[AliAdams]

If I'm reading this correctly, a keylogger or screen reader on each device would achieve the same result which wouldn't be as much 'reading encrypted messages' as it would be 'reading unencrypted information on a compromised device'.

[onion2k]

Or the app that gets installed manages to access WhatsApp's and Snapchat's private keys on the device. Or compromises a system library that those apps use to generate the user's keys when they're installed. They would be more interesting attacks.