> In order to store and exchange user public keys like identity keys and prekeys, we need to have a central key server. Unlike the RCS messaging servers, the key server is currently only hosted by Google.
* Ibid § Key Server
For interoperability there would (potentially) need to be a neutral third-party where everyone's public key(s) are present. ("Everyone" potentially being all folks who use RCS, which may be every phone on the planet.)
Android/Google encryption is proprietary as evidenced by the fact that the Content-Type header is "application/vnd.google.rcs.encryption":
* https://www.gstatic.com/messages/papers/messages_e2ee.pdf § Messages Encryption
They state:
> In order to store and exchange user public keys like identity keys and prekeys, we need to have a central key server. Unlike the RCS messaging servers, the key server is currently only hosted by Google.
* Ibid § Key Server
For interoperability there would (potentially) need to be a neutral third-party where everyone's public key(s) are present. ("Everyone" potentially being all folks who use RCS, which may be every phone on the planet.)