def client_id_is_known(provider, authentication_request):
    """
    Verifies the client identifier is known.
    :param provider: provider instance
    :param authentication_request: the authentication request to verify
    :raise InvalidAuthenticationRequest: if the client_id is unknown
    """
    if authentication_request['client_id'] not in provider.clients:
        logger.error('Unknown client_id \'{}\''.format(authentication_request['client_id']))
        raise InvalidAuthenticationRequest('Unknown client_id',
                                           authentication_request,
                                           oauth_error='unauthorized_client')