It will be open source, end to end encrypted using Signal’s double ratchet encryption protocol, and he plans to make it easy for fediverse platforms to integrate it. The beta will release later this month.
He’s also the creator of https://fedidb.org btw
I’ve not been on either platform long enough to use the DMs, but this is a good point.
After all, DMs aren’t actually private on either platform, as far as I’m aware.