Get WhatsApp messages

Whatsapp did not publish any official APIs.

There's this open source API for communicating with whatsapp, it's not official and might stop working if Whatsapp update their protocols.

https://github.com/venomous0x/WhatsAPI

Regarding the legality of using this or other non-official API, it depends on the service agreement that you agreed to with Whatsapp. Read it and see if they frown upon using their communication protocols with clients other than theirs. My guess would be they do not allow it.

UPDATE: This GitHub repository seems to have been taken offline due to legal threats.