Update: Be sure to read the followup discussion, which explains Facebook's point of view, that this is a deliberate compromise, and not a defect, that makes the app more usable for a wide variety of users, while putting them to little additional risk (namely, that Facebook might change its mind; or be forced to spy on its users; or suffer a security breach or internal rogue employee).
When Facebook implemented Open Whisper Systems' end-to-end encrypted messaging protocol for Whatsapp, they introduced a critical flaw that exposes more than a billion users to stealthy decryption of their private messages: in Facebook's implementation, the company can force Whatsapp installations to silently generate new cryptographic keys (without any way for the user to know about this unless a deep settings checkbox had been ticked), which gives the company the ability to decrypt user messages, including messages that have already been sent in the past.. (more…)