NSA proof app for Apple and similar devices is oxymoron. Until you know what exactly the vendor can do remotely with your device you cannot assume security.
These devices and OS-s are provided by companies that are part of PRISM so they have a track record of collaboration with law enforcement.
So running secure software on them is like putting a steel lock on a mosquito net.
All carrier locked phones, including Android and Firefox OS, can receive ota updates without knowledge or consent. Any open source phone OS can and will be modified to benefit the carrier before reaching consumers.
These devices and OS-s are provided by companies that are part of PRISM so they have a track record of collaboration with law enforcement.
So running secure software on them is like putting a steel lock on a mosquito net.