Sorry if this is a silly question, but I keep running into situations where a small CLI or GUI tool that could be handed a single file and hand me back an encrypted version would be useful. I've done some googling, but all I typically turn up is blogspam about random Windows-only tools that seem to be of dubious quality.

Anyone know of a good tool for this type of thing?

First, some context. The latest from the US justice department saying that they will be focusing on finding "ANTIFA leaders" is incredibly troubling for anyone involved in leftist groups. I foresee a lot of good activists, regardless of how far left they actually are, arrested on trumped up charges in order to squash opposition.

Organizing is essential to resist fascism. This is made more difficult by the pandemic, as in person meetings bring a huge, almost unacceptable risk. As such, many orgs have been turning to platforms like Slack instead. Trouble is, Slack logs are not encrypted and I am certain that as a business based in the US Slack will not put up a fight to keep user data safe if the feds come calling.

I'd like to collect a decent list of alternatives. Important factors include encryption, ownership, open source status, ease of use, federation, scalability, hosting, cross platform, and anything else you can think of.