The present invention relates to a security locking system of the type comprising a cylinder (3) provided with a set of pistons (8) that are able to be positioned by elastic means on the impression formed in the edge (7b, 7c) of a control key (7) perpendicularly to the direction in which said key (7) is introduced into the cylinder (3), these pistons (8) being able to take up two positions, namely a position retracted into the cylinder (3) when the key (7) is introduced into the latter and an extended position when the key (7) is withdrawn there from. This system is characterized in that:—the cylinder is made up of a locking block (3) of parallelepipedal shape that is able to be received in a receiving block (5),—it comprises locking means which are activated when the locking block (3) is introduced into the receiving block (5).