In the previous article I covered William Friedman and the "CSP 888" cipher machine, a highly secure, electromechanical rotor-based cipher machine used for top-level communications during World War II. Friedman was instrumental in leading the Army's cryptologic services during the 1920s and 1930s, which set the stage for the development of secure machines like the CSP 888. There is much reason to connect Edgar Allan Poe to the design of the Zodiac Killer's Z340 and Z13 through his cryptographic works, such as Poe's July 1841 essay "A Few Words on Secret Writing", which William Friedman mentioned in his 1937 bulletin entitled “Edgar Allen Poe, Cryptographer”. In Poe's essay, the alphabet was split A through M and N through Z, which was seemingly adopted by the Zodiac Killer on April 20th 1970, beginning and ending his ciphertext characters with the letter A and M, in an array of characters totalling thirteen (half an alphabet). To then find the three circled 8's present in the cipher and possibly mimicking the cipher machine "CSP 888", was extremely interesting (whether deliberate or accidental). Especially when you consider that William Friedman and Edgar Allan Poe are bound together by the words in “Edgar Allen Poe, Cryptographer” (1937), written by Friedman, whose career in the field of cryptology was originally inspired by Edgar Allan Poe. Was the Zodiac Killer influenced by the cryptographic work of William Friedman, and drew upon his knowledge of this individual when he was challenged by Donald C. B. Marsh of the American Cryptogram Association, who asked the Zodiac Killer to mail a cipher which truly included his name?  A challenge published in the San Francisco Examiner on October 22nd 1969, just 17 days before the Zodiac Killer mailed his infamous 340 cipher on November 8th 1969, which may also have been influenced by the works of William Friedman.

The 340 cipher was finally cracked in December 2020 by David Oranchak, Sam Blake and Jarl Van Eycke, who discovered that the cipher was a combination of substitution and transposition (using frequency analysis). The Riverbank Publications is a series of pamphlets written by the people who worked for millionaire George Fabyan in the multi-discipline research facility he built in the early 20th century near Chicago. They were published by Fabyan, often without author credit. The publications on cryptanalysis, mostly written by William Friedman, with contributions from Elizebeth Smith Friedman and others, are considered seminal in the field. In particular, Publication 22 introduced the Index of Coincidence, a powerful statistical tool for cryptanalysis. William Friedman's work on transposition ciphers at Riverbank Laboratories (circa 1916–1920) established foundational, mathematical techniques for solving complex reordering ciphers. Notably, Riverbank Publication No. 19, Formulae for the Solution of Geometrical Transposition Ciphers, provided analytical methods to solve transposition, which he compared to reassembling jigsaw puzzle pieces. It's quite ironical that the title of Friedman's work was called Riverbank Publication No. 19, Formulae for the Solution of Geometrical Transposition Ciphers, when a period 19 shift was required to break the Zodiac Killer's 340 transposition cipher. This was a fraction of the work issued by William Friedman. But was he the inspiration for the Zodiac Killer, who employed this type of cipher on November 8th 1969?