Discover the Copiale Cipher: The Mysterious 18th-Century Book That Took 260 Years to Decode

In the world of cryptography, substitution ciphers are child’s play. Indeed, we may remember literally playing with them as children, writing secret messages to our friends by replacing all the letters with numbers, say, or shifting them one or two places over in alphabetical order. Cracking such codes was a trivial matter even before the computer age, but certain simple variations could make them more robust. Take the document known as the Copiale cipher (downloadable as a two–part PDF), a 105-page bound manuscript that stayed undecipherable for more than 260 years. Its mystery finally yielded to the efforts of University of Southern California computer scientist Kevin Knight and Uppsala University linguists Beata Megyesi and Christiane Schaefer only in the early twenty-tens.

As Tommie Trelawny tells the story of the Copiale cipher in the Hochelaga video above, the manuscript, which was originally thought to date between 1760 and 1780, first had to be converted into machine-readable code. The text’s use of 88 unique symbols, one of them shaped like an eye, necessitated coming up with names for all of them apart from the Roman letters, which had no particular meaning in isolation.

When another scan searched for repeated letter combinations, its results shed light on probable similarities with the German language. This made sense, since the book was found in Germany in the first place. Could multiple symbols in this strange cipher have been substituted for single German letters? Could the code be, in cryptographic terms, a homophonic cipher?

Approaching the text under that hypothesis revealed meanings suggesting, tantalizingly, that it had been written by a secret society. It even describes an initiation ritual in which the inductee must first “read” a blank piece of paper, then try again with eyeglasses, then again after washing his eyes, and then, finally, undergo a symbolic “operation” involving the plucking of a single eyebrow. This society, the Oculists, turns out to have been composed entirely of ophthalmologists meeting in the seventeen-forties. That they did so covertly may owe to their having been Freemasons, whose rites had recently been banned by Pope Clement XII. The Copiale cipher suggests that Oculists appear to have had no aims more sinister than the pursuit of knowledge — not that, for most of us today, the notion of eighteenth-century eye surgery isn’t terrifying enough.

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Based in Seoul, Colin Marshall writes and broadcasts on cities, language, and culture. He’s the author of the newsletter Books on Cities as well as the books 한국 요약 금지 (No Summarizing Korea) and Korean Newtro. Follow him on the social network formerly known as Twitter at @colinmarshall.