The Zodiac Ciphers

Summary

A killer sent four ciphers to Californian newspapers. The first was broken in days, by a schoolteacher and his wife, at their kitchen table. The second resisted for fifty-one years and was finally broken in December 2020 by three men in three countries who had never met.

Two remain unsolved. And the most interesting fact in the case is that the hardest cipher may have been hard by accident.

What is documented

Z408. In July 1969 the Zodiac sent a cipher in three parts to three San Francisco newspapers, with a threat to kill more people if it were not printed. It was printed. It is a homophonic substitution cipher: several symbols may stand for the same letter, which flattens the frequency patterns a cryptanalyst would otherwise exploit.

It was solved within days. Donald Harden, a high school history and economics teacher, and his wife Bettye worked on it for roughly twenty hours and read it. The message is a statement of the killer's motives. It contains no name.

Z340. Sent to the San Francisco Chronicle on 8 November 1969 and published on 12 November. Three hundred and forty characters, drawn from sixty-three distinct symbols. It went to the FBI immediately and it defeated everyone, professional and amateur, for fifty-one years.

How Z340 was broken. In 2020 three people who had never met in person solved it: David Oranchak, a software developer in Virginia; Sam Blake, a mathematician in Melbourne; and Jarl Van Eycke, a programmer in Flanders who had written a cipher solver called AZdecrypt.

The insight was that Z340 was not one kind of cipher but two. It is a homophonic substitution and a transposition: the symbols were scrambled into a different order before being substituted. AZdecrypt could break homophonic substitutions of that length reliably. It could not break Z340, and that failure was the clue.

Blake generated roughly 650,000 rearrangements of the ciphertext, applying different transposition schemes. Oranchak fed them in batches through Van Eycke's solver. On 3 December 2020, two phrases surfaced out of the noise that were not noise: a taunt about being caught, and a reference to the gas chamber.

The gas chamber was the confirmation. Two weeks before the cipher was posted, a caller claiming to be the Zodiac had phoned into a live television programme in San Francisco and said he was sick and did not want to go to the gas chamber. The cipher's message opens by denying that the caller was him. The text is anchored to a specific fortnight in October and November 1969, and it could not have been fabricated by anyone afterward.

They sent the solution to the FBI on 5 December 2020. The Bureau independently verified it and amended its file.

What it says, and why that matters. It is a taunt. It says he hopes they are enjoying trying to catch him, denies the television call, and states that he is not afraid of execution. There is no name. Oranchak's own summary was that the message does not say very much, and is more of the same attention-seeking material.

Z13 and Z32. Sent in April and June 1970. Both are very short. Z13 followed the words "My name is". They remain unsolved, and their brevity is the problem: thirteen and thirty-two characters carry almost no statistical structure to attack, and multiple readings will always be consistent with them.

The killer. Five murders are attributed to the Zodiac, in the Bay Area between December 1968 and October 1969. He claimed thirty-seven. He was never identified. The case remains open with the FBI's San Francisco division.

Leading explanations

Z340 was probably harder than intended. This is the most interesting finding in the case, and it comes from the people who broke it.

Z408 was solved in days by two amateurs. The Zodiac would have known that, and would have wanted the next one to hold. Oranchak has argued that the computational effort required to break Z340 simply did not exist in 1969, and that some of the cipher's difficulty appears to be an unintended by-product of his encipherment process rather than a designed feature. There are irregularities in the text consistent with a man making mistakes while scrambling by hand.

He wanted a harder cipher. He appears to have built an accidentally unbreakable one, and then to have gone on writing to the newspapers as though it would be read.

Z13 will probably not identify him. It is thirteen characters and follows the words "My name is". Because it is so short, a very large number of plausible names can be fitted to it, and nothing in the cipher itself can distinguish between them. A solution that cannot be verified is not a solution. Researchers on the case, including Oranchak, have consistently said so.

Current status

Partially explained. Two of four ciphers are solved, one of them within days and one after half a century. Two remain, and the shorter is probably unsolvable in any meaningful sense.

The killer is unidentified.

The registry keeps this record for the shape of the thing. A man taunted the police with a puzzle for fifty-one years, and when it was finally opened it was empty. The cipher that everyone believed held the answer turned out to be a man saying, at length, that he had not been on television.

Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.