For more than half a century, governments all over the world trusted a single company to keep the communications of their spies, soldiers and diplomats secret.

The company, Crypto AG, got its first break with a contract to build code-making machines for U.S. troops during World War II. Flush with cash, it became a dominant maker of encryption devices for decades, navigating waves of technology from mechanical gears to electronic circuits and, finally, silicon chips and software.