I don't see why a fridge should talk directly do a server. That would be a lot of fridges to reflash or more likely replace the sensor if the server changes.

I don't see why a fridge should talk directly do a server. That would be a lot of fridges to reflash or more likely replace the sensor if the server changes. If they all go to a hub, all you have to do is update the hub, plus, the hub can do stuff like local alerts. Symmetric encryption is probably the way to go for use cases that directly connect. I wish there was something like an IPv9 (7 and 8 are probably taken!) with 48 byte addresses and a fixed layout. 8 byte ISP identifier, 4 bytes customer, 2 bytes subnet, 2 bytes device, and 32 bytes for a public key hash. No more certificates, no more insecure LAN connections, just IP that's always secure no matter what as long as you have the right address, and even if you change ISPs you keep the key part of the address.