Here I would like to return to an old topic with a fresh approach, partly inspired by 'Nobody,' a regular contributor to this site.
As early as August 10th 1969 Professor D.C.B. Marsh of the American Cryptogram Association (ACA) was commenting on the Zodiac ciphers. In the San Francisco Sunday Examiner & Chronicle he praised Donald Harden's deciphering skills and called the code "complicated and obviously drawn by somebody who knows his business". 
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The Zodiac Killer had already responded to prompts by Vallejo Police Chief Jack Stiltz to send more details to prove the letter writer and killer were one and the same. The more the Zodiac Killer wrote, not only lessened his time for killing, but may have provided additional clues to detectives regarding his identity. This tactic was likely adopted by Professor D.C.B. Marsh in the October 22nd 1969 San Francisco Examiner newspaper article by Will Stevens, which laid down a challenge to the Zodiac Killer to reveal his name. The newspaper stated "Dr Marsh told the Examiner today: "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in the cipher to established cryptogram experts. He knows, to quote Edgar Allen Poe, that any cipher created by man can be solved by man. Zodiac has not told the truth in his cipher messages to the Examiner, the Chronicle and the Vallejo Times-Herald. Zodiac has not done this, because to tell the complete truth in relation to his name -in cipher code - would lead to his capture. I invite Zodiac to send The American Cryptogram Association a cipher code - however complicated - which will truly and honestly include his name." 

Clearly, Dr. Marsh and the detectives would have considered it highly unlikely for the Zodiac Killer to respond by giving us his name, but while he was constructing ciphers he wasn't murdering innocent civilians. There also remained an outside chance that Zodiac would rise to the challenge and create a code, "however complicated", that Dr Marsh and his fellow cryptographers could unpick. This article, rather than a straightforward challenge to Zodiac, could have been deliberately manufactured with the assistance of detectives, to bait the Zodiac Killer into using a particular methodology in his cipher construction. If the police knew the source of Zodiac's inspiration, then it could unearth the cipher technique he employed when designing his next code. 

'Nobody' considered the idea that the Zodiac Killer may have used a foreign language in the cipher, or possibly a rotating key that switches at certain intervals. By employing such a technique, the Zodiac Killer could make the cipher virtually "crackproof". 
The article in the October 22nd 1969 San Francisco Examiner may have been specifically designed to lead the Zodiac Killer down a particular path, making his next cipher easier to crack. Dropping innocuous sections of text into an article such as "to quote Edgar Allen Poe, that any cipher created by man can be solved by man", can be specifically designed to guide the Bay Area murderer into using one of Edgar Allan Poe's cipher techniques or a model suggested by him. We know the Zodiac Killer was an avid reader of the newspapers and may have took this bait of using a publication by Edgar Allan Poe, thinking he was getting one over on Dr Marsh and the police. The hope from the standpoint of law enforcement, was that the Zodiac Killer would bite the hook and get reeled in. 

Sixteen days later the Zodiac Killer mailed the 340 cipher revealing, not his name, but his identity, carefully positioned at the base of his cipher. This catapulted the idea that the Zodiac Killer had indeed responded to the article by Dr Marsh, but wasn't quite ready to reveal his name to authorities. But it is my belief that the Zodiac Killer did create the 340 and 13 Symbol ciphers in tandem, with a view to inextricably link his identity to his name using the word "Zodiac" on the final row of the 340 cipher, with the three 8's as the conduit between the two. See here.

The Zodiac Killer, however, would hold back his '13 Symbol' cipher for approximately six months. Zodiac researcher Michael Cole stated "Clearly, the Zodiac did not send the cryptogram to Marsh, as had been requested (this non-public way of satisfying the challenge was apparently unacceptable to the attention-craving serial killer). Neither did he respond in a time frame that most would have expected. But these details matter little. In fact, the timing itself provides yet another reason to conclude that what we have here is a specific instance of cause and effect. As mentioned, this article was published in the Examiner on October 22, 1969.  Six months to the day later, April 22, 1970, the people of the San Francisco Bay Area were reading about the My Name Is cipher in the Chronicle."

But there may have been a much more cunning reason why the Zodiac Killer delayed his mailing of the April 20th 1970 communication, and it has everything to do with Edgar Allan Poe. Did the Zodiac Killer pick up on the wording of Dr. D.C.B. Marsh in the article, create and mail his '13 Symbol' cipher based on something in the writings of Edgar Allen Poe? 

We can see in the Edgar Allan Poe essay 'A Few Words on Secret Writing,' at the very beginning he shows the alphabet being split into two halves of 13 letters, A to M, and N to Z.
"Were two individuals, totally unpractised in cryptography, desirous of holding by letter a correspondence which should be unintelligible to all but themselves, it is most probable that they would at once think of a peculiar alphabet, to which each should have a key. At first it would, perhaps, be arranged that a should stand for z, b for y, c for x, d for w, &c. &c.; that is to say, the order of the letters would be reversed. Upon second thoughts, this arrangement appearing too obvious, a more complex mode would be adopted. The first thirteen letters might be written beneath the last thirteen, thus:
n o p q r s t u v w x y z
a b c d e f g h i j k I m; and, so placed, a might stand for n and n for a, o for b and b for a, &c. &c. This, again, having an air of regularity which might be fathomed, the key alphabet might be constructed absolutely at random." ​​

The Zodiac Killer not only created the April 20th 1970 cipher with 13 characters, but it too began with an A and ended with an M, just like the suggested encryption technique featured by Edgar Allan Poe. Notice the way the cipher is organized symmetrically using the alphabetical characters, the three 8's and the two symbols. The same symmetry was observed when placing half the alphabet alongside the 13 character cipher. See here.  

The Dr. Marsh newspaper article opened with "A Challenge perhaps unique in the annals of American crime was injected today into the bizarre case of the killer who begins his letters to newspapers 'This is the Zodiac Speaking'."
The article continued, and bemoaned the fact that Zodiac hadn't revealed his true identity: "Zodiac has not done this, because to tell the complete truth in relation to his name - in cipher code - would lead to his capture. I invite Zodiac to send The American Cryptogram Association a cipher code - however complicated - which will truly and honestly include his name." 

'A Few Words on Secret Writing' stated "This challenge has elicited but a single response, which is embraced in the following letter. The only quarrel we have with the epistle, is that its writer has declined giving us his name in full. We beg that he will take an early opportunity of doing this, and thus relieve us of the chance of that suspicion which was attached to the cryptography of the weekly journal above-mentioned–the suspicion of inditing ciphers to ourselves. The postmark of the letter is Stonington, Conn." ​link. 

If the Zodiac Killer was basing the '13 Symbol' cipher on the Dr. Marsh article and secret writing of Edgar Allan Poe, why did he wait approximately six months to mail it to the San Francisco Chronicle?
It may not have been the six month anniversary that was important to him, but the only date mentioned in the Edgar Allan Poe literature, 'A Few Words on Secret Writing'. The date was April 21st. 
Had the Zodiac Killer deliberately waited six months to ensure his '13 Symbol' cipher letter would be postmarked April 21st, or be published on April 21st, which unfortunately for him never materialized? He was slightly early on the mailing, resulting in the postmark of April 20th, and the San Francisco Chronicle published his cipher in the April 22nd edition, effectively thwarting any intentions to recreate the Connecticut letter.

However, the possibility remains, that the Zodiac Killer could have used one of the cipher techniques employed or suggested by Edgar Allan Poe in 'A Few Words on Secret Writing', and the idea proposed by 'Nobody' regarding the use of a second language as the key, a much more viable proposition than initially considered.
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 "It is not to be supposed that Cryptography, as a serious thing, as the means of imparting important information, has gone out of use at the present day. It is still commonly practised in diplomacy; and there are individuals, even now, holding office in the eye of various foreign governments, whose real business is that of deciphering. We have already said that a peculiar mental action is called into play in the solution of cryptographical problems, at least in those of the higher order. Good cryptographists are rare indeed; and thus their services, although seldom required, are necessarily well requited. An instance of the modern employment cipher is mentioned in a work lately published by Messieurs Lea & Blanchard, of this city–“Sketches of Conspicuous Living Characters of France.” In a notice of Berryer, it is said that a letter being addressed by the Duchess de Berri to the legitimists of Paris, to inform them of her arrival, it was accompanied by a long note in cipher, the key of which she had forgotten to give. “The penetrating mind of Berrver,” says the biographer, “soon discovered it. It was this phrase substituted for the twenty-four letters of the alphabet–Le,gouvernement provisoire.” 'A Few Words on Secret Writing' by Edgar Allan Poe.

