For 51 years, three lines of cryptic symbols sat in FBI files, defeating the agency's own cryptanalysts, the National Security Agency, the Navy, and thousands of amateur codebreakers who tried and failed in public view. Then, on December 5, 2020, a software developer in Virginia, a mathematician in Australia, and a programmer in Belgium, who had never met in person, submitted a solution to the FBI's Cryptanalysis and Racketeering Records Unit. Within days, the agency confirmed it. The message inside, when it finally came, was not a confession or a name. It was something stranger: a taunt, fully decoded, that still revealed nothing about who had written it.
Background: A Killer Who Wanted to Be Read
Between December 1968 and October 1969, an unidentified attacker killed at least five people in Northern California, targeting young couples and a lone taxi driver, while two additional victims survived their attacks. The killer, who was never caught and has never been officially identified, began writing letters to Bay Area newspapers, taking credit for the murders and embedding portions of his messages in cipher, daring police and the public to decode them. He signed his communications with a crosshairs symbol and adopted the name "Zodiac," by which the case has been known ever since.
The first of his ciphers, a 408-character message split across three newspapers in 1969, was solved within roughly a week by Donald and Bettye Harden, a married couple in Salinas, California with an amateur interest in codes. That early, rapid success set expectations that the killer's other ciphers would fall just as quickly. They did not. The second cipher, a 340-character message known as Z340, was mailed to the San Francisco Chronicle on November 8, 1969, two weeks after a caller claiming to be the Zodiac phoned into a Bay Area television talk show. Unlike its predecessor, Z340 would resist every attempt to break it for the next half-century.
Why Z340 Defeated Everyone Who Tried
Z340's resistance to decoding came down to a specific, deliberately layered design. The cipher used 63 distinct symbols to represent the 26 letters of the English alphabet, a technique called homophonic substitution, in which multiple different symbols can each stand in for the same letter, specifically to defeat the kind of statistical letter-frequency analysis that cracks simpler substitution ciphers. Layered on top of that, the cipher's solvers later determined, was a transposition element: the decoded letters did not read in a straightforward line, but had to be reassembled according to a separate, non-obvious pattern, compounding the difficulty far beyond a standard substitution puzzle.
The FBI's own cryptanalysts attempted to break Z340 and failed, as did codebreakers reportedly consulted from the National Security Agency and Navy cryptography units, though full records of those efforts remain classified. The American Cryptogram Association made repeated attempts over the decades, and an active community of amateur sleuths, organized partly through dedicated websites tracking the case since the late 1990s, produced what the eventual solvers described as hundreds or thousands of proposed solutions over fifty years, several of which went viral and received national media attention before being debunked.
| Element | Detail |
|---|---|
| Cipher name | Z340 (340 characters) |
| Date sent | November 8, 1969, to the San Francisco Chronicle |
| Cipher type | Homophonic substitution combined with transposition |
| Symbol count | 63 distinct symbols representing 26 letters |
| Years unsolved | 51 years (1969–2020) |
| Solved by | David Oranchak (US), Sam Blake (Australia), Jarl Van Eycke (Belgium) |
| Confirmed by | FBI Cryptanalysis and Racketeering Records Unit, December 2020 |
How Three Strangers Finally Broke It
The successful effort was a study in distributed, software-driven cryptanalysis rather than a single flash of insight. David Oranchak, a software developer who had been working on Zodiac's ciphers since 2006, partnered with mathematician Sam Blake in Australia and programmer Jarl Van Eycke in Belgium, the latter having built a specialized homophonic-cipher solving program called AZdecrypt capable of testing roughly 200 candidate solutions per second with reported 99 percent accuracy on cleanly structured ciphers. Working remotely during 2020, Blake generated 650,000 systematic rearrangements of the cipher's symbol order, testing different hypotheses about how Zodiac might have transposed his message, while Van Eycke's software ran each variation in search of recognizable English.
