Zero-knowledge virtual machine (zkVM) is a powerful infrastructure for proving the correctness of a program execution with a succinct proof, attracting significant interest from researchers, developers, and users. It has been widely used in applications such as blockchain rollups, privacy-preserving machine learning, and off-chain computation. As the field grows, a wide range of zkVMs have been proposed. However, they adopt different choices in instruction formats, trace layouts, and proving backends, which results in a highly heterogeneous design landscape and makes it difficult to understand the relations among these systems.To bridge this gap, we provide a comprehensive study of zkVMs that covers both their theoretical foundations and practical implementations. We decompose zkVMs into three layers: (1) the ISA layer, which defines instruction semantics and determines the structure of the execution trace, (2) the VM layer, which captures program execution and organizes constraints through modular circuit components, and (3) the proving layer, which converts execution traces into algebraic constraints and generates the final proofs. This decomposition allows us to isolate the role of each layer while also examining how they interact in real systems. To give readers a more direct understanding of how these design choices affect performance, scalability, and usability, we conduct a comprehensive experimental evaluation of representative zkVMs following this layered framework. Finally, we conclude the paper by summarizing the main observations from our analysis and outlining several potential directions for zkVM design and implementation.

Zero-knowledge Succinct Non-interactive Argument of Knowledge (zkSNARK) is a powerful cryptographic primitive that enables a prover to convince a verifier that something is true without leaking the private witness. Current zkSNARKs face significant computational costs in generating proofs, which restricts their use in areas like private payments, confidential smart contracts, and anonymous credentials. Private delegation offers a practical solution by outsourcing the heavy computation to powerful external workers without leaking any private information. In this work, we propose HyperSiniel, an efficient private delegation framework for general zkSNARKs that achieves a new feature called guaranteed output delivery (GOD). HyperSiniel is designed to be compatible with any universal zkSNARKs constructed from a polynomial interactive oracle proof (PIOP) and a polynomial commitment scheme (PCS). It enables a computationally limited delegator to outsource proof generation to several workers in a fully non-interactive and privacy-preserving manner. Compared to the most state-of-the-art frameworks (e.g., Siniel [NDSS'25]), HyperSiniel ensures that the delegator always receives a correct proof, regardless of malicious worker behavior. We implement HyperSiniel and compare the performance with Siniel across varying bandwidths and circuit sizes. Under low-bandwidth conditions (10MBps), HyperSiniel incurs only an additional 25% overhead compared with Siniel, while the total running time of HyperSiniel is almost identical to Siniel under high-bandwidth settings (1000MBps). These results show that the strong robustness guarantee of GOD in HyperSiniel comes almost for free, making it a practical and secure solution for real-world zkSNARK delegation.

Wildcard Keyword Searchable Encryption (WKSE) has grown into a ubiquitous tool. It enables clients to search desired files with wildcard expressions. Although promising, previous schemes confront three barriers: (1) An adversary can launch a correlation attack to acquire the similarity between keywords. (2) The WKSE schemes exhibit false positives which can lead to wrong search results. (3) Existing feature extraction strategies limit the flexibility of search expressions. In this paper, we propose a Multi-Character Searchable Encryption scheme (MCSE) that overcomes the aforementioned barriers. To resist correlation attacks, we design the randomize pad model to encrypt the vector. To eradicate false positives, we apply the vector space model and complete feature extraction strategies so that a feature set uniquely identifies a keyword or expression. To enhance search flexibility, we introduce three distinct feature extraction strategies for keyword expressions, wildcard expressions, and logical expressions, enabling effective multi-character search. These strategies enable indexes to accom modate the search of diverse expressions. Finally, we prove that MCSE is indistinguishable against chosen-feature attacks and implement MCSE on two real datasets. Compared with state-of the-art schemes, the experiment results show that MCSE achieves good performance.

Secure and efficient data sharing is essential in cloud environments, where data owners must delegate decryption rights without re-encrypting data for each user. Proxy Re-Encryption (PRE) addresses this by allowing a proxy to transform ciphertexts for authorized recipients without accessing the plaintext. As a variant, Attribute-Based Conditional PRE (AB-CPRE) enhances traditional PRE by incorporating two key features: (1) attribute-based access control, and (2) conditional ciphertext transformation based on a specified policy. Despite significant advancements, existing AB-CPRE schemes face a trilemma in balancing functionality and security, hindering their use in cloud data-sharing: (1) support limited to single-hop re-encryption, restricting multi-hop scenarios; (2) a weak security model relying on selective security without allowing the adversary to choose the target attributes or policies adaptively; and (3) an insufficient security guarantee only targeting chosen plaintext attacks (CPA), offering no protection against honest re-encryption attacks (HRA).In this paper, we propose the first AB-CPRE scheme tailored to the cloud environment that simultaneously supports multi-hop transformation, adaptive-policy security, and resistance to HRA. Our construction is based on the learning with errors (LWE) assumption in the standard model, making it also quantum-resistant. We prove security through a novel re-encryption key simulatability technique, allowing the simulation of the re-encryption key without knowing the corresponding secret key, which is of independent interest. Through a comprehensive performance comparison, our scheme demonstrates a lower decryption overhead and a comparable re-encryption key size, showing its practicality compared to the state-of-the-art schemes while offering stronger security and functionality.

Cloud computing, as a persuasive paradigm, offers on-demand data services. However, it faces various security threats during data sharing due to trust issues. To mitigate this problem, many cloud-based data-sharing systems employ cryptographic tools to guarantee the confidentiality of sensitive data. Nevertheless, fine-grained data sharing still suffers from many challenges, especially in complex cloud environments. In this paper, we introduce two cloud-based data-sharing systems with fine-grained access control. The first solution supports dynamic user groups, while the second solution further offers outsourced decryption, enabling compatibility with resource-constrained devices. To formalize our solution theoretically, we introduce the concept of ElGamal -type cryptosystem (ETC) and server-aided ETC with key encapsulation mechanism to generalize public-key encryption with specific features implicitly specified by ElGamal encryption. Through the application of ETC, we present generic constructions for revocable attribute-based encryption (RABE) and server-aided RABE (SR-ABE) with formal definitions and security analyses. These schemes serve as the fundamental mechanisms behind our proposed solutions. By applying the state-of-the-art attribute-based encryption scheme proposed in CCS'22, we introduce instantiations of RABE and SR-ABE with various desirable properties, including large universe, attribute multi-use, key exposure resistance, fast decryption, and more. Extensive experiments substantiate the superior performance of our proposed instantiations over previous solutions.

Hierarchical Deterministic Wallet (HDW) and Stealth Address (SA) are widely used in cryptocurrency communities due to their functionality and security. In the preliminary version of this work (ESORICS 2022), we formally define the syntax and security models of Hierarchical Deterministic Wallet supporting Stealth Address (HDWSA), capturing the functionality and security requirements imposed by the practice in cryptocurrency. We propose a concrete HDWSA construction and prove its security in the random oracle model. Note that when applied in blockchain, in practice, signature aggregation could reduce the cost of computation, storage, and communication dramatically. In this full version, we develop HDWSA definition to further support signature aggregation (referred to as HDWSA$^{2}$). In particular, we first formally define HDWSA$^{2}$, which, besides enjoying all the virtues of HDWSA on functionality and security, allows multiple signatures on different messages to be aggregated into one signature. We propose a concrete HDWSA$^{2}$construction and prove its security in the random oracle model. We implement the HDWSA$^{2}$construction and the experimental results show that verification of an aggregate signature is about 13$\boldsymbol{\times }$faster than sequential verification of all the individual signatures. We can reduce the size of signatures in a single block by about 60% after aggregation.

Password-based authentication is widely applied in Internet of Things (IoT). It allows IoT devices to identify users with passwords to resist unauthorized access. However, choices of weak passwords, especially popular ones, might violate users’ privacy and lead to large-scale network attacks. Collection of popular passwords among IoT devices to establish blocklists via a service provider can prevent use of weak passwords. To protect unpopular passwords during collection, existing privacy-preserving schemes rely on expensive cryptographic primitives (e.g., garbled circuits and zero-knowledge proofs), which would impose heavy communication and computation burdens on constrained devices and hinder wide deployment of these schemes. In this paper, we propose EAGER+, an efficient privacy-preserving scheme for weak password collection in IoT against perpetual leakage. EAGER+ is mainly built on secret sharing and symmetric encryption, thereby enabling lightweight computation and communication on IoT devices. In EAGER+, we conceive a password-locked encryption with conditional decryption mechanism to efficiently identify popular passwords, where a password is essentially locked under itself in the encryption to guarantee its security, and the password can be revealed from the ciphertext by the service provider only if a sufficient number of devices exploit it. The mechanism is integrated with a servers-aided password-hardening mechanism to resist offline dictionary guessing attacks. Moreover, EAGER+ uses a key renewal mechanism to periodically update secrets for password hardening on key servers to thwart perpetual leakage towards the secrets. We formally analyze the security of EAGER+, and conduct experimental evaluations to show that EAGER+ is more efficient than existing schemes.

Password-based threshold single-sign-on authentication (PbTA) allows multiple identity servers to in a threshold manner authenticate a user and issue a token, with which the user accesses relevant services. We analyze existing PbTA schemes and reveal a potential threat: vulnerability against perpetual credential leakage, in which “perpetual” adversaries could perpetually attempt to compromise long-lived credential databases maintained by identity servers. Compromising a threshold number of credential databases enables the adversaries to launch offline dictionary guessing attacks (DGA) or illegally obtain users’ tokens. To address these issues, we first propose a basic device-enhanced PbTA scheme (DE-PbTA), where an auxiliary device collaborates with identity servers in hardening a user’s password during authentication, such that perpetual adversaries cannot learn the password from compromised credentials via offline DGA. Using the hardened password, a private key can be derived to decrypt ciphertexts from identity servers for token construction, which protects the user’s tokens against perpetual adversaries. Then, we extend basic DE-PbTA to support dynamic usage of multiple devices, where a user can actively choose$t^{\prime } $devices out of$n^{\prime } $for authentication. Provable security and high efficiency of the basic/enhanced DE-PbTA scheme are demonstrated by comprehensive analysis and experimental evaluations.