Robert Michael Salem's body was discovered by friends on the evening of Sunday April 19th 1970 at his 745 Stevenson Street, San Francisco residence. The autopsy concluded he had died on Wednesday April 15th. The article below was published in the San Francisco Chronicle on the morning of April 20th 1970. This is the same day that Zodiac's '13 Symbol' cipher communication was postmarked, proclaiming "I hope you have fun trying to figgure out who I killed."​​​
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Bearing in mind this article was considering a Zodiac connection, did the Bay Area murderer respond by creating this letter and mailing it almost immediately. The killer hadn't written since the 'Melvin Belli' letter on December 20th 1969, exactly 122 days ago. Therefore, do we conclude that [1] the Zodiac Killer just happened to independently write the April 20th 1970 communication, which accidentally coincided with the April 20th 1970 San Francisco Chronicle article, but he wasn't the murderer of Robert Salem. [2] He was the murderer of Robert Salem and had prepared the letter from the time of his death on the 15th, before mailing it on the 20th, or [3] He immediately sprang into action on April 20th 1970 after reading the article, and created the '13 Symbol' cipher in double quick time, designed his 'photoelectric switched' bus bomb diagram, and recalled the murder of San Francisco police officer Brian McDonnell on February 16th 1970, all in time to have this letter postmarked on the same day. To answer this question we have to go back six months. 

Michael Cole wrote an excellent article entitled "My Name Is" Cipher Motivation that suggested the Zodiac Killer likely drew his inspiration for the '13 Symbol' cipher from an October 22nd 1969 newspaper article by Will Stevens, which laid down a challenge to the Zodiac Killer from the president of the American Cryptogram Association (ACA), Professor D.C.B. Marsh, to reveal his name. Apparently, several months later the killer duly obliged by opening his correspondence with the line "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...."

Michael Cole wrote "Clearly, the Zodiac did not send the cryptogram to Marsh, as had been requested (this non-public way of satisfying the challenge was apparently unacceptable to the attention-craving serial killer). Neither did he respond in a time frame that most would have expected. But these details matter little. In fact, the timing itself provides yet another reason to conclude that what we have here is a specific instance of cause and effect. As mentioned, this article was published in the Examiner on October 22, 1969.  Six months to the day later, April 22, 1970, the people of the San Francisco Bay Area were reading about the My Name Is cipher in the Chronicle. This relationship between the Examiner article and the My Name Is cipher may well mean there is an even higher likelihood that the cryptogram does indeed encipher some form of the killer's name"

If the April 20th 1970 letter was referring to the Robert Salem murder, by teasing us with the statement "I hope you have fun trying to figgure out who I killed"​​​, then any manufactured correlation between the dates by the Zodiac Killer would rely on the Zodiac Killer committing the murder just shy of the six month anniversary and then mailing the letter. This would mean the Zodiac Killer could prepare the letter in the days between the murder and the mailing date of April 19th or 20th, to enable it to be postmarked on the 20th. If he didn't kill Robert Salem, and was simply responding to the San Francisco Chronicle with an unprepared letter, then his code and bus bomb diagram must have been designed with little forethought, and mailed in time to be franked on the same day. The other possibility, is he had the code and bus bomb diagram already prepared in advance and just added the text around these designs. However, there is one major problem. The photograph of the crime scene wasn't shown until a follow up article by Paul Avery in the San Francisco Chronicle on April 21st 1970. By which time, the Zodiac Killer had already mailed the letter.               

It has previously been argued that the Zodiac Killer created the '13 Symbol' code in tandem with the 340 cipher, a matter of weeks after the October 22nd 1969 challenge by the American Cryptogram Association (ACA) and Professor D.C.B. Marsh. 
The reason behind this claim was the correlation between eight characters on the final row of the 340 cipher and eight characters from the '13 Symbol' cipher (six being the circled eights).   

On the left we have lined up the alphabet over the top of the cipher. Now calculate numerically the difference in each column. ie: C to N produces a difference of 11, whereas B and E produces a difference of 3. Non alphabetical symbols cannot produce a difference, so a zero is inserted. The resulting numbers produced have been color coded. The red section is before the first circled 8, the green section is after the last circled 8 and the blue section spans the first and last circled 8. Notice the perfect symmetry. The red and green numbers are identical read forwards or backwards, and the blue numbers again read identically in either direction, producing a mirror image. Is this a coincidence or is it by design. 
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The 340 Cipher was the Zodiac Killer's unsolved masterpiece, and being a man with an inflated ego, driven by self publicity, what better way to drop us a hint by placing his signature or pseudonym on the final line of the 340 Cipher. You will notice he placed the Zodiac crosshair symbol, followed by what could be interpreted as Zodiac and then an upside down Aries symbol, with a line on the top and a dot to the left. So what we shall do is place the correct spelling of Zodiac alongside the version he placed on the 20th line of the 340 Cipher and use the identical technique we used to create the symmetry on the '13 Symbol' cipher. The alphabetical difference of ZOD produces three zero's, the IAC produces 888. These two sets of figures incorporated gives us the three circled 8's found on his next cipher, as though he was carrying them forward. It is also apparent on the '13 Symbol' cipher that either side of the three circled 8's are the Zodiac crosshairs and an inverted Aries symbol, identical to the final line of the 340 Cipher. So, the author has effectively carried eight symbols from one cipher to another.

"In Christian numerology, the number 888 represents Jesus, or sometimes more specifically Christ the Redeemer. This representation may be justified either through gematria, by counting the letter values of the Greek transliteration of Jesus' name, or as an opposing value to 666, the number of the beast." Wikipedia.

The ZOD and IAC are separated. The first produces the 000. The second produces the 888. The two are then combined, to create the transition from the 340 cipher to the '13 Symbol' cipher. This was presumably designed by the Zodiac Killer when he created the 340 cipher sometime before November 8th 1969, with a view to his next cipher. 

When the Zodiac Killer mailed the April 20th 1970 letter and code (assuming he wasn't the killer), he hadn't seen the way Zodiac was written on the wall of Robert Salem's apartment. Therefore, did the killer of Robert Salem just accidentally separate the ZOD and IAC of ZODIAC, thereby mimicking the Zodiac Killer's 340 cipher creation, when he had ample room on the wall to write it normally, or was the Zodiac Killer present at the 745 Stevenson Street, San Francisco residence on April 15th 1970, giving him the opportunity to create the design, and at least five days to prepare his letter? The April 20th 1970 code being the confirmation of his involvement in the murder. 
Was this the reason for the wording "I hope you have fun trying to figgure out who I killed"?

The crucified image on the wall of Robert Salem's wall may well be the Egyptian ankh. The ankh appears frequently in Egyptian tomb paintings and other art, often at the fingertips of a god or goddess in images that represent the deities of the afterlife conferring the gift of life on the dead person's mummy; this is thought to symbolize the act of conception. 
​Wikipedia.

In the last three articles it was proposed that the author of the 408 cipher may have left his identity within the final 18 characters. He specifically told us that he would not give us his name, but assured us his identity was concealed within the cipher - reasoned to be something like "I am the Zodiac Killer." The theme of his identity and name were integral to the final third of the cipher and letter mailed to the San Francisco Chronicle on July 31st 1969, and may very well have manifested through both his second and third ciphers as well. The third cipher began with "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...."- therefore, the final line of the second cipher (340 cipher) may be the 'narrative bridge' between the first and third cryptic offerings. 

The first cipher achieved everything the Zodiac Killer had wished for. He had fashioned using threats and newspaper rivalry maximum exposure for his handiwork, which was very quickly published by the Vallejo Times-Herald, San Francisco Examiner and San Francisco Chronicle. The failure of Zodiac to give us his name did not satiate the desire of some in the quest for the real identity of the killer, and was championed by Dr D.C.B. Marsh, president of the American Cryptogram Association, who on October 22nd 1969 in an article published by the San Francisco Examiner, stated "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in a cipher to established cryptogram experts. "Zodiac" had not done this, Marsh suggested, because to tell the complete truth in relation to his name, in cipher code, would lead to his capture. I invite 'Zodiac' to send The American Cryptogram Association a cipher code-however complicated."

​This article was not to go unnoticed by the killer, who, two and a half weeks later mailed his infamous 340 cipher, unsolved to this day according to official sources. This cipher was to become the all-important bridge between cipher one and three. If he gave us his identity (the Zodiac Killer) in the unsolved 18 characters of cipher one, and stated "my name is" in cipher three (supposedly giving us his name), then by logical extension, cipher two should have been the promise of a name, and the conduit between the two.

If we can provide a compelling reason to show that the near-Zodiac on the final line of the 340 cipher was actually intended to be deciphered to 'Zodiac', then we are left with the final two characters of the cipher to decode, thereby bridging the gap between cipher two and three. 
Using the most powerful supercomputers in the world and the talent of some of the brightest minds in cryptography, the 340 cipher has failed to buckle. So, it's time to use the character of the Zodiac against himself, and his willingness to respond to the newspapers.

