I am assuming he means a long time to fix the trust btwn the US govt and people? Because I am not aware of any concrete evidence that shows damages specifically caused by the disclousres.
Pretty sure most terrosits already knew that they had to be careful with their comm. as it was most likley monitored. And the bombings in europe where not coordinated by advanced encryption, but normal social media accounts as i understand.
Other agencies are in charge of counter terrorism intelligence such as the FBI, DHS and NCTC.
Disclosures about their global surveillance program had much more to do with human intelligence (mass propaganda), industrial espionage, and diplomatic intelligence than terrorism.
The media seriously misinformed the American people about the duties and prerogatives on the NSA, mostly to link the NSA and its excesses to something equally controversial, however, untrue.
Anyway walking back from that line of reasoning: yes. Insurgencies around the world have had their operational security studied for decades and they are very sanitary wrt carrying around cellphones that have their batteries in, etc.
But do you really expect a nation state to say "hey, this is exactly how were are damaged. Please exploit us." That's just the nature of the intelligence business, it's hard to provide proof of damage without causing further harm.
Yes, if they want any rebuttal to Snowden's accusations. Snowden brought proof. If the state wants anybody to believe an alternative explanation, they need to provide at least some evidence. "Just trust us!" is not a useful defense.
> it's hard to provide proof of damage without causing further harm.
That isn't always true, but potential future harm hasn't stopped the government in the past. If revealing sensitive information is politically useful, the perceived realpolitik benefit can outweigh theoretical future harm. This is especially true in today's hypernormal "narrative based" political climate.