The Zodiac Killer made the 340 cipher more difficult by using a combination of substitution and transposition to disguise his message, but he added a third layer in the second 9 rows of the cipher by adding 14 deliberate spelling mistakes in an organized pattern 9 spaces apart (running diagonally across the cipher). The Zodiac Killer not only hid his message by using a 9 shift (and negative 8 shift) technique shown here, but he would further complicate matters for codebreakers by baking in these spelling mistakes using the same 9 shift technique. If you thought that was the last hurdle codebreakers had to overcome you would be mistaken, because a fourth layer was added, also 9 spaces apart. The Zodiac Killer placed the phrase "LIFE IS" into his message, inserted deliberately into certain positions so that it would appear at the end of the 10th row of the original 340 cipher when the key was applied. Below are the 14 deliberate spelling errors he placed into his message, running diagonally across the grid (black rectangles). The four green circles, I believe, are the accidental errors made by the Zodiac Killer. These concepts were fully explained in the previous article No Cipher Mistakes by Zodiac.
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