Digital Fortress is a science-fiction/suspense novel and author Dan Brown's first published work. It was published first as an ebook in 1996 and then as a paperback in 1998.

Plot

Susan Fletcher, a brilliant mathematician, head of National Security Agency's cryptography division, finds herself faced with an unbreakable code, resistant to brute-force attacks by a 3-million processor supercomputer. Along with her fiancé, a skilled linguist with eidetic memory, she must find a solution to stop the spreading of the code.

Criticism

The book has been criticized for its inaccurate depictions of cryptography. It has also been criticized for its short-attention-span writing style (128 chapters in 429 pages) and its unbelievable characters.

Code solution

The code that appears in the end of the book

128-10-93-85-10-128-98-112-6-6-25-126-39-1-68-78

is decrypted by looking at the first letter of the chapter for each number. For example, chapter 128 starts 'When Susan awoke'. The resulting text is

WECGEWHYAAIORTNU

Decryption is performed using a columnar transposition cipher, termed a "Caesar Square" cipher in the book (this is unrelated to the Caesar cipher). The letters are arranged into a square:

WECG
EWHY
AAIO
RTNU

and read from the top down.

WEAREWATCHINGYOU

Add spaces and you get the plaintext,

"We are watching you"

a reference to the NSA's monitoring systems.

External link

• A forum for discussing the book. The site also has an alternate solution to the code.