Aiming to provide people with great convenience and comfort, smart home systems have been deployed in thousands of homes. In this paper, we focus on handling the security and privacy issues in such a promising system by customizing a new cryptographic primitive to provide the following security guarantees: 1) fine-grained, privacy-preserving authorization for smart home users and integrity protection of communication contents; 2) flexible self-sovereign permission delegation; 3) forward security of previous messages. To our knowledge, no previous system has been designed to consider these three security and privacy requirements simultaneously. To tackle these challenges, we put forward the first-ever efficient cryptographic primitive called the Forward-secure Hierarchical Delegable Signature (FS-HDS) scheme for smart homes. Specifically, we first propose a new primitive, efficient Hierarchical Delegable Signature (HDS) scheme, which is capable of supporting partial delegation capability while realizing privacy-preserving authorization and integrity guarantee. Then, we present an FS-HDS for smart homes with the efficient HDS as the underlying building block, which not only inherits all the desirable features of HDS but also ensures that the past content integrity is not affected even if the current secret key is compromised. We provide comprehensively strict security proofs to prove the security of our proposed solutions. Its performance is also validated via experimental simulations to showcase its practicability and effectiveness.

Federated Learning (FL) allows multiple data owners to jointly train machine learning models by sharing local models instead of raw private data, alleviating data privacy concerns. However, as the local computation of data owners is unpredictable, it increases its vulnerability to Byzantine attacks, where compromised data owners submit abnormal local models that can severely degrade global model accuracy. Existing Byzantine-robust FL methods depend on a semi-honest server executing predefined Byzantine-robust aggregation rules (ByRules) to filter out abnormal local models, but these methods fail when the server is compromised. Although recent serverless Byzantine-robust FL approaches mitigate the risk of a compromised server, they suffer from challenges in achieving consensus on ByRules and impose a heavy burden on privacy protection. In this paper, we propose ROBY, a novel serverless FL framework that extends existing ByRules to a decentralized setting, effectively defending against Byzantine attacks and ensuring privacy protection for local models. ROBY introduces a shared, dynamically updated consensus dataset that serves as a reliable benchmark for applying ByRules and enabling efficient consensus on ByRules among decentralized data owners. Moreover, we design a dual-layer privacy shielding strategy in ROBY to protect local model privacy without sacrificing global model accuracy or incurring extra computational and communication overhead. Extensive evaluations demonstrate that ROBY substantially enhances both Byzantine robustness and privacy protection compared to server-based FL methods.

The evolution of decentralized identity (DID) and self-sovereign identity (SSI) frameworks, as endorsed by W3C Verifiable Credentials (VC) and eIDAS 2.0, underscores the need for secure, efficient, and privacy-preserving credential management. However, existing credential systems often depend on centralized issuers, lack efficient aggregation mechanisms, or fail to ensure unlinkability across authentication sessions. To address these challenges, we propose DISC (Decentralized Identity System with Self-Sovereign Credential Aggregation), a novel credential system that enables multi-authority credential issuance, user-controlled credential aggregation, and unlinkable authentication. DISC allows users to aggregate credentials from multiple issuers while maintaining constant-size authentication tokens and supporting batch verification for scalable authentication. Additionally, DISC ensures unlinkability of aggregated authentication tokens, preventing verifiers from correlating sessions even when credentials share attributes. Security analysis proves DISC’s unforgeability, anonymity, and unlinkability, while experimental results confirm its efficiency in credential issuance, aggregation, and verification. Compared to existing schemes, DISC offers a scalable, privacy-preserving, and efficient decentralized identity solution, making it well-suited for real-world applications requiring secure and privacy-preserving identity verification.

Cloud-based data sharing has emerged as a prevailing solution for enterprises and end users, supporting various online services in our daily lives. However, the current cloud security solutions are vulnerable to the “harvest now, decrypt later” threat imposed by future quantum computers. To encounter the threat, lattice-based cryptographic solutions for supporting cloud data encryption and search have been extensively investigated by both academia and industry. Despite these efforts, existing lattice-based schemes fall into a trilemma: (1) lack of efficient access control for data retrieval; (2) inadequate protection of keyword privacy in both ciphertext and search token; and (3) difficulty in realizing forward secrecy to safeguard historical data. These limitations result in a substantial burden for lattice-based solutions to be adopted in real-world cloud data sharing. To our knowledge, no prior work has comprehensively addressed these issues at the same time, motivating us to design a more flexible, efficient, and secure lattice-based solution. In this paper, we propose an efficient, privacy-preserving, and forward-secure data sharing framework centered around a novel primitive called Forward-Secure Authenticated Searchable Encryption (FS-ASE). Specifically, we first construct an Authenticated Searchable Encryption (ASE) scheme based on ideal lattices, enabling efficient one-to-many search functionality and ensuring keyword privacy in both ciphertext and search token. On top of this primitive, we present the FS-ASE scheme, which achieves forward secrecy through a highly efficient key evolution mechanism, thereby keeping the confidentiality of historical data even if the current secret key is compromised. Finally, the security of our construction is proven under the Ring Learning With Errors (RLWE) assumption, and experimental results show that it achieves performance improvements of 158× in data retrieval and 350× in token generation over state-of-the-art approaches, indicating its practicality in real use.

Single-sign-on authentication is widely deployed in mobile systems, which allows an identity server to authenticate a mobile user and issue her/him with a token, such that the user can access diverse mobile services. To address the single-point-offailure problem, threshold single-sign-on authentication (PbTA) is a feasible solution, where multiple identity servers perform user authentication and token issuance in a threshold way. However, existing PbTA schemes confront critical drawbacks. Specifically, these schemes are vulnerable to perpetual secret leakage attacks (PSLA): an adversary perpetually compromises secrets of identity servers (e.g., secret key shares or credentials) to break security. Besides, they fail to achieve popular password collection, which is an effective means of enhancing system security. In this paper, we propose a secure PbTA scheme with popular password collection, dubbed AugSSO. In AugSSO, we conceive an efficient key renewal mechanism that allows identity servers to periodically update secret key shares in batches, and require storage of hardened password-derived public keys in credentials for user authentication, thereby resisting PSLA. We also present a popular password collection mechanism, where an aggregation server is introduced to identify popular passwords without disclosing unpopular ones. We provide security analysis and performance evaluation to demonstrate security and efficiency of AugSSO

As an advanced one-to-many public key encryption system, attribute-based encryption (ABE) is widely believed to be a promising technology for achieving flexible and fine-grained access control of encrypted data on untrusted storage servers (e.g., public cloud servers). However, user revocation in ABE is a critical but challenging problem, and designing efficient revocable ABE has been an active research topic in the past decade. Almost all the existing revocable ABE schemes incorporate a timestamp in the encryption algorithm such that revoked users cannot decrypt ciphertexts generated in future time intervals. To prevent revoked users from decrypting past ciphertexts, the storage server needs to perform a process called ciphertext delegation (Sahai et al., CRYPTO’12) that periodically updates the timestamp for all ciphertexts. As the number of ciphertexts could be huge in a storage system, ciphertext delegation could pose a huge computation overhead to the server.Motivated by the popularity of commodity Trusted Execution Environment (TEE) technologies, this paper initiates the study on hardware-based revocable ABE (HR-ABE) to eliminate the (unscalable) ciphertext delegation and prevent collusion attacks between an untrusted storage server and revoked users. We formalize this new notion and present an efficient HR-ABE construction that also supports outsourced decryption for resource-constrained data users. Furthermore, HR-ABE is also designed to address the potential secret leakage problem suffered by TEE (e.g., due to side-channel attacks) so that the leakage of secrets possessed by TEE does not lead to leakage of user data. We prove HR-ABE’s security formally and benchmark its performance experimentally.

The Internet of Things (IoT) is widely recognized for its potential to enhance efficiency and productivity across various industries. However, its increasing prevalence has also made it a more attractive target for cybercriminals. While many advanced cryptographic solutions have been developed to secure IoT, some practical security and privacy issues such as self-sovereign delegation, flexible revocation, and lightweight access remain inadequately addressed in existing solutions. In this paper, we propose PLIC, a Privacy-aware Lightweight IoT Communication scheme, which not only enables any authorized user to flexibly delegate their lightweight access privileges to other delegatees, such that they can also access the authorized IoT targets in the same lightweight way, but also supports flexible revocation of access for specific users without affecting non-revoked users. Specifically, our solution leverages wildcard-based access control and tree-based encryption technologies to enable self-sovereign delegation, dynamic membership updates, and stably efficient decryption overhead in IoT. In addition, comprehensive security proofs are rendered to validate the robustness of our approach. Finally, experimental comparisons with similar methodologies demonstrate the practicality and superior performance of our solution, which indicates its effectiveness for practical IoT appli-cations.

Abstract Policy-based chameleon hash functions have been widely proposed for its use in blockchain rewriting systems. They allow anyone to create a mutable transaction associated with an access policy, while an authorized user who possesses sufficient rewriting privileges from a trusted authority satisfying the access policy can rewrite the mutable transaction. However, existing chameleon hash functions lack certain fundamental security guarantees, including forward security and backward security. In this paper, we introduce a new primitive called forward/backward-secure policy-based chameleon hash (FB-PCH for short). We present a practical instantiation. We prove that the proposed scheme achieves forward/backward-secure collision-resistance, and show its practicality through implementation and evaluation analysis.

Abstract In this paper, we introduce the first generic framework of policy-based remote user authentication from multiple biometrics. The proposed framework allows an authorized user to remotely authenticate herself to an authentication server using her multiple biometrics, which enhances both the security and usability of user authentications. The authentication server approves a user’s authentication request if and only if the user’s multiple biometrics satisfies an authentication policy. In particular, the authentication policy can be dynamically updated to satisfy different security and usability requirements in practice. We implement an instantiation of the proposed framework and report its performance under various authentication policies.

