Establishing a secret key and a secure channel does not require trust. It just requires a pair of keys so that you can be sure that the message gets to the othe

Establishing a secret key and a secure channel does not require trust. It just requires a pair of keys so that you can be sure that the message gets to the other party unintercepted and unmodified. However, just because you have a channel that can't be MitM-ed doesn't mean you can actually trust whoever is on the other end. The problem of "how do you know you trust the other end" requires some foundational source of trust, whether that's trusting the transportation medium (that the person you handed the USB key to is actually who he/she claims to be), the issuer (could be yourself), and so on.