In December 2020, the FBI's San Francisco field office quietly confirmed something that half a century of professional codebreakers had failed to deliver: the Zodiac killer's 340-character cipher had finally been read. The solvers were not agency cryptanalysts. They were a software developer in Virginia, a mathematician in Melbourne and a warehouse worker in Belgium, collaborating over the internet, and the message they extracted had been sitting in plain sight since November 1969 — fifty-one years of taunting silence, ended by three amateurs and a piece of home-made software.
The Zodiac murdered at least five people in Northern California in 1968 and 1969 — two teenagers near Vallejo, a second couple attacked seven months later, a couple stabbed at Lake Berryessa and a taxi driver shot in San Francisco — though in his letters he claimed a running tally that eventually reached thirty-seven. What made him a media obsession was the correspondence. He wrote to Bay Area newspapers demanding front-page publication, signed with a crossed-circle symbol, and enclosed ciphers. The first, a 408-character message sent in three parts to three newspapers in the summer of 1969, was cracked within about a week — not by the FBI or naval intelligence, both of which tried, but by Donald Harden, a schoolteacher from Salinas, working at his kitchen table with his wife Bettye. It was a homophonic substitution cipher, and its contents were chilling but useless: the killer liked killing "because it is so much fun", and he was collecting slaves for the afterlife. No name, despite his promise that the cipher contained his identity.
Fifty-one years of failure
The follow-up, posted to the San Francisco Chronicle on 8 November 1969 and known as Z340, was a different beast. It looked like the first cipher but resisted every substitution attack thrown at it, and over the decades it became one of the most famous unsolved cryptograms in the world, attracting generations of hobbyists and a thick sediment of false solutions.
The team that finally broke it had been grinding at the problem for years. David Oranchak, an American software developer, had run a website devoted to the Zodiac ciphers since the 2000s; Sam Blake, an Australian applied mathematician, generated hundreds of thousands of possible reading schemes; and Jarl Van Eycke, a Belgian warehouse worker and self-taught codebreaker, had written AZdecrypt, widely regarded as the best homophonic-cipher solver in existence. The breakthrough insight was that Z340 was not one puzzle but two stacked on top of each other: a transposition scheme concealed beneath the substitution. Read the grid diagonally — down one, across two — in three separate sections, and the substitution finally yields. The killer had also made small errors in his own scheme, which had helped keep it unbreakable. The team submitted the solution in December 2020, and the FBI confirmed it.
What it said — and didn't
The great anticlimax is the message itself. There is no name, no address, no verifiable detail. "I hope you are having lots of fun in trying to catch me," it begins, before denying that a man who had recently phoned a live television programme claiming to be the Zodiac was really him — a reference that dates the cipher's composition to the weeks after an infamous October 1969 call-in to a Bay Area morning show. He adds that he is not afraid of the gas chamber "because it will send me to paradice all the sooner" — the misspellings are his — and reprises the theme that his victims will become his slaves in the afterlife. As a confession it reveals psychology, not identity.
Two shorter ciphers remain unsolved: the thirteen-character message of 1970 that follows the words "My name is —", and a thirty-two-character cipher supposedly pointing to a bomb location. Cryptographers largely agree they may never be solved, not because they are fiendish but because they are too short — texts that brief admit many equally plausible readings, so no solution can prove itself. The case, meanwhile, remains open. The best-known suspect, Arthur Leigh Allen, died in 1992 without ever being charged, and partial DNA and fingerprint evidence has failed to tie him to the crimes; a much-publicised 2021 claim by a group of volunteer sleuths to have named the killer was dismissed by the FBI within days. The Zodiac promised his ciphers would unmask him. Fifty years of brilliance proved only that he lied — the codes could be beaten, but the man behind them still hasn't been.
Quiz nuggets
- The Zodiac's 340-character cipher, sent to the San Francisco Chronicle in November 1969, was solved in December 2020 — 51 years later.
- The 2020 solvers were amateurs: American David Oranchak, Australian Sam Blake and Belgian Jarl Van Eycke, whose AZdecrypt software cracked it.
- The killer's first cipher, 408 characters sent in three parts in 1969, was solved in about a week by schoolteacher Donald Harden and his wife Bettye.
- The Zodiac claimed 37 victims in his letters, but police confirmed five murders in 1968–69.
- Two Zodiac ciphers remain unsolved, including a 13-character message following the words "My name is".