If the Zodiac Killer is responding to Dr D.C.B. Marsh, then he clearly isn't prepared to give us his name immediately in the November 8th 1969 '340 cipher', as the opening line in his April 20th 1970 communication would testify to. It would be over five months later that he stated "my name is".  Therefore, the final two characters of the 340 cipher are the conclusion to an open-ended promise of something more to come, in similar fashion to "my name is."  The Zodiac Killer certainly wasn't going to miss the opportunity to keep Dr D.C.B. Marsh and the public waiting just a little bit longer. 
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The final line or sequence of the 340 cipher probably read "The next cipher will reveal who the Zodiac is" or "I will soon reveal who the Zodiac is" - or something along these lines. This 'promise' can be seen in his opening gambit of the April 20th 1970 communication: "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...." ​
The statement of "by the way have you cracked the last cipher I sent you" is superfluous, as he knows only too well the 340 cipher has not been cracked, yet, it also lacks his usual abrasive sarcasm. However, it is followed by "my name is", and therefore could be construed as the conclusion to his promise from the final line of his 340 cipher and to Dr D.C.B. Marsh, and the allegorical  bridge between the two ciphers.

But how do we know that the near-Zodiac on the final line of the 340 cipher was to be decoded to 'Zodiac'?  Because he fashioned a bridge between cipher two and three using his pseudonym. Placing the real spelling of Zodiac under the '340 cipher' version of Zodiac, and counting the alphabetical difference numerically between the columns (see here), we achieve the numbers 000888. When combined, the three circled 8's found on the '13 Symbol' cipher.
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These six characters of the 340 cipher were effectively carried forward numerically and placed front and center of the '13 Symbol' cipher design. You will also notice that the two characters either side of three 8's (shown on the right), are both visible either side of the near-Zodiac on the 340 cipher. A total of eight characters and three circled 8's the common link or bridge between cipher two and three - in which the name of Zodiac played a pivotal part in the formation of the 'My Name is Cipher'. The story is now complete. 

The Zodiac Killer stated on July 31st 1969 "I will not give you my name, but in this cipher is my identity." He promised in the 340 cipher, in a response to Dr D.C.B. Marsh, that he would soon reveal his name. Then, five and a half months later, on April 20th 1970 he stated "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...." 

Do ciphers two and three really contain a full and coherent message, not dissimilar to the 408 cipher solution, or had the Zodiac Killer taken Dr D.C.B. Marsh's advice "to send The American Cryptogram Association a cipher code - however complicated", and taken it a step too far?

The Zodiac mailed correspondence to the San Francisco Chronicle on April 20th 1970, in which he stated "I have killed ten people to date. It would have been a lot more except that my bus bomb was a dud.  I was swamped out by the rain we had a while back." Suisun City, Solano County, California can be seen via the link to border the city of Fairfield. But why is this relevant?
It has been suggested in previous articles that the Zodiac Killer may have resided near the payphone at Springs Road and Tuolumne Street, but we know that two letters were mailed from Fairfield to the San Francisco Chronicle on December 7th 1969, and to the San Francisco Examiner on December 16th 1969. If these were genuine Zodiac letters, then there is a possibility that he may have been a resident of Suisun City or Fairfield for the duration of the 7th to 16th of December and mailed his letters from this location. Was he a resident of this area beyond December 16th 1969, or did he reside here permanently? The Bay Area murderer may have had two residences, a vacation home, lockup or a business in this region of Solano County. 
Just a matter of weeks after the final Fairfield letter, floods in Suisun City/Fairfield, "from Laurel Creek and Union Avenue Creek caused flooding in January 1970" resulting in the evacuation of many "residences and businesses" in the area. The Zodiac Killer then wrote to the San Francisco Chronicle in April, stating "that my bus bomb was a dud.  I was swamped out by the rain we had a while back." But was he telling the truth?  

# The population of Suisun City was only 2,917 in 1970.
# The population of Fairfield was 44,146 in 1970.

It is important when we examine the writings of the Zodiac Killer to view them in context to the news breaking during the time in which he put pen to paper. The 1970s were a turbulent period in San Francisco and the Zodiac certainly monopolized on this fact in at least three of his communications pertinent to this story. The Zodiac Killer also had a penchant for adding historical references to his material in order to convey the message. The Mikado by Gilbert & Sullivan, a Tim Holt comic book, The Most Dangerous Game by Richard Connell, and surprisingly, Moby Dick by Herman Melville to name but a few. To believe the Zodiac Killer's bomb threats may have been inspired by a man who borrowed the Moby Dick author's name, may take a bit of selling.
Samuel Joseph Melville was the principal conspirator and bomb setter in the 1969 bombings of eight government and commercial office buildings in New York City. He was born Samuel Joseph Grossman, but changed his name with respect to novelist and poet Herman Melville.
​He had already committed four bombings prior to the November 9th 1969 Zodiac 'Bus Bomb' letter. It began with Grace Pier on July 27th, followed by August 20: Marine Midland Building, September 19: Federal Plaza and then an Army Induction Center on October 7th. These crimes received statewide coverage throughout America, but were they an inspiration to the Zodiac Killer in the power of large scale destruction as a tool for terror. The answer may have come five and a half months later.
But first we shall take a look at the April 20th 1970 letter, which stated
"I am mildly cerous as to how much money you have on my head now.  I hope you do not think that I was the one who wiped out that blue meannie with a bomb at the cop station.  Even though I talked about killing school children with one. It just wouldn't doo to move in on someone else's teritory.  But there is more glory in killing a cop than a cid because a cop can shoot back.  I have killed ten people to date. It would have been a lot more except that my bus bomb was a dud.  I was swamped out by the rain we had a while back."  
We know that the Zodiac Killer often referenced newspaper headlines - and this communication would be no different - denying his involvement in the murder of San Francisco police officer Brian McDonnell (44) on February 16th 1970. Person or persons unknown had placed a pipe bomb packed with fence staples on the window ledge of Park Police Station in Upper Haight (1899 Waller Street). Another eight police officers were injured. The Zodiac Killer was unlikely to have had any involvement in the bombing, however, he was keen to suggest that he was capable of such an act, but for "the rain we had a while back."
Two weeks prior to the April 20th 1970 Zodiac letter, on April 6th 1970, the deadliest day in the history of California law enforcement occurred, when two armed criminals managed to kill four California Highway Patrol officers in just under 5 minutes. Known as the Newhall Massacre, it hit the newspaper stands around the country.
One can't help thinking the Zodiac Killer was fully aware of this tragedy, when he cynically penned the words "there is more glory in killing a cop than a cid because a cop can shoot back."  Or maybe, we give the Zodiac too much credit.
The Zodiac Killer may not have killed Sgt. Brian McDonnell on February 16th 1970, but his following correspondence on April 28th 1970, just eight days later, would certainly coalesce both Sam Melville and Park Police Station under one banner, when he wrote "If you dont want me to have this blast you must do two things. Tell everyone about the bus bomb with all the details. I would like to see some nice Zodiac buttons wandering about town. Everyone else has these buttons like, black power, melvin eats bluber, etc. Well it would cheer me up considerably if I saw a lot of people wearing my buton. Please no nasty ones like melvin's. Thank you."  

The phrase "melvin eats bluber" was referring to the prominent lawyer and bon vivant Melvin Belli, who had earlier become the focus of one of Zodiac's letters. The Zodiac was referring to the 1960s fad buttons created by Irwin Weisfeld, but had altered the slogan from "Melville Eats Blubber" (Moby Dick) to "Melvin Eats Bluber".
Bearing in mind the introduction and tone of this card: "I hope you enjoy yourselves when I have my blast," and his choice of counterculture button, the following connection cannot be ignored.
​The Zodiac borrowed the Melville Eats Blubber button and changed it for purpose, as did Sam Melville, when altering his name from Grossman to Melville before his bombing campaign. The crucial question, however, is where did Zodiac intend to have this "blast". The clue may be staring us square in the face (see image on right).
The Zodiac Killer wrote "I hope you enjoy yourselves when I have my blast" around the picture of Don Quixote and Sancho Panza, the characters in a novel by Miguel de Cervantes Saavedra.
These characters are situated only 3,650 feet (0.68 miles) from the murder site of Sgt. Brian McDonnell, both of which are situated in Golden Gate Park. 
The Zodiac Killer, almost certainly, never had any intention of placing a bomb near the Don Quixote and Sancho Panza statue in Golden Gate Park, but the fact that this statue was in such close proximity to the bombing of Park Police Station on February 16th 1970, could be interpreted as a mischievous and insensitive offering to police. He was effectively rubbing their "noze in their booboos."
Then came the possible threat on Ingleside Police Station, two months later, on June 26th 1970, contained within the 'Button' letter and map code, and featured in the article 'The 32 Symbol Code-Hidden In Plain Sight'. The Zodiac Killer would again reference the slaying of another police officer - on this occasion Sgt Richard P. Radetich (25), who on June 19th at 5.25 am was gunned down by three shots from a .38 caliber revolver at point blank range through the driver side window of his vehicle, while in the process of serving a parking ticket. He was sat in his police car near 643 Waller Street, The same Waller Street as Park Police Station. 
The proximity of these three consecutive communications to the area of San Francisco shown below, certainly shows us where the Zodiac Killer's focus was between April 20th and June 26th 1970. However, he wasn't finished just yet. The 'Little List' letter mailed on July 26th 1970 was the fourth and final piece of the jigsaw.

