In vehicular ad hoc networks (VANETs), conditional privacy preserving authentication (CPPA) scheme is widely deployed to solve security and privacy issues. Existing CPPA schemes usually require ideal tamper-proof devices (TPDs) on vehicles which, however, might be infeasible or do not exist in reality due to high security requirements. To address this problem, we propose a practical framework of CPPA scheme that supports more realistic TPDs which are less secure correspondingly. We demonstrate that this framework also manages to achieve nonframeability in addition to other security objectives including nonrepudiation, conditional privacy preserving, and unlinkability. Moreover, performance analysis shows that our framework has better efficiency in authentication. All these features make our framework practical for VANETs.

To address the confidentiality concerns of malicious adversaries that fully compromise the message broker in pub/sub based IoT systems, several researchers use proxy re-encryption (PRE) to realize end-to-end encrypted message distribution (from publisher to subscriber). However, the all-or-nothing share feature of PRE poses a problem that the share cannot be efficiently revoked. The only way for publishers to revoke the access rights of subscribers is to pick a new public-private key pair and re-generate the re-encryption keys for all the remaining subscribers, which hampers the scalability in practice. To realize efficient user revocation, we present REEDS, an efficient revocable end-to-end encrypted message distribution system for IoT. The core of REEDS is a novel proxy-aided identity-based conditional proxy re-encryption (PIB-CPRE) scheme. Essentially, we use a binary-tree structure to organize re-encryption keys, so that the update of re-encryption keys is reduced from linear to logarithmic in the number of subscribers. We show that REEDS satisfies confidentiality, efficient immediate revocation, decentralized authorization, and maintains low overhead for publishers and subscribers. The prototype system is implemented and its performance is evaluated. The results show that REEDS is not only easy to deploy over existing message brokers but also highly efficient.

The Snowden revelations in 2013 showed that user machines running cryptographic protocols might be stealthily corrupted by attackers (e.g., manufacturers and supply-chain intermediaries) who could tamper cryptographic implementations to insert backdoors to undermine cryptographic tools. To formalize such attacks, in CRYPTO 2014, Bellare et al. proposed the notion of Algorithm-Substitution Attack (ASA) which has been extensively studied so far. In this work, we turn to investigate the security of anonymous communication (AC) protocol---a well-known tool to protect user privacy on the Internet---in the case when user machines are corrupted. Specifically, we give a formal treatment of ASAs on the universal mixnet-based AC (U-Mix-AC) protocols. We show that ASAs on U-Mix-AC protocols could be more dangerous than previously thought by presenting attacks that are extremely powerful. As the countermeasure, we adopt cryptographic reverse firewall (CRF), originally proposed by Mironov and Stephens-Davidowitz in EUROCRYPT 2015, to restore the security of U-Mix-AC protocols in the presence of ASAs. We also implement our proposed ASAs and CRFs for experimental evaluations, and the results show that the execution time of subverted algorithms is almost the same as that of faithful ones and our designed CRFs are effective to guard the security of U-Mix-AC protocol.

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.