When the NSA's invincible code-breaking machine encounters a mysterious code it cannot break, the agency calls 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 would cripple U.S. intelligence. Caught in an accelerating tempest of secrecy and lies, Susan Fletcher battles to save the agency she believes in. Betrayed on all sides, she finds herself fighting not only for her country but also for her life, and in the end, for the life of the man she loves.This is clearly a standard techno-thriller; although you can argue that it may have elements of science fiction, it can still fit comfortably into the fiction category. It is (I believe rightfully) designated as fiction.
Neal Stephenson's Cryptonomicon:
In 1942, Lawrence Pritchard Waterhouse-mathematical genius and young Captain in the U.S. Navy-is assigned to detachment 2702. It is an outfit so secret that only a handful of people know it exists, and some of those people have names like Churchill and Roosevelt. The mission of Waterhouse and Detachment 2702-commanded by Marine Raider Bobby Shaftoe-is to keep the Nazis ignorant of the fact that Allied Intelligence has cracked the enemy's fabled Enigma code. It is a game, a cryptographic chess match between Waterhouse and his German counterpart, translated into action by the gung-ho Shaftoe and his forces.This novel is as much a techno-thriller as Digital Fortress. Both emphasize technology, but neither slip far enough into fantastical elements that you would call them science fiction. However, the author of Cryptonomicon became famous for books like Snow Crash, in which a near-future humanity spends much of its time in a virtual-reality version of the Internet called the "Metaverse". Thus, since the author got his start in sci-fi, he is relegated to the ghetto and his book is automatically labeled as science fiction.
Fast-forward to the present, where Waterhouse's crypto-hacker grandson, Randy, is attempting to create a "data haven" in Southeast Asia-a place where encrypted data can be stored and exchanged free of repression and scrutiny. As governments and multinationals attack the endeavor, Randy joins forces with Shaftoe's tough-as-nails granddaughter, Amy, to secretly salvage a sunken Nazi submarine that holds the key to keeping the dream of a data haven afloat. But soon their scheme brings to light a massive conspiracy with its roots in Detachment 2702 linked to an unbreakable Nazi code called Arethusa. And it will represent the path to unimaginable riches and a future of personal and digital liberty... or to universal totalitarianism reborn.
Well-known sci-fi and fantasy authors like Orson Scott Card have been fighting this pigeonholing of their novels for years with very limited success. It's nearly impossible for them to write standard fiction when publishers want to put their book in the sci-fi and fantasy section where those who are looking for fiction novels won't see it and those looking for science fiction or fantasy won't want to read it.