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As a decentralized network infrastructure, the data sent to the blockchain are public and temper-evident. The cover of massive normal transactions in a blockchain network is ideal for constructing a stable and reliable covert channel to achieve one-to-many group covert communication. Existing blockchain-based covert communication schemes face challenges in balancing concealment, embedding rate and filtering efficiency, making them unsuitable for direct extension to group scenarios. Adopting a key-leakage scheme can increase the channel capacity while maintaining high concealment from external adversaries. However, it will also expose more knowledge to the receiver. A malicious receiver has the ability to steal a sender's identity or replay historical transactions to control the entire channel. In this paper, we define the capabilities of malicious receivers in blockchain-based group covert communication scenarios and propose a group covert communication scheme resistant to transaction forgery attacks. Theoretical analysis and experiments prove that our covert transactions do not have any transaction correlativity, ensuring the unique authenticity of the sender's identity while maintaining supreme concealment compared with the existing schemes. The precision and recall of machine learning detection results can reach 0.57–0.62 (0.5 is the ideal value). [ABSTRACT FROM AUTHOR]

Reporting helps to combat illegal activities and deters lawbreakers and potential lawbreakers. From ancient times to the present, public authorities have usually rewarded effective reporting information to build harmonious societies. In this process, protecting the privacy of the whistleblower is a very important issue. Existing blockchain-based anonymous reporting solutions help solve the problem of insufficient anonymity in traditional reporting solutions, but they do not address the issue of hiding the reporting behavior. The disclosure of reporting behavior may alert offenders in advance and negatively impact case handling. This paper proposes an anonymous and covert reporting scheme and rewarding mechanism based on blockchain, which realizes the covertness of the reporting behavior while protecting the privacy of the whistleblower. The proposed scheme uses ring signature and derived address technology to ensure anonymity and achieves covertness by embedding information in the ring signature based on the idea of covert communication. Theoretical analysis proves that the proposed scheme has covertness, anonymity, and unforgeability properties. Experiments show that the proposed scheme takes only 0.08 s to upload data and 0.07 s to verify while achieving covertness. [ABSTRACT FROM AUTHOR]

Blockchain technology is an immutable database that cannot be censored, tampered with, or erased, which can increase credibility and reduce interaction costs. Therefore, blockchain technology is expected to become the next generation Internet infrastructure, with broad application prospects in many industries, such as medicine, energy, and finance. The open network and publicly stored data in a blockchain system, however, raise serious risks of data theft and user privacy leakage, which has become a core issue that restricts the blockchain technology from becoming practical. This article uses kleptography technology to design a blockchain covert data transmission scheme to achieve high-concealment and high-performance data transmission under open network conditions. Experiments conducted in the Bitcoin network prove that the covert data transmission mechanism of this article has high compatibility and high performance, and can be practically applied to many popular blockchain systems. [ABSTRACT FROM AUTHOR]

As an integral part of V2G networks, EVs receive electricity from not only the grid but also other EVs and may frequently feed the power back to the grid. Payment records in V2G networks are useful for extracting user behaviors and facilitating decision-making for optimized power supply, scheduling, pricing, and consumption. Sharing payment and user information, however, raises serious privacy concerns in addition to the existing challenge of secure and reliable transaction processing. In this article, we propose a blockchain-based privacy preserving payment mechanism for V2G networks, which enables data sharing while securing sensitive user information. The mechanism introduces a registration and data maintenance process that is based on a blockchain technique, which ensures the anonymity of user payment data while enabling payment auditing by privileged users. Our design is implemented based on Hyperledger to carefully evaluate its feasibility and effectiveness. [ABSTRACT FROM AUTHOR]

Vehicles share information via Vehicular Ad-hoc Network (VANET) that is considered the foundation of the intelligent transportation system. Access control from functional encryption (FE) allows secure sharing over encrypted data with both utilization and privacy. However, low-powerful vehicles and the untrusted server incurs that FE-based access control mechanisms are less practical with client-side inefficiency and access unreliability. In this work, we put forth ORAC, an Optimized encrypted Access Control system with partial privacy and Reliability. ORAC is built on top of functional encryption that guarantees data partial privacy and data utilization in VANET, and separates the data access into the offline phase and the online phase, such that most of decryption computation could be done in the offline phase and vehicles could execute the real decryption efficiently. By embedding blockchain transactions into identity records and data storage, ORAC protects data accessed by vehicles from tampering and thereby manages data transfers with high reliability. It can be instantiated directly from pairing-based assumptions and features only the decryption complexity of two exponentiation and two multiplication operations. We give a formal security proof of the instantiation in the semantic security model. We also prototype our scheme by implementing ORAC over Ethereum with two types of smart contracts and the evaluation can demonstrate its efficiency optimization. [ABSTRACT FROM AUTHOR]

