A method includes receiving, in a first device, an access request. The method further includes measuring a motion of the first device to determine a first motion value, performing a pairing protocol with a second device, and granting the access request responsive to a successful pairing in accordance with the pairing protocol. The pairing protocol comprises a cryptographic commitment process. The successful pairing is based at least in part on a determination that a second motion value supplied by the second device substantially matches the first motion value. The cryptographic commitment process comprises sending a committed first motion value to the second device prior to receiving the second motion value from the second device.