Presumably there are still multiple ways for messages to be intercepted from the user's iPhone:
- Physical access or confiscation of the device
- Possible backdoor in iOS or the physical iPhone hardware?
- Compromise or physical access to a host machine where the user backs up their device.
Although, I'm not sure what can be done to stop this.
There's no such thing as perfect security in the real world. Instead, your best bet is to raise the cost of breaking the security that you do have. If you raise that cost above the value of that which you're securing, then you are in good shape.
I recently learned about Sancus [0]. It provides a way to protect an application from the operating system. It requires hardware support though. So not available in any mainstream hardware right now. It also allows very secure DRM.
- Physical access or confiscation of the device - Possible backdoor in iOS or the physical iPhone hardware? - Compromise or physical access to a host machine where the user backs up their device.
Although, I'm not sure what can be done to stop this.