It would be befitting that the Zodiac Killer's last correspondence may possibly be the 'Eureka' card mailed in the December of 1990, considering the last major work of Edgar Allan Poe was the non-fiction offering Eureka: A Prose Poem.    
"Adapted from a lecture he had presented, Eureka describes Poe's intuitive conception of the nature of the universe with no antecedent scientific work done to reach his conclusions. He also discusses man's relationship with God, whom he compares to an author. Similar to his theories on a good short story, Poe believes the universe is a self-contained, closed system. In coming to his conclusions, Poe uses ratiocination as a literary device, through his character C. Auguste Dupin, as if Poe himself were a detective solving the mystery of the universe. Eureka, then, is the culmination of Poe's interest in capturing truth through language, an extension of his interest in cryptography. Some modern critics believe Eureka is the key to deciphering meaning in all Poe's fiction, that all his works involve similar theories." Wikipedia. Strange therefore, that the Christmas card possibly engineered by Zodiac should contain a cryptic message in the form of photocopied keys mailed from Eureka, California.
This is not the only time that Edgar Allan Poe may have played a part in a cryptic message mailed by the Zodiac Killer. It could have been the essay 'A Few Words on Secret Writing' that inspired the creation of the '13 Symbol' cipher mailed on April 20th 1970 by the Bay Area murderer. A cipher that was devised sometime between October 22nd 1969 and November 8th 1969, and belatedly mailed by the killer some 6 months later, with the accompanying text about the murder of San Francisco police officer Brian McDonnell on February 16th 1970. Secret Writing from our Secret Pal.      

Michael Cole wrote an excellent article entitled "My Name Is" Cipher Motivation that suggested the Zodiac Killer likely drew his inspiration for the '13 Symbol' cipher from an October 22nd 1969 newspaper article by Will Stevens, which laid down a challenge to the Zodiac Killer from the president of the American Cryptogram Association (ACA), Professor D.C.B. Marsh, to reveal his name. Apparently, several months later the killer duly obliged by opening his correspondence with the line "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...." 
He followed this by placing 13 characters underneath, widely believed to be the Zodiac Killer offering us his name. 
One couldn't help considering whether this cipher was not only created because of the newspaper article, but whether this cipher was created based on what was contained within the article- in particular the works of Edgar Allan Poe. This would be right up Zodiac's alley, devising a cipher based in the text of the newspaper article. 
Here is an excerpt "Dr Marsh told the Examiner today: "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in the cipher to established cryptogram experts. He knows, to quote Edgar Allen Poe, that any cipher created by man can be solved by man. Zodiac has not told the truth in his cipher messages to the Examiner, the Chronicle and the Vallejo Times-Herald. Zodiac has not done this, because to tell the complete truth in relation to his name-in cipher code-would lead to his capture. I invite Zodiac to send The American Cryptogram Association a cipher code, which will truly and honestly include his name."

We can see in the Edgar Allan Poe essay 'A Few Words on Secret Writing,' at the very beginning he shows the alphabet being split into two halves of 13 letters:
"Were two individuals, totally unpractised in cryptography, desirous of holding by letter a correspondence which should be unintelligible to all but themselves, it is most probable that they would at once think of a peculiar alphabet, to which each should have a key. At first it would, perhaps, be arranged that a should stand for z, b for y, c for x, d for w, &c. &c.; that is to say, the order of the letters would be reversed. Upon second thoughts, this arrangement appearing too obvious, a more complex mode would be adopted. The first thirteen letters might be written beneath the last thirteen, thus:
n o p q r s t u v w x y z
a b c d e f g h i j k I m; and, so placed, a might stand for n and n for a, o for b and b for a, &c. &c. This, again, having an air of regularity which might be fathomed, the key alphabet might be constructed absolutely at random." 

Quite fortunate that the Zodiac Killer, after possibly reading this newspaper article, should then create a cipher of 13 characters beginning with an A (the first letter of the alphabet) and ending with an M (the 13th letter of the alphabet), just like Edgar Allan Poe's construction in his essay 'A few Words on Secret Writing.'  
Throughout the newspaper article Professor D.C.B. Marsh is urging the Zodiac Killer to give his name, which clearly he wasn't going to achieve with such a limited number of characters, but he may have played a game with D.C.B. Marsh by offering us another cryptic message within the cipher. We contended previously that the 340 cipher and '13 Symbol' cipher were both likely created at the same time because of the interconnectivity between the two. The name Zodiac appeared to be numerically carried forward from the 20th line of the 340 to the '13 Symbol' cipher in the form of three circled 8's. This was achieved by placing the correct spelling of Zodiac alongside the 'near Zodiac' and numerically counting the difference between the columns. This produced perfect numerical symmetry, as did placing A to M (half the alphabet) alongside the '13 Symbol' cipher. For this to be achieved, it was argued the two ciphers must have been created with each in mind, and therefore both devised before November 8th 1969. See image here for explanation or read article 'Heaven Sent'.  
"In Christian numerology, the number 888 represents Jesus, or sometimes more specifically Christ the Redeemer. This representation may be justified either through gematria, by counting the letter values of the Greek transliteration of Jesus' name, or as an opposing value to 666, the number of the beast. "  
Wikipedia.
Curiously, if you visit the English gematria site here and enter Edgar Allan Poe into the calculator, 666 is achieved.    
​Was the Zodiac Killer playing games, giving us the name Jesus in the form of a numerical value and depicting himself as the savior of souls for the afterlife.

Professor D.C.B. Marsh attempted to coerce the Zodiac into giving us his name by stroking his ego- so it seemed rather unusual that every alphabetical letter within the '13 Symbol' cipher apart from one, could be found in the word 'NAME'. The Zodiac Killer gave us A twice, M twice, N twice and E once. The only exception was the letter K- the only letter the Zodiac Killer scrubbed out in the 340 cipher. Removing this from the '13 Symbol' cipher only leaves us with letters contained in the word NAME.
The creation of these two ciphers together is the only reasonable explanation for a correlation between the two (if one exists), and the inspiration of Edgar Allan Poe in the construction of the '13 Symbol' cipher, based upon splitting the alphabet down its center, was ultimately born from the article by Will Stevens.
​Edgar Allen Poe stated that "any cipher created by man can be solved by man."
Could it be that, any cipher created by Zodiac can be solved by Edgar Allan Poe.

This is a combination of previous articles (with a little extra) under one banner, attempting to dispel the idea that the 340 code is a genuine cipher, and all attempts at trying to reveal a continuous and coherent message will ultimately fail. 
By overwhelmingly consensus, the 340 cipher will soon be entering its 50th year unbroken, remaining impervious to all attempts at releasing its secrets, despite innumerable attempts by the best codebreakers and codebreaking machinery thrown at it. Either the Zodiac Killer created a cipher so stunningly impossible and misjudged the complexity of his cipher, or he didn't create one at all. The '13 Symbol' and '32 Symbol' codes were simply too short, with too many variables to be broken by conventional means. The '32 Symbol' cipher has 29 unique characters, meaning you can literally fill in the blanks however you choose. The '13 Symbol' cipher is simply too short to be called a cipher, and any solution proffered can be readily dismissed without a valid key. The Zodiac certainly wasn't stupid and must have known this- pouring huge doubt on whether he ever intended them to be solved in a conventional manner. Therefore, why should we treat the 340 cipher any differently. The whole thing just appeared to be one big game for the Zodiac Killer, as we huffed and puffed long into the night trying to pry open his ultimate masterpiece.
Many observers have wondered whether the Zodiac Killer mailed the 'code key' to the Vallejo Police Department on August 10th 1969 under the guise of a "concerned citizen," unaware that it had already been solved by Donald Gene and Bettye June Harden, as well as the FBI in Washington. Unusual that he should then sit on the 340 cipher for many years to come, able to resist the temptation of revealing his subsequent ramblings. Keeping a secret for nearly 50 years would certainly take great restraint from a killer who bathed in the spotlight.
The 340 cipher may very well have been designed in the mind of the killer as early as August 4th 1969, or the moment his 408 cipher was cracked. The decrypted 408 read: "I like killing people because it is so much fun it is more fun than killing wild game in the forest because man is the most dangerous animal of all to kill something gives me the most thrilling experience it is even better than getting your rocks off with a girl the best part of it is that when I die I will be reborn in paradice and all the (people) I have killed will become my slaves I will not give you my name because you will try to slow down or stop my collecting of slaves for my afterlife." 
The answer to the 340 cipher probably already lay within the text of the 408 cipher- just designed in an altogether different format. The clue would later reveal itself in the design of the 'Halloween' card, mailed on October 27th 1970, where the killer placed "Paradice" and "Slaves" in a cross formation, and told us "sorry no cipher." This wouldn't be the only time the Zodiac Killer gave us clues, alluding to the fact we were chasing a solution to a cipher that didn't exist.        

