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The Solution of the Zodiac Killer's 340-Character Cipher

David Oranchak, Sam Blake, Jarl Van Eycke

Abstract

The case of the Zodiac Killer is one of the most widely known unsolved serial killer cases in history. The unidentified killer murdered five known victims and terrorized the state of California. He also communicated extensively with the press and law enforcement. Besides his murders, Zodiac was known for his use of ciphers. The first Zodiac cipher was solved within a week of its publication, while the second cipher was solved by the authors after 51 years, when it was discovered to be a transposition and homophonic substitution cipher with unusual qualities. In this paper, we detail the historical significance of this cipher and the numerous efforts which culminated in its solution.

The Solution of the Zodiac Killer's 340-Character Cipher

Abstract

The case of the Zodiac Killer is one of the most widely known unsolved serial killer cases in history. The unidentified killer murdered five known victims and terrorized the state of California. He also communicated extensively with the press and law enforcement. Besides his murders, Zodiac was known for his use of ciphers. The first Zodiac cipher was solved within a week of its publication, while the second cipher was solved by the authors after 51 years, when it was discovered to be a transposition and homophonic substitution cipher with unusual qualities. In this paper, we detail the historical significance of this cipher and the numerous efforts which culminated in its solution.
Paper Structure (64 sections, 1 equation, 57 figures, 1 algorithm)

This paper contains 64 sections, 1 equation, 57 figures, 1 algorithm.

Table of Contents

  1. Background
  2. Homophonic Substitution Ciphers
  3. Transposition Ciphers
  4. The Zodiac Killer's First Cipher: Z408
  5. The Zodiac Killer's Second Cipher: Z340
  6. The Zodiac Killer's remaining ciphers
  7. Historical efforts to break Z340
  8. Initial months (1969--1970):
  9. Donald C. B. Marsh:
  10. FBI's cryptanalysis:
  11. Attempts from other agencies:
  12. American Cryptogram Association (ACA), 1970:
  13. Robert Graysmith (1986):
  14. American Cryptogram Association (ACA), 1988--1989:
  15. Gareth Penn, 1981--1985:
  16. ...and 49 more sections

Figures (57)

  • Figure 1: The Zodiac Killer's 340-character cipher voigt3402007
  • Figure 2: The Zodiac Killer's 408-character cipher voigt408
  • Figure 3: The Hardens' handwritten solution of Z408. (graysmith2007zodiac pp. 224--225)
  • Figure 4: Z408 substitution key. Note: There are no substitutions for the letters J, Q, and Z since none appear in the plaintext.
  • Figure 5: Filler conjecture. Symbols along the last line are possibly copied from above. cisco2009
  • ...and 52 more figures