Preliminary Overview to the 340 Cipher

   The ZODIAC’s uniqueness as a serial killer and his enduring fame to this day is based largely on his ability to market his crime spree. An integral part of this are his ciphers. They reflect his taunting nature and his desire to turn his crime spree into a game of murder and seek. He even teased us, saying his identity lay within decoding the ciphers. Although this is unlikely, he may unwittingly have betrayed himself within his coded messages. To this day we still try to find a solution to his ciphers in hopes we will find that one, tenuous clue.

   His first cipher, called the 408 cipher because it was made up of 408 symbols, is the only cipher that has been decoded to where all agree that the decipherment is correct. His subsequent ciphers, namely the 340 cipher (because, naturally, it is made up of 340 symbols) has never been decoded. This is due to a number of factors. First and foremost is that The ZODIAC entirely changed his code. He may have used many of the same symbols that he used in his previous 408 Cipher, but he changed what letter they represented in our alphabet so that they no longer corresponded to the letter to which they corresponded in his 408 Cipher. Moreover, he added many more symbols, making a total of 63 symbols used in this shorter cipher. This makes it certain that perhaps a dozen of these symbols could all represent the same letter in our alphabet. That makes the task of deciphering quite daunting since frequency analysis will not turn up common letters like E, A, T and S. This has proven the case. Some 43 years after he sent his second longest cipher— the 340— it still has not been cracked.

   His ciphers were homophonic substitution ciphers, meaning that the letters and symbols he used represented letters in our alphabet. But the “B” in his cypher text would not represent the “B” in our alphabet.  He could use it for “S” or for “G” or any letter he wanted. The applies to the unique symbols he used. They make the cipher more decorative, but each one had a practical purpose inasmuch as it stood for a letter in our alphabet.

     This is typical coding. The difference with the ZODIAC’s code is that no one to whom he sent the ciphers had the key by which to decode them. In war, ciphers are naturally sent between allies who have the key to decode the cryptogram. But the ZODIAC was at war with us all. He sent his ciphers to his enemies. Cryptographers had to guess what he was saying by analyzing the frequency of certain characters. This is standard decoding practice, of course, known as “known-plain text attack.” (“E” is the most common letter in the English language, for instance. Cryptographers would look for the frequency of symbols or letters he used and deduce the one that must represent the letter “E”.) The ZODIAC, however, had enough knowledge of code to use a number of symbols to represent “E”. Therewith he was able to hide its frequency.

     After his 408 Cipher was published in the newspaper, per his demand, one section each in 3 separate newspapers (San Francisco Chronicle, Examiner and Vallejo Times) it was decoded by a couple in Salinas, California. The Hardens decoded it in the only way possible: in understanding the circumstances in which it was written and guessing about what the ZODIAC would be saying. They deduced this guy must be such an egomaniac that he would begin the cipher with “I” and somewhere within it would be “killed,” “killing” or “kill.” They indeed were correct. Once they found these words, they definitely had decoded some of the letters in his alphabet. With these, more words could be found, more blanks could be filled in, more symbols could be decoded.

     The ZODIAC was possibly a bit surprised that his code was broken in a week by amateurs using “known plain text attack.” When on November 8, 1969, he sent his 340 Cipher to the San Fran Chronicle, he had had a couple of months in which to invent a new code. He used the same symbols mostly, but he added many new ones. He now had 63 unique symbols and letters by which to represent the 24 letter English alphabet. To this day it has never been cracked. This has caused many to condemn it as gibberish; that it was a fake code that says nothing. Its sole purpose is to taunt us with the idea there is a real message of import there.

     The problem with some of this is that many tried to decode it the “lazy way.” They tried to use mathematics, computers, frequency, everything the Hardens didn’t do. The Hardens did it like Rochefort’s team at Honolulu in order to anticipate the attack on Midway. Frankly, they did it like any cryptologist must do. The circumstances of the writing are everything. “Known plaintext attack” was the only way. Rochefort knew the geography he was dealing with. He second-guessed AF in Japanese code meant Midway.

     Before we get into the details of The ZODIAC Case as it stood in October and early November, let’s have a look at the construction of the 340 Cipher itself.   

The San Francisco Police photograph of the actual 340 Cipher written by the ZODIAC and received at the San Fran Chronicle on November 8, 1969.

   There are 63 symbols and letters used to construct the 340 Cipher. Classifying them reveals the above frequency. It is logical to deduce that many of the lesser used symbols represent common vowels, E, I, O, and A and that they are used to hide frequency. Some may also represent very rare letters like X, Y, and Z that need only be used a couple of times. One symbol immediately stands out. It is the + symbol. In the 408 Cipher ZODIAC used this + symbol for “E” along with 6 other symbols. He never used one of the 7 symbols representing “E” more than 9 times in that cipher. He had shown enough knowledge to hide his “E” by using several symbols. It is basically Code 101 to do that. However, here we are greeted by a symbol that is used way out of proportion. If this is gibberish, it might represent “E” since it really wouldn’t matter. If this cipher does contain a coherent message, then I suspected he wasn’t hiding the frequency of a common consonant, like T, S, R, or N.

     Further scrutiny reveals the consistent use of doubling certain symbols and letters and even transposing them.

