Lock receives signal from Key, writes down time and picks a random key and uses these to create a ciphertext, encrypts that with the public key of Key to create a second ciphertext and sends. Key receives message, decrypts with private key to first ciphertext and encrypts that with the public key of Lock and sends back. Lock decrypts message with private key and earlier random key, compares to current time and if it has taken more than a set time period does not unlock.

The clock has to be pretty fast, but you can get a secure time of flight measurement, so you can absolutely know the distance of the radio signal path.