Shared vehicle security

A vehicle comprising: a locking mechanism for securing the vehicle; a user input device exposed to the exterior of the vehicle; a transceiver for communicating with a network external to the vehicle; and a processor configured for implementing a public/private key security algorithm and coupled to the transceiver for sending and receiving data by means of the transceiver, and coupled to the locking mechanism for disengaging the locking mechanism to permit access to the vehicle; the processor being configured to: receive from the transceiver a data block including a first token, the first token representing a public key of a user; receive from the user input device a second token; compare the first and second tokens to determine whether the second token is consistent with having been derived from a private key that forms a key pair with the public key; and if that determination is positive, cause the locking mechanism to be disengaged.