AnoPas: Practical anonymous transit pass from group signatures with time-bound keys

An anonymous transit pass system allows passengers to access transport services within fixed time periods, with their privileges automatically deactivating upon time expiration. Although existing transit pass systems are deployable on powerful devices like PCs, their adaptation to more user-friendly devices, such as mobile phones with smart cards, is inefficient due to their reliance on heavy-weight operations like bilinear maps. In this paper, we introduce an innovative anonymous transit pass system, dubbed $AnoPas$, optimized for deployment on mobile phones with smart cards, where the smart card is responsible for crucial lightweight operations and the mobile phone handles key-independent and time-consuming tasks. Group signatures with time-bound keys (GS-TBK) serve as our core component, representing a new variant of standard group signatures for the secure use of time-based digital services, preserving users’ privacy while providing flexible authentication services. We first constructed a practical GS-TBK scheme using the tag-based signatures and then applied it to the design of AnoPas. We achieve the most efficient passing protocol compared to the state-of-the-art AnoPas/GS-TBK schemes. We also present an implementation showing that our passing protocol takes around 38.6 ms on a smart card and around 33.6 ms on a mobile phone.