Last month, the cryptographer and coder known as Moxie Marlinspike was getting settled on an airplane when his seatmate, a midwestern-looking man in his 60s, asked for help. He couldn't figure out how to enable airplane mode on his aging Android phone. But when Marlinspike saw the screen, he wondered for a moment if he was being trolled: Among just a handful of apps installed on the phone was Signal.

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