The Zodiac Killer

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The Zodiac Killer

• Background on
• Unsolved 340-cipher
• Ryan Garlick

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Background

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

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July 4, 1969

• Mike Mageau and Darlene Ferrin
• Parked at a Lovers Lane
• Darlene was killed and Mike seriously wounded
• Gunman approached and shot without saying a word

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July 31, 1969

• Zodiac mails very similar handwritten letters to
  3 newspapers in northern California

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(No Transcript)
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July 31, 1969

• Each newspaper also received a different cipher
• Vallejo Times-Herald received

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Solved by

• CIA?
• NSA?
• The California Department of Justice?
• FBI?
• Office of Naval Intelligence?

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Solved by

• High school history teacher Donald Harden and his
  wife Betty at their breakfast table.

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Cipher Details

• The Cipher was a homophonic substitution cipher
• ? S, etc.
• Homophones more than one symbol represents S.
• Throws off statistical analysis

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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
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Oct. 11, 1969

• Cab driver Paul Stine killed in San Francisco

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• 2 Days later
• Another letter

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Nov. 8, 1969
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The Dripping pen card included this 340
character cipher
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Assumptions

• Another substitution cipher?
• Left to right?
• Meaningless?
• Polyalphabetic?
• Symbols mean different things for each occurrence
  in the cipher
• Book cipher?

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Naïve Brute Force

• 2663 possible keys
• Then all possible word combinations
• x min number of words 1, y max 1

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We can do much better

• Not all symbols represent A
• Max number of mappings per symbol
• Dont need to generate every possible word
• Scan for digrams, trigrams
• Total word count
• Use his previous vocabulary 22 letters

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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

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Still a lot of work

• Distribute the problem
• Other optimizations
• Your ideas

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Genetic Algorithm

• Heuristic method
• Directed search
• Evolve the key
• Many parameters population size, mutation rate,
  of pop to discard, etc.

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EM Algorithm

• Expectation Maximization
• Bigram probabilities for the entire cipher

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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!

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Distributed Key processing
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Sources

• zodiackiller.com
• Robert Graysmith, Zodiac
• Kevin Knight _at_ USC EM algorithm