Summary: In classical public‐key infrastructure (PKI), the certificate authorities (CAs) are fully trusted, and the security of the PKI relies on the trustworthiness of the CAs. However, recent failures and compromises of CAs showed that if a CA is corrupted, fake certificates may be issued, and the security of clients will be at risk. As emerging solutions, blockchain‐ and log‐based PKI proposals potentially solved the shortcomings of the PKI, in particular, eliminating the weakest link security and providing a rapid remedy to CAs' problems. Nevertheless, log‐based PKIs are still exposed to split‐world attacks if the attacker is capable of presenting two distinct signed versions of the log to the targeted victim(s), while the blockchain‐based PKIs have scaling and high‐cost issues to be overcome. To address these problems, this paper presents a secure and accountable transport layer security (TLS) certificate management (SCM), which is a next‐generation PKI framework. It combines the two emerging architectures, introducing novel mechanisms, and makes CAs and log servers accountable to domain owners. In SCM, CA‐signed domain certificates are stored in log servers, while the management of CAs and log servers is handed over to a group of domain owners, which is conducted on the blockchain platform. Different from existing blockchain‐based PKI proposals, SCM decreases the storage cost of blockchain from several hundreds of GB to only hundreds of megabytes. Finally, we analyze the security and performance of SCM and compare SCM with previous blockchain‐ and log‐based PKI schemes. [ABSTRACT FROM AUTHOR]

Currently, decentralized redactable blockchains have been widely applied in IoT systems for secure and controllable data management. Unfortunately, existing works ignore policy privacy (i.e., the content of users' redaction policies), causing severe privacy leakage threats to users since users' policies usually contain large amounts of private information (e.g., health conditions and geographical locations) and limiting the applications in IoT systems. To bridge this research gap, we propose PFRB, a policy-hidden fine-grained redactable blockchain in decentralized blockchain-based IoT systems. PFRB follows the decentralized settings and fine-grained chameleon hash-based redaction in existing redactable blockchains. In addition, PFRB hides users' policies during policy matching such that apart from successful policy matching, users' policy contents cannot be inferred and valid redactions cannot be executed. Some main technical challenges include determining how to hide policy contents and support policy matching. Inspired by Newton's interpolation formula-based secret sharing, PFRB converts policy contents into polynomial parameters and utilizes multi-authority attribute-based encryption to further hide these parameters. Theoretical analysis proves the correctness and security against the chosen-plaintext attack. Extensive experiments on the FISCO blockchain platform and IoT devices show that PFRB achieves competitive efficiency over current redactable blockchains. [ABSTRACT FROM AUTHOR]

Machine learning has become increasingly popular in academic and industrial communities and has been widely implemented in various online applications due to its powerful ability to analyze and use data. Among all the machine learning models, decision tree models stand out due to their great interpretability and simplicity, and have been implemented in cloud computing services for various purposes. Despite its great success, the integrity issue of online decision tree prediction is a growing concern. The correctness and consistency of decision tree predictions in cloud computing systems need more security guarantees since verifying the correctness of the model prediction remains challenging. Meanwhile, blockchain has a promising prospect in two-party machine learning services as the immutable and traceable characteristics satisfy the verifiable settings in machine learning services. In this paper, we initiate the study of decision tree prediction services on blockchain systems and propose VDT, a Verifiable Decision Tree prediction scheme for decision tree prediction. Specifically, by leveraging the Merkle tree and hash function, the scheme allows the service provider to generate a verification proof to convince the client that the output of the decision tree prediction is correctly computed on a particular data sample. It is further extended to an update method for a verifiable decision tree to modify the decision tree model efficiently. We prove the security of the proposed VDT schemes and evaluate their performance using real datasets. Experimental evaluations show that our scheme requires less than one second to produce verifiable proof. [ABSTRACT FROM AUTHOR]

