Danny Yadron:

In the past decade, Moxie Marlinspike has squatted on an abandoned island, toured the U.S. by hopping trains, he says, and earned the enmity of government officials for writing software.

Mr. Marlinspike created an encryption program that scrambles messages until they reach the intended reader. It’s so simple that Facebook Inc.’s WhatsApp made it a standard feature for many of the app’s 800 million users.

The software is effective enough to alarm governments. Earlier this year, shortly after WhatsApp adopted it, British Prime Minister David Cameron called protected-messaging apps a “safe space” for terrorists. The following week, President Barack Obama called them “a problem.”