Briar is a messenger app that worked on local networks, over Bluetooth, and over Tor if traveling the Internet. Fully encrypted and the purpose was decentralized, serverless messaging.
I liked the concept, and tested it out a little on my Android devices. But it looked straight out of 2009, and it had the issues described in the post. Still. Thanks for the work. I hope it can get revived or inspire others some day.
P.S. feature request! If Alice, Bob and Charlie are all contacts with each other, and Alice writes an offline message to Charlie, Alice should be able to opportunisticly hand the encrypted message to Bob on their shared network, and Bob can deliver it to Charlie.
This P2P system would probably only work if implemented by Google/Apple themselves and they have zero desire to do so since it's a feature almost no one would want.
This is intentionally only included in Forum-mode chats in Briar. Over direct message, leaking contacts is considered a breach of security. (Your definition of "leak" may differ.) In group messages, only the group admin is considered trusted, and every message must go directly to or come from them.
In every security tradeoff, Briar chose the option that maximizes security, even considering how the airwave transmission times might be fingerprintable as Briar traffic.
Not saying any of this is a good way to make a useable app for wide adoption, but it is intentional and highly opinionated.