Federated Learning (FL), an innovative distributed paradigm, has attracted significant interest for its inherent privacy preservation in collaborative model training. However, recent studies demonstrate that publicly shared gradients are vulnerable to malicious reconstruction of sensitive client data. While countermeasures like differential privacy and homomorphic encryption exist, they typically compromise model accuracy or computational efficiency, hindering practical deployment. This work simultaneously addresses two critical challenges in the FL training process: 1) efficient protection of client privacy, and 2) guaranteeing the authenticity of client gradients while ensuring the verifiability of the server's aggregation result. To this end, we propose an efficient and authentication-enhanced privacy-preserving protocol. Our solution allows clients to mask their local gradients and furnish corresponding proofs. The aggregation server subsequently verifies all submissions, aggregates only the valid masked gradients, and generates a proof for clients to verify the correctness of the aggregation result. Furthermore, the protocol is designed to be robust against client dropout. We provide formal proof that our protocol meets all security requirements in a semi-trusted environment. Both comprehensive theoretical analysis and extensive experimental evaluations confirm that our approach achieves more robust security, better dropout resilience, and superior overall efficiency compared to state-of-the-art protocols such as PSA, VerifyNet, and EVP.

Timed signatures are cryptographic primitives that enable senders to predefine the validity period of a signature. Currently, two primary types of timed signatures have been developed. The first type, known as Verifiable Timed Signatures (CCS'2020), implements a delay before a signature becomes effective. The second type is Short-Lived Signatures (ASIACRYPT'2022), which allows for the setting of an expiration time for signatures upon creation. However, certain applications requiring time-sensitive authorization demand both activation and expiration times to be set, a requirement not fulfilled by the existing timed signature schemes. To overcome this limitation, we propose a novel flexible timed signature scheme called Time Interval Signatures (TIS). TIS combines Verifiable Delay Functions and Short-Lived Signatures with our Zero-Knowledge Proof of Product, facilitating the flexible setting of both activation and expiration times for the signature. Building on TIS, we present TimeGuardian, a time-bound NFT rights protocol that enables presetting authorization and revocation periods for NFT usage rights. Experimental results show that TIS achieves signature size reductions of 98.67% and 57.14% compared to existing verifiable timed signature solutions.

Blockchain transaction verification is fundamental to decentralized financial applications, ensuring both transaction authorization and integrity. Most existing verification schemes utilize an equal weight model that grants identical rights to all participants. However, these schemes fail to capture real-world scenarios like Proof-of-Stake blockchains, where participants have right discrepancies. Additionally, many schemes depend on trusted third parties for key generation, introducing single points of failure and compromising security. To address these limitations, we propose WBlock that is a weighted and decentralized verification scheme. WBlock involves a distributed key generation protocol that embeds each entity's weight into its secret share, eliminating the need for trusted third parties. It further employs a Schnorr-type weighted threshold signing protocol, enabling distributed transaction authorization while reflecting participant weight in signature shares. Security analysis shows that WBlock achieves both correctness and unforgeability. Comparative theoretical analysis shows that our key generation and signing protocols outperform existing methods in terms of round complexity, communication overhead and data size. Experimental results confirm that WBlock achieves a balance between security and efficiency, with acceptable overhead for real-world blockchain deployment.

Public key encryption with equality test (PKEET) has been widely adopted in applications such as private health record management, secure outsourced data processing, and email filtering, owing to its ability to test equality on ciphertexts encrypted under different public keys. However, existing PKEET schemes often fall short in specialized settings, such as case-insensitive matching. Moreover, their security guarantees remain inadequate. In particular, many existing schemes are vulnerable to offline message recovery attacks (OMRA), which present a significant security challenge to PKEET. Furthermore, the existing IND-CCA security model is incomplete, as it fails to model all potential attacks that an adversary could exploit to execute the OMRA. To address these challenges, we propose a new variant of PKEET, termed public key encryption with case-insensitive fuzzy equality test (PKE-CIFET). To analyze security, we propose a unified security model that more closely aligns with IND-CCA security than previous works. This model includes more comprehensive oracles and considers adversaries launching the OMRA through multi-hop testing. Based on this model, we provide a comprehensive and rigorous security proof. Furthermore, experimental results demonstrate that the proposed PKE-CIFET scheme is efficient in terms of computational cost.

