Thank you for the detailed explanation. 

Some US websites will only accept US credit card numbers - a Canadian or British VISA number will be refused even though the company accepts VISA, and even if I am in the US. Will these cards work in such a situation?

Hi, first post here. I'm glad to see there are some interesting topics at hand! I've been wondering about this kind of thing for a while (and with recent "for your own good" activities making news as of late, who isn't?) But I haven't have a chance to look up (or cook up) anything yet. So thanks for the links - Cryptocat in particular looks like a good solution for times you don't need something more involved (OTPs) or for corresponding with people who are less tech-savvy.