The relevance (to Nash's death):
"Books one and two were clearly systems for encoding messages and were the first books written on cryptography, Dr. Reeds said." Very interesting if true.
The joke:
Trithemius offered plenty of hints, Dr. Reeds said. "He says that
there are elaborate calculations that you have to do, and he tells you
that you have to read the tables," he said. When Trithemius said that
people can send messages without using letters he probably just meant
they could use number codes instead.
The cipher used in book III is very simple, however. Throw out null values, reduce modulo 25, and map in reverse alphabetical order to Latin letters. The first two books of the Steganographia contain a "mind numbing variety" of other encoding methods.
The joke:
Here's the article describing the solution:Reeds, Jim. "Solved: the ciphers in book III of Trithemius's steganographia." Cryptologia 22.4 (1998): 291-317 http://profs.sci.univr.it/~giaco/download/Watermarking-Obfus...
The cipher used in book III is very simple, however. Throw out null values, reduce modulo 25, and map in reverse alphabetical order to Latin letters. The first two books of the Steganographia contain a "mind numbing variety" of other encoding methods.