Dynamic Searchable Symmetric Encryption (DSSE) plays a crucial role in secure cloud-based database systems, as it enables efficient keyword search and dynamic updates over encrypted data. However, practical deployment of DSSE schemes faces two significant challenges. First, clients may inadvertently perform faulty updates—such as re-adding an existing keyword-identifier pair or attempting to delete a non-existent one—which can compromise the correctness of subsequent search results. Second, even with correctly issued updates, malicious servers may return incorrect or incomplete search results, undermining data integrity. To address these challenges, we propose FVDSSE, the first fault-tolerant DSSE scheme that tolerates client-side operational faults and provides result verifiability against malicious servers. Moreover, it simultaneously ensures strong privacy by guaranteeing forward and backward privacy—two essential properties for any practical DSSE. To further optimize performance, we present FVDSSE-C, an enhanced variant that leverages caching techniques. Experimental evaluations on a real-world dataset show that FVDSSE-C achieves up to 130× improvement in search efficiency and 3× reduction in communication overhead compared to the state-of-the-art scheme (YCR22-C).

Recent developments in the field of Dynamic Searchable Symmetric Encryption (DSSE) with forward and backward privacy have attracted much attention from both research and industrial communities. However, most DSSE schemes with forward and backward privacy schemes only support single keyword queries, which impedes its prevalence in practice. Although some forward and backward private DSSE schemes with expressive queries (e.g., conjunctive queries) have been introduced, their backward privacy either essentially corresponds to single keyword queries or forward privacy is not comprehensive. In addition, the deletion of many DSSE schemes is achieved by addition paired with a deletion mark (i.e., lazy deletion). To address these problems, we present two novel DSSE schemes with conjunctive queries (termed SDSSE-CQ and SDSSE-CQ-S), which achieve both forward and backward privacy. To analyze their security, we present two new levels of backward privacy (named Type-O and Type-O-, more and more secure), which give a more comprehensive understanding of the leakages of conjunctive queries in the OXT framework. Eventually, the security analysis and experimental evaluations show that the proposed schemes achieve better security with reasonable computation and communication increase.

Ring signatures enable a user to sign a message on behalf of a group while preserving both anonymity and unforgeability. Despite these strong privacy guarantees, they present regulatory challenges. To address these issues, we introduce a novel cryptographic primitive:timed anonymous ring signatures(TARS). Unlike group signatures, which rely on a trusted third party, TARS maintains the decentralization and unforgeability of traditional ring signatures while incorporatingtimed anonymity, allowing the signer’s identity to be disclosed by any user after a predetermined time period, denoted asT. To realize this, we propose a new CCA-securetimed public key encryption(TPKE) scheme that ensures correct decryption without the secret key after the timeT. Building upon TPKE, we present two concrete TARS constructions that guarantee anonymity until timeTand unforgeability at all times. To demonstrate its applicability, we apply TARS to a decentralized bidding system that is anticollusion between the auctioneer and the bidders. The system ensures anonymous bidding while disclosing the winner’s identity after the bid announcement, maintaining a transparent, fair, and decentralized bidding process. Finally, experimental evaluations confirm the practicality and efficiency of the proposed schemes. Crucially, the TARS scheme extends conventional ring signature schemes with timed anonymity by introducing only a moderate computational overhead (experimentally measured at ≈1.37 seconds under our configuration) while preserving their cryptographic robustness.

