Substitution

Simple substitution by hand or automatically: map letters by click, keyboard or drag beside frequency bars, take word-pattern suggestions, test a probable word, or let simulated annealing find the key by English quadgrams. Homophonic substitution, many symbols to a letter, is solved the same way with an index-of-coincidence check, as for the Zodiac 408. Al-Kindi's ninth-century treatise is the earliest known frequency analysis; Poe's “The Gold-Bug” (1843) turns on one.

• Ibrahim A. Al-Kadi, “Origins of Cryptology: The Arab Contributions”, Cryptologia 16 (2), 1992, pp. 97–126
• Edgar Allan Poe, “The Gold-Bug”, Dollar Newspaper, Philadelphia, 1843 — Edgar Allan Poe Society of Baltimore transcription
• David Oranchak, Sam Blake and Jarl Van Eycke, “The Solution of the Zodiac Killer’s 340-Character Cipher”, arXiv:2403.17350 (2024)