'Generate a check_authentication request message given an
id_res message.'

def _createCheckAuthRequest(self, message):

    signed = message.getArg(OPENID_NS, 'signed')
    if signed:
        for k in signed.split(','):
            oidutil.log(k)
            val = message.getAliasedArg(k)
            if (val is None):
                oidutil.log(('Missing    signed    field    %r' % (k,)))
                return None
    check_auth_message = message.copy()
    check_auth_message.setArg(OPENID_NS, 'mode', 'check_authentication')
    return check_auth_message