On December 3, 2020, one of those hundreds of thousands of variations finally produced legible fragments. Oranchak has described the moment less as a sudden breakthrough than as a familiar false alarm that, this time, kept holding up under closer inspection, with team members hand-correcting the remaining gaps once the underlying transposition pattern became clear. Two days later, they submitted the completed solution to the FBI, which confirmed its validity publicly within days. Decoded, the message read, in the killer's own broken spelling: "I hope you are having lots of fun in trying to catch me. That wasn't 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."
Theories and Explanations
The most direct interpretation of the solved cipher is also the most anticlimactic: Z340 was not, in the end, a confession, a coordinate, or a name, but an extended taunt confirming Zodiac's awareness of media coverage and reiterating themes already present in his earlier letters, namely a stated lack of fear toward execution and a self-mythologizing belief that his victims would serve him in an afterlife. Oranchak himself noted that the most disappointing part of the breakthrough was the complete absence of any personally identifying information, meaning the cipher's solution closed a 51-year cryptographic puzzle without advancing the separate, still-open question of the killer's identity.
A second line of analysis has focused on what the cipher's construction reveals about its author rather than its content. The combination of misspellings, irregular symbol substitution, and a transposition scheme complex enough to defeat professional cryptanalysts for decades suggests someone with genuine, if self-taught, skill in cryptography, alongside writing patterns that some later commentators have speculated, without any clinical basis, might reflect dyslexia or an unconventional cognitive profile. A third, more skeptical position treats this kind of psychological profiling from cipher structure alone as inherently unreliable, noting that the same complexity could simply reflect deliberate effort by someone determined to be difficult to catch, independent of any underlying neurological explanation.
The Curious Connection
Z340's solution offers something genuinely rare in unsolved-mystery coverage: an ending that is verified, government-confirmed, and still unsatisfying. Most of the cases this series covers stay open because the evidence needed to close them was destroyed, classified, or never collected. Z340 stayed open for an entirely different reason: the evidence was sitting in public view the entire time, fully solvable with patience and computation, and it simply took until 2020 for the right combination of tools and persistence to arrive. That distinction matters for how we think about mystery itself, since not every unsolved case represents a hidden conspiracy or a permanently unknowable truth; some are simply unfinished math problems waiting for someone with the right software.
What makes the case linger, even after the cipher's defeat, is the gap between solving a puzzle and solving a crime. Breaking Z340 satisfied a half-century of cryptographic curiosity without moving the actual murder investigation forward by a single verified fact, a reminder that the parts of a mystery that fascinate the public, codes, symbols, and elaborate constructed puzzles, are not always the same parts that lead toward justice. The Zodiac's other surviving cipher, known as Z13 or the 13-character message, remains unsolved to this day, and his identity remains, more than five decades later, entirely unknown.
FAQ
What is the Z340 cipher?
Z340 is a 340-character coded message sent by the unidentified Zodiac Killer to the San Francisco Chronicle on November 8, 1969. It uses 63 distinct symbols in a homophonic substitution cipher combined with a transposition scheme, and it remained unsolved for 51 years.
Who solved the Z340 cipher and when?
Software developer David Oranchak, mathematician Sam Blake, and programmer Jarl Van Eycke solved the cipher using custom decryption software and submitted their solution to the FBI on December 5, 2020. The FBI publicly confirmed the solution within days.
What did the decoded Z340 message say?
The decoded message was a taunt directed at investigators, referencing a television show, expressing a lack of fear of execution, and describing a belief in an afterlife where the killer would have "slaves." It contained no information identifying the killer.
Did solving the cipher reveal the Zodiac Killer's identity?
No. The decoded message contained no personally identifying information, and the Zodiac Killer's identity remains officially unknown. Solving Z340 closed a cryptographic puzzle but did not advance the unsolved murder investigation.
Are any of the Zodiac Killer's ciphers still unsolved?
Yes. A separate, shorter cipher known as Z13, sent by the Zodiac in 1970, remains unsolved as of this writing, alongside the larger question of the killer's identity, which has never been officially confirmed.