In this article, we propose a secure fine-grained task allocation scheme with bilateral access control (FTA-BAC) for intelligent transportation systems. To enhance the security, we formulate bilateral access control in task allocation, by adopting the matchmaking encryption (ME) to encrypt the task requirements/interests for secure task matching. In this way, both task requesters and workers can specify their match policies simultaneously, without revealing their sensitive information (i.e., attributes and geographical location). To realize fine-grained task allocation, we use a linear integer secret sharing (LISS) scheme to represent task requirements/interests, supporting theAND/ORoperation on match policies. To further improve the efficiency, we design a delegation mechanism to reduce the computation burden on resource-limited end devices, by diverting the high-frequency matching operations to edge nodes. Then, we prove the security of FTA-BAC under formally defined security model. Finally, we analyze the performance of FTA-BAC through theoretical analysis and experimental evaluation, demonstrating that FTA-BAC can provide practical task allocation for intelligent transportation systems compared with the state-of-the-art works.

The Internet of Things (IoT) has greatly changed our lives and generated a large amount of data. Cloud storage helps IoT limited-resource IOT devices process the massive data. However, cloud servers are untrusted in most scenarios as they may illegally obtain sensitive data. Although existing symmetric searchable encryption (SSE) schemes can protect the privacy of outsourced data while preserve data availability, Most of them leak access and search patterns to the cloud server to gain better performance. Such leakages will be used to recover private information. Meanwhile, semi-honestly secure searchable encryption cannot prevent attacks done by the malicious server such as returning the false search result. Therefore, it is still a challenge to prevent malicious cloud server misbehavior, and preserve patterns, simultaneously. This paper proposes OpenSE to solve the aforementioned problems. First, this paper constructs FastOPE as a major building block. With the OPE protocol, the verifiable searchable encryption OpenSE can be trivially realized. After that, security proofs show that OpenSE is secure against malicious cloud servers with access and search pattern hidden. Finally, we implement experiments on real datasets to compare OpenSE with some state-of-the-art works in terms of running time of setup phase and search phase as well as storage overhead. The experimental results show that OpenSE outperforms the state-of-the-art works in terms of setup phase and storage overhead. In addition, the theoretic comparison shows that OpenSE outperforms most existing works in terms of security, in which OpenSE enjoys both verifiability and pattern hidden.

Time series data analytics technology significantly benefits modern scientific research, especially in fields such as medical health, financial investment, and transportation. Unfortunately, privacy issues hinder people from handing over the data to a third party for various analytical tasks; because the data may reveal much more individual sensitive information, e.g., disease information from medical data, investment tendency from financial data, or the daily trajectory from transportation data. To break down this barrier, secure computation approaches have shown their importance in processing sensitive data, and have attracted much attention from the industry and research communities. However, when considering the case of secure time-series data analytics (e.g., DTW similarity), we are still far from achieving high efficiency due to high round complexity in communication or expensive computational complexity. We observe that DTW involves a lot of comparison operations and existing approaches in dealing with the comparison require higher communication costs. To this end, this paper studies secure DTW-based analytics with practical efficiency over time series data. Specifically, we propose the framework of secure time series data analytics (STDA) and formulate the problem of top-$k$query for outsourced time series data. Based on threshold Paillier encryption, we present a top-$k$query protocol utilizing the DTW distance as a metric and its security analysis, optimizations, and performance evaluation. The experimental results demonstrate that in a wide-area network with a 10 ms latency, our top-$k$approach outperforms the state-of-the-art by 3x times, while DTW calculation outperforms by 9x times. Correspondingly, the optimized$\mathcal {F}_{\text {DTW}}$achieves 17x times better, and optimized top-$k$achieves 4-10x times better.

Wildcard Keyword Searchable Encryption (WKSE) enables users to search desired encrypted files with wildcard queries. Previous schemes only enabled single-character wildcard queries or restricted multi-character wildcard queries. Even if the two types of queries are supported by several schemes, they are vulnerable to correlation attacks and composition attacks. In this paper, we propose a WKSE scheme Secure Flexible Wildcard Queries (SFWQ) that supports highly flexible wildcard queries and resists correlation and composition attacks. Specifically, we adopt the interval matching method instead of traditional position matching, so that SFWQ supports a variety of queries, including single-character wildcard queries, multi-character wildcard queries, and mixed wildcard queries that the combination of both single-character and multi-character wildcards within the same query. Moreover, the number and position of wildcards within wildcard keywords are adjustable according to user preference. To resist the correlation attack and composition attack, we leverage key aggregate searchable encryption (KASE) and key exchange protocol to process characters so that even the same characters of the same keyword behave as different ciphertexts. We define a security model for WKSE which catches the correlation attack and composition attack. Our proof validates SFWQ is secure under the security model. Finally, we implement SFWQ and compare it with state-of-the-art schemes. The experimental results demonstrate that our scheme is feasible and efficient.

Federated learning (FL) allows multiple parties, each holding a dataset, to jointly train a model without leaking any information about their own datasets. In this paper, we focus on vertical FL (VFL). In VFL, each party holds a dataset with the same sample space and different feature spaces. All parties should first agree on the training dataset in the ID alignment phase. However, existing works may leak some information about the training dataset and cause privacy leakage. To address this issue, this paper proposes OpenVFL, a vertical federated learning framework with stronger privacy-preserving. We first propose NCLPSI, a new variant of labeled PSI, in which both parties can invoke this protocol to get the encrypted training dataset without leaking any additional information. After that, both parties train the model over the encrypted training dataset. We also formally analyze the security of OpenVFL. In addition, the experimental results show that OpenVFL achieves the best trade-offs between accuracy, performance, and privacy among the most state-of-the-art works.

Space Information Network (SIN) enables universal Internet connectivity for any object, even in remote and extreme environments where deploying a cellular network is difficult. Access authentication is crucial for ensuring user access control in SIN and preventing unauthorized entities from gaining access to network services. However, due to the complex communication environment in SIN, including exposed links and higher signal delay, designing a secure and efficient authentication scheme presents a significant challenge. In this paper, we propose a secure communication protocol for SIN with periodick-time anonymous authentication (named PkT-SIN) that allows satellite users to anonymously authenticate to ground stations at mostktimes in each single time period. An efficient handover mechanism is designed to ensure seamless communication for satellite users to communicate with different satellites and ground stations, taking into account the dynamic topology of SIN. As a core component of PkT-SIN, we propose a novel primitive, periodick-time keyed-verification anonymous credential (PkT-KVAC), that enables users to derivektokens from a credential for anonymous and unlinkable authentication. On the other hand, a verifier can always recognize a reused token from a dishonest user. PkT-KVAC is of independent contribution to anonymous authentication in pay-per-use business scenarios. Formal security proofs confirm that PkT-SIN and PkT-KVAC have desired security features. The supremacy of their computing features is demonstrated through comprehensive comparison and rigorous performance analysis.

Outsourcing storage has emerged as an effective solution to manage the increasing volume of data. With the popularity of pay-as-you-go payment models in outsourcing storage, data auditing schemes that prioritize timeliness can be valuable evidence for elastic bill settlement. Unfortunately, existing data auditing schemes do not sufficiently consider timeliness during auditing. Furthermore, practical data auditing schemes should have the capability to check the integrity of scalable data. In this paper, we propose a blockchain-based dynamic data auditing scheme with strong timeliness to ensure that data stored in outsourcing storage systems remain intact. Our scheme encapsulates timestamps into homomorphic verifiable tags to simultaneously check data integrity and timestamp validity. To achieve dynamicity, we utilize the Merkle hash tree to store the tags, allowing for block-level dynamic operations. Additionally, by leveraging the transparency, non-repudiation, and tamper resistance of blockchain technology, we design a blockchain-based data auditing framework to prevent malicious behavior from all entities. We then formally prove the soundness and privacy of our scheme. Finally, we conduct theoretical analysis and experimental evaluation to demonstrate that the performance of our scheme is of acceptable efficiency to existing works in terms of computation cost, communication overhead, and storage overhead.

Privacy-preserving data evaluation is one of the prominent research topics in the Big Data era. In many data evaluation applications that involve sensitive information, such as the medical records of patients in a medical system, protecting data privacy during the data evaluation process has become an essential requirement. Aiming at solving this problem, numerous fuzzy encryption systems for different similarity metrics have been proposed in literature. Unfortunately, the existing fuzzy encryption systems either fail to achieve attribute-hiding or achieve it, but are impractical. In this article, we propose a new fuzzy encryption scheme for privacy-preserving data evaluation based on overlap distance, which can work in an integer domain while achieving attribute-hiding. In particular, we develop a novel approach to enable an accurate overlap distance to be fast calculated. This technique makes the number of pairing operations during decryption stage negative correlation with the size of the threshold, which is pretty practical for some applications especially with a large threshold. Additionally, we provide a formal security analysis of the proposed scheme, followed by a comprehensive experimental. Also we show that our scheme can be well applied to some scenarios, such as fuzzy keyword searchable encryption and attribute-hiding closest substring encryption.

The Internet of Things and cloud services have been widely adopted in many applications, and personal health records (PHR) can provide tailored medical care. The PHR data is usually stored on cloud servers for sharing. Weighted ABE is a practical and flexible technique to protect PHR data. Under a weighted ABE policy, the data user's attributes will be “scored”, if and only if the score reaches the threshold value, they can access the data. However, while this approach offers a flexible access policy, the data owners have difficulty controlling their privacy, especially sharing PHR data in collaborative e-health systems. This paper aims to find a balance between privacy and flexibility and proposes an AND-weighted ABE scheme in cloud-based personal health records sharing systems. The proposed scheme can meet both privacy and flexibility. Only when the data user satisfies the scored-based policy and is in the specified organization(s), can the data user access the PHR data. Besides, we give the security proof and the performance evaluation of the proposed scheme. The security proof and performance analysis show that the proposed scheme can efficiently and securely share PHR data in cloud service.