One of the best finds in recent years was by Tahoe27, a regular contributor to the Zodiac Killer Site forum, who discovered a connection between a Tim Holt comic and the 'Halloween' card. Many features on the cover of the comic parallel the 'Halloween' card, including the wording By Fire, By Gun, By Rope and By Knife, along with "Redmask" (the skeleton eyes) and "Lady Doom" (peek-a-boo you are doomed). However, I believe Tahoe27 is not convinced that the 'Halloween' card was authored by the Zodiac Killer. 
If the 'Halloween' card was authored by an impostor, who based the 'Halloween' card on the Tim Holt comic, then are we to believe he was extremely fortunate that a year earlier, the real Zodiac Killer just happened to create a 340 cipher containing the word "BY" in all four corners of the cipher, along with PARADICE and SLAVES being a possibility, running down and across the cipher on the midpoint vertically and horizontally, just like the configuration on the 'Halloween' card.
Admittedly, the words paradice and slaves have not been proven to exist in the 340 cipher, but the fact they are a distinct possibility by comparison to the 'Halloween' card appears another very fortunate stroke of luck- particularly if we believe the author of the 'Halloween' card wasn't the Zodiac Killer- and only considered the Tim Holt comic in his design of the 'Halloween' card. If the 'Halloween' card was crafted by the Zodiac Killer using only the Tim Holt comic, then the above still applies.
The more likely conclusion we could draw from this, is that the 340 cipher was designed with the Tim Holt comic in mind, and the 'Halloween' card (one year later) was giving us a clue.  

​On the right, we can see how the four words "BY" fit into each quadrant of the 340 cipher just like the 'Halloween' card offering. What are the odds that in each quadrant of 80 characters we would find the letters B and Y together either horizontally or vertically every time. Not to mention the fortunate appearance of paradice and slaves bisecting the cipher perfectly. The killer may even have given us his signature in the bottom right corner, along with the running total, which was seven and counting on November 8th 1969.
If this wasn't the intended design of the 340 cipher, then either the Zodiac Killer or an impostor got extremely lucky, in just happening upon a Tim Holt comic that mirrored the 340 cipher unintentionally. But we may just be able to prove a definite link between these two communications and confidently state that both were designed by the Zodiac Killer, and with a fair degree of confidence, argue that the 340 cipher is not a cipher in the true sense of the word. The majority of the characters likely filler, but in this instance, far more than the 18 characters at the foot of the 408 cipher. The reason we may be able to contend the 340 cipher is not a continuous and uniform message like the 408 cipher (and not a cipher as we believe one to be), is because the Zodiac Killer lined us up for a fall and then admitted to it a year later.
On the 'Dripping Pen' card accompanying the 340 cipher, the killer stated
"This is the Zodiac speaking. I though you would nead a good laugh before you hear the bad news. You won't get the news for a while yet. Could you print this new cipher in your frunt page? I get aufully lonely when I am ignored, so lonely I could do my Thing." Was that bad news to come on October 27th 1970, when he mailed the 'Halloween' card.
If we believe that the 'Halloween' card design is referring back to the 340 cipher, then the message concealed on the envelope inner of "sorry no cipher," is the Zodiac Killer finally admitting that the 340 cipher is not the real cipher we had thought.
He is declaring that the 340 is "no cipher," and is in fact, based upon the paradice and slaves configuration. This is why he crosses the phrase "sorry no cipher" to mimic the design of paradice and slaves, using 13 letters in both instances.
The Zodiac Killer could easily have written "sorry no cipher" just the once, but deliberately fashioned it to form a link between the 'Halloween' card design and the 340 cipher.
After searching for solutions to the 340 cipher for nearly half a century, the Zodiac Killer may have given us the answer to its design after just one year, but it was overlooked. We have took the phrase "sorry no cipher" as an apology, that he wasn't giving us a cipher in the 'Halloween' card, when in actual fact, he was apologizing for stringing us along for the best part of a year, and offered us the real solution to the 340 cipher on October 27th 1970, contained within the 'Halloween' card. You could say it was a "trick" and "treat" befitting of Halloween.

​You will notice that the paradice and slaves configuration from the 'Halloween' card fits nicely into the 340 cipher.
The two dashes on the 10th line are represented by the letter S. 
The left-facing V on the 9th column and 10th line are represented by the letter E. 
​The two addition signs on the 9th column are represented by the letter A.
When the letter R of paradice is placed into its only conceivable position, along with the L and V from slaves, it is reminiscent of the 'Halloween' card. What are the chances that this could be achieved accidentally?  

The word paradice travels vertically 17 lines, exactly matching the 17 columns that slaves travels horizontally.The Zodiac Killer placed crosshairs beneath the 9th column, the midsection of the cipher (shown here).
The intersection on the 'Halloween' card configuration forms RAD and LAV, exactly the same as the bisecting center point of the 340 cipher. The Zodiac Killer also underlined the LAV of Paul Averly on the Halloween card envelope, on which he also placed the intersecting "sorry no cipher" on the envelope inner. 
The 'Halloween' card stamp depicts the surface of the moon with a half-earth in the distance and the words "In the beginning God." This phrase fits perfectly on the 1st line (the beginning) of the 340 cipher, with the 15th, 16th and 17th characters uncannily similar to the word GOD (the O in half phase just like the image on the stamp). 

But this wouldn't be the only correspondence that suggested the 340 cipher was designed as a puzzle rather than a 408 style message, If the paradice and slaves connection does exist between the 340 cipher and 'Halloween' card, then it makes more sense that the design of paradice and slaves was created in the 340 cipher first, and the 'Halloween' card was to "clue us in". If not, then the observation is just happenstance.
​If the 340 cipher doesn't actually contain a message, it will never be broken, and paradoxically will remain unsolved despite no solution being possible. 
The cipher has many other issues- in that, the Zodiac Killer may have had 'cipher three' formulated or drafted before he designed 'cipher two', to enable the interconnectivity between both. He had to think forwards to create backwards.
If we assume the 340 cipher has a solution and therefore a key (no matter how complex), then one may assume the Zodiac created the key, then created the message, and then applied the message to the key, so as to encrypt it. If this were the case, we must believe that the 'near Zodiac' that emerged on line 20 was by sheer chance. We would also have to believe, that had he crafted 'cipher three' completely independent of 'cipher two', with no intention of connecting both, then the following observations also happened completely by chance.

​Although covered many times before, the following will help explain the conundrum. 
Because the '13 Symbol' cipher was thirteen characters in total (half the alphabet) and began with an A and ended with an M, it was considered that by placing the first half of the alphabet alongside the '13 Symbol' cipher and numerically counting the difference between the columns, something may emerge- It did.
For example: C to N produces a difference of 11, whereas B and E produces a difference of 3. Non alphabetical symbols cannot produce a difference, so a zero is inserted. The resulting numbers produced have been color coded. The red section is before the first circled 8, the green section is after the last circled 8 and the blue section spans the first and last circled 8. Notice the perfect symmetry. The red and green numbers are identical read forwards or backwards, and the blue numbers again read identically in either direction, producing a mirror image.
An equally symmetrical pattern was created using the identical technique, when placing the correct spelling of Zodiac alongside the 'near Zodiac' on line 20 of the 340 cipher. 
Furthermore, the three 0's and three 8's created, can be combined and carried forward onto his '13 Symbol' cipher, which seemingly is what he did.
It is also apparent on the '13 Symbol' cipher, that either side of the three circled 8's are the Zodiac crosshairs and an inverted Aries symbol, also present on either side of the 'near Zodiac' on the final line of the 340 cipher. So, the author has effectively carried eight symbols from one cipher to another.
The point being, that when the Zodiac Killer sat down at his desk and began to create a unique key for his 340 cipher, into which he was to place his message, it accidentally created the 'near Zodiac' on the 20th line, accidentally created the correlation between 'near Zodiac' and Zodiac to produce three circled 8's, accidentally created the symbols either side of 'near Zodiac,' that fully or in part, carried forward to the '13 Symbol' cipher in the correct position, and accidentally created symmetry in both instances. Or, the Zodiac crafted the '13 Symbol' cipher before or alongside the 340 cipher, which I would argue nobody believes.
The Zodiac Killer may have began his 340 cipher key from the bottom first, deliberately allocating specific letters in his message to create the 'near Zodiac' characters and continuing from there. But, if the interconnectivity exists between the 340 cipher and '13 Symbol' cipher, he effectively began with 'cipher three' first and worked backwards, and worked forwards with the words ​paradice and slaves from the 408 cipher. 

