Sleuths who cracked Zodiac Killer's cipher thank the crowd

Fifty-one years of community contributions, software, and clever cryptanalysis contributed

Three men received recognition in December 2020 for cracking the Zodiac Killer's 340-character cipher (Z340) – but they want to share credit with the community of sleuths who helped with the 51-year code breaking effort.

"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," wrote David Oranchak, a US-based software developer and cryptologist, Sam Blake, a mathematician based in Australia, and Jarl Van Eycke, a cipher expert based in Belgium, in a paper released last week that documents the history of the Z340 solution.

The Zodiac Killer is known to have murdered at least five people in the San Francisco Bay Area between December 1968 and October 1969 – although the perpetrator claimed to have killed 37. A suspect has still not been officially identified.

After attacking two couples separately in December 1968 and July 1969, resulting in the deaths of three of the four victims, the serial killer mailed his first cipher, Z408 – named for the number of characters in the ciphertext – in three parts to three local newspapers: the Vallejo Times-Herald, the San Francisco Examiner, and the San Francisco Chronicle.

Eight days later, it was solved by Donald and Bettye Harden – a couple living in Salinas, California, with an interest in ciphers. The code was, according to Oranchak, Blake, and Van Eycke, a homophonic substitution cipher, alternately described as a monoalphabetic substitution cipher with variants. This means the characters in the original message could be replaced with several possible cipher characters, in order to resist analysis based on the statistical frequency of characters in English words.

The Zodiac Killer sent his second cipher – Z340 – to The San Francisco Chronicle on November 8, 1969. It remained unsolved until Saturday, December 5, 2020, when Oranchak, Blake, and Van Eycke presented their solution to the FBI and had their work confirmed.

Z340's cipher alphabet consists of 63 different symbols – enough that several could represent the same English letter. That complexity, along with misspellings and transpositions in the actual message, made breaking the code rather complicated.

Refined homophonic substitution key to decipher entire Z340 ciphertext

"Refined homophonic substitution key to decipher entire Z340 ciphertext" – Click to enlarge

The trio's solution – made possible in part by software created by Van Eycke called AZdecrypt – was the culmination of "hundreds, if not thousands," of purported solutions.

"Shortly after Zodiac mailed [Z340], many people raced to decipher it, and some asserted their decryptions were correct," the authors state in their paper. "But none were definitively endorsed by law enforcement, so these efforts continued for decades."

Oranchak, Blake, and Van Eycke recount the history of these attempts in their paper. They note that the FBI tried and failed to break the code. And they cite reports that the NSA and Navy cryptographers also gave it a shot – with the caveat that lack of confirmation about these claims may be because the records are classified. Other attempts – notably by the American Cryptogram Association, Chronicle cartoonist Robert Graysmith, and author Gareth Penn (pen name George Oakes) – also contributed.

Around 2012, Oranchak became involved, followed by Van Eycke, and Blake.

"The popularity of the case and noteworthy nature of Z340 led many amateur codebreakers to join the efforts to solve the cipher," the trio observe. "Many so-called solutions were developed that 'went viral' and received significant attention in the news media."

Various websites – starting with the Tom Voigt's in 1998 – helped gather information about the killings over the years.The authors note that, while the online community that coalesced around the Zodiac case had varying levels of experience and analytical ability, the various viewpoints represented were ultimately helpful.

"Taken as a whole, the combined efforts of the entire group of enthusiastic investigators led to crowdsourced knowledge about Z340, providing many useful details that made the discovery of its solution possible, as long as researchers could navigate around numerous unusable ideas and information," write Oranchak, Blake, and Van Eycke.

Part of what made the trio's breakthrough possible was an update to AZdecrypt that helped significantly. "A new feature that turned out to be instrumental was the software's ability to add whitespace characters automatically to candidate plaintexts," the authors explain.

"Both Z408 and Z340 lack word divisions, so decrypted plaintexts also lack them. But AZdecrypt's new feature could automatically insert them where they likely belonged, based on language n-gram statistics. This made partial solutions much more comprehensible."

Oranchak, Blake, and Van Eycke conclude by dedicating their work to the victims of the Zodiac Killer, their families, and descendants: "We hope that one day justice will prevail." ®

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