RCS uses a set of standard internet protocols like Session Initiation Protocol (SIP) to establish a connection between two clients
Google wants to ensure that the communication of Android users is protected even if they do not use secure messenger platforms. The SMS successor RCS is to get end-to-end encryption for this.
Rich Communication Services (RCS) is
the globally standardized successor to SMS, which also allows the exchange of
multimedia content. In contrast to the purely text-based SMS, it can be
used in the same way as the various self-contained messenger platforms, but is
equally available on all newer cellular devices. So you don't have to ask
the desired communication partner how he can be reached.
However, the standards for the communication channel do not provide for encryption from scratch. This means that these messages are basically just as easy to intercept as is the case with conventional SMS. Google wants to improve this and encrypt all RCS messages on the sender's device at least on Android platform and only translate them back into plain text at the recipient.
PKI with signal
Google wants to fall back on the
proven method of the signal protocol, which is already used on various other
messaging platforms and provides strong encryption according to the public key
concept. Google would operate its own key server on which the public keys
of the participants can be provided.
The procedure would then be limited to the RCS communication between users who use Android's messaging app in an appropriately equipped version. In principle, the use of the signal protocol could also guarantee that the developers of other apps or smartphone platforms would follow suit. If the other person is not adequately equipped, the entire system automatically reverts to unencrypted communication.