'Request that the agent sign the given C{data} with the private key
which corresponds to the public key given by C{blob}.  The private
key should have been added to the agent already.
@type blob: L{bytes}
@type data: L{bytes}
@return: A L{Deferred} which fires with a signature for given data
created with the given key.'

def signData(self, blob, data):

    req = NS(blob)
    req += NS(data)
    req += '\x00\x00\x00\x00'
    return self.sendRequest(AGENTC_SIGN_REQUEST, req).addCallback(self._cbSignData)
