[greendude29]

Neat; but if I were non-technical person, the next question I would ask would be, why can't Mallory just forge Ted's certificate and use that to fool Alice or Bob? In this scenario, Alice or Bob would receive a box which has a certificate which looks very similar to what they remember Ted having.

What would be a good non-technical metaphor for that?

In reality of course, we use trustworthy browsers such as Firefox which (relatively invisibly) enforce certificate validity for us. I'm wondering where that would fit in the metaphor.