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.
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.