[friend_and_foe]

Sometimes who someone is talking to is enough to cause them problems. If we all had a unique ID for every person we talk to, there's no way to build a social graph. Even if IDs are pseudonymous, there's no forward secrecy there, once you've identified a participant in one social connection you've identified them in all of their social connections. Simplex solves this. Who everyone knows, even if you don't know the surname of everyone involved, is useful information to an adversary.