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.
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.
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.
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