In an effort to confuse the decoders, the sender will sometimes place deliberate errors into ciphertext in a classic cryptographic technique known as nulls, trapdoors, or intentional noise, aimed at disrupting automated frequency analysis. The Zodiac Killer likely did this in his 408 cipher on July 31st 1969, but took this technique to a new level when placing 14 deliberate errors, running diagonally across the second 9 rows of his 340 cipher on November 8th 1969. These manufactured errors were all situated 9 positions apart (shown by the red squares below), as were the blue squares spelling "LIFE IS". This is explained in greater detail in the article "The 340 Cipher: The Final Conclusion". But if the Zodiac Killer was to confuse cryptanalysts further, it would have benefited him to replace higher frequency letters within his message. If we take a look at the letters he replaced in his second 9 rows, we may get an insight into his thinking.
THE RED SQUARES ARE DELIBERATE ERRORS IN THE CIPHER
THE BLUE SQUARES SPELLING "LIFE IS" ARE ADDED LETTERS RATHER THAN SPELLING MISTAKES
THE BROWN LETTERS REPRESENT THE CORRECT SPELLINGS IN THE MESSAGE
The individual 10 letters he replaced were A, E, H, N (2 times), O, R (3 times), S (2 times), T, V and Y. In the total count of 14 letters he replaced with misspellings, all but V and Y fall in the top nine most frequently occurring letters in the English alphabet. This means that 12 of the correct letters he replaced (to produce errors) could be found in the nine most frequently used letters in normal text. Only the letter "I" was not selected in the nine most commonly used letters replaced on any diagonal (it was replaced on the 10th row in position 14, when the Zodiac Killer used the letter "E" instead. But this was not on a diagonal). The four green dots in the images above (showing misspellings) have yet to be fully explained. The frequency table on the left was based on a sample of 40,000 words. This frequency of letters is pretty much replicated on Wikipedia.
It could be of interest to go back to his 408 cipher on July 31st 1969 and examine which letters he chose to misspell in that instance. The majority of the letters replaced in the 340 cipher along the diagonal are of higher frequency in everyday text---but I cannot say whether this was a deliberate choice on behalf of the Zodiac Killer.
It could be of interest to go back to his 408 cipher on July 31st 1969 and examine which letters he chose to misspell in that instance. The majority of the letters replaced in the 340 cipher along the diagonal are of higher frequency in everyday text---but I cannot say whether this was a deliberate choice on behalf of the Zodiac Killer.
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