1. Cryptography and Murder – The Zodiac Killer

2. 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

3. This is the Zodiac speaking...
Wrote many letters (at least 18) to the police and to newspapers in the SF area

4. 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

5. 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

6. 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

7. Part 1 Character Cipher
Cipher Key
Cipher Solution

8. 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.

9. FORENSIC SCIENCE USES CRYPTOGRAPHY TO SOLVE CRIMES.B C D E F G H I J K L M N O P
Plain
Cipher N O P Q R S T U V W X Y Z A B C
Solution:
IRUHQVLF VFLHQFH XVHV FUBSWRJUDSKB WR VROYH FULPHV.

10. Caesar Ciphers Answer 3: Do number four! Answer 4:
ZKBD YP DRO JYNSKM MYNO RKC LOOX LBYUOX
The Zodiac killer is still at large.

11. 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

12. 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.