You should really consider using a separate offline machine for processing withdrawals. Sign the transactions on the offline machine, and then broadcast said transactions with another PC that's connected to the internet. That way you keep a vacuum between the network and your private keys. Also, the transaction signing machine shouldn't be used for anything else, since that could get it compromised.
Actually, we're thinking of two possibilities:
-the first is having a dedicated machine, connected to the network although totally behind a firewall, whose task would be sign and broadcast all transactions as they come (I described it here:
https://bitcointalksearch.org/topic/m.1289607). That would allow "instant" bitcoin withdrawal but of course wouldn't be as secure as the second proposal
-this second proposal is yours: having a dedicated machine totally offline, with manual transaction signing once a day every business day.
This will be up to you which one you prefer, when we're ready will ask all of our users what they prefer.
Is that one encrypted as well? If not, where is that one stored?
It's not encrypted (you still need the passphrase to use the wallet though), and stored with our important business paper at our house (where our current company is). There is no lock to access these papers, but it's in a room closed when visitors come.
Where is that paper stored?
In my partner's wallet (the physical one I mean), with all her others important stuff.
Sounds good! Does this backup also contain the database which holds the information how much USD and BTC each account holder has, plus open positions if any? I wouldn't want an evil hacker deleting your database like he did with Bitcoinica, with you not knowing who owns what.
Yep, it contains user balance (usd and btc), open positions, user addresses,... So we know exactly who owns what from these dumps. Also we keep 7 days of dumps history, in case a corrupted/empty db is dumped.
Do you use 2-factor authentication on Mtgox?
We have a yubikey.
And are waiting for mtgox to set up an unlink page, I wouldn't want for this key to stop working as it happened to someone else a few days ago.
Thanks for all the answers! Looking very good so far. I might just try your site out
Thanks to you. While the engines and algorithms still needs optimization and will go through lots of change during the beta phase, the security is already implemented as I feel this is the most important feature in this kind of platform.