Internet of Things (IoT) devices upload their data into the cloud for storage because of their limited resources. However, cloud storage data has been subject to potential integrity threats, and consequently auditing techniques are demanded to ensure the integrity of stored data. Unfortunately, existing auditing approaches require owners to undertake expensive tag calculations, which is unsuitable for resource-constrained IoT devices. To resolve the issue, we present aFairCloudAuditing proposal by employing theBlockchain (FCAB). We combine certificateless signatures with the designed dynamic structure to constructively offload the cost of tag computation from the IoT device to the introduced fog node, significantly reducing the local burden. Considering that fog nodes may behave dishonestly during auditing, FCAB enables the IoT device to verify the audit result's authenticity by extracting reliable checking records from the blockchain, thereby achieving auditing fairness, which ensures that thehonestcloud and fog node will gain the corresponding reward. Finally, FCAB is proved to satisfy tag unforgeability, proof unforgeability, privacy preserving, and auditing fairness. Experiment evaluations affirm that FCAB is computationally and communicationally efficient and retains a smaller and fixed computation locally at the data processing stage (mainly including tag computation) than existing auditing methods.

With the prevalence of outsourced computation, such as Machine Learning as a Service, protecting the privacy of sensitive data throughout the whole computation is a critical yet challenging task. The problem becomes even more tricky when multiple sources of input and/or multiple recipients of output are involved, who would encrypt/decrypt data using different keys. Considering many computation tasks demand binary operands and operations but there are only outsourced computation constructions for arithmetic calculations [1], in this paper, the authors propose a privacy-preserving outsourced computation framework for Boolean circuits. The proposed framework can protect sensitive data throughout the whole computation, i.e., input, output and all the intermediate values, ensuring privacy for general outsourced tasks. Moreover, it compresses the ciphertext domain of [1] and attains secure protocols for four logic gates (AND, OR, NOT, and XOR) which are the basic operations in Boolean circuits. With the proposed framework as a building block, a novel Privacy-preserved (encrypted) Bloom Filter and a Multi-keyword Searchable Encryption scheme under the multi-user setting are presented. Security proof and experimental results show that the proposal is reliable and practical.

Cloud computing has emerged as a popular choice for distributing data among both individuals and companies. Ciphertext-policy attribute-based encryption (CP-ABE) has been extensively used to provide data security and enable fine-grained access control. With this encryption technique, only users whose attributes satisfy the access policy can access the plaintext. In order to mitigate the computational overhead on users, particularly on lightweight devices, partial decryption has been introduced, where the cloud assists in performing the decryption computations without revealing sensitive information. However, in this process, the cloud obtains the user's attributes, thus infringing on the user's privacy. To address this issue, this article proposes a privacy-preserving cloud-based data distribution system with filtering (PPDF) to enable partial decryption without revealing the user's attributes. The proposed system also employs an edge server to assist the user in filtering out invalid ciphertexts, i.e., ciphertexts where the user's attributes do not satisfy the access policy, and transmit only the valid partially decrypted ciphertexts to the data receiver. Consequently, the proposed PPDF scheme achieves constant decryption cost for the data receiver. We provide a security proof and a performance evaluation of the proposed scheme, which confirms its effectiveness and practicality in various real-world applications.

The Internet of Things (IoT) is experiencing explosive growth and has gained extensive attention from academia and industry in recent years. However, most of the existing IoT infrastructures are centralized, which may cause the issues of unscalability and single-point-of-failure. Consequently, decentralized IoT has been proposed by taking advantage of the emerging technology called blockchain. Voting systems are widely adopted in IoT, for example a leader election in wireless sensor networks. Self-tallying voting systems are alternatives to unsuitable, traditional centralized voting systems in decentralized IoT. Unfortunately, self-tallying voting systems inherently suffer from fairness issues, such as adaptive and abortive issues caused by malicious voters. To address these issues, in this article, we introduce a framework of the self-tallying voting system in decentralized IoT based on blockchain. We propose a concrete construction and prove that the proposed system satisfies all the security requirements, including fairness, dispute-freeness, and maximal ballot secrecy. We simulate the algorithms on a laptop, an Android phone, and a Raspberry Pi to test the time consumption and evaluate the gas cost of each algorithm in a private blockchain as well. The implementation results demonstrate the practicability of our system.

Deterministic Wallet (DW) and Stealth Address (SA) mechanisms have been widely adopted in the cryptocurrency community, due to their virtues on functionality and privacy protection, which come from a key derivation mechanism that allows an arbitrary number of derived keys to be generated from a master key. However, these algorithms suffer a vulnerability that, when one derived key is compromised somehow, the damage is not limited to the leaked derived key only, but to the master key and in consequence all derived keys are compromised. In this article, we introduce and formalize a new signature variant, called Key-Insulated and Privacy-Preserving Signature Scheme with Publicly Derived Public Key (PDPKS), which fully captures and improves the functionality, security, and privacy requirements of DW and SA. We propose a PDPKS construction and prove its security and privacy in the random oracle model. Furthermore, we implement the construction with parameters for 128-bit security, and the results show that it is practically efficient for the setting of cryptocurrencies. With its solid guarantee on functionality, security and privacy, as well as its practical efficiency, our PDPKS construction provides a practical cryptographic tool that refines DW and SA, without security vulnerability.

Cloud computing is considered as one of the most prominent paradigms in the information technology industry, since it can significantly reduce the costs of hardware and software resources in computing infrastructure. This convenience has enabled corporations to efficiently use the cloud storage as a mechanism to share data among their employees. At the first sight, by merely storing the shared data as plaintext in the cloud storage and protect them using an appropriate access control would be a nice solution. This is assuming that the cloud is fully trusted for not leaking any information, which is impractical as the cloud is owned by a third party. Therefore, encryption is mandatory, and the shared data will need to be stored as a ciphertext using an appropriate access control. However, in practice, some of these employees may be malicious and may want to deviate from the required sharing policy. The existing protection in the literature has been explored to allow only legitimate recipients to decrypt the contents stored in the cloud storage, but unfortunately,no existing workdeals with issues raised due to the presence of malicious data publishers. Malicious data publishers construct data following the given policy, but the ciphertexts can actually be decrypted by unauthorized users without valid keys, or simply, anyone else who is unauthorized. The impact of the involvement of malicious data publishers is detrimental, as it may damage intellectual properties from the corporations. Therefore, it remains an elusive research problem on how to enable a sound approach to resolve the issue when malicious data publishers are involved in the system, which is a very practical question. In this work, we presenta new direction of researchthat can cope with the presence of malicious data publishers. We resolve the aforementioned problem by proposing the notion of Sanitizable Access Control System (SACS), which is designed for a secure cloud storage that can also resist against malicious data publishers. We define the threat model and its formal security model, as well as its design and scheme which is based on$q$q-Parallel Bilinear Diffie-Hellman Exponent Assumption. We provide the security proof of our construction as well as its performance analysis. We believe that this work has opened a new area of research which has never been explored before, even though it is very practical. Therefore, this work will enhance the adoption of secure cloud storage in practice.

To improve data availability and durability, cloud users would like to store multiple copies of their original files at servers. The multi-copy auditing technique is proposed to provide users with the assurance that multiple copies are actually stored in the cloud. However, most multi-replica solutions rely on Public Key Infrastructure (PKI), which entails massive overhead of certificate computation and management. In this article, we propose an efficient multi-copy dynamic integrity auditing scheme by employing certificateless signatures (named MDSS), which gets rid of expensive certificate management overhead and avoids the key escrow problem in identity-based signatures. Specifically, we improve the classic Merkle Hash Tree (MHT) to achieve batch updates for multi-copy storage, which allows the communication overhead incurred for dynamics to be independent of the replica number. To meet the flexible storage requirement, we propose a variable replica number storage strategy, allowing users to determine the replica number for each block. Based on the fact that auditors may frame Cloud Storage Servers (CSSs), we use signature verification to prevent malicious auditors from framing honest CSSs. Finally, security analysis proves that our proposal is secure in the random oracle model. Analysis and simulation results show that our proposal is more efficient than current state-of-the-art schemes.

Message-Locked Encryption (MLE) is a useful tool to enable deduplication over encrypted data in cloud storage. It can significantly improve the cloud service quality by eliminating redundancy to save storage resources, and hence user cost, and also providing defense against different types of attacks, such as duplicate faking attack and brute-force attack. A typical MLE scheme only focuses on deduplication. On the other hand, supporting search operations on stored content is another essential requirement for cloud storage. In this article, we present a message-locked searchable encryption (MLSE) scheme in a dual-server setting, which achieves simultaneously the desirable features of supporting deduplication and enabling users to perform search operations over encrypted data. In addition, it supports both multi-keyword and negative keyword searches. We formulate the security notions of MLSE and prove our scheme satisfies all the security requirements. Moreover, we provide an interesting extension of our construction to support Proof of Storage (PoS). Compared with the existing solutions, MLSE achieves better functionalities and efficiency, and hence enables more versatile and efficient cloud storage service.

Equivocation is one of the most fundamental problems that need to be solved when designing distributed protocols. Traditional methods to defeat equivocation rely on trusted hardware or particular assumptions, which may hinder their adoption in practice. The advent of blockchain and decentralized cryptocurrencies provides an auspicious breakthrough paradigm to resolve the problem above. In this paper, we propose a blockchain-based solution to address contractual equivocation, which supports user-defined fine-grained policy-based equivocation. Specifically, users will be de-incentive if the statements they made breach the predefined access rules. The core of our solution is a newly introduced primitive named Policy-Authentication-Preventing Signature (PoAPS), which combined with a deposit mechanism allows a signer to make conflict statements corresponding to a policy to be penalized. We present a generic construction of PoAPS based on Policy-Based Verifiable Secret Sharing (PBVSS) and demonstrate its practicality via a concrete implementation in the blockchain. Compared with the existing solutions that only handle specific types of equivocation, our proposed approach is more generic and can be instantiated to deal with various kinds of equivocation.

