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Duan, Huayi, Zheng, Yifeng, Du, Yuefeng, Zhou, Anxin, Wang, Cong, Au, Man Ho.  2019.  Aggregating Crowd Wisdom via Blockchain: A Private, Correct, and Robust Realization. 2019 IEEE International Conference on Pervasive Computing and Communications (PerCom. :1—10.

Crowdsensing, driven by the proliferation of sensor-rich mobile devices, has emerged as a promising data sensing and aggregation paradigm. Despite useful, traditional crowdsensing systems typically rely on a centralized third-party platform for data collection and processing, which leads to concerns like single point of failure and lack of operation transparency. Such centralization hinders the wide adoption of crowdsensing by wary participants. We therefore explore an alternative design space of building crowdsensing systems atop the emerging decentralized blockchain technology. While enjoying the benefits brought by the public blockchain, we endeavor to achieve a consolidated set of desirable security properties with a proper choreography of latest techniques and our customized designs. We allow data providers to safely contribute data to the transparent blockchain with the confidentiality guarantee on individual data and differential privacy on the aggregation result. Meanwhile, we ensure the service correctness of data aggregation and sanitization by delicately employing hardware-assisted transparent enclave. Furthermore, we maintain the robustness of our system against faulty data providers that submit invalid data, with a customized zero-knowledge range proof scheme. The experiment results demonstrate the high efficiency of our designs on both mobile client and SGX-enabled server, as well as reasonable on-chain monetary cost of running our task contract on Ethereum.

Lu, Xingye, Au, Man Ho.  2016.  Anonymous Identification for Ad Hoc Group. Proceedings of the 11th ACM on Asia Conference on Computer and Communications Security. :583–591.
An anonymous identification scheme for ad hoc group allows a participant to identify himself as a member of a group of users in a way that his actual identity is not revealed. We propose a highly efficient construction of this cryptographic primitive in the symmetric key setting based on the idea of program obfuscation. The salient feature of our scheme is that only hash evaluations are needed. Consequently, our scheme outperforms all existing constructions for a reasonably large ad hoc group size (of around 50000 users) since no exponentiation nor pairing operation is involved. Technically, the participant only needs to evaluate one hash operation to identify himself. While the time complexity of the verifier is linearly in the size of the ad hoc group, the actual running time is rather insignificant since the constant factor of this linear dependence is the time of a single hash evaluation. To analyse the security of our proposal, we develop a security model to capture the security requirements of this primitive and prove that our construction satisfies these requirements in the random oracle model against unbounded attackers. Similar to other identification schemes secure in the random oracle model, our proposed protocol requires only two message flow.
Xu, Cheng, Xu, Jianliang, Hu, Haibo, Au, Man Ho.  2018.  When Query Authentication Meets Fine-Grained Access Control: A Zero-Knowledge Approach. Proceedings of the 2018 International Conference on Management of Data. :147-162.

Query authentication has been extensively studied to ensure the integrity of query results for outsourced databases, which are often not fully trusted. However, access control, another important security concern, is largely ignored by existing works. Notably, recent breakthroughs in cryptography have enabled fine-grained access control over outsourced data. In this paper, we take the first step toward studying the problem of authenticating relational queries with fine-grained access control. The key challenge is how to protect information confidentiality during query authentication, which is essential to many critical applications. To address this challenge, we propose a novel access-policy-preserving (APP) signature as the primitive authenticated data structure. A useful property of the APP signature is that it can be used to derive customized signatures for unauthorized users to prove the inaccessibility while achieving the zero-knowledge confidentiality. We also propose a grid-index-based tree structure that can aggregate APP signatures for efficient range and join query authentication. In addition to this, a number of optimization techniques are proposed to further improve the authentication performance. Security analysis and performance evaluation show that the proposed solutions and techniques are robust and efficient under various system settings.

Yu, Zuoxia, Au, Man Ho, Yang, Rupeng, Lai, Junzuo, Xu, Qiuliang.  2018.  Achieving Flexibility for ABE with Outsourcing via Proxy Re-Encryption. Proceedings of the 2018 on Asia Conference on Computer and Communications Security. :659-672.

Outsourcing the decryption of attribute-based encryption (ABE) ciphertext is a promising way to tackle the question of how users can perform decryption efficiently. However, existing solutions require the type of the target ciphertext to be determined at the setup of the outsourcing scheme. As such, making the target cryptosystems (or the clients) to be versatile becomes an issue that warrants investigations. In this paper, the problem we wish to tackle is to transform an ABE ciphertext to any client who is using the same, or possibly different, public-key encryption (PKE) system with the sender. The problem is of practical interest since it is hard to require all clients to use the same PKE, especially in the case of remote and cross-system data sharing. In addition, we also consider whether robust client-side decryption scheme can be adopted. This feature is not supported in the existing ABE with outsourcing. We introduce cross-system proxy re-encryptions (CS-PRE), a new re-encryption paradigm in which a semi-trusted proxy converts a ciphertext of a source cryptosystem (\$\textparagraphi\_0\$) into a ciphertext for a target cryptosystem (\$\textparagraphi\$). We formalize CS-PRE and present a construction that performs well in the following aspects. (1)Versatility: \$\textparagraphi\_0\$ can be any attribute-based encryption (ABE) within Attrapadung's pair encoding framework. \$\textparagraphi\$ can be any public-key encryption. Furthermore, the keys and public parameters can be generated independently. (2) Compatibility: CS-PRE does not modify the public parameters and keys of \$\textparagraphi\_0\$ and \$\textparagraphi\$. Besides, input for the conversion is an ordinary ciphertext of \$\textparagraphi\_0\$. (3) Efficiency: The computational cost for re-encryption and decryption of the re-encrypted ciphertext are roughly the same as a decryption in \$\textparagraphi\_0\$ and \$\textparagraphi\$ respectively. We prove that our construction is fully secure assuming \$\textparagraphi\_0\$ is secure in Attrapadung's framework and \$\textparagraphi\$ is IND-CPA secure. Furthermore, it remains secure when there are multiple target cryptosystems. As with other proxy re-encryption, CS-PRE enables flexible sharing of cloud data, as the owner can instruct the cloud server to re-encrypt his ciphertext to those for the intended recipient. In addition, it allows lightweight devices to enjoy access to remote data encrypted under powerful but possibly costly encryption, such as functional encryption, by utilizing the server's power in converting the ciphertext to a simpler encryption, such as RSA. Finally, instances of CS-PRE can be viewed as new proxy re-encryption schemes, such as a PRE supporting ABE for regular language to Hierarchical IBE or Doubly Spatial Encryption to lattice-based encryptions (e.g. NTRUCCA).