Composable Anonymous Proof-of-Location With User-Controlled Offline Access

A proof-of-location ( $pol$ ) is a digital credential issued to a user after proving their location to an issuer. The user can use the $pol$ at a later time to prove to a verifier that they have been present at a claimed location. A secure Proof-of-Location (POL) system requires that $pol\text{s}$ be unforgeable and non-transferable to other users. POL systems can be used to provide fine-grained authentication and authorization and must ensure the privacy of the $pol$ owner against the issuer and the verifier while allowing efficient presentation of $pol\text{s}$ combined with other credentials when needed. Efficiency is in terms of communication overhead in user-verifier POL sessions, which has particular significance in high-volume $pol$ verification scenarios. We first propose a POL system that (i) is provably secure in a simulation-based framework, allowing a $pol$ to be securely used with other credentials, and (ii) provides anonymity against the issuer and the verifier. We then extend the system to allow $pol\text{s}$ to be stored on a public distributed ledger system and selectively be presented to the verifiers by the user. This is the first POL system that satisfies the above properties. We implement POL algorithms on a mobile phone and present our experimental results showing the practicality of the system. Our proposed scheme is highly scalable compared to existing systems, reducing the user-verifier POL communication overhead by up to a factor of 94.