I'm looking for the free or open source groupware solution which is best suited to run in a non-trustworthy cloud. My intention is to run the software myself, e.g. on Google Kubernetes Engine or AWS.
Running such a complex software involving database persistence and network traffic in the cloud means that there's no guarantee of security. However, there're features like encryption with powerful key management which decreases changes of intrusion and increases the cost for it.
I'd like to not engage in an unnecessarily deep discussion about reasons for the untrustworthiness of clouds in general and of different providers.