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squidlogicon Five Open-Source Slack Alternatives

Maybe give balboa.io a try, it's the closest I can think of. Not just DMs but also file storage and transfer. Kind of similar UI to slack.

Also, peerio.com, but they don't have a 'channels-like' interface, so maybe not what you're looking for.

squidlogicon Tor Messenger Beta: Chat Over Tor, Easily

Good to have another E2EE chat option. End to end encryption needs to be the new normal. It's not enough to just encrypt the connection to your service anymore. That might have been a feather in the caps of eCommerce sites in the 90s, but today we can and should do better.

squidlogicon Sustaining Digital Certificate Security

Couldn't you have a browser plugin that would verify your js is being served up from the correct source?