The Zodiac's encrypted messages

At the turn of the 1970s, a serial killer was at large in California. What set him apart was that he communicated with the police through seventeen letters, four of them encrypted. The first was deciphered in 1969; the second has only just been cracked. These cryptograms put a new spin on classic cryptographic methods.

The serial killer who called himself Zodiac terrorized California (United States) in the late 1960s and early 1970s, keeping the police on tenterhooks for many years. He is believed to have killed at least five people, though he himself claimed thirty-seven victims.

The four cryptograms he sent through the media are known as Z408, Z340, Z32 and Z13, after their respective numbers of characters. In theory, the longest, Z408, should have been the easiest to decipher—and indeed it was. It was sent in three sections, each eight lines by seventeen columns.

The first third of Z408, the first encrypted message sent by Zodiac.

A quick examination shows that this is not a monoalphabetic substitution, in which each letter of the alphabet is always replaced by the same symbol: it contains around fifty different symbols, far more than 26. The simplest hypothesis is that frequent letters such as "e" were encrypted in several different ways. In other words, this is probably a homophonic cipher, like the one used by Mary, Queen of Scots, and other Renaissance sovereigns.