Without any additional information the April 20th 1970 code is unsolvable. The Zodiac Killer was many things, but he wasn't a half-witted imbecile with the brain of a gibbering lunatic when he designed this short 13 character code. In a newspaper article authored by Will Stevens on October 22nd 1969 in the San Francisco Examiner, Dr. D.C.B. Marsh challenged the Zodiac Killer no less than four times to supply us with his name in a cipher. Dr. Marsh stated in one section "The killer wouldn't dare...as he claimed in letters to the newspapers...to reveal his name in a cipher to established cryptogram experts". He added: "I invite Zodiac to send The American Cryptogram Association, care of Dr. D.C.B. Marsh, a cipher code - however complicated - which will truly and honestly include his name", quoting Edgar Allan Poe "that any cipher devised by man can be solved by man". Therefore, it was no great surprise when Zodiac's April 20th 1970 letter landed on the desk of the San Francisco Chronicle with the contemptuous opening line of "By the way have you cracked the last cipher I sent you? My name is------". He knew his last cipher had not been solved, yet in mocking fashion was replying to the claim "that any cipher devised by man can be solved by man". The phrase "by the way" is used to introduce a new subject to be considered or to give further information. The Zodiac was effectively saying to Dr. Marsh he was wrong, now try this new cipher for size.
The Zodiac Killer chose to include eight characters from the English alphabet in his cipher, with 26 to choose from. Bearing in mind the challenge by Dr. Marsh was for Zodiac to reveal his name in the cipher, what are the odds of "NAM" forming consecutively, along with seven of the eight letters chosen from the alphabet being present in the word "name", being unintentional? Thereby leaving only K surplus to requirements. Although not impossible that this was just coincidental, one could argue that the N, A, M and E were chosen deliberately, considering the challenge by Dr. Marsh and the mocking introduction to the cipher by the Zodiac Killer. Did the Bay Area murderer, responding to the proposition of the eminent cryptographic expert to supply his "true identity" and "name", include his "identity" on line 20 of the 340 cipher, and use the word "name" in the Z13 just five months later?
Was the Zodiac Killer was just playing games with The American Cryptogram Association, determined not to have a further two ciphers cracked in a matter of days? - and thereby avoiding the acute embarrassment of creating "a cipher designed by man that can be solved by man". After the challenge on October 22nd 1969, the Zodiac Killer was going to design the remaining ciphers on his terms, to which he would ultimately reveal the answers. He wasn't going to allow Dr. Marsh the satisfaction of using cryptographic techniques to pry open his ciphers, so used the wording of Dr. Marsh "to send a cipher code, however complicated" against him, by sending two of the most infantile ciphers possible.
If we strike out the seven letters that have now formed "NAME" we are left with "NAME K". If this was the beginning letter of his surname, then the first circled 8 could represent his forename. So, if we are looking for an expanded declaration of his name using both forename and surname in later communications, we would be looking for a two letter combination, ending in K. But in addition, this would have to be accompanied by some pertinent text alongside the two letter declaration. This occurs in only one subsequent communication of the Zodiac Killer.
Was the Zodiac Killer was just playing games with The American Cryptogram Association, determined not to have a further two ciphers cracked in a matter of days? - and thereby avoiding the acute embarrassment of creating "a cipher designed by man that can be solved by man". After the challenge on October 22nd 1969, the Zodiac Killer was going to design the remaining ciphers on his terms, to which he would ultimately reveal the answers. He wasn't going to allow Dr. Marsh the satisfaction of using cryptographic techniques to pry open his ciphers, so used the wording of Dr. Marsh "to send a cipher code, however complicated" against him, by sending two of the most infantile ciphers possible.
If we strike out the seven letters that have now formed "NAME" we are left with "NAME K". If this was the beginning letter of his surname, then the first circled 8 could represent his forename. So, if we are looking for an expanded declaration of his name using both forename and surname in later communications, we would be looking for a two letter combination, ending in K. But in addition, this would have to be accompanied by some pertinent text alongside the two letter declaration. This occurs in only one subsequent communication of the Zodiac Killer.
