Version Control System (VCS) plays an essential role in software supply chain, as it manages code projects and enables efficient collaboration. For a private repository, where source code is a high-profile asset and needs to be protected, VCS’ security is extremely important. Traditional (unencrypted or encrypted) VCS solutions rely on a trusted service provider to host the code and enforce access control, which is not realistic enough for real-world threats. If the service provider peep in or the hackers break into the repository, the read & write privilege to the sensitive code is totally lost. Therefore, we consider whether one can relax the assumption on the server by introducing acovert adversary, namely, it may act maliciously, but will not misbehave if it can be caught doing so. However, protecting sensitive code and enforcing access control on a covert adversarial server is a challenging task. Existing encryption-based VCS solutions failed to address this challenge, as they offered limited access control functionalities, introduced heavy key management overhead or storage overhead. Moreover, the crucial feature of compression of the source files were missing in an encrypted and versioned storage. To address these problems, we introduceGringotts, an end-to-end encrypted VCS, tailored for read & write access control, version control and source file compression. We present a formal model and propose a scheme with detailed analysis. We also implement and evaluateGringottson top-10 most starred code projects on GitHub. The results demonstrate thatGringottsintroduces low latency (less than 0.3 s) for commit encryption and decryption, supports fine-grained access control and rich version control functionalities with practical performance.

Personally identifiable information (PII) refers to any information that links to an individual. Sharing PII is extremely useful in public affairs yet hard to implement due to the worries about privacy violations. Building a PII retrieval service over multi-cloud, which is a modern strategy to make services stable where multiple servers are deployed, seems to be a promising solution. However, three major technical challenges remain to be solved. The first is the privacy and access control of PII. In fact, each entry in PII can be shared to different users with different access rights. Hence, flexible and fine-grained access control is needed. Second, a reliable user revocation mechanism is required to ensure that users can be revoked efficiently, even if few cloud servers are compromised or collapse, to avoid data leakage. Third, verifying the correctness of received PII and locating a misbehaved server when wrong data are returned is crucial to guarantee user's privacy, but challenging to realize. In this paper, we propose Rainbow, a secure and practical PII retrieval scheme to solve the above issues. In particular, we design an important cryptographic tool, called Reliable Outsourced Attribute Based Encryption (ROABE) which provides data privacy, flexible and fine-grained access control, reliable immediate user revocation and verification for multiple servers simultaneously, to support Rainbow. Moreover, we present how to build Rainbow with ROABE and several necessary cloud techniques in real world. To evaluate the performance, we deploy Rainbow on multiple mainstream clouds, namely, AWS, GCP and Microsoft Azure, and experiment in browsers on mobile phones and computers. Both theoretical analysis and experimental results indicate that Rainbow is secure and practical.

The immutability of blockchains is critical for cryptocurrencies, but an imperative need arises for the redaction of on-chain data due to privacy-protecting laws like GPDR. Recently, Ateniese et al. (EuroS&P 2017) proposed an elegant solution to this problem based on chameleon hash functions, followed by many subsequent works. While these works offered a solution to the permissioned blockchain, the approaches were not efficient enough for the permissionless setting, in terms of either security (which may cause inconsistent historical transactions) or performance (only up to a few hundred nodes). In this paper, we investigate this problem and present Wolverine, a redactable permissionless blockchain. First, we present a formal redactable blockchain model, carefully considering transaction consistency. Next, towards a practical scheme, we introduce the novel concept of non-interactive chameleon hash (NITCH). NITCHs dynamically distribute a trapdoor key among a group and each party in the group can compute its partial share without communicating with others. Anyone who possesses enough shares can then find a valid hash collision. To prevent the static group from being compromised after a sufficiently long time, we provide a generic transform from NITCHs to decentralized random beacons (DRBs) and design a committee evolution protocol based on DRBs that refresh the group after every fixed interval of time. Based on NITCH and the committee evolution protocol, we construct Wolverine which offers important features such as scalability, transaction consistency, and public accountability. Finally, we demonstrate the practicality of Wolverine by giving a proof-of-concept implementation based on Bitcoin in Golang.

