jm !,ù¬Z¥Wÿö[znó[ºK|th.ÑZäH«ÕLDon't have to reply on a central service or know the location of the other person. Also async - neither party has to be online simultaneously.
https://whatsonchain.com/tx/35f8f0f0d52b7217c67a825d6cf71250ad7e9aa3f897f9630224fd175a2fcee5