'Called back when the private key is returned.  Sign the data and
return the signature.
@param privateKey: the private key object
@type publicKey: L{keys.Key}
@param signData: the data to be signed by the private key.
@type signData: L{bytes}
@return: the signature
@rtype: L{bytes}'

def _cbSignData(self, privateKey, signData):

    return privateKey.sign(signData)