Abstract In this paper, we introduce a new construction for unlinkable secret handshake that allows a group of users to perform handshakes anonymously. We define formal security models for the proposed construction and prove that it can achieve session key security, anonymity and affiliation hiding. In particular, the proposed construction ensures that (i) anonymity against protocol participants (including group authority) is achieved since a hierarchical identity-based signature is used in generating group user’s pseudonym-credential pairs and (ii) revocation is achieved using a secret sharing-based revocation mechanism.

In this paper, we introduce a new construction of reusable fuzzy signature based remote user authentication that is secure against quantum computers. We investigate the reusability of fuzzy signature, and we prove that the fuzzy signature schemes provide biometrics reusability (aka. reusable fuzzy signature). We define formal security models for the proposed construction, and we prove that it achieves user authenticity and user privacy. The proposed construction ensures: 1) a user’s biometrics can be securely reused in remote user authentication; 2) a third party having access to the communication channel between a user and the authentication server cannot identify the user.

Monero provides a high level of anonymity for both users and their transactions. However, many criminal activities might be committed with the protection of anonymity in cryptocurrency transactions. Thus, user accountability (or traceability) is also important in Monero transactions, which is unfortunately lacking in the current literature. In this paper, we fill this gap by introducing a new cryptocurrency named Traceable Monero to balance the user anonymity and accountability. Our framework relies on a tracing authority, but is optimistic, in that it is only involved when investigations in certain transactions are required. We formalize the system model and security model of Traceable Monero. We present a detailed construction of Traceable Monero by overlaying Monero with two types of tracing mechanisms, tracing the one-time addresses with money flows and tracing the long-term addresses. We prove the security of Traceable Monero and implement a prototype of the system, which demonstrates that Traceable Monero incurs merely a very small overhead in generating and verifying a transaction compared to Monero transactions.

With the ubiquity of mobile devices and rapid development of cloud computing, mobile cloud computing (MCC) has been considered as an essential computation setting to support complicated, scalable and flexible mobile applications by overcoming the physical limitations of mobile devices with the aid of cloud. In the MCC setting, since many mobile applications (e.g., map apps) interacting with cloud server and application server need to perform computation with the private data of users, it is important to realize secure computation for MCC. In this article, we propose an efficient server-aided secure two-party computation (2PC) protocol for MCC. This is the first work that considers collusion between a malicious garbled circuit evaluator and a semi-honest server while ensuring privacy and correctness. Also, it can guarantee fairness when collusion does not exist. The security analysis shows that our protocol can securely compute any function f(x, y) against different types of adversaries in the malicious model. Also, the experimental performance analysis shows that this work outperforms the previous works for at least 10 times with the same security level.

Proof of Storage (PoS) enables a cloud storage provider to prove that a client's data is intact. However, existing PoS protocols are not designed for the pay-as-you-go business model in which payment is made based on both storage volume and duration. In this paper, we propose two PoS protocols suitable for the pay-as-you-go storage business model. The first is a time encapsulated Proof of Retrievability (PoR) protocol that ensures retrievability of the original file upon successful auditing by a client. Considering the large size of outsourced data, we then extend the protocol to a privacy-preserving public auditing protocol which allows a third party auditor to audit outsourced data on behalf of its clients without sacrificing the privacy of the data or the timestamp (i.e., time of storage). We formalize the definition, system model and security model of the proposed PoS system and prove the security of the proposed protocols by a sequence of games in the algebraic group model with a random oracle. We analyze the performance of the protocols both theoretically and experimentally and show that the protocols are practical.

As an essential element of the next generation Internet, Internet of Things (IoT) has been undergoing an extensive development in recent years. In addition to the enhancement of people's daily lives, IoT devices also generate/gather a massive amount of data that could be utilized by machine learning and big data analytics for different applications. Due to the machine-to-machine communication nature of IoT, data security and privacy are crucial issues that must be addressed to prevent different cyber attacks (e.g., impersonation and data pollution/poisoning attacks). Nevertheless, due to the constrained computation power and the diversity of IoT devices, it is a challenging problem to develop lightweight and versatile IoT security solutions. In this article, we propose an efficient, secure, and privacy-preserving message authentication scheme for IoT. Our scheme supports IoT devices with different cryptographic configurations and allows offline/online computation, making it more versatile and efficient than the previous solutions.

As cloud storage has been widely adopted in various applications, how to protect data privacy while allowing efficient data search and retrieval in a distributed environment remains a challenging research problem. Existing searchable encryption schemes are still inadequate on desired functionality and security/privacy perspectives. Specifically, supporting multi-keyword search under the multi-user setting, hiding search pattern and access pattern, and resisting keyword guessing attacks (KGA) are the most challenging tasks. In this article, we present a new searchable encryption scheme that addresses the above problems simultaneously, which makes it practical to be adopted in distributed systems. It not only enables multi-keyword search over encrypted data under a multi-writer/multi-reader setting but also guarantees the data and search pattern privacy. To prevent KGA, our scheme adopts a multi-server architecture, which accelerates search response, shares the workload, and lowers the key leakage risk by allowing only authorized servers to jointly test whether a search token matches a stored ciphertext. A novel subset decision mechanism is also designed as the core technique underlying our scheme and can be further used in applications other than keyword search. Finally, we prove the security and evaluate the computational and communication efficiency of our scheme to demonstrate its practicality.

Abstract In this paper, we introduce a new construction for linkable secret handshake that allows authenticated users to perform handshake anonymously within allowable times. We define formal security models for the new construction, and prove that it can achieve session key security, anonymity, untraceability and linkable affiliation-hiding. In particular, the proposed construction ensures that (i) anyone can trace the real identities of dishonest users who perform handshakes for more than k times; and (ii) an optimal communication cost between authorized users is achieved by exploiting the proof of knowledges.

Abstract Subversion of cryptography has received wide attentions especially after the Snowden Revelations in 2013. Most of the currently proposed subversion attacks essentially rely on the freedom of randomness choosing in the cryptographic protocol to hide backdoors embedded in the cryptosystems. Despite the fact that significant progresses in this line of research have been made, most of them mainly considered the classical setting, while the research gap regarding subversion attacks against post-quantum cryptography remains tremendous. Inspired by this observation, we investigate a subversion attack against existing protocol that is proved post-quantum secure. Particularly, we show an efficient way to undetectably subvert the well-known lattice-based encryption scheme proposed by Regev (STOC 2005). Our subversion enables the subverted algorithm to stealthily leak arbitrary messages to the outsider who knows the backdoor. Through theoretical analysis and experimental observations, we demonstrate that the subversion attack against the LWE encryption scheme is feasible and practical.

Abstract Hierarchical identity-based signature (HIBS) plays a core role in a large community as it significantly reduces the workload of the root private key generator. To make HIBS still available and secure in post-quantum era, constructing lattice-based schemes is a promising option. In this paper, we present an efficient HIBS scheme in polynomial rings. Although there are many lattice-based signatures proposed in recent years, to the best of our knowledge, our HIBS scheme is the first ring-based construction. In the center of our construction are two new algorithms to extend lattice trapdoors to higher dimensions, which are non-trivial and of independent interest. With these techniques, the security of the new scheme can be proved, assuming the hardness of the Ring-SIS problem. Since operations in the ring setting are much faster than those over integers and the new construction is the first ring-base HIBS scheme, our scheme is more efficient and practical in terms of computation and storage cost when comparing to the previous constructions.

Outsourcing helps relocate data from the cyber-physical system (CPS) for efficient storage at low cost. Current server-based outsourcing mainly focuses on the benefits of servers. This cannot attract users well, as their security, efficiency, and economy are not guaranteed. To solve with this issue, a hybrid outsourcing model that exploits both cloud server and edge devices to store data is needed. Meanwhile, the requirements of security and efficiency are different under specific scenarios. There is a lack of a comprehensive solution that considers all of the above issues. In this work, we overcome the above issues by proposing the first hybrid user-centric data outsourcing (HUCDO) scheme. It allows users to outsource data securely, efficiently, and economically via different CPSs. Brielly, our contributions consist of theories, implementations, and evaluations. Our theories include the first homomorphic collision-resistant chameleon hash (HCCH) and homomorphic designated-receiver signcryption (HDRS). As implementations, we instantiate how to use our proposals to outsource small- or large-scale data through distinct CPS, respectively. Additionally, a blockchain with proof-of-discrete-logarithm (B-PoDL) is instantiated to help improve our performance. Last, as demonstrated by our evaluations, our proposals are secure, efficient, and economic for users to implement while outsourcing their data via CPSs.

In a cloud data storage system, symmetric key encryption is usually used to encrypt files due to its high efficiency. In order allow the untrusted/semi-trusted cloud storage server to perform searching over encrypted data while maintaining data confidentiality, searchable symmetric encryption (SSE) has been proposed. In a typical SSE scheme, a users stores encrypted files on a cloud storage server and later can retrieve the encrypted files containing specific keywords. The basic security requirement of SSE is that the cloud server learns no information about the files or the keywords during the searching process. Some SSE schemes also offer additional functionalities such as detecting cheating behavior of a malicious server (i.e., verifiability) and allowing update (e.g., modifying, deleting and adding) of documents on the server. However, the previous (verifiable) SSE schemes were designed for single users, which means the searching can only be done by the data owner, whereas in reality people often use cloud storage to share files with other users. In this paper we present a multi-user verifiable searchable symmetric encryption (MVSSE) scheme that achieves all the desirable features of a verifiable SSE and allows multiple users to perform searching. We then define an ideal functionality for MVSSE under the Universally Composable (UC-) security framework and prove that our ideal functionality implies the security requirements of a secure MVSSE, and our multi-user verifiable SSE scheme is UC-secure. We also implement our scheme to verify its high performance based on some real dataset.

