A safety lock includes an interchangeable lock core unit which may indifferently be of the type comprising a double-bit mechanism or of the type comprising a cylinder-lock mechanism. Provided on the outer face of the casing of the lock is a member (38) for protecting the area where the key is introduced, said protective member comprising a fixed body (39) permanently connected to the rigid casing, and an interchangeable internal element (A2, B2) which has a different conformation according to whether the core installed in the lock is of the double-bit mechanism type or of the cylinder-lock mechanism type.