The present incarnation of this site is dedicated to the
solution of what is commonly called:
"The Bomb Cipher"
The Zodiac Killer.
The Zodiac Killer is credited with killing at least 5 persons in the area of San Francisco in the late 1960s. He harangued and taunted the police through letters to the local newspapers. One persistent threat starting in November of 1969 and lasting through 1970 was that he would plant a bomb to kill a bus load of school children. He went so far as to send diagrams of the bomb, a list of the components and even his formulation of the charge. He then sent a map and a cipher to the paper saying that the cipher concealed the bearing and distance from Mount Diablo (on the map) to the bomb site.
For over thirty years this cipher has not been cracked, and the threatened bomb site has remained a mystery. I believe that I have finally discovered the secret of this cipher and know the presumably targeted community.
As a cautioning word to avert needless alarm: after thirty years and particularly considering the material the Zodiac mentions, I do not believe that even if a bomb had been planted that to-day it would be a working bomb. For example, he said the battery for his trigger mechanism had a life of only one year.
This is a cipher of 32 characters, 29 of which are unique. Without going into the details of ciphers in general, let me explain how the only other cipher of the Zodiac that has ever been cracked works. The Zodiac used what is called a homophonic substitution cipher. Without using fifty-cent words this means that he substituted different symbols for the letters of the alphabet in the plain text to get a text that was illegible. But this is not all. He also made a short list of several different symbols that would stand for single letters of the alphabet being substituted. For example, he might say that E will be substituted randomly by Q, W, R or T. This makes it harder to decipher since you are never quite sure if QW are representing two different letters or in truth represent EE. If you have the key, however, you simply substitute backwards and the text flows out. In any respect, this earlier cipher was cracked and the clear text is plain English and quite coherent, if not extremely deranged.
Now we are immediately met with a problem in this particular cipher. The earlier cipher had enough text to play with and to observe patterns. In 32 characters, this is impossible for a substitution cipher. With 29 out of 32 characters unique, there is no pattern. If this were a homophonic cipher, I would have to pass on it as being completely impossible to decipher. But I believe it is decipherable, because a lunar month after he sent the cipher and the map, and there was apparently no solution forthcoming from the police, the Zodiac sent clues. And later in a Halloween card he sent more clues. So I believe he fully expected and intended this cipher to be cracked or at least made crackable.
Given that this is a cipher with a reasonable solution, I looked at the characters and noted that all characters are unique except for three and these are each doubled. They are "C", "O", and Triangle. I reasoned that if within a stream of text in which we have all unique characters we observe three characters with two and only two occurrences, that this was supposed to catch our attention. This indeed was the pattern we needed. I highlighted these six characters on the cipher and tried to make something of the intervening characters. It was then the thought dawned on me that perhaps to convey digits all he intended was for us to count the characters -- like counting small piles of stones. Doing this and using these sentinel characters as separators, I got these digits.3,7,11,5. Since the Zodiac said that the cipher told us "radians & # inches along the radians," I was not happy to see 11 in there since the map is not that large. Also, because I was expecting single digits. Then I thought to look at the expected number of digits to give me radians and inches to a reasonable precision on the map. To get to one degree precision I would need one unit and two decimals in radians. Then I thought a corresponding precision in inches should be one unit and one decimal. This is 5 digits. There should be another sentinel character -- one to separate radians and inches and giving a 5th digit. Looking again to the cipher I noticed the circle with a dot. It was a very reasonable character and in a very reasonable spot to break and separate the radians from the inches. [N.B. I have further argument to this point below.] Interpreting it in this fashion gave me: 3.74 radians and 6.5 inches.
Next I needed to plot this on the map and see whether I am on the top of a mountain or in the middle of the Bay, in which case my theory would be all wet. :-) After getting feedback from Tom Voigt of www.zodiackiller.com that the scale of the map was about 6.4 miles per inch, I calculated the bearing and distance and came up with a very real location: I got 37° 30' 10.501"N 122° 30' 12.790"W, which lands me with slightly wet feet at Pillar Point of Half Moon Bay. This is well within the error of his map and the Zodiac's probable equipment. Another source of a little error could be that I switched to a more accurate topographical map and a very precise location of the Mount Diablo bench mark rather than continuing to use the Zodiac's relatively clumsy Phillips 66 "area insert" road map of which I only had a poor copy. Furthermore, I used clean and precise mathematics instead of a classroom protractor, ruler and blue felt-tip pen. I believe that my calculations are more precise and accurate than what the Zodiac performed. Later, I double checked with classroom equipment, and the pen's point was right on the shore.
It doesn't take much imagination to see the humor the Zodiac would have enjoyed in choosing this site since his persona reflected the night sky and he murdered and sent letters timed to the phases of the moon.
