Popular chained Byzantine Fault Tolerant (BFT) consensus protocols adopt a pipelined paradigm to achieve efficiency. However, these protocols remain vulnerable to critical attacks, such as forking and partitioning. While the two-phase commit rule can prevent forking, it often requires complex or expensive view-change mechanisms. Moreover, our study confirms that partitioning attacks are feasible in many existing chained BFT protocols. This paper devised NunChuck, a fully two-phase chained BFT protocol that ensures linear authenticator complexity, optimistic responsiveness, and resilience to partitioning attacks. NunChuck employs a lightweight certificate-based view-change mechanism to maintain efficiency, and introduces a unified message structure that enables leaders to make progress even under partitioned network conditions. We formally prove its safety and liveness properties. Furthermore, we develop a prototype implementation in Golang and conduct extensive evaluations. The results demonstrate that NunChuck consistently outperforms HotStuff and FastHotStuff, particularly in adversarial environments, with simulated delay analysis further corroborating our experimental findings.

Secure aggregation becomes a major solution to providing privacy for federated learning. Secure aggregation for mobile devices typically relies on Shamir secret sharing (SSS) to achieve dropout robustness, but limits the system’s corruption and dropout tolerance. Although Prio+, a state-of-the-art method utilizing two non-colluding servers, avoids such limitations, its effectiveness is only against honest-but-curious servers. Thus, this paper presents a novel secure aggregation protocol in the malicious model. The proposed protocol uses a non-colluding server and initiator to achieve almost full (up ton-2) corruption and dropout tolerance, and exploits our discrete-logarithm (DL) extractable and equivocable commitment scheme to achieve malicious security. The proposed protocol’s security is proven in two models: malicious users colluding with the server and malicious users colluding with the initiator. Finally, a prototype of the developed protocol is implemented, with the experimental results demonstrating that our protocol is efficient and suitable for both cross-device and cross-silo federated learning scenarios. Compared with the sum protocol of Prio+, the proposed protocol achieves malicious security with affordable additional overhead, i.e., 4.8 to 6.1 times more computation cost and 2.8 to 2.9 times more communication cost for a single user.

We introduce the notion of complementary-witness-soundness for a witness indistinguishable proof system. We then present a new general framework for public key encryption schemes, denoted as CWSWI-type encryption, based on a complementary-witness-sound witness indistinguishable (CWSWI) proof system. The construction of a CWSWI-type encryption scheme is relatively straightforward, while the security thereof against adaptive chosen ciphertext attack (denoted CCA2) can be rigorously proven. Many existing CCA2 public key encryption schemes can be interpreted as being of CWSWI-type, including the schemes of Lindell, Cramer-Shoup and Peikert-Waters. Thus, our CWSWI approach provides a novel means for designing and analysing CCA2 secure public key encryption schemes.

We study the problems of outsourcing computation where a computational weak client outsources its computation task to a powerful server. Our work focuses on the research about outsourcing polynomial functions. Based on Zhang and Safavi-Naini's work, we construct a private outsourcing computation scheme for polynomial functions. Our scheme can keep privacy of the outsourced data and function. It is efficient in the sense of amortization and satisfies public verification. Compared with Zhang and Safavi-Naini's work, our construction uses a new efficient verify-method and makes the verification process public (any other clients can verify the correctness of the results returned by the server using the public key).