The Zodiac ciphers

The tool above does to your text what Z340 did to its message before any symbol was drawn. It writes the letters into rows 17 wide, in sections of up to nine rows, starting top left and then moving two squares right and one down for each letter, wrapping round the edges. Reading the result row by row gives a jumble with every letter still there. Swap the boxes and paste a grid to read it back along the same route.

It only shows the transposition. The real cipher then swapped each letter for one of several symbols, and the two steps together are what kept it unsolved.

Z408, 408 symbols, mailed on 31 July 1969 in three parts to the San Francisco Chronicle, the San Francisco Examiner and the Vallejo Times. Solved within a week.

Z340, 340 symbols in 20 rows of 17, mailed to the Chronicle on 8 November 1969. Solved on 5 December 2020, 51 years later.

Z13, 13 symbols after the words "My name is", in a letter postmarked 20 April 1970. Unsolved.

Z32, 32 symbols in a letter postmarked 26 June 1970, with a map of the Bay Area. Unsolved.

Z408 was a homophonic substitution: each letter of the message could be written with any of several symbols, so the common letters didn't stand out the way they do in a simple substitution. The FBI and the CIA worked on it. It was solved on 5 August 1969 by Donald and Bettye Harden, a couple in Salinas, neither of them a cryptologist. Bettye guessed that the word KILL would appear, and that one guess, a crib, unlocked the rest. The message did not contain the name its author had promised.

Z340 used the same kind of homophonic substitution, and the obvious attacks failed for half a century. The reason was a second layer. Before the symbols went on, the message had been shuffled: the first nine rows were written along the two-right, one-down route you can try above, the next nine the same way, and the last two rows on their own. Read in the ordinary order, the letters never formed words, however the symbols were assigned.

The clue was statistical. Pairs of symbols repeated unusually often at a spacing of 19, the distance the route covers in one step through a 17-wide grid. Sam Blake, an Australian mathematician, generated huge numbers of possible transpositions. David Oranchak, an American software engineer, ran them through AZdecrypt, a solver written by Jarl Van Eycke, a Belgian programmer, and on 5 December 2020 the first section came out as English. A skipped symbol on one line, a mistake by the cipher's author, had garbled part of the second section, and Van Eycke found and corrected it. The FBI confirmed the solution.

The team's own account of how they did it, The Solution of the Zodiac Killer's 340-Character Cipher, is on arXiv, and it's the source for the method here.

The two short ciphers are a different problem. With only 13 and 32 symbols there isn't enough text for statistics to separate a right answer from a wrong one, and hundreds of solutions have been proposed for each. Oranchak has said it's practically impossible to tell whether any of them is correct. Z32 came with the claim that it gave the location of a bomb, and no bomb was ever found.

Neither solved cipher names anyone. Z408 boasts and Z340 taunts the police, denying a call to a television programme and claiming not to fear the gas chamber. The killer of at least five people in Northern California in 1968 and 1969 has never been identified. The ciphers are remembered because they were a public challenge, and because two of them were eventually met by ordinary people working with the newspaper's printed copy.