Not that difficult, but it'd almost certainly have major security flaws that GHCQ and the NSA could exploit, because homebrew end-to-end encryption tends to. The bigger problem is that merely using an Isis-branded chat application is the equivalent of sending a text message to the security services saying "hello, I'm a potential terrorist, please pay special attention to my every movement". Even the Tor project hasn't actually managed to reliably disguise their protocol as something else, and they have a bunch of smart people working on it with the advantage that countries tend to reveal the fact they've been detected through blocking rather than just quietly monitoring the people using it and rounding them up the moment they try something.