Trithemius
Fred also mentioned a 15th- to 16th-century abbot named Johannes Trithemius (much less fictional, as far as I can tell, than the monk described in the previous entry) who wrote a trilogy of books titled Steganographia (see a Strange Horizons article for more info on steganography); the third book consisted of a cryptic set of tables of numbers, which people for centuries thought had to do with black magic.
In the late 1600s, a guy named Heidel claimed to that the book was in code, and claimed to have cracked the code, but he encrypted his solution in a code of his own. It wasn't until 1996 that a professor in Pittsburgh published a plain-text solution to the Trithemius code; a couple years later, a New Jersey mathematician (unaware that the code had been broken) independently found and published the solution. And cracked Heidel's code as well.
So all of that is kinda interesting; but more interesting to me is that Trithemius was a contemporary of John Dee, and did some of the same kinds of things Dee did (cryptography, magic, advising the powerful). And then there's this interesting bit from the article:
Trithemius was an adept practitioner of fictionally-enhanced nonfiction. "He wrote histories, chronicles, even fake chronicles," said Dr. Gerhard F. Strasser, a historian at Pennsylvania State University in State College. "He invented people who were only uncovered in the 19th century as being fictitious Germanic heroes," Dr. Strasser added.
Trithemius also was a magician. "Everyone who was interested in magic emulated him," Dr. Reeds said.
I'm now really curious as to whether Trithemius is going to show up in later volumes of John Crowley's Aegypt series.