — but then, if there’s any correspondent we shouldn’t rush to take at his word, it’s this one.

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Based in Seoul, Colin Marshall writes and broadcasts on cities, language, and culture. His projects include the Substack newsletter Books on Cities, the book The Stateless City: a Walk through 21st-Century Los Angeles and the video series The City in Cinema. Follow him on Twitter at @colinmarshall, on Facebook, or on Instagram.

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