Digital Fortress   by Dan Brown

Quick comparison: Digital Fortress is even better than the Da Vinci Code. For a long time while I was reading this book I was making plans to modify the BookBlog software so I could award six stars. Unfortunately it loses its brilliant plausibility at the end when the computer science becomes plain wrong, and a clue to a dead man's password comes from nowhere, and is more obvious than the book makes out (at least to a computer programmer with a degree in physics as I do; perhaps it is more convincing to others), but at the same time the correctness of the solution is dubious.

The book is about an attack on the NSA by a former employee, who convinces them that he has created an unbreakable encryption algorithm, but actually it is just a cover to get the experts to drop their guards and let a worm into the system. Against a backdrop of sophisticated insights into the workings of one of the United State's most secret agencies, a simple action adventure takes place on the other side of the Atlantic where a civilian is doing the secret service's dirty work to hunt down a ring that belonged to the dead man (which turns out to be a red herring), but he himself is pursued by others from the secret service despatched by a double-crossing supremo.

So it has fallen short of the impossible six stars, but is still as good as a book gets, and comfortably achieves the maximum rating.