​The Zodiac may have presented us with a fictitious negro male on August 4th 1969, who by inference, provided us with some valuable information about the Zodiac Killer driving his brown car by the payphone at Springs and Tuolumne. Was this a red herring of a clue, much like the correction on line 6 of the 340 cipher.
It can be clearly seen on the left, that the Zodiac Killer was conscientious enough to correct an alphabetical K to a reversed K on line 6, even though spelling correctly was never much of an issue with the Zodiac Killer. To validate that this was a genuine mistake, he made sure that the original K had not been fully obscured. This may give the impression it is easy to mix up your K's when implementing your encrypted text. Or maybe this is what he wanted us to believe. Who in their right mind would correct a fake cipher? Therefore it must be genuine.
Did the Zodiac Killer implement this correction, conveniently leaving the blacked out character still visible (and just reversing it), to give us the impression that the 340 cipher is undeniably genuine. Was this move just another red herring, to again validate something that never was. 
Alternatively, the final line of the 340 cipher may contain just a part message, yet incomplete, bearing in mind that many believe the '13 Symbol' cipher may contain some form of a name. The 340 cipher may end with: "the Zodiac is".... to be continued on the next cipher. The anticipation of the newspapers and readers alike, for the sequel to be released, would be a Zodiac dream.
The Zodiac Killer may have been pushing us in the right direction on October 27th 1970, but his attempt at revealing the workings of the 340 cipher may have begun far earlier, on December 16th 1969- just 38 days after his 'so called' masterpiece- in which he would be claiming a potential 38 more victims.
The December 16th 1969 'Fairfield' letter, mailed to the San Francisco Examiner, has been considered dubious Zodiac correspondence, however, it may have been Zodiac's first foray into revealing the mechanics of the 340 cipher, albeit using a rudimentary code as a tool. 
Clearly, the Zodiac must have known that offering just five different symbols in a code of nine is open to many different interpretations, and hence practically unsolvable. Therefore, one could suggest this wasn't his intention- he was simply teasing us with the mechanics of how the 340 cipher was designed, by using a rudimentary diagram based on the Tim Holt comic- and something he would expand upon nearly a year later.
One can see how he uses the Zodiac symbol around the crosshairs in the December 16th 1969 'Fairfield' letter code, implying a uniformity of message, but aligns five symbols as an introduction to this design. This marries perfectly with 'Death By Gun', 'Death By Knife', 'Death By Fire' and 'Death By Rope.' Death is the introduction, following by the methods of death around the crosshairs. This is exactly what he offered us on October 27th 1970, when he placed the methods of death in the four quadrants of the bisecting Paradice and Slaves.  Here is a representation. 

Another Fairfield letter was mailed prior to this correspondence on December 7th 1969, mailed to the San Francisco Chronicle. Was the Zodiac Killer highlighting pertinent sections of the 340 code in as little as one month after his November 8th 1969 'Dripping Pen' card and cipher, negating the idea he could resist the temptation of giving us clues to its design.
The final offering to the mechanics of the 340 cipher may have come with his final confirmed correspondence- that of the 'Exorcist' letter mailed to the San Francisco Chronicle on January 29th 1974. ​         

The symbols at the foot of the 'Exorcist' letter were rearranged by Kevin Robert Brooks to reveal the words "To Kill". The message would then read "If I do not see this note in your paper, I will do something nasty, which you know I'm capable of doing...To kill."  
This threatening message was very similar to the trinity of communications that began the Zodiac's letter writing campaign on July 31st 1969, where he stated "I want you to print this cipher on the front page of your paper. In this cipher is my idenity. If you do not print this cipher by the afternoon of Fry.1st of Aug 69, I will go on a kill ram-Page Fry. night. I will cruse around all weekend killing lone people in the night then move on to kill again, until I end up with a dozen people over the weekend."  
In both the 'Exorcist' letter and July 31st communications the author would threaten "to kill'- as he would in the decoded 408 cipher. This is not particularly unusual for a serial murderer, but linking the unarranged symbols on the 'Exorcist' letter to the 340 code may connect yet another communication to the design of the cipher.
Key features of the 'Halloween' card, '13 Symbol' cipher, Fairfield communications and 'Exorcist' letter can be found in the 340 cipher, suggesting the Zodiac Killer was constantly pushing us towards the design of the 340 puzzle.
The following presentation will show a perceived correlation of the 'Exorcist' letter symbols to the only corrected section of the 340 cipher.

​​In the image on the left I have moved the two symbols on the extreme left of the 'Exorcist' letter to the right side as shown, without altering their position on the vertical axis. Then I have turned the extreme right symbol on the 'Exorcist' letter 90 degrees clockwise to form the new arrangement, shown in the bottom left image. This can now be compared to certain characters on the fifth, sixth, seventh line of the 340 cipher. The area in the proximity of the corrected sixth line.
Five of the six characters on the 'Exorcist' letter now bear a noticeable similarity to this area of the 340 cipher.
Did the Zodiac Killer sit down to create a uniform and continuous message using a code key in the 340 cipher, just like the message contained within the 408 cipher? If so, then the above findings were all created accidentally and unintentionally by the killer.
He placed an intersecting symbol at the center of the 340 cipher (+ sign), with two dashes at the periphery of the center line, to create the illusion of intersecting crosshairs.
Paradice and Slaves (uniformly of 17 characters) was created as a viable possibility bisecting the 340 cipher along the X and Y axis, along with "RAD" and "LAV" on the intersection of the cipher. The word "BY" (horizontally) was present in all four quadrants of the cipher. The 'near Zodiac' allied with the correct spelling of Zodiac on the 20th line produced the symmetry of three circled 8's. Using the identical technique and placing half the alphabet alongside the '13 Symbol' cipher, yet again, perfect symmetry was observed.
The three circled 8's, along with the symbols either side, all carried forward from the second cipher to the third cipher, and the 'Exorcist' letter symbols aligned with the corrected sixth line of the 340 cipher. This of course all happened by chance and the Zodiac Killer knew nothing about it.  
This would indicate that the Zodiac Killer had deliberately placed Paradice and Slaves into the 340 cipher, to which he would give us a clue one year later. The machinations of the '13 Symbol' cipher were most likely constructed in the mind of the killer before November 8th 1969, as to create a continuity between the two ciphers. The idea therefore, that the author of all these communications could interweave these designs together and still create a rambling uniform message to co-exist alongside them within the 340 cipher, would probably be a step too far.
The idea of a killer who sat down one evening to write a rambling 340 character message, to which he enciphered using a code key, who would then accidentally and inadvertently create so many by-products in his cipher without his knowledge, seems rather fortuitous. The Zodiac seemingly couldn't resist showing us the workings of his great masterpiece, and would eventually tell us that the 340 code was "no cipher", by giving us a portion of its true design in the 'Halloween' card on October 27th 1970. However, it's far more fun chasing chasing a real cipher, that will long continue for another 50 years.      

Here we will revisit an article published by the San Francisco Examiner on October 22nd 1969, in which Professor D.C.B. Marsh, president of the American Cryptogram Association laid down a challenge to Zodiac. The words he chose to use in this article may be the bedrock to much of the subsequent Zodiac communications, beginning with the 340 cipher, mailed only seventeen days after the San Francisco Examiner article. Here are the crucial extracts from that article.
'Dr Marsh told the Examiner today: "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in the cipher to established cryptogram experts. He knows, to quote Edgar Allen Poe, that any cipher created by man can be solved by man. Zodiac has not told the truth in his cipher messages to the Examiner, the Chronicle and the Vallejo Times-Herald. Zodiac has not done this, because to tell the complete truth in relation to his name-in cipher code-would lead to his capture. I invite Zodiac to send The American Cryptogram Association a cipher code, which will truly and honestly include his name." It is difficult to believe that Zodiac, an avid reader of the newspapers, could resist such a challenge, particularly as he had promised to reveal his identity in the 408 cipher two months earlier, but failed to do so.  

