Background on Unsolved 340-cipher Ryan Garlick 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 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 Key server EM client GA client Human Reader Brute client

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