The monk Johannes Trithemius, considered one of the founders of modern cryptography, had ingenuity in spades. His three-volume work, Steganographia, written around 1500, describes an extensive system for concealing secret messages within innocuous texts. On its surface, the book seems to be a magical text, and the initial reaction in the sixteenth century was so strong that Steganographia was circulated only privately until publication in 1606. But less than five years ago, Jim Reeds of AT&T Labs deciphered mysterious codes in the third volume, showing that Trithemius' work is more a treatise on cryptology than demonology. Reeds' fascinating account of the code-breaking process is quite readable.
One of Trithemius' schemes was to conceal messages in long invocations of the names of angels, with the secret message appearing as a pattern of letters within the words, for example, as every other letter in every other word:
padiel aporsy mesarpon omeuas peludyn malpreaxo
which reveals "prymus apex."
Another clever invention in Steganographia was the "Ave Maria" cipher. The book contains a series of tables, each of which has a list of words, one per letter. To code a message, the message letters are replaced by the corresponding words. If the tables are used in order, one table per letter, then the coded message will appear to be an innocent prayer.
The modern version of Trithemius' scheme is the Spam Mimic program.