The result is that if you want to send a secure RCS message between Android devices, you need to use Google’s own Messages app at both ends; it implements E2EE using the well-worn Signal protocol. Similarly, Apple adopted RCS in iMessage last year, but with a proprietary implementation of E2EE.
In short, it’s a confusing jumble. This is one reason why alternatives such as WhatsApp and Signal, both of which also use the Signal protocol, have become so popular; you get E2EE out of the box without compatibility worries, and they allow groups of up to 1,024 members.
Having a single protocol, MLS, covering E2EE changes the story. Now RCS with MLS can offer a range of advanced features including large groups, which are critical for businesses which need many-to-many communication. Right now, if even one user in a group is using an RCS app without compatible E2EE, the security of the whole group chat can be compromised. MLS gives every app maker one IETF standard to aim for.
