The present invention provides a key blank and a key set. The key blank includes one or more telescopic members inserted through corresponding one or more channels of the key shank for fitting to release a security mechanism of a lock unit. The lock set has one or more fitting keys and a lock unit including: a stator; a rotor; a first locking mechanism; and an additional security mechanism including one or more wafer units. The key shank has one or more telescopic members each inserted in a channel perforated in the key shank. The wafer unit of the security mechanism is located and configured for engaging an edge of the telescopic member when the fitting key is used, such that each telescopic member will move the respective wafer unit it engages for releasing the additional security mechanism for allowing the rotor to rotate in the stator.