A key-operable security lock, of the type comprising: - a stator body; - a rotor body rotatably mounted within said stator body and provided with a passage for insertion of a key for operating the lock; and - coding means for preventing rotation of said rotor body with respect to said stator body and for enabling, instead, said rotation when an authorised key is inserted in said passage; wherein said coding means comprise: - at least one first pin, which is carried by said rotor body and is able to slide along its own axis, projecting within said passage for insertion of the key; - at least one second pin, which is carried by said rotor body in a fixed position along its own axis, with the possibility of rotation about said axis, projecting within said passage for insertion of the key; - at least one third pin, which is carried by said stator body and is able to slide along its own axis, co-operating with said first pin for determining a condition of blocking of the lock in which said first pin or third pin sets itself in a position of interference straddling the interface between said rotor body and said stator body; and - at least one blocking member, which is carried by said rotor body and is mobile between an operative position where it projects from said rotor member and engages a seat made in said stator body, and a position where said member is entirely contained within said rotor body, wherein said second pin has an outer surface configured for co-operating with said blocking member so as to determine the position of said member on the basis of the orientation of said second pin about its own axis, said lock being characterized in thatsaid rotor body comprises a protective body, which projects within said passage for insertion of the key so as to obstruct it, in order to hinder possible attempts at effraction made through said passage, for example by way of a drill, and wherein said protective body is slidably mounted along said ideal axis of rotation between two opposite end positions. - a stator body; - a rotor body rotatably mounted within said stator body and provided with a passage for insertion of a key for operating the lock; and - coding means for preventing rotation of said rotor body with respect to said stator body and for enabling, instead, said rotation when an authorised key is inserted in said passage; - at least one first pin, which is carried by said rotor body and is able to slide along its own axis, projecting within said passage for insertion of the key; - at least one second pin, which is carried by said rotor body in a fixed position along its own axis, with the possibility of rotation about said axis, projecting within said passage for insertion of the key; - at least one third pin, which is carried by said stator body and is able to slide along its own axis, co-operating with said first pin for determining a condition of blocking of the lock in which said first pin or third pin sets itself in a position of interference straddling the interface between said rotor body and said stator body; and - at least one blocking member, which is carried by said rotor body and is mobile between an operative position where it projects from said rotor member and engages a seat made in said stator body, and a position where said member is entirely contained within said rotor body, wherein said second pin has an outer surface configured for co-operating with said blocking member so as to determine the position of said member on the basis of the orientation of said second pin about its own axis, said lock being characterized in thatsaid rotor body comprises a protective body, which projects within said passage for insertion of the key so as to obstruct it, in order to hinder possible attempts at effraction made through said passage, for example by way of a drill, and wherein said protective body is slidably mounted along said ideal axis of rotation between two opposite end positions.