How to authenticate with challange and private keys? [Java]

[membersound]

I'd like to translate the following JS code into Java code:

Code:

var key = new Bitcoin.ECKey(Crypto.util.hexToBytes(SHA256(password))); //password = private user password
var signed_challenge = Crypto.util.bytesToHex(key.sign(Crypto.util.hexToBytes(SHA256(challenge_key)))); //challenge_key = generated key from server

The challenge_key is the desired output and must be send back to the server.

I'm want to use bitcoinj for this. Maybe anyone has already worked with it and can help how the calls in java have to be constructed? I came up with the following, but don't know how to continue:

Code:

String password; //the private user pw
String challenge; //a string from the server

BigInteger privkey = new BigInteger(Hex.decode(Sha256Hash.create(password.getBytes()).toString()));
ECKey key = new ECKey(privkey);
key.sign(new Sha256Hash(challenge));

If what I did is correct: how do I get the challange_key that has to be send back to the server out of the ECkey that I signed?

[Mike Hearn]

key.sign returns an object with the r/s components in it. You can send them back to the server.