Reference
The Zodiac Killer Ciphers
The Zodiac Killer sent a series of encrypted messages to newspapers in the late 1960s and early 1970s. The most famous, a 408-symbol cipher, was a substitution cipher solved within days by a schoolteacher and his wife. A later 340-symbol cipher resisted solution until 2020.
Written and maintained by the CipherPad team at FusionStudios. Last updated 14 July 2026.
How the ciphers worked
The 408-symbol cipher was a homophonic substitution cipher: each letter could be represented by several different symbols, which flattens letter frequencies and makes the message harder to crack than a plain substitution.
Even so, the codebreakers guessed that a vain writer might begin with the word I, and used likely words to break in. The later 340-symbol cipher added a transposition step, reading the symbols in a zig-zag order, which is why it held out for half a century.
What you can learn from it
The Zodiac ciphers are a vivid lesson in how substitution and transposition combine, and how human habits (predictable openings, favourite words) give cryptanalysts a way in.
You can explore the same building blocks with our classic cipher tools. Try a Caesar or Atbash substitution, then a keyword-based Vigenere, to feel how each layer adds difficulty.
Frequently asked questions
Was the Zodiac Killer cipher ever solved?
Yes. The 408-symbol cipher was solved within days in 1969. The harder 340-symbol cipher was finally cracked by a team of codebreakers in 2020.
What kind of cipher did the Zodiac Killer use?
A homophonic substitution cipher, where each letter maps to several symbols. The 340 cipher also used a transposition, reading the symbols in a zig-zag pattern.
Sources
What the factual claims on this page rest on. Entries marked as not independently checked name the standard rather than linking to it, because a citation that looks authoritative and turns out to be wrong is worse than an honest gap.
- The 408-symbol cipher (Z408), sent to the San Francisco Chronicle, San Francisco Examiner and Vallejo Times-Herald in three parts, July 1969. Supports: That the first cipher was 408 symbols and was sent to newspapers in three parts.
- Donald Harden, a schoolteacher, and Bettye Harden solved Z408 in August 1969, within about a week of publication. Supports: Who solved the 408 cipher and when.
- The 340-symbol cipher (Z340), sent to the San Francisco Chronicle in November 1969, remained unsolved for 51 years. Supports: That the second cipher was 340 symbols and resisted solution for decades.
- Z340 was solved in December 2020 by David Oranchak, Sam Blake and Jarl Van Eycke, whose solution was confirmed by the FBI. Supports: Who solved the 340 cipher, when, and that the solution was independently confirmed.
- Both ciphers use homophonic substitution: several different symbols stand for the same plaintext letter, which flattens the letter frequencies that break a simple substitution cipher. Supports: The cipher type, and why the 340 was so much harder than the 408.