Cryptography and Murder –The Zodiac Killer

The Case

Serial killer active in the San Francisco area 1967 – 1974 (at least) Linked to 7 murders, but claimed many more Never caught Police investigation closed in 2004 Lots of theories and speculations around the

case

This is the Zodiac speaking...

Wrote many letters (at least 18) to the police and to newspapers in the SF area

Ciphers

Four letters contained ciphers

Appear to be substitution ciphers, but with more cipher text symbols than ordinary letters

Two of them contain too little cipher text to be solved

408 Cipher

First cipher the Zodiac sent

Was divided in three parts and sent to three

different newspapers All three parts published

on the front pages Solved in a week by a

high school teacher and his wife

How Did They Do It?

Knew the killer had a big ego, assumed he would start with ’I’

Assumed the word ’kill’ would appear several times

This was 1969 – only pencil and paper!

Donald Harden

http://www.zodiackiller.com/donaldharden.mp3

Part 1 Character Cipher

Cipher Key

Cipher Solution

Caesar Ciphers

Method used by Romans for hiding messages; called “Caesar Ciphers”

An easy way to hide messages is to shift each letter of the alphabet forward or backward a number of spaces.

Let’s shift the alphabet backward by three and code the following message:FORENSIC SCIENCE USES

CRYPTOGRAPHY TO SOLVE CRIMES.

FORENSIC SCIENCE USES CRYPTOGRAPHY TO SOLVE CRIMES.

Plain

Cipher

A B C D E F G H I J K L M

D E F G H I J K L M N O P

N O P Q R S T U V W X Y Z

Q R S T U V W X Y Z A B C

Solution:

IRUHQVLF VFLHQFH XVHV FUBSWRJUDSKB WR VROYH FULPHV.

Caesar Ciphers

Answer 3: Do number four! Answer 4:

ZKBD YP DRO JYNSKM MYNO RKC LOOX LBYUOX

The Zodiac killer is still at large.

This is quite cumbersome so let’s look at letter frequencies to speed this up.

Most commonly used letters in descending order of frequency: E-T-A-O-N-R-I-S-H-D-L-F-C-U- M-G-P-Y-W-B-V-K-X-J-Q-Z

Do number 5! Answer 5:

The Zodiac killer uses complex ciphers.

HVS NCRWOQ YWZZSF IGSG QCADZSL QWDVSFG

Substitution Ciphers It would have been simple to break the code if the

Zodiac killer had used a Ceasar cipher, but he didn’t.

He used a substitution cipher. In a substitution cipher, either the alphabet is not in order or each letter of the alphabet is represented by a symbol.

Do number 6! Answer 6: Donald Harden solved a zodiac cipher.