From Zodiac Ciphers

Zodiac, the serial killer who terrorized Northern California in the late 1960s, sent four ciphers to local newspapers. The first cipher was separated into three different parts and each part was sent to Vallejo Times-Herald, the San Francisco Chronicle, and the San Francisco Examiner. The combination of all three parts formed a 408-letter cipher, which was decrypted one week after it was received. Zodiac also sent a 340-letter cipher, mailed on a card on November 8 1969, that remains unsolved to this day.

Articles and information about the 340 cipher

High-resolution scan of the 340-character cipher

Cipher as it appeared in the SF Chronicle on Nov 13, 1969 (Credit: Deborah Silva, who posts as Seagull on the Zodiac forums)

Homophone sequences

The "Pivots"

Transpositions and other possible variations

Is the 340 cipher unsolved because it is not a simple homophonic substitution cipher like the 408 cipher? I began an attempt to rule out specific encipherment schemes here:

Hypothesis Testing

Here is an older collection of considered possibilities:

Halloween Card