How would ISIS go about distributing and training hundreds and thousands of grunts to use said app. Assuming most of their force is technically illiterate and/or literally illiterate. And, manages to do so without leaking the information to opposition intel.

For every developer willing to create such a tool for financial incentives there are likely an order of magnitude more grunts who would be willing to leak the information for a cash payout.

Make no mistake. For as much as ISIS loves to preach that their cause is purely ideologically driven, their existence depends on the ability to pay their troops a living wage.

You know what form of communication suffers from none of those issues? What their grunts likely already use? Gaming systems and voice chat. Considering the demographic of their fighting force, that's the first place I'd look.

Don't get me wrong. I respect John McCain for his service. I just think his motivations lately are focused more on policy and less on the war effort. The end goal being, to once again outlaw the non-government sanctioned use of encryption.