Messaging Layer Security. It's a protocol for encrypted group messages, so far it has required central servers, though nostr:nprofile1qqspwwwexlwgcrrnwz4zwkze8rq3ncjug8mvgsd96dxx6wzs8ccndmcpzemhxue69uhhyetvv9ujuurjd9kkzmpwdejhgqgcwaehxw309aex2mrp0yh8xmn0wf6zuum0vd5kzmqpzfmhxue69uhhqatjwpkx2urpvuhx2uctyqxu5 has figured out how to do it with nostr and we have whitenoise which is kind of like signal on nostr.