You will notice how he begins this section of text, with "There are reports city police pig cops are closeing in on me". The text refers to the notion that law enforcement could possibly be close to capturing him, with the last word of "me" followed by a comma. One would therefore expect the next line to be a reaction by him in relation to this news. This has long been thought to be the abbreviated word "fuck" - an approach he didn't take in the August 4th 1969 Debut of Zodiac letter, when he wrote "When I hung the phone up, the damn X@ thing began to ring & that drew his attention to me + my car". The two letters of Fk may very well have been his initials - and a declaration that he "Fk, was crackproof". It may also be the answer to the 13 Symbol cipher, when responding to the challenge of Dr. Marsh.
If "Fk, I'm crackproof" was the answer to the 13 Symbol cipher, then one would have expected the Zodiac Killer to have designed the cipher with some markers present, that the answer could be fitted around later. Especially if no cryptographic technique was required. The phrase of "Fk, I'm crackproof" was the perfect response to Dr. Marsh, in that Zodiac could satisfy the request of Dr. Marsh by giving us his name, albeit in the shortened format. He began the April 20th 1970 letter by stating "By the way have you cracked the last cipher I sent you? My name is------". He responded immediately with his name (FK), before rubbing Dr. Marsh's "noze in his booboos" by declaring he was crackproof, having also just taunted the cryptographic expert with the mocking introduction of "have you cracked the last cipher I sent you?". The Zodiac Killer knew that Dr. Marsh would never solve his Z340 and Z13 ciphers when he designed them, because they couldn't be solved using standard cryptographic methods - and more importantly - the answers were 6 months in the making.
The circled eights (The number 8 in the Bible represents a new beginning) seemed to have extra significance compared to the other characters in the cipher. Because the Celtic Cross and Anchor have relevance in Christianity, it was considered that the number 8 was the beginning of each word. The three circled eights, along with the K and M inside them, were the markers I described earlier, so that his later solution would be understood. All we had to do now, was place the phrase "Fk I'm crackproof" into the 13 Symbol cipher, with each number 8 beginning each element of the phrase.
If "Fk, I'm crackproof" was the answer to the 13 Symbol cipher, then one would have expected the Zodiac Killer to have designed the cipher with some markers present, that the answer could be fitted around later. Especially if no cryptographic technique was required. The phrase of "Fk, I'm crackproof" was the perfect response to Dr. Marsh, in that Zodiac could satisfy the request of Dr. Marsh by giving us his name, albeit in the shortened format. He began the April 20th 1970 letter by stating "By the way have you cracked the last cipher I sent you? My name is------". He responded immediately with his name (FK), before rubbing Dr. Marsh's "noze in his booboos" by declaring he was crackproof, having also just taunted the cryptographic expert with the mocking introduction of "have you cracked the last cipher I sent you?". The Zodiac Killer knew that Dr. Marsh would never solve his Z340 and Z13 ciphers when he designed them, because they couldn't be solved using standard cryptographic methods - and more importantly - the answers were 6 months in the making.
The circled eights (The number 8 in the Bible represents a new beginning) seemed to have extra significance compared to the other characters in the cipher. Because the Celtic Cross and Anchor have relevance in Christianity, it was considered that the number 8 was the beginning of each word. The three circled eights, along with the K and M inside them, were the markers I described earlier, so that his later solution would be understood. All we had to do now, was place the phrase "Fk I'm crackproof" into the 13 Symbol cipher, with each number 8 beginning each element of the phrase.
The completed introduction to the April 20th 1970 letter now read perfectly with respect to the Zodiac Killer giving us his name: "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you? My name is Fk, I'm crackproof". And so did the phrase "There are reports city police pig cops are closeing in on me, Fk I'm crackproof". Placing his real name in the 13 Symbol cipher was never a realistic proposition - and even if this had been done - it would never have been revealed and could never be proven. The only way a solution to the 13 Symbol cipher could be proven beyond any doubt, was for the Zodiac Killer to give us the answer. He obliged on October 5th 1970, as he did for the Z32 on July 26th 1970 and the Z340 on October 27th 1970. Proving this is the answer to the Z13 in isolation, is nigh on impossible, but when all three ciphers can be answered in three consecutive communications using only the words of Zodiac, it exposes a killer unable to resist temptation. The Zodiac Killer simply didn't have the temperament to take his secrets to the grave.
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