Whereas with Textsecure. Well it just works...
By modifying your own device? I don't think so.
In the United States even if an artifact or process is protected by trade secrets, reverse-engineering the artifact or process is often lawful as long as it has been legitimately obtained.
[0] https://en.wikipedia.org/wiki/Reverse_engineering#United_Sta...
As far as I can see that article says that reverse engineering is legal in the case that: (1) the EULA doesn't mention it (I've no idea what Whatsapp EULA says - do you?). (2) it is done for the purpose of interoperability. What is being proposed by the GP is in fact not interoperability but security testing.
As I said before I think that the laws on this are stupid. But why worry about this when there is a great FOSS program in the same space?
There's a much easier way. Turn off your phone's cellular connection, but turn on wifi and connect it to a wifi network you control. Then just sniff the packets.
1) add authentication with other users
2) make a public statement about it (believe it or not, that hasn't happened yet. Perhaps it will come when the iOS versions supports it - or perhaps it never will)
3) commit to the new encryption system in their privacy policy (make it at least somewhat legally binding - which could also be used against abusive law enforcement orders)
If anyone out there does it, feel free to post your findings to http://imfreedom.org/.
I'd be willing to bet that WhatsApp has some competent programmers, and looks very similar to how Apple's built iMessage. I think everyone is entitled to the most security possible, but unfortunately when you're at the scale of WhatsApp, perfect security would make all that ultra-tantalizing data pretty hard to analyze. They're a business, they have a responsibility to their investors to grow the business, and data right now is a _big_ business.