An apparatus and a method for an authentication protocol. In the aforesaid lowered, inoperative, position, the table is inclined with respect to a vertical plane parallel to the longitudinal plane of symmetry of the chair, in such a way that two or more chairs of the same type can be set longitudinally against one another, with the tables of two adjacent chairs partially overlapping one another. The primary winding is adapted to be electrically connected to a low-voltage ignition signal. Each edge in the input graph is described based on the shape, orientation and pen lift of the edge trajectory that the edge represents.