Blockchain-based cloud storage is one of the areas which have been developing rapidly in both academia and industry in recent years. As a key feature to improve the usability of such systems, keyword search is yet missing in existing blockchain-based cloud storage systems due to the encryption of uploaded files. To enable this important feature for blockchain-based cloud storage systems while preserving privacy, blockchain-based Symmetric Searchable Encryption schemes have attracted a lot of research interest. In this research, we pinpoint three important requirements for blockchain-based SSE systems to be practical. The first two requirements are fast query response and low on-chain storage cost. They are the key for a blockchain-based SSE system to be usable and feasible. The third requirement is soundness, which guarantees that dishonest behaviours from both users and service providers can be detected and prevented. To the best of our knowledge, none of the existing studies achieves these properties at the same time. To fill in this gap, we present a novel construction of the blockchain-based SSE system. We provide both theoretical analysis and empirical evaluation to show that the system achieves all the desired properties.

Dynamic searchable symmetric encryption (DSSE) enables the data owner to outsource its database (document sets) to an untrusted server and make searches and updates securely and efficiently. Conjunctive DSSE can process conjunctive queries that return the documents containing multiple keywords. However, a conjunctive search could leak the keyword pair result pattern (KPRP), where attackers can learn which documents contain any two keywords involved in the query. File-injection attack shows that KPRP can be utilized to recover searched keywords. To protect data effectively, DSSE should also achieve forward privacy, i.e., hides the link between updates to previous searches, and backward privacy, i.e., prevents deleted entries being accessed by subsequent searches. Otherwise, the attacker could recover updated/searched keywords and records. However, no conjunctive DSSE scheme in the literature can hide KPRP in sub-linear search efficiency while guaranteeing forward and backward privacy. In this work, we propose the first sub-linear KPRP-hiding conjunctive DSSE scheme (named HDXT) with both forward and backward privacy guarantees. To achieve these three security properties, we introduce a new cryptographic primitive: Attribute-updatable Hidden Map Encryption (AUHME). AUHME enables HDXT to efficiently and securely perform conjunctive queries and update the database in an oblivious way. In comparison with previous work that has weaker security guarantees, HDXT shows comparable, and in some cases, even better performance.

In this article, we introduce a new mechanism for constructing multi-client searchable encryption (SE). By tactfully leveraging the RSA-function, we propose the first multi-client SE protocol that successfully avoids per-query interaction between data owner and client. Therefore, our approach significantly reduces the communication cost by eliminating the need for data owner to authorize client queries at all times. To be compatible with the RSA-based approach, we also present a deterministic and memory-efficient ‘keyword to prime’ hash function, which may be of independent interest. Further, to improve efficiency, we put forward a more generic construction from set-constrained PRFs. The construction not only inherits the merits of our first protocol, but also achieves an enhanced security (against untrusted clients), where colluding attack among clients is also taken into account. Both protocols are instantiated via the recent representative SE protocol by Cashet al.with the support of boolean queries. At last, we implement our proposed protocols and comprehensively evaluate their performance to demonstrate their practicability and scalability.

Due to its capabilities of searches and updates over the encrypted database, the dynamic searchable symmetric encryption (DSSE) has received considerable attention recently. To resist leakage abuse attacks, a secure DSSE scheme usually requires forward and backward privacy. However, the existing forward and backward private DSSE schemes either only support single keyword queries or require more interactions between the client and the server. In this article, we first give a new leakage function for range queries, which is more complicated than the one for single keyword queries. Furthermore, we propose a concrete forward and backward private DSSE scheme by using a refined binary tree data structure. Finally, the detailed security analysis and extensive experiments demonstrate that our proposal is secure and efficient, respectively.