Sharing digital medical records on public cloud storage via mobile devices facilitates patients (doctors) to get (offer) medical treatment of high quality and efficiency. However, challenges such as data privacy protection, flexible data sharing, efficient authority delegation, computation efficiency optimization, are remaining toward achieving practical fine-grained access control in the Electronic Medical Record (EMR) system. In this work, we propose an innovative access control model and a fine-grained data sharing mechanism for EMR, which simultaneously achieves the above-mentioned features and is suitable for resource-constrained mobile devices. In the model, complex computation is outsourced to public cloud servers, leaving almost no complex computation for the private key generator (PKG), sender and receiver. Additionally, the communication cost of the PKG and users is optimized. Moreover, we develop an extensible library called libabe that is compatible with Android devices, and the access control mechanism is actually deployed on realistic environment, including public cloud servers, a laptop and an inexpensive mobile phone with constrained resources. The experimental results indicate that the mechanism is efficient, practical and economical.

Since the introduction of Bitcoin in 2008, cryptocurrency has been undergoing a quick and explosive development. At the same time, privacy protection, one of the key merits of cryptocurrency, has attracted much attention by the community. A deterministic wallet algorithm and a stealth address algorithm have been widely adopted in the community, due to their virtues on functionality and privacy protection, which come from a key derivation mechanism that an arbitrary number of derived keys can be generated from a master key. However, these algorithms suffer a vulnerability. In particular, when a minor fault happens (say, one derived key is compromised somehow), the damage is not limited to the leaked derived key only, instead, it spreads to the master key and all derived keys are compromised. In this paper, to provide a formal treatment for the problem, we introduce and formalize a new signature variant, called Key-Insulated and Privacy-Preserving Signature Scheme with Publicly Derived Public Key (PDPKS), which forms a convenient and robust cryptographic tool for offering the virtues of deterministic wallet and stealth address, while eliminating the security vulnerabilities. Specifically, PDPKS allows anyone to derive new signature verification keys for a user, say Alice, based on her long-term public key, while only Alice can derive the signing keys corresponding to those verification keys. In terms of privacy, given a derived verification key and valid signatures with respect to it, an adversary is not able to tell which long-term public key, out of a set of known long-term public keys, is the one from which the verification key was derived. A distinguishing security feature of PDPKS, with the above functionality and privacy features, is that the derived keys are independent/insulated from each other, namely, compromising the signing key associated with a verification key does not allow an adversary to forge a valid signature for another verification key, even if both verification keys are derived from the same long-term public key. We formalize the notion of PDPKS and propose a practical and proven secure construction, which could be a convenient and secure cryptographic tool for building privacy-preserving cryptocurrencies and supporting promising use cases in practice, as it can be used to implement secure stealth addresses, and can be used to implement deterministic wallets and the related appealing use cases, without security concerns.

A multi signature scheme allows a group of signers to produce a joint signature on a common message, which is more compact than a collection of distinct signatures from all signers. Given this signature and the list of signers' public keys, a verifier is able to check if every signer in the group participated in signing. Recently, a multisignature scheme with public key aggregation has drawn a lot of attention due to their applications into the blockchain technology. Such multisignatures provide not only a compact signature, but also a compact aggregated public key, that is both the signature size and the public key size used to verify the correctness of the signature are independent from the number of signers. This is useful for a blockchain because of its duplication over a distributed network, and thus it is required to be as compact as possible. In this paper, we introduce a new multisignature scheme with such a feature. Our scheme is proven secure under the Decisional Diffie-Hellman assumption. In addition, in the presence of rogue key attacks, the security of our scheme is proven in the plain public key model.

Abstract Thanks to the ease of access and low expenses, it is now popular for people to store data in cloud servers. To protect sensitive data from being leaked to the outside, people usually encrypt the data in the cloud. However, management of these encrypted data becomes a challenging problem, e.g. data classification. Besides, how to selectively share data with other users is also an important and interesting problem in cloud storage. In this paper, we focus on ciphertext-policy attribute based encryption with equality test (CP-ABEET). People can use CP-ABEET to implement not only flexible authorization for the access to encrypted data, but also efficient data label classification, i.e. test of whether two encrypted data contain the same message. We construct an efficient CP-ABEET scheme, and prove its security based on a reasonable number-theoretic assumption. Compared with the only existing CP-ABEET scheme, our construction is more efficient in key generation, and has shorter attribute-related secret keys and better security.

An Internet of Things (IoT) system generally contains thousands of heterogeneous devices which often operate in swarms-large, dynamic, and self-organizing networks. Remote attestation is an important cornerstone for the security of these IoT swarms, as it ensures the software integrity of swarm devices and protects them from attacks. However, current attestation schemes suffer from single point of failure verifier. In this paper, we propose an Efficient and Secure Distributed Remote Attestation (ESDRA) scheme for IoT swarms. We present the first many-to-one attestation scheme for device swarms, which reduces the possibility of single point of failure verifier. Moreover, we utilize distributed attestation to verify the integrity of each node and apply accusation mechanism to report the invaded nodes, which makes ESDRA much easier to feedback the certain compromised nodes and reduces the run-time of attestation. We analyze the security of ESDRA and do some simulation experiments to show its practicality and efficiency. Especially, ESDRA can significantly reduce the attestation time and has a better performance in the energy consumption comparing with list-based attestation schemes.

We introduce the first deniable attribute-based key exchange (DABKE) framework that is resilient to impersonation attacks. We define the formal security models for DABKE framework, and propose a generic compiler that converts any attribute-based key exchanges into deniable ones. We prove that it can achieve session key security and user privacy in the standard model, and strong deniability in the simulation-based paradigm. In particular, the proposed generic compiler ensures: 1) a dishonest user cannot impersonate other user’s session participation in conversations since implicit authentication is used among authorized users; 2) an authorized user can plausibly deny his/her participation after secure conversations with others; 3) the strongest form of deniability is achieved using one-round communication between two authorized users.

Applying consortium blockchain as a trust layer for heterogeneous industrial Internet-of-Things devices is cost-effective. However, with an increase in computing power, some powerful attacks (e.g., the 51% attack) are inevitable and will cause severe consequences. Recent studies also confirm that anonymity and immutability of blockchain have been abused to facilitate black market trades, etc. To operate controllable blockchain for IIoT devices, it is necessary to rewrite blockchain history back to a normal state once the chain is breached. Ateniese et al. proposed redactable blockchain by using chameleon hash (CH) to replace traditional hash function, it allows blockchain history to be written when needed (EuroS&P 2017). However, we cannot apply this idea directly to IIoT without solving the following problems: (1) achieve a decentralized design of CH; (2) update the signatures accordingly to authenticate the redacted contents; (3) satisfy the low-computing need of the individual IIoT device. In this paper, we overcome the above issues by proposing the first threshold chameleon hash (TCH) and accountable-and-sanitizable chameleon signature (ASCS) schemes. Based on them, we build a redactable consortium blockchain which is efficient for IIoT devices to operate. It allows a group of authorized sensors to write and rewrite blockchain without causing any hard forks. Basically, TCH is the first TCH and ASCS is a public-key signature supporting file-level and block-level modifications of signatures without impairing authentications. Additionally, ASCS achieves accountability to avoid abuse of redaction. While security analysis validates our proposals, the simulation results show that redaction is acceptably efficient if it is executed at a small scale or if we adopt a coarse-grained redaction while sacrificing some securities.

Homomorphic signature (HS) is a novel primitive that allows an agency to carry out arbitrary (polynomial time) computation f on the signed data m→ and accordingly gain a signature σh for the computation result f(m→) with respect to f on behalf of the data owner (DO). However, since DO lacks control of the agency’s behavior, receivers would believe that DO did authenticate the computation result even if the agency misbehaves and applies a function that the DO does not want. To address the problem above, in this paper we introduce a new primitive called authorized function homomorphic signature (AFHS). In AFHS, the agency has to obtain a confidence key skf from DO in order to evaluate a function f on the data m→ and to obtain a signature with which one can check whether the agency acts in accordance with DO’s instructions. A black-box construction of AFHS based on HS is given in this paper, and we show that if the underlying primitives are secure, so is our construction under the given security model. Moreover, we provide a somewhat concrete construction that offers stronger security guarantee.

We present a new type of public-key encryption called Criteria-based Encryption (or CE, for short). Different from Attribute-based Encryption, in CE, we consider the access policies as criteria carrying different weights. A user must hold some cases (or answers) satisfying the criteria and have sufficient weights in order to successfully decrypt a message. We then propose two CE Schemes under different settings: the first scheme requires a user to have at least one case for a criterion specified by the encryptor in the access structure, while the second scheme requires a user to have all the cases for each criterion. We prove that both schemes are secure under the Decisional q-Bilinear Diffie Hellman Exponent assumption without random oracles. In addition, we also present two special CE schemes for the above two settings without considering the weight requirement. We show that under this special case CE schemes can be constructed much more efficiently.

Cloud computing is an emerging computing paradigm that enables users to store their data in a cloud server to enjoy scalable and on-demand services. Nevertheless, it also brings many security issues, since cloud service providers (CSPs) are not in the same trusted domain as users. To protect data privacy against untrusted CSPs, existing solutions apply cryptographic methods (e.g., encryption mechanisms) and provide decryption keys only to authorized users. However, sharing cloud data among authorized users at a fine-grained level is still a challenging issue, especially when dealing with dynamic user groups. In this paper, we propose a secure and efficient fine-grained access control and data sharing scheme for dynamic user groups by: 1) defining and enforcing access policies based on the attributes of the data; 2) permitting the key generation center to efficiently update user credentials for dynamic user groups; and 3) allowing some expensive computation tasks to be performed by untrusted CSPs without requiring any delegation key. Specifically, we first design an efficient revocable attribute-based encryption (ABE) scheme with the property of ciphertext delegation by exploiting and uniquely combining techniques of identity-based encryption, ABE, subset-cover framework, and ciphertext encoding mechanism. We then present a fine-grained access control and data sharing system for on-demand services with dynamic user groups in the cloud. The experimental data show that our proposed scheme is more efficient and scalable than the state-of-the-art solution.

