India, Feb. 7 -- It was Yogesh Panchal's first overseas trip. So, naturally, one of the first things the 33-year-old entrepreneur did after checking into his hotel was take a tour of the sights. He took the mandatory selfies, clicked pictures of the big-ticket tourist attractions, and sent them via WhatsApp to his wife, relatives and friends back home in India.

Then Yogesh returned to his hotel room. "I was talking to my wife over a WhatsApp call, when suddenly some plainclothes people came to my room, blindfolded me, and took me away me for an inquiry."

What Yogesh hadn't factored in was that he was visiting Tehran, capital of one of the world's most secretive nations, where censorship is a way of life. On his phone were images of what...