I understand that securing your bit coins is basically about protecting the private keys for your addresses. This makes sense from my past dealings with public/private key encryption. How do exchanges handle and store your bit coins for you? Is it basically a secured list of your keys? Can you get access to the keys either to back them up, spend them outside the exchange, or move them to another exchange? What would happen if an exchange was compromised (i.e. siezed). I can imagine that I'd lose any dollars but my bitcoins are still accessible to me outside the exchange as long as I have the private keys, right?