Proxy signature is a useful cryptographic primitive that has been widely used in many applications. It has attracted a lot of attention since it was introduced. There have been lots of works in constructing efficient and secure proxy signature schemes. In this paper, we identify a new attack that has been neglected by many existing proven secure proxy signature schemes. We demonstrate this attack by launching it against an identity‐based proxy signature scheme which is proven secure. We then propose one method that can effectively prevent this attack. The weakness in some other proxy signature schemes can also be fixed by applying the same method.

It is widely acknowledged that the collaborations with more users increase productivity. Secure cloud storage is a promising tool to enhance such a collaboration. Access control system can be enabled with attribute-based encryption. In this system, a user encrypts and uploads his/her data to the cloud with an access policy, such that only people who satisfy that access policy can decrypt the data. When a recipient would like to enable another person who is originally unauthorized by the original access policy, this recipient will need to extend the access policy by adding a new policy that includes the new person hence, the notion of extendable access control system. Admitting new users to access the uploaded data is an important requirement in enhancing collaborations. The main issue is with regards to the integrity protection during the process of extending the access policy. When a new access policy is added, the cloud has to be sure that the extended access policy remains guarding the same encrypted data as the original access policy, even though the cloud cannot decrypt this ciphertext, which is a challenging problem to solve. In this paper, we answer the above problem affirmatively by introducing an extendable access control system with Integrity Protection (EACSIP), which is suitable to enhance collaboration in the cloud. The construction of EACSIP is built on top of a novel cryptographic primitive, namely functional key encapsulation with equality testing. The security proof and the performance evaluation of EACSIP are provided in this paper.

In this paper, we present the notion of recipient-revocable identity-based broadcast encryption scheme. In this notion, a content provider will produce encrypted content and send them to a third party (which is a broadcaster). This third party will be able to revoke some identities from the ciphertext. We present a security model to capture these requirements, as well as a concrete construction. The ciphertext consists of k+3 group elements, assuming that the maximum number of revocation identities is k. That is, the ciphertext size is linear in the maximal size of R, where R is the revocation identity set. However, we say that the additional elements compared to that from an IBBE scheme are only for the revocation but not for decryption. Therefore, the ciphertext sent to the users for decryption will be of constant size (i.e.,3 group elements). Finally, we present the proof of security of our construction.

Anonymous proxy signatures are very useful in the construction of anonymous credential systems such as anonymous voting and anonymous authentication protocols. As a basic requirement, we should ensure an honest proxy signer is anonymous. However, in order to prevent the proxy signer from abusing the signing right, we should also allow dishonest signers to be traced. In this paper, we present three novel anonymous proxy signature schemes with different levels of (namely, public, internal and original signer) traceability. We define the formal definitions and security models for these three different settings, and prove the security of our proposed schemes under some standard assumptions.

Grouping proof is a very useful security primitive that can be used to prove the co-existence of multiple entities in an identification protocol. It can be incorporated into radio frequency identification protocols and used in many practical applications such as pharmaceutical distribution and manufacturing. However, the existing grouping proofs do not support secure key establishment, which is required in order to allow secure communication between the reader and the radio frequency identification tags after the identification process. In this paper, we propose a novel grouping proof with key exchange that allows the reader to establish a secure communication channel with the tags. We define the formal security models for grouping proof with key exchange and prove that the proposed generic protocol can achieve grouping proof soundness, session key security, contributiveness, and tag identity privacy.

Public key encryption with keyword search (PEKS) is a well-known cryptographic primitive for secure searchable data encryption in cloud storage. Unfortunately, it is inherently subject to the (inside) offline keyword guessing attack (KGA), which is against the data privacy of users. Existing countermeasures for dealing with this security issue mainly suffer from low efficiency and are impractical for real applications. In this paper, we provide a practical and applicable treatment on this security vulnerability by formalizing a new PEKS system named server-aided public key encryption with keyword search (SA-PEKS). In SA-PEKS, to generate the keyword ciphertext/trapdoor, the user needs to query a semitrusted third-party called keyword server (KS) by running an authentication protocol, and hence, security against the offline KGA can be obtained. We then introduce a universal transformation from any PEKS scheme to a secure SA-PEKS scheme using the deterministic blind signature. To illustrate its feasibility, we present the first instantiation of SA-PEKS scheme by utilizing the Full Domain Hash RSA signature and the PEKS scheme proposed by Boneh et al. in Eurocrypt 2004. Finally, we describe how to securely implement the client-KS protocol with a rate-limiting mechanism against online KGA and evaluate the performance of our solutions in experiments.

Searchable encryption is of increasing interest for protecting the data privacy in secure searchable cloud storage. In this paper, we investigate the security of a well-known cryptographic primitive, namely, public key encryption with keyword search (PEKS) which is very useful in many applications of cloud storage. Unfortunately, it has been shown that the traditional PEKS framework suffers from an inherent insecurity called inside keyword guessing attack (KGA) launched by the malicious server. To address this security vulnerability, we propose a new PEKS framework named dual-server PEKS (DS-PEKS). As another main contribution, we define a new variant of the smooth projective hash functions (SPHFs) referred to as linear and homomorphic SPHF (LH-SPHF). We then show a generic construction of secure DS-PEKS from LH-SPHF. To illustrate the feasibility of our new framework, we provide an efficient instantiation of the general framework from a Decision Diffie-Hellman-based LH-SPHF and show that it can achieve the strong security against inside the KGA.

We propose two new ciphertext policy attribute-based encryption (CP-ABE) schemes where the access policy is defined by AND-gate with wildcard. In the first scheme, we present a new technique that uses only one group element to represent an attribute, while the existing ABE schemes of the same type need to use three different group elements to represent an attribute for the three possible values (namely, positive, negative, and wildcard). Our new technique leads to a new CP-ABE scheme with constant ciphertext size, which, however, cannot hide the access policy used for encryption. The main contribution of this paper is to propose a new CP-ABE scheme with the property of hidden access policy by extending the technique we used in the construction of our first scheme. In particular, we show a way to bridge ABE based on AND-gate with wildcard with inner product encryption and then use the latter to achieve the goal of hidden access policy. We prove that our second scheme is secure under the standard decisional linear and decisional bilinear Diffie-Hellman assumptions.

Recently, a practical public integrity auditing scheme supporting multiuser data modification (IEEE TRANSACTIONS ON INFORMATION FORENSICS AND SECURITY, DOI 10.1109/TIFS.2015.2423264) was proposed. Although the protocol was claimed secure, in this paper, we show that the proposal fails to achievesoundness, the most essential property that an auditing scheme should provide. Specifically, we show that a cloud server can collude with a revoked user to deceive a third-party auditor (TPA) that a stored file keeps virgin even when the entire file has been deleted.

We study the problem of key establishment for secure many-to-many communications. The problem is inspired by the proliferation of large-scale distributed file systems supporting parallel access to multiple storage devices. Our work focuses on the current Internet standard for such file systems, i.e., parallel Network File System (pNFS), which makes use of Kerberos to establish parallel session keys between clients and storage devices. Our review of the existing Kerberos-based protocol shows that it has a number of limitations: (i) a metadata server facilitating key exchange between the clients and the storage devices has heavy workload that restricts the scalability of the protocol; (ii) the protocol does not provide forward secrecy; (iii) the metadata server generates itself all the session keys that are used between the clients and storage devices, and this inherently leads to key escrow. In this paper, we propose a variety of authenticated key exchange protocols that are designed to address the above issues. We show that our protocols are capable of reducing up to approximately 54 percent of the workload of the metadata server and concurrently supporting forward secrecy and escrow-freeness. All this requires only a small fraction of increased computation overhead at the client.

In a recent paper (IEEE Trans. Wireless Commun., vol. 14, no. 1, 2015), Heet al.proposed an accountable and privacy-enhanced access control (APAC) protocol, which aimed to provide privacy for honest users against network owners and accountability against misbehaving users without the involvement of any trusted third party. However, the level of trust on the network owner has not been clearly defined in Heet al.’s paper, and we demonstrate in this letter that in the case where the network owners cannot be trusted to correctly generate the system parameters, then the APAC protocol cannot ensure user privacy.

With a rapid growth of data storage in the cloud, data integrity checking in a remote data storage system has become an important issue. A number of protocols, which allow remote integrity checking by a third party, have been proposed. Although those protocols are provably secure, the data privacy issues in those protocols have not been considered. We believe that these issues are equally important since the communication flows of integrity proofs from the cloud server should not reveal any useful information of the stored data. In this paper, we introduce a new definition of data privacy called ‘IND-Privacy’ by an indistinguishability game. It is found that many existing remote integrity proofs are insecure under an IND-Privacy game. It is also found that by adopting witness indistinguishable proofs, the IND-Privacy is achievable. We provide an instantiation that captures data integrity, soundness and IND-privacy.

Deduplication is a popular technique widely used to save storage spaces in the cloud. To achieve secure deduplication of encrypted files, Bellare et al. formalized a new cryptographic primitive named message-locked encryption (MLE) in Eurocrypt 2013. Although an MLE scheme can be extended to obtain secure deduplication for large files, it requires a lot of metadata maintained by the end user and the cloud server. In this paper, we propose a new approach to achieve more efficient deduplication for (encrypted) large files. Our approach, named block-level message-locked encryption (BL-MLE), can achieve file-level and block-level deduplication, block key management, and proof of ownership simultaneously using a small set of metadata. We also show that our BL-MLE scheme can be easily extended to support proof of storage, which makes it multi-purpose for secure cloud storage.

We propose a new Ciphertext Policy Attribute Based Encryption(CP-ABE) scheme where access structures are defined by AND-Gates with wildcards. One major difference between our scheme and the existing ones is that we can use a single element to represent one attribute, while the previous schemes require three different elements to represent the three possible values (namely positive, negative, and wildcard) of an attribute. Our proposed scheme also achieves both constant-size ciphertext and constant number of decryption operations, and is proven secure under the standard Decision Linear Assumption.