Cloud-edge computing is a new paradigm for data sharing. Many computation tasks are assigned to multiple edge nodes to mitigate the computing burden of the cloud and data is also outsourced to them to provide real-time services for IoT devices. However, two major issues remain, namely data privacy and real-world deployment. According to the data privacy rights and principles that stated by General Data Protection Regulation (GDPR), data access control, restriction of data processing and finding inaccuracy data are critical issues that should be tackled in cloud-edge computing. Besides, since there are various types of devices and many of them are resource-constrained, how to efficiently apply deployment in cloud-edge computing is challenging for practice. In this work, we propose a new cryptographic primitive Controllable Outsourced Attribute-Based Proxy Re-Encryption (COAB-PRE) and a universal WebAssembly-based implementation framework for cross-platform deployment. In particular, COAB-PRE achieves bilateral and distributed access control whereby data producers and data consumers can both specify policies the other party must satisfy without a centralized access control server. The property, that we called controllable delegation, restricts the data processing on the edge nodes. COAB-PRE also supports comprehensive verifiability to find out a wrong result produced by the edge nodes and locate the misbehaved one. Moreover, we further discussed the potential property of COAB-PRE and put forward an improved scheme with high efficiency on devices. We also implemented our scheme using the approach and deployed it on different devices for experiment. All theoretical and experimental results indicate that our solution is secure and practical, and our implementation is suitable for cloud-edge computing.

With more and more data moving to the cloud, privacy of user data have raised great concerns. Client-side encryption/decryption seems to be an attractive solution to protect data security, however, the existing solutions encountered three major challenges: low security due to encryption with low-entropy PIN, inconvenient data sharing with traditional encryption algorithms, and poor usability with dedicated software/plugins that require certain types of terminals. This work designs and implements WebCloud, a practical browser-side encryption solution, leveraging modern Web technologies. It solves all the above three problems while achieves several additional remarkable features: robust and immediate user revocation, fast data processing with offline encryption and outsourced decryption. Notably, our solution works on any device equipped with a Web user agent, including Web browsers, mobile and PC applications. We implement WebCloud based on ownCloud for basic file management utility, and utilize WebAssembly and Web Cryptography API for complex cryptographic operations integration. Finally, comprehensive experiments are conducted with many well-known browsers, Android and PC applications, which indicates that WebCloud is cross-platform and efficient. As an interesting by-product, the design of WebCloud naturally embodies a dedicated and practical ciphertext-policy attribute-based key encapsulation mechanism (CP-AB-KEM) scheme, which can be useful in other applications.

As an innovative technique for cloud storage services, attribute based encryption (ABE) enables fine-grained access control over encrypted data in many cloud computing applications. However, there exist two main drawbacks that restrict the development of ABE. One is that most of the existing user revocation mechanisms cannot achieve high efficiency, immediacy and robustness simultaneously. Another is that the decryption of ABE contains expensive pairing operations which often grow with the complexity of access policy. In this work, we propose a practical server-aided revocable fine-grained access control mechanism with the help of cloud’s storage, computing and management capabilities, which not only achieves efficient fine-grained attribute based access control, but also actualizes immediate and robust user revocation. Moreover, most of the complicated operations in decryption are outsourced to the public cloud server, leaving one exponentiation for the users. At last, we implement our proposed mechanism with$\mathsf{Charm}$Charmframework. The benchmark results demonstrate the high efficiency and practicality of our proposed mechanism.

Many enterprises and individuals prefer to outsource data to public cloud via various pricing approaches. One of the most widely-used approaches is the pay-as-you-go model, where the data owner hires public cloud to share data with data consumers, and only pays for the actually consumed services. To realize controllable and secure data sharing, ciphertext-policy attribute-based encryption (CP-ABE) is a suitable solution, which can provide fine-grained access control and encryption functionalities simultaneously. But there are some serious challenges when applying CP-ABE in pay-as-you-go. First, the decryption cost in ABE is too heavy for data consumers. Second, ABE ciphertexts probably suffer distributed denial of services (DDoS) attacks, but there is no solution that can eliminate the security risk. At last, the data owner should audit resource consumption to guarantee the transparency of charge, while the existing method is inefficient. In this work, we propose a general construction named fully accountable ABE (FA-ABE), which simultaneously solves all the challenges by supporting all-sided accountability in the pay-as-you-go model. We formally define the security model and prove the security in the standard model. Also, we implement an instantiate construction with the self-developed library$\mathsf{ libabe}$. The experiment results indicate the efficiency and practicality of our construction.

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.