Suddenly I’m more confused. How can you agree on a secret key, remotely, without some foundational source of trust? I don’t trust the default CA packages anymore, given the prevalence of untrustworthy vendors like Comodo.
(For everyone else: just because I'm playing along with this misissuance scenario doesn't mean I don't think ~everyone should trust WebPKI.)
> that’s much safer than using public key cryptography.
It's much harder to screw up than using PKC (thereby making it safer). It's something that is really important for UX - you want to make the easiest path the safest path.
Mail is another option, depending on how secure you think that is.
Cryptography is often not about "uncrackable" but about confidence in security over a foreseen interval.
On the 4th of June 1943 the details of Operation Neptune ("D-Day") were a priceless secret of the Allied Command, a break that allowed Berlin to know the plan could have resulted in the invasion force being expected by a heavily prepared and well-reinforced German defence. Those who made it back to England alive would have no reward for their efforts.
By the 7th of June 1943 plans for Neptune were a historical curiosity of little military value.
And I did mean Neptune, the Overlord plans would have remained valuable for several weeks as they included details of the immediate objectives in Normandy and how resupply would be done, Neptune was just the invasion and related actions.
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.