For almost two years, Poitras had been working on a documentary about surveillance, and she occasionally received queries from strangers. She replied to this one and sent her public key — allowing him or her to send an encrypted e-mail that only Poitras could open, with her private key — but she didn’t think much would come of it.

 

The stranger responded with instructions for creating an even more secure system to protect their exchanges. Promising sensitive information, the stranger told Poitras to select long pass phrases that could withstand a brute-force attack by networked computers. “Assume that your adversary is capable of a trillion guesses per second,” the stranger wrote.

Before long, Poitras received an encrypted message that outlined a number of secret surveillance programs run by the government. She had heard of one of them but not the others. After describing each program, the stranger wrote some version of the phrase, “This I can prove.”