Digital Fortress by Dan Brown

National Security Agency, Fort Meade, Maryland: when the most powerful intelligence organization on earth's invincible code-breaking machine encounters a mysterious code it cannot break, it calls for its head cryptographer, Susan Fletcher, a brilliant and beautiful mathematician. What she uncovers sends shock waves throuugh the corridors of power. The NSA is being held hostage...not by guns or bombs, but by a code so ingeniously complex that if released it would cripple US intelligence. Seville, Spain: the creator of the code, Ensei Tankado, is found dead. And with him has died the secret to an impregnable code-writing formula that threatens to obliterate the post-cold war balance of power. Forever. From the underground hallways of power to the skyscrapers of Tokyo to the towering cathedrals of Spain, a desperate race unfolds. Caught in an accelerating tempest of secrecy and lies, betrayed on all sides, Susan Fletcher finds herself fighting not only for her country but for her life, and in the end, for the life of the man she loves.