Wire app. Not sure if Riot has a web app right now, but they plan to have one when they overhaul the UI at least:

https://medium.com/@RiotChat/a-sneak-peek-at-a-whole-new-rio...

Neither is P2P, but Riot is federated.

Please make this an Ask HN, with a question mark at the end,so as to not confuse other readers. Right now, it seems you're talking about an app that does this.