Analysis of the Zodiac Killer’s Three Part Cipher (Z 408)

The three segments of the Zodiac Killer’s Three Part Cipher Z 408 comprise 8 columns, 17 rows and 136 characters each. In total, the cipher contains 408 characters represented by 54 different cipher symbols , covering 24 letters of the English alphabet. The remaining letters "J", “Q” and “Z” are not present in the text of the cipher solution and therefore not represented by additional cipher symbols.
It has been commonly said that the Zodiac Killer's Three Part Cipher is a homophonic substitution cipher, where multiple cipher symbols have been allocated to single letters of the English alphabet. Having worked with the cipher and after analysis of the cipher key, we arrive at a different conclusion in part. In our opinion, the Three Part Cipher is not a full homophonic substitution cipher but a hybrid between a simple substitution cipher and a homophonic substitution cipher.
It is our impression that originally, this Zodiac cipher has been a simple substitution cipher that has been enhanced with homophonic substitution properties afterwards, in order to flatten letter frequencies and to aggravate letter frequency analysis. In other words, we believe that the Zodiac Killer increased the difficulty level of a simple substitution cipher by adding more symbols to some yet not all of the most frequent letters. For instance, the letter "M" is represented by only one single cipher symbol 17 times. Same applies for the letters "B", "J", "G" and "U", though to lesser extent.
The Z 408 cryptogram includes inaccuracies and errors, made by the Zodiac Killer himself when he transcribed the cipher symbols from draft to final version, and additionally through misinterpretation of the cipher symbols when reprinted in the media. After careful analysis, we believe that the original cipher must have been mostly correct, with the intentional or unintentional omission of the letter ‘I’ in the word ‘EXPERIENCE’ and the intentional or unintentional misspelling of the word ‘PARADISE’ as ‘PARADICE’ as only exceptions.
The root cause for the other cipher errors may have been the thickness of the Zodiac Killer’s writing tool. For instance, if one take a thick felt pen and try to draw a small triangle with a large dot inside, there is a high risk that it rather looks like a zodiac cipher than a zodiac cipher. Or if you write the letter zodiac cipher with a rather diagonal instead of a horizontal middle bar, one may mistake it for an zodiac cipher. Or if you would draw the letter zodiac cipher with a very low middle bar, it would possibly look more like a zodiac cipher. As it looks, exactly this is what happened, and the Zodiac Killer’s presumed misspellings were actually misinterpretations of his own Zodiac’s handwriting when he transcribed the cpher symbols from draft to final version.
Obviously, the cipher symbol  zodiac cipher at F13  should read zodiac cipher, and  the cipher symbol zodiac cipher at G9 should read zodiac cipher. The cipher symbol  zodiac cipher at I11 should read zodiac cipher, and the cipher symbol zodiac cipher at P10 should read zodiac cipher. Furthermore, the cipher symbol zodiac cipher at N12 should in fact be an zodiac cipher, and the cipher symbol  zodiac cipher at U6 should be an zodiac cipher.
 
 
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