If it is true end to end encryption, NSA or anyone listening on the wire cannot read it unless they resort to brute force or some sort of vulnerability in the system. If they can get access to either endpoint they can read it there, or harvest the keys from there.

There were some recent congressional hearings where the FBI, etc wanted to make companies provide a back door into the system. Law enforcement may like it, but anyone who works in security will tell you it is a bad idea. In theory the backdoor key would be provided with a warrant, but there is not a good way of preventing bad guys from exploiting it. Additionally these companies operate globally. Do we want Facebook to provide a backdoor on the legally binding order of a Chinese or Russian court?