1. Background on Unsolved 340-cipher Ryan Garlick
The Zodiac Killer
Background on
Unsolved 340-cipher
Ryan Garlick

2. 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

3. 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

4. July 31, 1969
Zodiac mails very similar handwritten letters to 3 newspapers in northern California…

6. July 31, 1969 Each newspaper also received a different cipher
Vallejo Times-Herald received

7. Solved by… CIA? NSA? The California Department of Justice? FBI?
Office of Naval Intelligence?

8. Solved by
High school history teacher Donald Harden and his wife Betty at their breakfast table.

9. Cipher Details The Cipher was a homophonic substitution cipher
Δ = ‘S’, etc.
Homophones – more than one symbol represents ‘S’.
Throws off statistical analysis

10. 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

11. Oct. 11, 1969
Cab driver Paul Stine killed in San Francisco

12. 2 Days later
Another letter

13. Nov. 8, 1969

14. The “Dripping pen” card included this 340 character cipher

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

16. Naïve Brute Force 26^63 possible keys
Then all possible word combinations
x = min number of words + 1, y = max + 1

17. 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

18. 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

19. Still a lot of work Distribute the problem Other optimizations
Your ideas

20. Genetic Algorithm Heuristic method Directed search “Evolve” the key
Many parameters – population size, mutation rate, % of pop to discard, etc.

21. EM Algorithm Expectation Maximization
Bigram probabilities for the entire cipher

22. 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!

23. Distributed Key processing
Key server
EM client
GA client
Human Reader
Brute client

24. Sources zodiackiller.com Robert Graysmith, Zodiac
Kevin USC – EM algorithm