Context-aware recommendation systems are of increasing popularity in the digital era to recommend personalized items to users. However, how to ensure user data privacy while remaining high recommendation accuracy is widely considered a challenge. In this work, we propose a privacy-preserving method for the context-aware recommendation system in the two-cloud model. In particular, we first adjust the standard additive secret sharing scheme to support secure negative integers computation, based on which we manage to design secure comparison protocol and division protocols that enjoy desirable security and efficiency. By using these new protocols, we propose a secure and efficient context-aware recommendation system that also supports offline users. Compared with the state-of-the-art, our scheme achieves stronger data privacy preservation by further protecting the intermediate data calculated during the system training. Experimental results on real-world datasets indicate that our scheme is efficient. Notable, our system could achieve more significant performance improvement by running the underlying schemes in parallel.

TLS 1.3 is a crucial protocol for securing modern internet communications. To facilitate a smooth transition to post-quantum security, hybrid key exchange, which combines classical key exchange algorithms with post-quantum key encapsulation mechanisms (KEMs), is proposed to enhance the security of the current TLS 1.3 handshake. However, existing drafts and implementations of hybrid key exchange for TLS 1.3 primarily rely on CCA-secure KEMs (i.e., secure against chosen-ciphertext attacks) based on the Fujisaki-Okamoto (FO) transform. The re-encryption step in their decapsulation algorithms not only introduces additional performance overhead but also raises the risk of side-channel attacks. Although Huguenin-Dumittan and Vaudenay (Eurocrypt 2022) and Zhou et al. (Asiacrypt 2024) demonstrated that the weaker CPA-secure KEMs (i.e., secure against chosen-plaintext attacks) suffice for constructing a secure TLS 1.3 handshake, their analyses were limited to single-KEM settings and did not consider the hybrid key exchange scenario. This work challenges the necessity of CCA security by proving that CPA-secure KEMs are sufficient for the TLS 1.3 handshake even in the hybrid key exchange setting. We provide the first formal security proofs for this claim, covering both the classical random oracle model (ROM) and the quantum random oracle model (QROM), thereby ensuring security against quantum adversaries. To validate the practical benefits, we conduct an extensive performance evaluation based on the latest OpenSSL implementation. Our results show that using CPA-secure KEMs yields up to 44.8% performance improvement at the key exchange layer and up to approximately 9% acceleration for the full TLS 1.3 handshake. Beyond performance gains, this approach reduces the codebase’s attack surface by eliminating the re-encryption step, thereby mitigating a class of side-channel vulnerabilities. Our work positions CPA-secure KEMs as a secure, efficient, and practical alternative for standardizing and deploying post-quantum TLS 1.3 even with hybrid key exchange.