   The most common doubled letters in English are, according to frequency: TH, HE, AN, IM, ER, ON, RE, ED, ND, HA, AT, EN, ES, OF, NT, EA, TI, TO, IO, LE, IS, OU, AR, AS, DE, RT, VE.

   The most common tripled letters are, according to frequency: THE, AND, THA, ENT, ION, TIO, FOR, NDE, HAS, NCE, TIS, OFT, MEN.

   Further examining the 340, however, brings to light some very interesting things. First, the ZODIAC does not repeat a letter until the second line. By the third line, he’s only repeated a few letters. He basically is throwing the majority of his arsenal of symbols out in the very first lines. Therefore it’s logical to deduce that the symbols in the first few lines contain many symbols that represent the most common letters in English, like T, R, S, N, I, E, O, A.  When coding his message, when he came to the second occurrence of one of these letters in his plain text message, he went along his alphabet and used the next alternate symbol for that letter. For example, if he had decided to represent “B” in our language with the letters C, A, F, G, in his coded alphabet, every time he needed an “B” he would use “C” and then next time, “A”, then next time “F”, then next time “G,” and so on.

     With this in mind, the backward “P,” the first symbol he repeats, becomes quite interesting. It is used 11 times altogether in the whole cipher, but it is the first repeated symbol. Does this reflect a rare letter like Y and he had no alternate symbols for it? Or does this represent an incredibly common letter like E and by the second line he had already expended all the symbols allocated to represent E and must already therefore start at the beginning of his table of “E” again?

   Another thing becomes interesting. If this is correct, then the doubling and transposing of letters in these first few lines also becomes significant, because they could represent the first occurrence of significant letter groups in English. The usage of G at the end of the first line doesn’t mean much until one realizes he repeats this twice, the next time on the 8th line, and the last on the line 5th line from the bottom. The and are on the first line but transposed on the 4th line already.

     Another significant thing is that the symbol W is not used after the 3rd line throughout the body of the cipher until the 4th to last line, where it is used 4 times until the end. It’s as though he realized he didn’t take advantage of that symbol enough throughout the body of the text.

     Very interesting is the fact that a symbol used only twice in the whole cipher, the backward semaphore , is used on the second line. Does this represent the first usage of some rare letter like Z or X?

     Another interesting doubling is + with a solid “P” .  This last symbol is used only twice in the whole cryptogram, and it is always following + .

   The ZODIAC starts repeating himself quantitatively only on the 3rd line. The first doubling of the same letter is on the 4th. Does this doubling reflect words that have 2 vowels or consonants together like TT, EE, LL, OO. . .or was he too clever for that?

     Contextualizing this, I would say that since no one has satisfactorily deciphered this in 43 years, and the fact he uses 63 symbols, a number more than enough to hide double letter groups, The ZODIAC’s double letter groups actually represent two words, those that end and begin with the same letter— e.g “lately you have even been naughty. . .”

     The cipher does not seem random. It seems like a carefully constructed message in English, coded specifically to hide doubles, vowel frequency, and worded in such a way that a common consonant’s frequency (+) cannot be easily exposed. The unhidden frequency of + is frightening. Its purpose is to mask the frequency of E with another common letter. But there is something provocative here. The intentional masking of frequency or even confusing it by not masking a letter are all seen in homophonic and mono-alphabetic ciphers. It is not necessary in a more complex cipher, such as a Vigenere Cipher because the latter uses a tabula recta by which frequency can be hidden by shifting, and this can be made even more secure by using a keyword.  A homophonic substitution cipher is many symbols bearing the same letter meaning. As such, they do not need keywords. A mono- and poly-alphabetic cipher need keywords to make them more secure. The one receiving the cipher obviously must have the keyword or deciphering is so difficult that only the most advanced cryptographers in the world could break it by another, involved method of guessing the length of the keyword.

     This is not a lesson in cryptography, so suffice it for me to say that what my ultimate point here is that The ZODIAC has given us an indication that he is using a keyword. The indication is that he gave it to us openly, right under our noses. It almost reminds me of the popular Cary Grant movie Charade (1964), when the object of their search had been under their noses the whole time but cleverly disguised.

     On the envelope in which the 340 Cipher arrived at the San Francisco Chronicle there is a clue. It is the only time The ZODIAC did this on an envelope. He circled the name of his favorite newspaper.

   My, my, there it was. Thanks to Robert Graysmith preserving everything he could, we have what is probably the keyword. It is Chronicle, without the “e”. This means is could be CHRONIL, ECHRONIL (using “e” as a letter preceding) of the full monty of CHRONILE, which really would amount to the same as ECHRONIL; or it could be SFCHRONIL(E).

     The following is the card and letter that was contained therein.     

   At the same time that the ZODIAC wrote the tongue-in-cheek dripping pen letter with the cipher contained therein, he wrote a plain letter. It was postmarked on the day after, November 9, 1969. He was quite the pen pal all of a sudden.

     The above gives us a good sample of ZODIAC’s writing, but more importantly it give us some background on the events he is still recalling and is therefore influenced by on November 9. All of this will prove invaluable in decoding his most enigmatic cipher.