The key section to note in the challenge by Professor D.C.B. Marsh is: "I invite Zodiac to send The American Cryptogram Association a cipher code, which will truly and honestly include his name."
Seventeen days later, the Zodiac Killer duly obliged with the November 8th 1969 340 cipher. Could this cipher have included the killer's name, but not in the format envisaged by Professor D.C.B. Marsh?
In a previous article entitled 'Zodiac Admitted 340 Not a Real Cipher' we examined the idea that the 340 cipher was inextricably linked to the Zodiac's 'Halloween' card, mailed on October 27th 1970. One can notice that the word "By" is located in each of the four quadrants of the 340 cipher. The author of the cipher also placed the letters F and K adjacent to one another no less than seven times (situated in each quadrant). But of particular interest was the corrected section on the sixth line of the 340 cipher, where the author chose to scrub out a forward facing K and replace it with a backwards K, elevated above the blacked out section.
The Zodiac Killer was known for numerous spelling errors in his communications, so why was it seemingly imperative to correct this one mistake? Was this correction crucial to the interpretation of the cipher, or was he drawing our attention to the letters in this section of the cipher? Are the F and K characters the initials of the Zodiac Killer?
This observation carries little merit on its own, so to bolster any argument for the initials of the Zodiac Killer being present in the 340 cipher, we have to explore the rest of his communications, particularly from around the time of the 'Halloween' card, in which we suggested was his admission that the 340 was unfortunately "no cipher". 
Linking the 'Halloween' card and 340 cipher through a Tim Holt comic book and explaining to us that the solution was "sorry no cipher." left us with the possibility that the 340 cipher was just a collection of puzzles, in which may have contained his name. The Zodiac Killer "wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in the cipher to established cryptogram experts", but he may have given us the abridged version.
We now enter the crucial phase of Zodiac communications, from the '13 Hole' postcard mailed on October 5th 1970, to the 'Exorcist' letter mailed on January 29th 1974. This critical period of four consecutive communications, is probably key to understanding the message the Zodiac Killer was trying to push. That the 340 was "sorry no cipher."      

In the '13 Hole' postcard the Zodiac Killer stated "There are reports city police pig cops are closeing in on me. Fk I'm crackproof. What is the price tag now?"
This section of pasted letters was placed upside down. The question one should be asking is 'what is the purpose behind this'. If its significance had no relevance to the actual postcard, did it have relevance to another communication.
The wording "FK I'm crackproof" could indicate that the killer thought he was immune from capture, however, it could also be a reference to his 340 cipher once again. The word "crackproof" is something that is unable to be cracked.
The 340 cipher would certainly be impervious to cracking, and certainly resistant to the attempts of The American Cryptogram Association had the killer devised a cipher not of conventional understanding, of which Professor D.C.B. Marsh was hoping when he laid down the challenge. The F and K text being flipped on the '13 Hole' postcard reminds one of the flipped K (next to F) on the only corrected portion of the 340 cipher. If FK were the Zodiac Killer's initials, then he certainly could be declaring that he was crackproof. 
​Professor D.C.B. Marsh stated  "He knows, to quote Edgar Allen Poe, that any cipher created by man can be solved by man. Zodiac has not told the truth in his cipher messages to the Examiner, the Chronicle and the Vallejo Times-Herald. Zodiac has not done this, because to tell the complete truth in relation to his name-in cipher code-would lead to his capture." However, the Zodiac Killer could give his name in the form of initials without any fear of revealing his identity, and without risking his immediate capture. This may have manifested itself when he mailed his following cipher on April 20th 1970, when he opened with the gambit "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...."  The code placed underneath was likely an immediate rebuttal to this question. 

All he is probably doing, is stating "My name is FK," and once again telling us he is crackproof, just like the '13 Hole' postcard, which would be mailed five and a half months later. The phrase "FK I'm crackproof" fits perfectly in circular formation within the '13 Symbol' cipher- which itself contains a series of circled 8's. Structured in this fashion, the F and K yet again align, as does the K and M in both the cipher and phrase. 
Continuing with our four consecutive communications from October 5th 1970 onward, we now have the October 27th 1970 'Halloween' card promising to yet again reveal his name: "I feel it in my bones, You ache to know my name, And so I'll clue you in." Again we feel disappointed only to see the word "Boo" on the card inner- but we know the Zodiac Killer isn't going to make this a conventional and straightforward offering. He offers us the phrase "sorry no cipher," but can we find his name or initials within this communication. This is where it gets more difficult, nevertheless, we are aware that within the '13 Hole' postcard he reversed or flipped the text containing "FK I'm crackproof" and he flipped the K (next to the F) on the sixth line of the 340 cipher. So the obvious conclusion, is to look for flipped or reversed text within the 'Halloween' card.  

The only place that F and K exist within one word on the 'Halloween' card, is in the word "knife." either side of the reversed letter N.
If we flipped the reversed N back to its appropriate position, the F and K of "knife" would be reversed yet again.
Now let us turn our attention to the peculiar configuration at the foot of the card inner. It would be extremely satisfying for the 'Halloween' card to read "I feel it in my bones, You ache to know my name, And so I'll clue you in... FK,"- so that is what we will create, by manipulating this card to read whatever we want it to.
The strange symbol, reading it as true as possible, appears like a number 7, followed by four dots and the letter F.
7+4=11=K (followed by F).
Without further ado, we shall move on to the next confirmed Zodiac communication on March 13th 1971, in which the Zodiac stated "This is the Zodiac speaking. Like I have allways said, I am crack proof. If the Blue Meannies are evere going to catch me, they had best get off their fat asses + do something. Because the longer they fiddle + fart around, the more slaves I will collect for my after life. I do have to give them credit for stumbling across my riverside activity, but they are only finding the easy ones, there are a hell of a lot more down there. The reason I'm writing to the Times is this, They don't bury me on the back pages like some of the others."
You will notice that the Zodiac Killer refers to "crackproof" once again, parroting his October 5th 1970 communication five months earlier. However, he doesn't just state he is crackproof, he reminds us that he has always been telling us he is crackproof, and quite possibly we just haven't got the message.

Roll on the January 29th 1974 'Exorcist' letter, signed "yours truley," nearly three years later, giving us our final installment of FK contained within the array of symbols at the foot of the letter. 
On this occasion he mirrors the sixth line of the 340 cipher by elevating the K above the letter F, and to its left. In fact, five characters from the Exorcist letter can be found in and around the corrected sixth line of the 340 cipher, as shown here in a previous article. This was possibly the fourth consecutive communication inferring an FK link and the suggestion that the author was indeed "crackproof."  
Was this a persistent and deliberate attempt to be true to the challenge laid down by The American Cryptogram Association and Professor D.C.B Marsh, by inserting his name (albeit initials) into the 340 cipher and subsequent communications, or just a case that my mind is F@@Ked after six exhausting years of looking for patterns where none exist.

The following observations are crafted with little, to no knowledge of cryptology, in respect to the validity or otherwise of the Zodiac 340 cipher. The cipher has withstood nearly 50 years of computer assaults, as well as some of the greatest minds in the field of codebreaking, yet as of writing, nothing has been officially verified as a solution despite many claims to the contrary.
The criteria for a definitive solution to the 340 cipher, is not when one person or few believe a solution has been achieved, it is when there is overwhelming consensus across the board, removing subjectivity from the equation.   
If the 340 cipher doesn't actually contain a message, it will never be broken, and paradoxically will remain unsolved despite no solution being possible. Having examined this cipher innumerable times, the only solution I can come to- is it has no solution.
This revolves around two key issues: In that, the Zodiac Killer may have had 'cipher three' formulated or drafted before he designed 'cipher two', to enable the interconnectivity between both. He had to think forwards to create backwards.
If we assume the 340 cipher has a solution and therefore a key (no matter how complex), then one may assume the Zodiac created the key, then created the message, and then applied the message to the key, so as to encrypt it. If this were the case, we must believe that the 'near Zodiac' that emerged on line 20 was by sheer chance. We would also have to believe, that had he crafted 'cipher three' completely independent of 'cipher two', with no intention of connecting both, then the following observations also happened completely by chance.     

