I always enjoy a novel with an academic protagonist but the Da Vinci Code reads like a bad conspiracy theory using the roles of the professor and other experts to give the theory some weight. But I bring up this book in this weblog because it spends considerable time on various cryptographic schemes.
I don't blame the author for not using modern cryptography but the methods described would be laughable 50 or 100 years ago. Imagine using the password 1123581321 to guard the biggest secret in the history of religion. It only gets worst: backwards writing, simple anagrams, substitution ciphers, riddles. I suppose these make for fun puzzles for the reader but do not make for a safe secret.
The book describes one intriguing device supposedly invented by Leonardo Da Vinci called a cryptex, a small cylinder with a combination lock that will destroy its written contents if broken. However the book calls it a rudimentary form of public-key cryptography, which only tells me Dan Brown has no idea what that term means.
For a far better novel dealing with cryptography and the related paranoia, check out Neal Stephenson's Cryptonomicon.