The big difference between WhatsApp and RCS is the level of encryption. The messages on WhatsApp are end-to-end encrypted, i.e., you and your WhatsApp buddy have access to the messages, whereas, in the case of RCS messages, they are encrypted in transit but are not end-to-end encrypted. This means that Google or the carrier can read your messages, i.e., no privacy is maintained.
WhatsApp provides additional security in the form of fingerprint lock, whereas RCS does not support this system. Finally, WhatsApp even offers a new group chat privacy setting which is just amazing!
RCS messaging even has some positive points, though. RCS features are added to the existing applications like SMS/MMS, so the application can operate without any internet connectivity, whereas WhatsApp works with an internet connection. So no internet connectivity implies no WhatsApp. RCS also promises to be available on every Android mobile set shortly.
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