Yes, you need some foundational source of trust, but you were going to communicate that public key one way or another. If you don't trust that medium (let's say

Yes, you need some foundational source of trust, but you were going to communicate that public key one way or another. If you don't trust that medium (let's say it's WebPKI TLS and an adversary gets a bad cert from Comodo somehow), the adversary could just lie about the public key too. A MITM adversary that's in a position to unseal and reseal libsodium secretbox messages can also do that with libsodium box. (For everyone else: just because I'm playing along with this misissuance scenario doesn't mean I don't think ~everyone should trust WebPKI.)