The Americans had tried almost everything, but they just couldn't crack an encrypted message they came across while investigating the 9/11 attacks. Finally, they approached a 17-year-old boy in Delhi about whom The New York Times had done a feature. Over the next 10 days, Ankit Fardia hunkered down in his room in Delhi and came up with the key to crack the message. The worst fears of American investigators came true — Al-Qaeda was using a sophisticated technology, called steganography, to communicate. It involved sending encrypted messages concealed in a photograph or series of photographs. “I was lucky in...