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Fantastic. Fast, comprehensive, and right! What more can you ask for?

I would have required a clarification to the answer as given, and I'm
sure I would have received it if I had asked.  But what I was really
looking for was in the first comment (by Iaint), or more precisely one
of the cryptography pages he linked to, which led me to other places
that answered my question.  The key (no pun intended) was looking for
sites that explained cryptography rather than ones that explained
encryption (even though they're very closely related).  What I hadn't
understood -- even though it's obvious now when I read explanations of
SSL -- is the asymmetric nature of the encryption and decryption keys.
 Now that I understand that, everything's obvious.  Thanks to both
Dandrick and Iaint!

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Awesome, thanks for the quick answer.