340 Letters, 51 Years

What was the 340 cipher?

On November 8, 1969, an envelope reached the San Francisco Chronicle with a greeting card inside and a single sheet of symbols folded behind it. The sender had already murdered five people in the Bay Area and had already made the press decode him once. The new sheet held 340 characters arranged in a neat block, 17 across and 20 down, drawn from an alphabet of 63 distinct symbols: crossed circles, half-filled squares, letters flipped on their backs. The count mattered. Twenty-six letters written with sixty-three symbols means several symbols stand in for the same letter, a homophonic substitution built to flatten the letter frequencies that crack a simple cipher. The Chronicle printed it. The FBI opened a file on it. And Z340, as it came to be catalogued, began its half-century of silence.

Why did it hold for 51 years?

Because the first cipher had fallen so fast. Z408, mailed in three parts that July, was broken within days by Bettye and Donald Harden. It was Bettye who read the killer’s craving for attention and called both cribs: the message would open with I, and KILL or KILLING or even I LIKE KILLING would sit somewhere inside it. Donald, a high school history and economics teacher in North Salinas, worked the substitution out from her cribs, about twenty hours at their kitchen table. So when Z340 arrived three months later, every solver on earth attacked it the same way: as a better version of the same lock. Frequency tables were compiled, symbol maps guessed, decades of candidate solutions produced and discarded. They all failed for the same invisible reason. In Z340 the letters had been moved before they were swapped. A substitution attack assumes the letters sit in reading order, and these did not, so every attack was correct in method and aimed at the wrong text. The world spent 51 years solving a puzzle the cipher was not.

How it was finally broken

The team that ended it never met in a room. David Oranchak, a software developer in Virginia, had been working the 340 for over a decade. Sam Blake, an applied mathematician in Melbourne, proposed the idea that unlocked it: if the letters had been rearranged, generate the rearrangements. Reading the grid diagonally, splitting it into segments, varying every parameter, they produced roughly 650,000 candidate unscramblings. Each one went through AZdecrypt, a homophonic-substitution solver written by the third man, Jarl Van Eycke, a Belgian codebreaker. In early December 2020 one candidate came back with phrases standing in the noise: HOPE YOU ARE, TRYING TO CATCH ME. Pulling on those threads exposed the true scheme, a diagonal transposition applied to the grid in three sections. Within days the full text was out and the FBI’s cryptanalysis unit confirmed it. Fifty-one years, ended by three amateurs, on their own time, with software they wrote themselves. Their own account of the break runs to a full paper.

What the message said

Almost nothing. No name, no address, no next victim. The message mocks a television call-in show that had staged a phone call with someone claiming to be the killer: that was not him, he says. He wrote that he was not afraid of the gas chamber. He spelled paradise with a c. He wrote 340 symbols to deny a phone call. The cipher that consumed thousands of solver-hours across five decades protected a taunt that could have fit on a postcard, and that is the cold joke at the center of the story: the strength of a cipher tells you nothing about the worth of what it hides.

What it teaches a solver

The lesson is the order of operations. When a substitution refuses to fall, when the frequencies look right but no key produces words, stop asking which symbol is which letter and start asking whether the letters are where they should be. Movement before substitution is exactly the trick the manual’s transposition tier trains: the rail fence moves letters along a zig-zag, and columnar transposition moves them by keyword, and in both the letters keep their identity while their order lies to you. The Zodiac stacked the two ideas, movement under substitution, and bought 51 years with it. It is the same instinct that broke Mary, Queen of Scots’ nomenclator four centuries earlier: the solver who wins is the one who stops fighting the cipher as presented and asks what the maker actually did. There is a case on the desk every day to practice that instinct on.