Digital Fortress

The National Security Agency owns a machine called TRANSLTR, a huge computer able to break almost any code in minutes. Then it meets one it cannot crack. The code, called Digital Fortress, was written by Ensei Tankado, a gifted former employee who opposed the agency's spying and now threatens to release his creation to the world. When Tankado suddenly dies in Seville, the only key to the code seems to have disappeared with him.

Commander Trevor Strathmore calls in Susan Fletcher, the agency's top cryptographer, to help deal with the crisis. He also sends her fiancé, David Becker, a professor who speaks several languages, to Spain to recover the dead man's belongings, including a gold ring engraved with strange letters. In Seville, Becker follows the ring from a hospital to a hostel and through the narrow streets of the old town, while a silent hired killer follows him. Back at the NSA, Susan begins to suspect that the problem inside the codebreaking dome is much bigger than anyone admits.

Fast and light, with plenty of puzzles to solve along the way. About 384 pages. It was Dan Brown's first novel, published long before The Da Vinci Code.