Crypto is almost always implemented as a library, and probably ultimately as a kernel module in most cases. There is no reason at all you should need to call out to a remote service to encrypt and decrypt data. That wouldn't even make sense, as it would need to be encrypted before traversing the network to be of any use. And there are plenty of commodity chat applications like Mattermost and RocketChat that you can self-host, again not necessarily needing to use someone else's cloud service but also not need to roll your own application.