We present a locking-based design-for-security methodology to prevent piracy of RF transceiver integrated circuits. The solution is called SyncLock as it locks the synchronization of the transmitter with the receiver. If a key other than the secret key is applied, synchronization and, thereby, communication fail. SyncLock is implemented using a novel locking concept consisting of two spatially separated mechanisms. A hard-coded error is hidden into the design to break synchronization while error correction, i.e., unlocking, takes place in another part of the design by applying the secret key. SyncLock offers several advantages: the secret key is unique, i.e., any incorrect key causes a denial-of-service, there is no performance penalty, it can be seemingly integrated into the digital design flow, area and power overheads are negligible, and it achieves maximum provable security thwarting all known counterattacks. SyncLock is demonstrated with hardware measurements.