Proxy re-encryption (PRE) allows a proxy to transform one ciphertext to another under different encryption keys while keeping the underlying plaintext secret. Because of the ciphertext transformability of PRE, there are many potential private communicating applications of this feature. However, existing PRE schemes are not as full-fledged as expected. The lack of necessary features makes them hard to apply in real-world scenarios. So far, there does not exist a unidirectional multihop PRE scheme with constant decryption efficiency and constant ciphertext size without extensions. Impractical performance and weak scalability also hinder PRE from most real-world applications. In this work, we present a new PRE scheme with secure hardware enclave namedhPRESS(hardware-enhanced PRE scheme using secure enclave). To the best of our knowledge,hPRESSis the first unidirectional multihop PRE scheme which achieves both constant decryption efficiency and constant ciphertext size without extensions. A detailed security analysis demonstrates that our proposal is CCA secure based on the security of the underlying encryption schemes and the secure enclave. We also implement a prototype based on Intel SGX, one of the most popular secure enclave techniques in recent years, and evaluate its performance. The experimental results show that, compared with previous PRE schemes, ourhPRESSis almost one order of magnitude faster in terms of the decryption and transformation.

We propose a multi-client Symmetric Searchable Encryption (SSE) scheme based on the single-user protocol [3] . The scheme allows any user to generate a search query by interacting with any θ is a threshold parameter) `helping' users. It preserves the privacy of a database content against the server assuming a leakage of up to θ-1 users' keys to the server while hiding the query from the θ-1 `helping users'. To achieve the query privacy, we design a new distributed key-homomorphic pseudorandom function (PRF) that hides the PRF input (search keyword) from the `helping' users. We present a concrete construction of our randomizable distributed PRF. By distributing the utilized keys among the users, the need for a constant online presence of the data owner to provide services to the users is eliminated, while providing resilience against a user key exposure. We extended our scheme to support user revocation in two different scenarios. In addition, we give a solution for fast update of the encryption key with the overhead significantly smaller than the re-encryption and re-uploading the database. Moreover, our scheme is secure against passive and active collusion between the server and a subset of users.

The recently proposed Oblivious Cross-Tags (OXT) protocol (CRYPTO 2013) has broken new ground in designing efficient searchable symmetric encryption (SSE) protocol with support for conjunctive keyword search in a single-writer single-reader framework. While the OXT protocol offers high performance by adopting a number of specialised data-structures, it also trades-off security by leaking 'partial' database information to the server. Recent attacks have exploited similar partial information leakage to breach database confidentiality. Consequently, it is an open problem to design SSE protocols that plug such leakages while retaining similar efficiency. In this paper, we propose a new SSE protocol, called Hidden Cross-Tags (HXT), that removes 'Keyword Pair Result Pattern' (KPRP) leakage for conjunctive keyword search. We avoid this leakage by adopting two additional cryptographic primitives - Hidden Vector Encryption (HVE) and probabilistic (Bloom filter) indexing into the HXT protocol. We propose a 'lightweight' HVE scheme that only uses efficient symmetric-key building blocks, and entirely avoids elliptic curve-based operations. At the same time, it affords selective simulation-security against an unbounded number of secret-key queries. Adopting this efficient HVE scheme, the overall practical storage and computational overheads of HXT over OXT are relatively small (no more than 10% for two keywords query, and 21% for six keywords query), while providing a higher level of security.

Data sharing in cloud storage is receiving substantial attention in information communications technology because it can provide users with efficient and effective storage services. To protect the confidentiality of the shared sensitive data, cryptographic techniques are usually applied. However, the data protection is still posing significant challenges in cloud storage for data sharing. Among them, how to protect and revoke the cryptographic key is the fundamental challenge. To tackle this, we propose a new data protection mechanism for cloud storage, which holds the following properties. First, the cryptographic key is protected by the two factors. Only if one of the two factors works, the secrecy of the cryptographic key is held. Second, the cryptographic key can be revoked efficiently by integrating the proxy re-encryption and key separation techniques. Finally, the data is protected in a fine-grained way by adopting the attribute-based encryption technique. Furthermore, the security analysis and performance evaluation show that our proposal is secure and efficient, respectively.