Physical unclonable keys for smart lock systems using Bluetooth Low Energy

Nowadays, several smart lock systems use Bluetooth Low Energy (BLE) to stablish a wireless communication between the physical key (key fob, card, smartphone, etc.) and the lock. Security is based on creating and storing secret digital keys to establish a cryptographically secure communication. The problem is that several attacks can break such security, particularly the copy of the physical key. In order to increase the difficulty of the attacks, the physical keys described in this paper do not store the secret cryptographic keys but reconstruct them when they are needed and remove them when they are not used. Only the trusted physical keys are able to reconstruct the secrets with the public data stored in them. This is possible by using the start-up values of the SRAM in the BLE chip of the physical key, which acts as a physical unclonable function (PUF), so that if the physical key is copied, the lock cannot be opened. The idea has been proven with the development of a smart lock system with key fobs based on the CC2541 BLE system on chip from Texas Instruments. Experimental results are included to illustrate the performance.