Although covered many times before, the following will help explain the conundrum. 
Because the '13 Symbol' cipher was thirteen characters in total (half the alphabet) and began with an A and ended with an M, it was considered that by placing the first half of the alphabet alongside the '13 Symbol' cipher and numerically counting the difference between the columns, something may emerge. It did.
For example: C to N produces a difference of 11, whereas B and E produces a difference of 3. Non alphabetical symbols cannot produce a difference, so a zero is inserted. The resulting numbers produced have been color coded. The red section is before the first circled 8, the green section is after the last circled 8 and the blue section spans the first and last circled 8. Notice the perfect symmetry. The red and green numbers are identical read forwards or backwards, and the blue numbers again read identically in either direction, producing a mirror image.
An equally symmetrical pattern was created using the identical technique, when placing the correct spelling of Zodiac alongside the 'near Zodiac' on line 20 of the 340 cipher. 
Furthermore, the three 0's and three 8's created, can be combined and carried forward onto his '13 Symbol' cipher, which seemingly is what he did.
It is also apparent on the '13 Symbol' cipher, that either side of the three circled 8's are the Zodiac crosshairs and an inverted Aries symbol, apparent either side of the 'near Zodiac' on the final line of the 340 Cipher. So, the author has effectively carried eight symbols from one cipher to another.
The point being, that when the Zodiac Killer sat down at his desk and began to create a unique key for his 340 cipher, into which he was to place his message, it accidentally created the 'near Zodiac' on the 20th line, accidentally created the correlation between 'near Zodiac' and Zodiac to produce three circled 8's, accidentally created the symbols either side of 'near Zodiac,' that fully or in part, carried forward to the '13 Symbol' cipher in the correct position, and accidentally created symmetry in both instances. Or, the Zodiac crafted the '13 Symbol' cipher before the 340 cipher, which I would argue nobody believes.
The Zodiac Killer may have began his 340 cipher key from the bottom first, deliberately allocating specific letters in his message to create the 'near Zodiac' characters and continuing from there. But, if the interconnectivity exists between the 340 cipher and '13 Symbol' cipher, he effectively began with 'cipher three' first and worked backwards. However, this doesn't seem the likeliest of answers. The alternative, one could argue, is that the 340 contains only his pseudonym and nothing more.

The Zodiac may have presented us with a fictitious negro male on August 4th 1969, who by inference, provided us with some valuable information about the Zodiac Killer driving his brown car by the payphone at Springs and Tuolumne. Was this a red herring of a clue, much like the correction on line 6 of the 340 cipher.
It can be clearly seen on the left, that the Zodiac Killer was conscientious enough to correct an alphabetical K to a reversed K on line 6, even though spelling correctly was never much of an issue with the Zodiac Killer. To validate that this was a genuine mistake, he made sure that the original K had not been fully obscured. This may give the impression it is easy to mix up your K's when implementing your encrypted text. Or maybe this is what he wanted us to believe. Who in their right mind would correct a fake cipher? Therefore it must be genuine.
Did the Zodiac Killer implement this correction, conveniently leaving the blacked out character still visible (and just reversing it), to give us the impression that the 340 cipher is undeniably genuine. Was this move just another red herring, to again validate something that never was. 
Alternatively, the final line of the 340 cipher may contain just a part message, yet incomplete, bearing in mind that many believe the '13 Symbol' cipher may contain some form of a name. The 340 cipher may end with: "the Zodiac is".... to be continued on the next cipher. The anticipation of the newspapers and readers alike, for the sequel to be released, would be a Zodiac dream.

The following will provide an alternative to an earlier article 'The Lake Berryessa Greeting Card,' regarding the inspiration of the 340 character cipher and the two week period it was designed by the Zodiac Killer. 
On October 22nd 1969 Will Stevens wrote an article for the San Francisco Examiner entitled 'Cipher Expert Dares Zodiac to Tell Name.' Dr D.C.B. Marsh, president of the American Cryptogram Association challenged Zodiac to reveal his name by creating a cipher for them to crack. He stated "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in a cipher to established cryptogram experts. "Zodiac" had not done this, Marsh suggested, because to tell the complete truth in relation to his name, in cipher code, would lead to his capture. I invite 'Zodiac' to send The American Cryptogram Association a cipher code-however complicated."
​Michael Cole authored an excellent article detailing how Zodiac would take up this challenge exactly six months later, when he mailed the '13 Symbol' cipher to the newspapers, beginning with the line "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...."
However, we will slightly deviate from the Michael Cole article by suggesting that both the 340 character cipher and '13 Symbol' cipher were responses initiated by the challenge of Dr D.C.B. Marsh. He issued the challenge- however complicated- and Zodiac would certainly oblige with his most complex cipher of all, just 17 days later. The Zodiac Killer, knowing he was dealing with cryptogram experts such as  Dr D.C.B. Marsh, professor of mathematics at the Colorado School of Mines in Golden, would certainly not disappoint, mailing the 340 character cipher just over two weeks later. He would then follow up on April 20th 1970 with a smaller code, implying he was now prepared to reveal his name.  

In his first correspondence to the newspapers the Zodiac Killer promised that the 408 cipher would contain his name, by stating "In this cipher is my idenity." This notion would soon be quashed, when the three-part cipher was cracked by Donald Gene and Bettye June Harden of Salinas, California, in a matter of days.
But this challenge on October 22nd 1969 was laid down by cryptogram experts, and probably the reason the Zodiac Killer went to the extreme, creating a cipher that remains unbroken nearly half a century later. Dr D.C.B. Marsh​ stated "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in a cipher." But the Zodiac wasn't going to play ball that easily, yet he did likely answer this challenge directly on the final line of the 340 character cipher by revealing his name in part. He placed the 'near Zodiac' from character 333 to 338. 
In the 408 cipher, the final section of the decoded message read "I will not give you my name because you will try to slow down or stop my collecting of slaves for my afterlife." The '13 Symbol' cipher opened with the line "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...."
Therefore, this should give us a clue to how the Zodiac Killer constructed the final two lines of the 340 character cipher, bearing in mind it was a response to the challenge proffered by Dr D.C.B. Marsh, and in part, revealed his pseudonym Zodiac. 
Here is one example of how the Zodiac may have signed off the 340 cipher, when you consider there are only two symbols after the 'near Zodiac,' in positions 339 and 340. There cannot be many reasonable options to consider for a two letter word.  

There are of course alternatives, such as "I will soon tell you who the Zodiac is," "My next cipher will reveal who the Zodiac is" or "The day will come when you'll know who Zodiac is." There are a myriad of possibilities, but always with the final two words being "Zodiac is." 
If the Zodiac began the 340 character cipher top down, are we to believe the 'near Zodiac' fell out by chance, and was not created specifically as a response to the American Cryptogram Association to reveal his name. The cipher may have signed off with something similar to the wording above, but could have been created from the bottom up, so as to lead us into his next cipher on April 20th 1970. The 408 cipher stated "I will not give you my name because you will try to slow down or stop my collecting of slaves for my afterlife." The Zodiac Killer was then laid down a challenge on October 22nd 1969, and we know his propensity to reply to individuals when challenged. Seventeen days later, a cipher arrived at the San Francisco Chronicle. Can this be a coincidence such a short time after the challenge by Dr D.C.B. Marsh? He signs off the 340 character cipher, now promising he will soon reveal who the Zodiac is. Then 5 months later he seemingly is true to his promise, stating "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...."
The second and third ciphers may be linked by the name Zodiac and a simple trick of mathematics, shown here. 
​           

Did the fact that Dr D.C.B. Marsh, being a professor of mathematics, play any part in this possible numerical solution, or the Zodiac Killer's  circled three eights (if indeed that is what they were). The killer would later state in the April 20th 1970 letter "PS I hope you have fun trying to figgure out who I killed."​​​   
Dr Marsh told the Examiner "That first code was complicated... and the maker obviously knew what he was doing. But it never included the true identity of Zodiac." But can we be sure the Zodiac read this article and responded from his basement residence.
Look at the accompanying news article to the Dr D.C.B. Marsh article, entitled  "Tunisia Hit by Heavy Rains." It reads "New torrential rains pounded Tunisia all last night and today, nearly isolating Tunis and Bezerte from the rest of the country. Broadcasts warned residents of low-lying suburbs to head for higher ground. Basements filled, and some ancient buildings in the Medina showed signs of crumbling."
The Zodiac Killer replied in the April 20th 1970 offering, by stating
"I have killed ten people to date. It would have been a lot more except that my bus bomb was a dud.  I was swamped out by the rain we had a while back." This however, is most likely too cryptic even for Zodiac. 
But the fact remains, that Dr D.C.B. Marsh set the Zodiac Killer a challenge, and a little over two weeks later he responded with an even more "complicated" cipher than the 408, possibly promising the professor of mathematics a solution to his identity in due course. One that arrived on April 20th 1970- but whether it held true to the promise of his name is another matter.​ 

Michael Cole- http://zodiacrevisited.com/name-cipher-motivation