How web vendors do it: PKI. PKI (root certs) was very controversial at the time it was proposed. It has endured because it's easy for things that are happening

How web vendors do it: PKI. PKI (root certs) was very controversial at the time it was proposed. It has endured because it's easy for things that are happening to keep happening. It's the idea that because a company with a root cert (eg. Verisign) says you are who you say you are, they will vouch for you. But it's not that secure, and plenty of people have demonstrated over the years it's easy to impersonate companies/websites by swindling Verisign first. Microsoft has been a victim on numerous occasions. Diffie Hellman isn't really susceptible to a MITM. It just means the party at the other end of your secure channel could be a hostile actor. So you need a way to discriminate on who's naughty/nice. Like Santa. For your use case you could distribute a usb stick of blessings to the computers you have deemed virtuous. But a nice computer can turn into a naughty computer, etc. So if the two machines can communicate at least once over a hostile network, then they can ensure future conversations are secure (co-authenticated) and not susceptible to mitm. The problem is, they might not be communicating that first time. I hope this captures the nuance.