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					The Zodiac Killer
   Background on
 Unsolved 340-cipher

    Ryan Garlick
              Background
• The Zodiac was a serial killer active in
  California during the late 1960‟s and early
  1970‟s.
• He definitely killed 5 people and claimed
  37 victims.
• He taunted the police in over 20 letters
  and ciphers
                  July 4, 1969



•   Mike Mageau and Darlene Ferrin
•   Parked at a Lover‟s Lane
•   Darlene was killed and Mike seriously wounded
•   Gunman approached and shot without saying a
    word
             July 31, 1969
• Zodiac mails very similar handwritten
  letters to 3 newspapers in northern
  California…
            July 31, 1969
• Each newspaper also received a different
  cipher
• Vallejo Times-Herald received
               Solved by…
•   CIA?
•   NSA?
•   The California Department of Justice?
•   FBI?
•   Office of Naval Intelligence?
               Solved by
• High school history teacher Donald
  Harden and his wife Betty at their
  breakfast table.
              Cipher Details
• The Cipher was a homophonic substitution
  cipher
• Δ = „S‟, etc.
• Homophones – more than one symbol
  represents „S‟.
  – Throws off statistical analysis
I LIKE KILLING PEOPLE
BECAUSE IT IS SO MUCH FUN
IT IS MORE FUN THAN KILLING
WILD GAME IN THE FORREST
BECAUSE MAN IS THE MOST
DANGEROUE ANIMAL OF ALL TO
KILL SOMETHING GIVES ME THE
MOST THRILLING EXPERENCE
IT IS EVEN BETTER THAN
GETTING YOUR ROCKS OFF
WITH A GIRL
THE BEST PART IS THAE WHEN I
DIE I WILL BE REBORN IN
PARADICE AND ALL THE I HAVE
KILLED WILL BECOME MY
SLAVES
I WILL NOT GIVE YOU MY NAME
BECAUSE YOU WILL TRY TO
SLOI DOWN OR STOP MY
COLLECTING OF SLAVES FOR
MY AFTERLIFE
EBEORIETEMETHHPITI
             Oct. 11, 1969
• Cab driver Paul Stine killed in San
  Francisco
• 2 Days later
• Another letter
Nov. 8, 1969
The “Dripping pen” card included this
340 character cipher
                Assumptions
•   Another substitution cipher?
•   Left to right?
•   Meaningless?
•   Polyalphabetic?
    – Symbols mean different things for each
      occurrence in the cipher
• Book cipher?
          Naïve Brute Force
• 26^63 possible keys

• Then all possible word combinations

              y
                  339 
             n 
                 
            n x 
                       
                       
• x = min number of words + 1, y = max + 1
      We can do much better
• Not all symbols represent „A‟
  – Max number of mappings per symbol


• Don‟t need to generate every possible
  word
  – Scan for digrams, trigrams
  – Total word count
  – Use his previous vocabulary – 22 letters
                 Scoring
• What is a „good‟ solution?
• Word count
• Bigrams and trigrams
• Frequency distribution – Zodiac may have
  thrown this off with junk at the end like on
  408 (solved) cipher
• Combination
           Still a lot of work
• Distribute the problem

• Other optimizations
  – Your ideas
          Genetic Algorithm
• Heuristic method
• Directed search
• “Evolve” the key




• Many parameters – population size,
  mutation rate, % of pop to discard, etc.
             EM Algorithm
• Expectation Maximization
• Bigram probabilities for the entire cipher
              Brute Force
• Intractable
• Many computers running keys
• The “testbed” for our key scoring
• Keeping good keys, return them to a key
  server
• We could get (very) lucky - stranger things
  have happened!
Distributed Key processing
              Sources
• zodiackiller.com
• Robert Graysmith, Zodiac
• Kevin Knight @ USC – EM algorithm

				
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posted:6/14/2011
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