>
> I'm curious - how can one completely guard against a MITM attack
> without using encryption?
>
AFAIK, you don't. I mean, you could setup some type of data checksum with a seed. But how do you tell bob without eve knowing without encrypting the seed. BTW, ipsec has a feature like this.
The net was never designed with security as a primary goal. Even today, that is readily apparent.