Paper 2025/539 Original: eprint.iacr.org/2025/539 ↗

Aegis: Scalable Privacy-preserving CBDC Framework with Dynamic Proof of Liabilities

Abstract

Blockchain advancements, currency digitalization, and declining cash usage have fueled global interest in Central Bank Digital Currencies (CBDCs). The BIS states that the hybrid model, where central banks authorize intermediaries to manage distribution, is more suitable than the direct model. However, designing a CBDC for practical implementation requires careful consideration. First, the public blockchain raises privacy concerns due to transparency. While zk-SNARKs can be a solution, they can introduce significant proof generation overhead for large-scale transactions. Second, intermediaries that provide user-facing services on behalf of the central bank commonly performs Proof of Liabilities on customers' static liabilities. However, in real-world scenarios where user liabilities can arbitrarily increase or decrease, the static nature poses such as window attacks. In this paper, we propose a new smart contract-based privacy-preserving CBDC framework based on zk-SNARKs, called $\textbf{Aegis}$. our framework introduces a transaction batching technique to enhance scalability and defines a new dynamic PoL which is near-real time. We formally define the security models for our system and provide rigorous security proofs to demonstrate its robustness. To evaluate the system’s performance, we instantiate our proposed framework and measure its efficiency. The result indicates that, the end-to-end process, including proof generation for 512 transactions, takes approximately 2.8 seconds, with a gas consumption of 74,726 per user.