Apple could supply the user with a "fed key" and MITM, when user asks for their friend's key. This is the "public key distribution problem". It's ultimately a

Apple could supply the user with a "fed key" and MITM, when user asks for their friend's key. This is the "public key distribution problem". It's ultimately a social problem (how do I identify that string of bits B corresponds with person P?) and can't be solved entirely in software. Some solutions: 1) Certificate authorities 2) Web of trust 3) Key transparency 4) Gossiping 5) Eschew automation, OOB manual verify There more, but ultimately all of them are ultimately trading off: a) Centralised versus decentralised trust b) Convenience versus security c) Opacity versus transparency And in particular, whether users need to understand what public keys are, and how to verify them.