Polygraphia

On its surface, Polygraphia presents itself as a work of angelic communication: page after page of tables assigning long strings of Latin words to individual letters, organized under the names of angels and the days and planets those angels were said to govern, apparently offering a method for composing messages an angel could read. A reader working only from that surface presentation, and many of Trithemius's contemporaries did, would take the book for a species of ceremonial magic, elaborate, systematic and dense, but magic nonetheless.

Worked correctly, the tables do something else entirely: each word in a given table's column stands for the same letter, so that a message written by selecting, letter by letter, an appropriately meaningful-sounding word from the correct column produces an ordinary looking passage of invented prose that in fact spells out a hidden message beneath its surface, a technique modern cryptography calls steganography combined with a substitution cipher. Because the finished message reads as continuous, if odd, prose rather than as an obvious code, it is concealed twice over: once by not looking like a cipher at all, and once by requiring a reader to already know which angelic and planetary framework governs which table before extracting anything from it.

Trithemius wrote a second, related work, Steganographia, which he chose not to publish, circulating it only in manuscript among a small circle of correspondents, and its reputation suffered accordingly: rumors that it taught genuine demonic communication attached to it for over a century, and it was formally placed on the Catholic Church's Index of Forbidden Books in 1609, decades after Trithemius's own death. Polygraphia, by contrast, Trithemius published openly in 1518, dedicated to a reigning Holy Roman Emperor, and its more overtly angelic and planetary framing, closer to the conventions of Renaissance natural magic that educated readers already found intellectually respectable, let it escape the suspicion that dogged its unpublished sibling.

That difference in reception outlasted both books' authors by centuries. Steganographia's true cryptographic content was not conclusively demonstrated until the cryptographer Jim Reeds published a decipherment of its most notorious passages in 1998, showing that what had read for four hundred years as literal instructions for commanding spirits was, in fact, an elaborate coded cipher manual, vindicating Trithemius against the demonic reputation his unpublished work had carried since the sixteenth century. Polygraphia, having always been readable as a legitimate work of learned angelology by those who did not look further, never generated the same controversy, and stands today as the more historically secure of the two: openly printed, correctly credited in its own time as an innovation in secret writing by later cryptographic scholars, and free of the centuries of suspicion its companion book only recently shed.