Equipped with various sensors and intelligent systems, modern cars turn into entities with connectivity, autonomy, and safety. Car rental/car sharing is an innovative transportation concept and integral in today's urban living. It enables users to access a fleet of vehicles located throughout cities. Complementing public transportation, the car-sharing service helps people to meet their transportation needs economically and in an environmentally responsible manner. When a customer wants to rent a car from a rental company or an owner wants to share a private car with his/her friends or family members, the customer or the user should gain admission to the car, such as unlocking the door and starting the engine. In this paper, we proposed a novel and secure key-sharing system named hierarchicalidentity-based signature key sharing (HIBS-KSharing), which consists of key generation, key transmission, and key management (e.g., remote issuing, revocation of access rights, and their delegation to other users or sharers). We implemented our proposed system based on Nexus smartphones and near-field communication devices. Compared with existing key-sharing schemes of car rental/sharing, our proposed HIBS-KSharing system is secure and easily extended.