Dan Brown - Digital Fortress

Dan Brown's first novel.
"When the NSA's most classified technological wonder--an 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." (Dan Brown's site.) http://www.danbrown.com/novels/digital_fortress/plot.html