Book 3 of Trithemius's Steganographia (written ca. 1500) contains hidden cipher messages within what is ostensibly a magic work. After almost 500 years these ciphers have been detected and solved. (Since 1606 it was known that similar ciphers were present in Books 1 and 2.) This has implications not only for the history of cryptography but also for the history of 16th century magic. Trithemius has been considered one of the main figures in the rise of the occult movement in the 1500's, but it now turns out that the evidence is less clear.
Read my long-titled paper, Solved: The Ciphers in Book III of Trithemius's Steganographia which was published in the October 1998 issue of Cryptologia, available here in PDF form. (Thanks to Clay Holden for the PDF version.)
Read the 14 April 1998 New York Times story.
Read Ivars Peterson's account of the story, also here.
Read the 29 June 1998 Pittsburg Post Gazette version of this story.
Read the version in La Stampa, 21 April 1999.
Read about it in a University of Leiden newsletter.