by Neil Sareen (Vanderbilt University)
The 1960s of the Bay Area of California are often remembered as a time of love and social expansion, but there remains a terrible and unexplained stain on the otherwise illustrious history. A lone and extremely elusive killer wandered the Bay Area streets at night. Known as the Zodiac killer, because of his messages signed with a zodiac symbol, he became one of the most infamous and terrifying killers in history. While he claimed to be responsible for the killings of 37 people, investigators were only able to confirm only seven victims (five were murdered, two survived). Throughout his serial killings, the Zodiac killer would write letters to the Bay Area press in an attempt to brag and taunt his pursuing officers. But these weren’t any ordinary letters. They were ciphers. From the late 1960s to the early 1970s the Zodiac killer sent four coded letters. Of the four ciphers, only one has actually been solved.
His letters were written in two parts. The first part was usually written in plain text, while the other was in cipher text, in which he claimed contained his identity. In the plain text part, he threatened newspapers to publish his letters or else he would kill more innocent people. In other segments of his letters, he listed the names of his next victims, creating havoc amongst the Bay area. His goal was to use the media to instill fear in Bay area citizens, and it worked. As cryptographers dug deeper into his letters, they were able to find out what drove the Zodiac killer to keep killing.
Of the four ciphers letters he sent, one was a three-part coded message, sent to three different press companies, making a 408-symbol cipher. His other famous cipher letter contains a 340-character cipher, which still doesn’t have a definitive solution. After the Zodiac killer sent his 408-symbol cipher (Z408), he sent another message to the police stating that if they could solve that cipher “they will have me.” To understand what the Zodiac killer meant by this, we will have to first see how the message was deciphered.
In 1969, two schoolteachers Donald and Bettye Harden managed to crack the Z408 cipher. The Z408 cipher consisted of random symbols corresponding to a plain text message. While Zodiac killer’s ciphers made him seem to be a genius, his Z408 cipher was not all that difficult to solve. It was a homophonic simple substitution cipher. In a basic simple substitution cipher, each ciphertext letter corresponds to a plaintext letter. However, in a homophonic substitution cipher, more than one ciphertext letter could correspond to a plaintext letter. This may seem tricky, but historically speaking, this cipher was far easier than any cryptanalyst would have expected (“Zodiac Killer Ciphers,” 2012).
To decipher the Z408 cipher, Donald and Betty Harden essentially looked for common patterns, and plugged in letters that might fit into the ciphertext. After analyzing the text, they noticed certain symbols appeared more frequently than others. For example, there were a high number of double symbols (double letters) found in the ciphertext. In frequency analysis, the letter “L” is doubled most frequently in English. Because the message came from a serial killer, they figured that the double letter “L” must be followed by the letter “I” creating the word “KILL”. In cryptography terms, the word “KILL” served as the “crib,” a word that could be plugged into other parts of the message to determine other phrases. While the message had a few misspellings, the meaning of the message was clear.
The cracked code offers frightening insight into the Zodiac killer’s mind. According to the plain text message, he was attempting to collect slaves for the afterlife. While the plain text message gave police the reason for his serial killing, the message never mentioned his name. According to the message, he refused to give up his identity because it would “slow down or stop [his] collection of slaves” (“Zodiac Killer Ciphers,” 2012).
While the Hardens managed to solve the Z408 cipher, the last 18 letters of the plaintext were “EBEORIETEMETHHPITI.” Seemingly jumbled text, cryptanalysts believe these letters are filler letters, used make the cipher three equally sized parts. Others believe that the letters can be rearranged to the name of the Zodiac killer (“Filler Theory,” 2009). These last 18 letters could be rearranged to make 741,015,475,200 transpositions, making the anagram almost impossible to decipher. Maybe the Zodiac killer was referring to the last 18 letters when he stated that the police would “have him.” Could the last 18 letters actually contain his name, or was it simply another method employed by the Zodiac killer to drive society insane? While the Z408 cipher has been solved, the mystery behind the last 18 letters still remains.
Adding to the fear that he once caused, the Zodiac killer still remains uncaught to this day. Recently however, Corey Starliper from Tewksbury, Massachusetts, claims to have cracked the Z340 cipher, by recognizing that Z340 is Caesar shift cipher (or a cipher where each plain text letter is shifted 3 letters down the alphabet). You may be wondering why the police didn’t decrypt a simple shift cipher earlier? Before Starliper actually applies the Caesar shift, he arbitrarily converts each Zodiac symbol to a Latin letter. The reason why the police were unable to solve the cipher was because the cipher consisted entirely of Zodiac symbols. This process seems to be a bit unreliable, considering Starliper’s method relies on his own assumptions. Surprisingly, the cipher message yields a message with English phrases, but not full English sentences (Muessig, 2011).
Interestingly enough, the last few phrases of the plaintext yield the words “MYNAMEISLEIGHALLEN.” Leigh Allen was a suspect while police were still investigating the case, however, his DNA did not match the DNA found in the envelopes of the Zodiac letters (Winkles, 2011). Many cryptanalysts question the accuracy of the decipherment but one thing is for sure: if a simple shift cipher of three was used, and it yielded a name as well as other phrases that would be used by a serial killer, it may just be an accurate decipherment. There is no proof that Leigh Allen is the Zodiac Killer. For all we know, the Zodiac killer could be framing Leigh Allen (Winkles, 2011).
Cryptography has made this case even more mysterious, and surprisingly, never actually helped investigators catch the Zodiac killer. However, cryptography has truly helped us discover more about one of the most elusive serial killers in history.
This post is part of a series of essays on the history of cryptography produced by students at Vanderbilt University. The students wrote these essays for an assignment in a first-year writing seminar taught by mathematics instructor Derek Bruff. The essays are shared here, in part, to give the students an authentic and specific audience for their writing. For more information on this cryptography seminar, see the course blog.
Muessig, Ben. (2011, July 7). Corey Starliper, Massachusetts Man, Claims He Cracked Zodiac Killer’s Code. The Huffington Post.
Sifakis, Carl. (2001). Zodiac Killer. The Encyclopedia of American Crime.
Winkles, Regina. (2011, February 4). Zodiac Killer: Identity Solved?
“Z 408 Cipher – Filler Theory.” (2009). Zodiologists.com.
“Zodiac Killer Biography.” (2012). Biography.com.
“Zodiac Killer’s 408 Character Cipher.” (2009). Zodiologists.com.
“Zodiac Killer Ciphers.” (2012, June 24). Throw the Book at Him.