Now, how can we say that this is a solution with any degree of certainty? Initially I was was satisfied with the notion that I could point to this and say that it is obviously a very possible solution to the cipher, but we will never know for sure. However, after looking at the clues and the timing of the letters I think the Zodiac gave us enough clues that we can have better than a wavering certainty that this is not just a possible solution, but the likely solution.
I mentioned before that Half Moon Bay matches the flavor of the Zodiac to a T, but I also observed that the three mailings that mentioned clues to this bomb were mailed in three different months precisely on the day of the last quarter of the moon -- the half moon. When the Zodiac mailed these letters he would have looked into the black sky and seen a half moon floating there. What brings this into something of a contrast with his other letters is that in mailing those he favored full moons and new moons. Here, with the bomb letters, he chose the half moon -- the last quarter.
Interestingly, when the Zodiac mailed letters he only used the most common stamps of the day: FDR, Jefferson, Eisenhower. This was true on all but the Halloween clue card. This card had the famous Earthrise stamp from Apollo 8 where the slightly gibbous "half Earth" hangs over the lunar horizon. The imagery is hauntingly suggestive of the half moon. These details are all available on Tom Voigt's web site mentioned above.
Now, does this location otherwise fit the Zodiac's MO? The primary element of his MO was that his killings either were near water or had water in their name. I guess Half Moon Bay fits that one.
He killed on a full moon or a new moon. This bomb on the surface would seem to deviate from his previous MO in this respect. He was choosing to kill school children as the sun rose. However, his bomb design very well could have selected a lethal time period of a desired lunar phase. Because sun light had to travel down a long tube, it would only be effective in a small window of days in a year.
Under a new moon the Zodiac sent two letters a day apart on November 8 and 9 1969. The first letter hinted to a bomb and presented an as yet un-cracked 340-character cipher. The second letter was a long diatribe against the police which included a description of the bus bomb and a diagram of how it worked. At the bottom of the letter the Zodiac signed it in his usual manner by drawing his logo of a circle with an overlying Greek cross. This time he was a little different. This time he placed eight extra tick marks around the circle as though it were the face of a clock. Next he placed small Xs at the clock positions of 6, 8, 9, 10, and 11. Along the left side of the "clock" the only open position was at 7 o'clock. This position from 12 o'clock is 214 degrees. The magnetic compass position of my calculated bearing from Mount Diablo is 210 degrees. This clock position is the closest to the calculated bearing and could have been a leading clue to the location. A further link is made by the fact that when the Zodiac sent in the map, he drew his Zodiac logo around Mount Diablo and marked the arms of the cross 0, 3, 6, and 9 combining a compass with the face of a clock. These are the only two times he did such a thing and so it naturally links them. Looking back at my reasoning and calculations, it made perfect sense to me that he would give such a hint because this would lead an investigator to look for a method of finding a radians/inches separator that was correct, or alternatively, validate his choice. I have tried all other separator positions and none other yield a location that makes any sense. And none other match so well with his clues.
Now I'll discuss the Halloween Clue Card in more detail. I believe this card was a combination of clues to his two unsolved ciphers: the 340-character cipher and the Bomb Cipher. I will only talk about what I believe to be the Bomb Cipher clues here. The part that concerns us the most is the bottom of the second side. Here we find the following:
Seeing the nature of this cipher, the only thing making sense to me is that the Zodiac would give us clues that would allow us to know that we were interpreting him correctly. Calling back to the original cipher we notice that it begins with a crescent "C" and these symbols on the bottom begin with a crescent -- harking to the final solution of Half Moon Bay. It is followed by three small crescents. My first digit is a 3. Next we have this cryptic emblem that has been variously interpreted. I prefer to take it very plainly at face value and in relation to the cipher. The left portion of the emblem is a 7 and the second digit in my calculation is a 7. Next we have four dots and the next digit in my calculation is 4. Now leaning the other way, as if implying that this marks a point of separation, is an F. On the cipher the first character after the circle with a dot and the first character in the inches count is an F. Finally we have a "Z" and the Zodiac's signature.
Finally, a cute observation: is it a coincidence? The Zodiac's first two killings were near Vallejo, CA. Just south of Pillar Point the beach on Half Moon Bay is called Vallejo Beach -- and it's directly in front of a school.
After all the cleverness, we are left with circumstantial arguments. In the end, we will perhaps always be left with arguments of a circumstantial nature. It would be my desire that we have closed the book on this cipher. However, maybe this will be a book that has been lying open for so long that there will always be that little crack marking the spot and making it easy to re-open back to the very page and start again. Who knows?
My challenge to-day is to one and all interested in this case to examine my thought processes and see if we can come up with a better approach. Either that, or find more evidence and connections that put more nails in the coffin. Until then, I aver that this is the most plausible, elegant, and compelling attempt at a solution thus far.
You may correspond with me at firstname.lastname@example.org