Steganographia

A work by Johannes Trithemius
From the Wikipedia page (at one point incorporated into the article on Trithemius).
Trithemius' most famous work, Steganographia (written c. 1499; published Frankfurt, 1606), was placed on the Index Librorum Prohibitorum in 1609 and removed in 1900. This book is in three volumes, and appears to be about magic—specifically, about using spirits to communicate over long distances. Since the publication of the decryption key to the first two volumes in 1606, they have been known to be actually concerned with cryptography and steganography. Until recently, the third volume was widely still believed to be solely about magic, but the "magical" formulae have now been shown to be covertexts for yet more cryptography content. However, mentions of the magical work within the third book by such figures as Agrippa and John Dee still lend credence to the idea of a mystic-magical foundation concerning the third volume. Additionally, while Trithemius's steganographic methods can be established to be free of the need for angelic–astrological mediation, still left intact is an underlying theological motive for their contrivance. The preface to the Polygraphia equally establishes, the everyday practicability of cryptography was conceived by Trithemius as a "secular consequent of the ability of a soul specially empowered by God to reach, by magical means, from earth to Heaven".
The work has lent its name to the modern field of steganography.
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