I wonder if this is similar to Snapchat. Snapchat uses AEC-ECB encryption (which you should never use) with a single key that is not-so-carefully hidden in every binary file they've ever distributed to a mobile device. The kind folks at Gibsonsec revealed Snapchat's "security by obscurity" and documented the API so well that I was able to make a Java Snapchat client in a few hours.

I hope that's not the case for WhatsApp, they have a lot more to lose.