diffie hellman. as in the guys that discovered rsa/dsa/ec asymmetric cryptography, specifically associated with generating shared encryption keys. we haven't even started to come close to what is possible. private replies are just the tip of the iceberg. the whole point of nostr threw out the whole notion of central authorities. we can prove who we are, all this other stuff is above this level. nostr identity is self sovereign. you throw the dice, you get a key, and that key is your identity. it is hard to fake that, and that's the point.