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.