Highly Reliable Anonymous Cloud Synchronization

Password-based cloud storage (PBCS) allows users to outsource datasets to the cloud server and synchronize the corresponding data. However, there are two key challenges. First, they lack anonymity, making user activities linkable to public identities. Although integrating anonymous password-authenticated key exchange (APAKE) can achieve anonymity, it relies on private information retrieval (PIR), which causes high communication and computation overhead. Additionally, existing APAKE protocols are vulnerable to online password-guessing attacks (PGA). Second, PBCS cannot ensure reliable synchronization, where malicious/victimized servers may roll back user data undetectably. To address these issues, we propose HRACS, a highly reliable and anonymous cloud synchronization scheme, based on our newly designed PIR-free APAKE. Specifically, PIR-free APAKE replaces PIR through a lightweight pseudonym-signature mechanism, reducing communication overhead from linear to constant. The deterministic pseudonym also enables anonymous rate-limiting, mitigating online PGA. Moreover, HRACS simultaneously supports reliable synchronization of datasets by encrypting each item with authenticated encryption and computing a fingerprint of the entire dataset. Each item carries a binding proof to the fingerprint, allowing the user to detect any unauthorized modifications (e.g., reordering or rollback). Experimental results demonstrate that our PIR-free APAKE achieves competitive computation efficiency, reducing the overhead by up to 68.29% compared with existing APAKE protocols. Regarding communication, the bandwidth requirement remains constant and is reduced by up to 99.96% relative to PIR-based APAKE protocols when scaling to 20,000 users. Moreover, HRACS strengthens security without sacrificing performance, incurring overhead comparable to existing PBCS schemes.