Cloud platforms provide a low-cost and convenient way for users to share data. One important issue of cloud-based data sharing systems is how to prevent the sensitive information contained in users' data from being disclosed. Existing studies often utilize cryptographic primitives, such as attribute-based encryption and proxy re-encryption, to protect data privacy. These approaches generally rely on a centralized server which may cause a single point of failure problem. Blockchain is known for its ability to solve such a problem. Some blockchain-based approaches have been proposed to realize privacy-preserving data sharing. However, these approaches did not fully explore the auditability provided by the blockchain. The dishonest cloud server can share data with a requester without notifying the data owner or being logged by the blockchain. In this paper, we propose a blockchain-based privacy-preserving data sharing system with enhanced auditability. The proposed system follows the idea of hybrid encryption to protect data privacy. The data to be shared are encrypted with a symmetric key, and the symmetric key is encrypted with a joint public key which is the sum of multiple blockchain nodes' public keys. Only if a data requester is authorized, the blockchain nodes will be triggered to execute a verifiable key switch protocol. By using the output of the protocol, the data requester can get the plaintext of the symmetric key. The blockchain nodes participate in both the authorization process and the key switch process, which means the behavior of the data requester is witnessed by multi-parties and is auditable. We implement the proposed system on Hyperledger Fabric. The simulation results show that the performance overhead is acceptable. [ABSTRACT FROM AUTHOR]

Edge-Integrated Internet-of-Vehicles (IoV) sinks service to edge nodes, which responses quickly to vehicles' requests and alleviates the burden of cloud servers. In such scenario, the entities of IoV need to be mutually authenticated since potential attacks can impersonate edge nodes to send false instructions to vehicles, or impersonate legitimate service subscribers to get free-ride IoV services. Furthermore, due to the high mobility characteristics of vehicles, frequent authentication is required between vehicles and edge nodes. We hope that the authentication process can be conducted efficiently to ensure the continuous service. Existing works cannot balance the security and efficiency well. This is still an issue worthy of discussion. In this article, we propose SEA, a secure and efficient blockchain-assisted authentication scheme for IoV. SEA achieves mutual authentication among vehicles, edge nodes and cloud servers. Specifically, the cloud server is only involved when vehicles are initially authenticated. And edge nodes realize the authentication of vehicles by querying the authentication result recorded by cloud on the blockchain, which significantly reduces the cryptographic computation overhead and eliminates network communication delay. Besides, session keys between any two entities involved are negotiated, which can secure sensitive data of vehicles. Extensive experiments have been conducted to show the security and efficiency of SEA. [ABSTRACT FROM AUTHOR]

With rise of 5G/6G network, low-storage devices usually outsource data for higher rate and less latency. Non-controlled outsourcing and complex communications incur security issues for IoT applications. Specially, user privacy and access reliability pose technical challenges for sensitive data outsourcing and sharing. In this paper, we harmonize functional encryption and blockchain to propose a r eliable and p rivacy-aware a ccess c ontrol system named R-PAC. It allows a result-form access without learning raw information and fair interaction against malicious users. With a combination of all-or-nothing encapsulation technology, R-PAC supports users' dynamic joining and key leakage resistance. We design R-PAC from Boneh–Franklin identity-based encryption, with forward-coverable encryption and reverse-discoverable decryption, and formally prove its indistinguishability security. We implement a R-PAC prototype and deploy it to a simulated Ethereum network to evaluate its performance. Experiments from both data access and transaction overhead show that R-PAC is with reasonable cost and has a trade-off between efficiency and strong security/functionality. • We propose R-PAC, a reliable and privacy-aware access control system. • We design R-PAC from Boneh–Franklin IBE, and formally prove its IND security. • We implement a R-PAC prototype and deploy it to a simulated Ethereum network. [ABSTRACT FROM AUTHOR]

• A Privacy-preserving contact Tracing scheme in 5G-integrated and Blockchain-based Medical applications, named PTBM, is proposed. • Everyone can perform location checking with his mobile phones to find whether they have been in possible contact with a diagnosed patient without violating their privacy. • A trusted medical center can effectively trace the patients and their corresponding close contacts. • Security and performanc e analysis show that the proposed PTBM scheme achieves privacy protection, traceability, reliability, and authentication, with high efficiency and low latency. The current pandemic situation due to COVID-19 is seriously affecting our daily work and life. To block the propagation of infectious diseases, an effective contact tracing mechanism needs to be implemented. Unfortunately, existing schemes have severe privacy issues that jeopardize the identity-privacy and location-privacy for both users and patients. Although some privacy-preserving systems have been proposed, there remain several issues caused by centralization. To mitigate this issues, we propose a P rivacy-preserving contact T racing scheme in 5G-integrated and B lockchain-based M edical applications, named PTBM. In PTBM, the 5G-integrated network is leveraged as the underlying infrastructure where everyone can perform location checking with his mobile phones or even wearable devices connected to 5G network to find whether they have been in possible contact with a diagnosed patient without violating their privacy. A trusted medical center can effectively trace the patients and their corresponding close contacts. Thorough security and performance analysis show that the proposed PTBM scheme achieves privacy protection, traceability, reliability, and authentication, with high computation & communication efficiency and low latency. [ABSTRACT FROM AUTHOR]