For what it’s worth, it’s the analogy I use, and for the twenty or so college sophomores I taught on Tuesday, the analogy is sufficiently understandable that they were able to generate and use SSH key pairs to do their homework.

I think the reason we (the HN crowd) don’t like it is that the analogy starts to break down when you start thinking through all of the operations you can do with public keys. But this does not matter much for somebody learning to use them for the first time. I’ve had to grow comfortable with the idea of giving people imperfect explanations so that they can build an intuition. Once that happens I can return with the mathematics so that they can really understand what is going on.