Digital FortressBy: Dan Brown When the National Security Agency's invincible code breaking machine encounters a mysterious code it cannot break, the agency calls in its head cryptographer, Susan Fletcher, a brilliant, beautiful mathematician. What she uncovers sends shock waves through the corridors of power. The NSA is being held hostage not by guns or bombs, but by a code so complex that if released would cripple U. S. intelligence. Caught in an accelerating