  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.
