Abstract: A transponder embedded in a key and a reader/exciter coil surrounding the lock cylinder are inductively linked to provide energization to the transponder, causing it to emit a digital identification code. A control circuit associated with the coil supplies energizing coil current and analyzes the emitted code to verify the authenticity of the key. The coil and control circuit are contained in a planar multi-layer hardboard printed circuit having high circuit integrity and being very thin for accommodation in the lock assembly. Alternatively, the coil and control circuit are contained in a flexible printed circuit. There the coil includes two opposed elongated legs which are positioned on opposite sides of the lock cylinder and folded in a way to conform to the cylinder surface, forming in effect a very thin cylindrical coil. An extension on the printed circuit form a path to the control circuit which is remote from the cylinder.