DOPS: A Practical Dual Offline Payment Scheme of CBDC for Mobile Devices

Dual offline payment has a great practical value for the transactions without network connection. Unfortunately, it is a puzzle in the view of central bank digital currency (CBDC) which should keep the centralized banking system and simultaneously avoid double-spending, counterfeiting, relay attack and other issues. There is limited literature about securely designing the dual offline payment solution specifically for CBDC. One public method that establishes the mobile payment on both trusted execution environment and secure element (SE), is too heavy to promote the general application. In response to this issue, we propose DOPS, a feasible dual offline payment scheme for two equipotent e-wallets of mobile device users. By only adopting SE, the secure architecture of the mobile device is constructed. Based on it, DOPS comes with the complete data structures and the core procedures for offline payment. We list the exceptional scenarios that might arise during dual offline transactions and provide explicit protocols for addressing them. A security analysis of the approach is presented based on realistic assumptions. Subsequently, we develop and evaluate a prototype system using feasible parameters. Our assessment demonstrates that DOPS fulfills the practical needs of offline CBDC payments for mobile users, excelling in both security and efficiency aspects.