I'm not convinced there is any ultimate way to build a secret type app that allows for perfect secrecy. Ultimately anything build on top of the internet has a chain of IP address that have to exist in order to send message back and forth. Add a system with a server in the middle and it doesn't seem like there is any way to avoid someone (or a chain of someones) to figure out the far endpoint. If you can get at least some information leakage (cell phone connections, other IP accesses, etc) I would think you could eventually figure out the device on the other end. The only hope you have to keep the source secret is to make it too difficult or expensive to uncover so no one bothers.