Signer anonymity is an important security requirement for many digital signature schemes due to the need of user privacy in many applications. In this paper, we study signer anonymity for proxy signature which allows a signer to delegate his/her signing right to another user (or proxy). Since the proxy signer is the actual singer in a proxy signature scheme, we are interested in protecting the proxy signer's identity in this paper. However, one potential problem in an anonymous proxy signature scheme is that the proxy signer may abuse the delegated signing right due to the anonymity property of a proxy signature. In this paper, we propose a novel anonymous proxy signature scheme with restricted traceability, which allows the original singer to trace dishonest proxy signers. However, if the proxy signer is honest, then his/her identity is well protected against any user (including the original signer). Our scheme will be useful in many applications such as anonymous authentication protocols and anonymous voting schemes.

SUMMARY Designated verifier signatures (DVS) allow a signer to create a signature whose validity can only be verified by a specific entity chosen by the signer. In addition, the chosen entity, known as the designated verifier, cannot convince any body that the signature is created by the signer. Multidesignated verifiers signatures (MDVS) are a natural extension of DVS in which the signer can choose multiple designated verifiers. DVS and MDVS are useful primitives in electronic voting and contract signing. In this paper, we investigate various aspects of MDVS and make two contributions. Firstly, we revisit the notion of unforgeability under rogue key attack on MDVS. In this attack scenario, a malicious designated verifier tries to forge a signature that passes through the verification of another honest designated verifier. A common counter‐measure involves making the knowledge of secret key assumption in which an adversary is required to produce a proof‐of‐knowledge of the secret key. We strengthened the existing security model to capture this attack and propose a new construction that does not rely on the knowledge of secret key assumption. Secondly, we propose a generic construction of strong MDVS. Copyright © 2013 John Wiley & Sons, Ltd.

ABSTRACT Anonymous user authentication is an important but challenging task for wireless communications. In a recent paper, Das proposed a smart card‒based anonymous user authentication protocol for wireless communications. The scheme can protect user privacy and is believed to be secure against a range of network attacks even if the secret information stored in the smart card is compromised. In this paper, we reanalyze the security of Das' scheme, and show that the scheme is in fact insecure against impersonation attacks. We then propose a new smart card‒based anonymous user authentication protocol for wireless communications. Compared with the existing schemes, our protocol uses a different user authentication mechanism, which does not require different entities to maintain a synchronized clock. We show that the proposed new protocol can provide stronger security and better efficiency and scalability than previous schemes.

In this paper, for the first time, we define a general notion for proxy re-encryption (PRE), which we call deterministic finite automata-based functional PRE (DFA-based FPRE). Meanwhile, we propose the first and concrete DFA-based FPRE system, which adapts to our new notion. In our scheme, a message is encrypted in a ciphertext associated with an arbitrary length index string, and a decryptor is legitimate if and only if a DFA associated with his/her secret key accepts the string. Furthermore, the above encryption is allowed to be transformed to another ciphertext associated with a new string by a semitrusted proxy to whom a re-encryption key is given. Nevertheless, the proxy cannot gain access to the underlying plaintext. This new primitive can increase the flexibility of users to delegate their decryption rights to others. We also prove it as fully chosen-ciphertext secure in the standard model.

We revisit the problem of secure cross-domain communication between two users belonging to different security domains within an open and distributed environment. Existing approaches presuppose that either the users are in possession of public key certificates issued by a trusted certificate authority (CA), or the associated domain authentication servers share a long-term secret key. In this article, we propose a generic framework for designing four-party password-based authenticated key exchange (4PAKE) protocols. Our framework takes a different approach from previous work. The users are not required to have public key certificates, but they simply reuse their login passwords, which they share with their respective domain authentication servers. On the other hand, the authentication servers, assumed to be part of a standard PKI, act as ephemeral CAs that certify some key materials that the users can subsequently use to exchange and agree on as a session key. Moreover, we adopt a compositional approach. That is, by treating any secure two-party password-based key exchange (2PAKE) protocol and two-party asymmetric-key/symmetric-key-based key exchange (2A/SAKE) protocol as black boxes, we combine them to obtain generic and provably secure 4PAKE protocols.

We revisit the problem of cross-domain secure communication between two users belonging to different security domains within an open and distributed environment. Existing approaches presuppose that either the users are in possession of public key certificates issued by a trusted certificate authority (CA), or the associated domain authentication servers share a long-term secret key. In this paper, we propose a four-party password-based authenticated key exchange (4PAKE) protocol that takes a different approach from previous work. The users are not required to have public key certificates, but they simply reuse their login passwords they share with their respective domain authentication servers. On the other hand, the authentication servers, assumed to be part of a standard PKI, act as ephemeral CAs that “certify” some key materials that the users can subsequently exchange and agree on a session key. Moreover, we adopt a compositional approach. That is, by treating any secure two-party password-based key exchange protocol and two-party asymmetric-key based key exchange protocol as black boxes, we combine them to obtain a generic and provably secure 4PAKE protocol.

In certificateless cryptography, a user secret key is derived from two partial secrets: one is the identity-based secret key (corresponding to the user identity) generated by a Key Generation Center (KGC), and the other is the user self-generated secret key (corresponding to a user self-generated and uncertified public key). Two types of adversaries are considered for certificateless cryptography: a Type-I adversary who can replace the user self-generated public key (in transmission or in a public directory), and a Type-II adversary who is an honest-but-curious KGC. In this paper, we present a formal study on certificateless key exchange (CLKE). We show that the conventional definition of Type-I and Type-II security may not be suitable for certificateless key exchange when considering the notion of forward secrecy which is important for key exchange protocols. We then present a new security model in which a single adversary (instead of Type-I and Type-II adversaries) is considered. We also construct a strongly secure certificateless key exchange protocol without expensive pairing operations. As far as we know, our proposed protocol is the first proven secure CLKE protocol without pairing.

A signcryption scheme allows a sender to produce a ciphertext for a receiver so that both confidentiality and non-repudiation can be ensured. It is built to be more efficient and secure, for example, supporting insider security, when compared with the conventional sign-then-encrypt approach. In this paper, we propose a new notion called heterogeneous signcryption in which the sender has an identity-based secret key while the receiver is holding a certificate-based public key pair. Heterogeneous signcryption is suitable for practical scenarios where an identity-based user, who does not have a personal certificate or a public key, wants to communicate securely with a server which has a certificate with its public key. We propose two constructions and show their security under the model we define in the random oracle model. The model we define captures the insider security for both confidentiality and unforgeability. Both of the schemes also support public verifiability and key privacy, that is, an adversary cannot find out who the sender and receiver are from a ciphertext in the insider security model. The second scheme is the most efficient one computationally among all key-privacy-preserving signcryption schemes even when compared with schemes in an identity-based cryptographic setting or certificate-based public key setting.

In a recent paper (IEEE Trans. Wireless Commun., vol. 9, no. 11, 2010), Chang and Tsai presented a self-verified mobile authentication scheme for large-scale wireless networks. In this letter, we show that there is a serious security flaw in the key delegation phase of the scheme: two colluding mobile users can retrieve the long-term secret key of their home server without performing any active attacks. We then present a suggestion to fix the problem without losing any features (such as high efficiency and scalability) of the original scheme.

In Information Processing Letters (2006), Tan pointed out that the anonymous signcryption scheme proposed by Yang, Wong and Deng (YWD) in ISC 2005 provides neither confidentiality nor anonymity. However, no discussion has been made on how a secure scheme can be made and there is no secure scheme available to date. In this paper, we propose a modification of YWD scheme which resolves the security issues of the original scheme without sacrificing its high efficiency and simple design. Indeed, we show that our scheme achieves confidentiality, existential unforgeability and anonymity with more precise reduction bounds. We also give a variation of our scheme and extend it to a ring signcryption scheme by using the technique due to Boneh, Gentry, Lynn and Shacham.

In a recent paper, Dutta and Barua presented a group key agreement protocol in dynamic setting. The protocol allows users to efficiently join or leave a group and was believed to be provably secure. In this letter, we point out a flaw in the Dutta-Barua dynamic group key agreement protocol.

A secure roaming protocol allows a roaming user U to visit a foreign server V and establish a session key in an authenticated way such that U authenticates V and at the same time convinces V that it is a legitimate subscriber of some server H, called the home server of U. The conventional approach requires the involvement of all the three parties. In this paper, we propose a new approach which requires only two parties, U and V, to get involved. We propose two protocols: one provides better efficiency and supports user anonymity to an extent comparable to that provided by current mobile systems; and the other one achieves strong user anonymity that protects U's identity against both eavesdroppers and foreign servers and is currently the strongest notion of user anonymity defined for secure roaming. Both protocols are universal in the sense that the same protocol and signaling flows are used regardless of the domain (home or foreign) that U is visiting. This helps reducing the system complexity in practice. We also propose a practical user revocation mechanism, which is one of the most challenging problems for two-party roaming supporting strong user anonymity. Our solutions can be applied in various kinds of roaming networks such as cellular networks and interconnected wireless local area networks.

User privacy is a notable security issue in wireless communications. It concerns about user identities from being exposed and user movements and whereabouts from being tracked. The concern of user privacy is particularly signified in systems which support roaming when users are able to hop across networks administered by different operators. In this paper, we propose a novel construction approach of anonymous and authenticated key exchange protocols for a roaming user and a visiting server to establish a random session key in such a way that the visiting server authenticates the user's home server without knowing exactly who the user is. A network eavesdropper cannot find out the user's identity either (user anonymity). In addition, visited servers cannot track the roaming user's movements and whereabouts even they collude with each other (user untraceability). Our construction approach is generic and built upon provably secure two-party key establishment protocols. Merits of our generic protocol construction include eliminating alias synchronization between the user and the home server, supporting joint key control, and not relying on any special security assumptions on the communication channel between the visiting server and the user's home server. Our protocol can also be implemented efficiently. By piggybacking some message flows, the number of message flows between the roaming user and the visiting server is only three. As of independent interest, we describe a new practical attack called deposit-case attack and show that some previously proposed protocols are vulnerable to this attack.