The cascading effects have made themselves felt in Congress, the courts, popular culture, Silicon Valley and world capitals. The basic structure of the Internet itself is now in question, as Brazil and members of the European Union consider measures to keep their data away from U.S. territory and U.S. technology giants including Google, Microsoft and Yahoo take extraordinary step to block the collection of data by their government.
On Dec. 16, in a lawsuit that could not have gone forward without the disclosures made possible by Snowden, U.S. District Judge Richard Leon described the NSA's capabilities as "almost Orwellian" and said its bulk collection of U.S. domestic telephone records was probably unconstitutional.
The next day, in the Roosevelt Room, an unusual delegation of executives from old telephone companies and young Internet firms told President Barack Obama that the NSA's intrusion into their networks was a threat to the U.S. information economy. The following day, an advisory panel appointed by Obama recommended substantial new restrictions on the NSA, including an end to the domestic call-records program.
Snowden grants that NSA employees by and large believe in their mission and trust the agency to handle the secrets it takes from ordinary people — deliberately, in the case of bulk records collection, and "incidentally," when the content of American phone calls and emails are swept into NSA systems along with foreign targets.
"I asked these people, 'What do you think the public would do if this was on the front page?' " he said. He noted that critics have accused him of bypassing internal channels of dissent. "How is that not reporting it? How is that not raising it?" he said.
Each year, NSA systems collected hundreds of millions of email address books, hundreds of billions of cellphone location records and trillions of domestic call logs.
In the view of the NSA, signals intelligence, or electronic eavesdropping, was a matter of life and death, "without which America would cease to exist as we know it," according to an internal presentation in the first week of October 2001 as the agency ramped up its response to the al-Qaida attacks on New York and Washington.
Using PRISM, the cover name for collection of user data from Google, Yahoo, Microsoft, Apple and five other U.S.-based companies, the NSA could obtain all communications to, from or "about" any specified target. The companies had no choice but to comply with the government's request for data.
That operation, which used the cover name MUSCULAR, tapped into U.S. company data from outside U.S. territory. The NSA therefore believed it did not need permission from Congress or judicial oversight. Data from hundreds of millions of U.S. accounts flowed over those Google and Yahoo links, but classified rules allowed the NSA to presume that data ingested overseas belonged to foreigners.
On June 29, Gilles de Kerchove, the European Union's counterterrorism coordinator, awoke to a report in Der Spiegel that U.S. intelligence had broken into E.U. offices, including his, to implant surveillance devices.
Comparable reactions, expressed less politely in private, accompanied revelations that the NSA had tapped the cellphones of German Chancellor Angela Merkel and President Dilma Rousseff of Brazil. The blowback roiled relations with both allies, among others. Rousseff canceled a state dinner with Obama in September.
"We're seeing al-Qaida and related groups start to look for ways to adjust how they communicate," said Matthew Olsen, director of the National Counterterrorism Center and a former general counsel at the NSA.
In a previous assignment, Snowden taught U.S. intelligence personnel how to operate securely in a "high-threat digital environment," using a training scenario in which China was the designated threat. He declined to discuss the whereabouts the files now, but he said he is confident he did not expose them to Chinese intelligence in Hong Kong and did not bring them to Russia at all.
It would be odd if Russian authorities did not keep an eye on him, but no retinue accompanied Snowden and his visitor saw no one else nearby. Snowden neither tried to communicate furtively nor asked that a visitor do so. He has had continuous Internet access and talked to his lawyers and journalists daily, from his first day in the transit lounge at Sheremetyevo airport.