Congratulations for your great work!
You say no private data is sent to the server. How can it be? I mean, the client must at least ask the server how much money it has on its addresses, what means the server will know which are your addresses, thus it will know how much money you've got on your wallet and how you spend them. Your "private data" is entirely known by the server.
Or by "private data" you meant only the private keys to the addresses? If that's the case, maybe you should correct your statement to make it clearer?
Thanks.
the client must at least ask the server how much money it has on its addresses, correct, but in bitcoin system, the btc addresses are not private data. Every transaction a user made, is seen by all the other peer, including involved addresses and amount of coins and where from/to. Check
http://www.blockexplorer.com to see more details.
So, it means that the "private data" you quoted is entirely known by every one in bitcoin network not only the server.
However, that is safe for everyone. Even you known an btc address and track it from the past to the future, you can't tell who is behind the btc address. Bitcoin ensures everyone is anonymous and safe. It's very difficult to know which person owns what amount. See
https://en.bitcoin.it/wiki/Introduction , "Anonymity" section.
"private data" here is only referred to the private keys to the addresses.