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Cryptography and Murder – The Zodiac Killer

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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

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

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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

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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

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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

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Part 1 Character Cipher Cipher Key Cipher Solution

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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.

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Plain Cipher ABCDEFGHIJKLM DEFGHIJKLMNOP NOPQRSTUVWXYZ QRSTUVWXYZABC Solution: IRUHQVLF VFLHQFH XVHV FUBSWRJUDSKB WR VROYH FULPHV.

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

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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

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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.

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