Digital Fortress By Dan Brown

The story is set in the year of 1996. When the United States National Security Agency's code-breaking supercomputer TRANSLTR encounters a revolutionary new code, Digital Fortress, that it cannot break, Commander Trevor Strathmore calls in head cryptographer Susan Fletcher to crack it. She is informed by Strathmore that it was written by Ensei Tankado, a former NSA employee who became displeased with the NSA's intrusion into people's private lives. If the NSA doesn't reveal TRANSLTR to the public, Tankado intends to auction the code's algorithm on his website and have his partner, "North Dakota", release it for free if he dies, essentially holding the NSA hostage. Strathmore tells Fletcher that Tankado has in fact died in Seville at the age of 32, of what appears to be a heart attack. Strathmore intends to keep Tankado's death a secret because if Tankado's partner finds out, he will upload the code. The agency is determined to stop Digital Fortress from becoming a threat to national security.

Strathmore asks Fletcher's fiancé David Becker to travel to Seville and recover a ring that Tankado was wearing when he died. The ring is suspected to have the passcode that unlocks Digital Fortress. However, Becker soon discovers that Tankado gave the ring away just before his death. Unbeknown to Becker, a mysterious figure, named Hulohot, follows him, and murders each person he questions in the search for the ring. Unsurprisingly, Hulohot's final attempt would be on Becker himself.