Comment by 10000truths

Establishing secrecy is easy if you've already established authenticity - do a key exchange and use the shared secret for your AEAD. Establishing the authenticity is the tricky part, and most encrypted communications protocols offload that to the user in some way:

SSH - the client pubkey has to somehow reach the server's authorized_keys file, and the server fingerprint has to somehow reach the client's known_hosts file.

HTTPS - The root certs have to be present in the client cert store in order for the client to authenticate the servers they connect to.

PGP - People exchange their public keys in key signing parties after verifying each other's identities in person.