'Return the signature of a message.
If I am not a sign-all association, the message must have a
signed list.
@return: the signature, base64 encoded
@rtype: str
@raises ValueError: If there is no signed list and I am not a sign-all
type of association.'

def getMessageSignature(self, message):

    pairs = self._makePairs(message)
    return oidutil.toBase64(self.sign(pairs))
