Ben Lovejoy on 9to5mac.com:
Handling end-to-end encryption on multiple devices is tricky, as the message is encrypted on the device itself. Apple has a solution for its Messages service, which is also end-to-end encrypted. Instead of a single public key used to identify you to other chat participants, Apple uses a cluster of these: one per device. WhatsApp will likely take the same approach.
There is a slightly increased security risk with this approach, in that a new device could join a chat if it had access to the public key. This is why Apple sends a message to all participants whenever a new device is added to a chat.
Emphasis mine. Not sure how I feel about this new feature yet. On one hand, I would really like the convenience of sending stuff from the iPad, but given the added security risk — it sounds like a spoiled whim. The current desktop implementation works well enough, and it’s inherently safer. Will keep an eye out for a better explanation of what’s sacrificed.