They aren't backdooring modern open-source encryption. They may have some elite knowledge about some esoteric corner of the code that allows them to theoretically throw a data center at the problem for a month or two, but the days of easy backdoors to decrypting everything in real time are gone imho. It is just too easy to implement mathematically-strong encryption these days. Too many people know how to do it from scratch. The NSA's real job is keeping american systems safe. That is done through creating the best encryption possible. They are very good at that job.