DIGITAL FORTRESS. (D.Brown)

When the NSA's invincible code-breaking machine - encounters a mysterious code it cannot break, the agency calls in its head cryptographer, Susan Fletcher, a brilliant and 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 ingeniously complex that if released it will cripple U.S. intelligence.