Ask HN: PKI and/or cryptography learning resources for the busy

The comments on this recent thread (https://news.ycombinator.com/item?id=19128097) pointed out many apparent flaws and oversimplifications in the explanation of PKI.

I'm sure many people may not understand these in depth, but at the same time may want to understand a little more (without getting too mathematical or getting a PhD).

Please list any resources for others to learn, and also state the difficulty level (if you are able to).