[FactolSarin]

Google's RCS encryption scheme isn't really proprietary, it's the Signal e2ee scheme. Granted, it's bolted into RCS and not part of the standard, but it's not 100% closed either. It's like a standard added to another standard in a nonstandard way.

[kelnos]

The key management is proprietary, for some reason. Can't build an interoperable impl without that.