On Lattice-Based Signatures with Advanced Functionalities

Lattice-based cryptography is a prominent class of cryptographic systems that has been emerged as one of the main candidates replacing classical cryptography in future computing environments such as quantum computing. Quantum computers exploit quantum mechanical phenomena to solve computational problems, on which the security of currently deployed (classical) cryptographic systems is based. While these computational problems, e.g., factoring integers and computing discrete logarithms, are intractable for conventional (classical) computers, it is meanwhile known that they can be easily solved on quantum computers (Shor 1997). However, lattice problems, such as finding short non-zero vectors, seem to withstand attacks having quantum computing power. In the last two decades we have seen many cryptographic proposals based on lattices. In particular, lattice-based (ordinary) signature schemes were greatly improved with respect to efficiency and security. This can be observed from the post-quantum standardization process initiated by the National Institute of Standards and Technology (NIST). In fact, from the five signature schemes that have been submitted to this process, there are currently three finalists, where two of them are lattice-based submissions. In this thesis, we are specifically interested in lattice-based signature schemes with advanced functionalities. In addition to the basic security goals that an ordinary signature scheme ensures, i.e., authentication, non-repudiation, and integrity, these schemes provide features that are application-specific. While ordinary signature schemes based on lattices are ready to be deployed in practice, this statement cannot be made for lattice-based signature schemes with advanced functionalities. This thesis makes a significant progress towards deploying the aforementioned type of signature schemes in practice. With focus on privacy-preserving applications in future computing environments, we particularly facilitate the protection of secret keys in cryptocurrencies such as Bitcoin and Ethereum. We provide practical solutions to anonymous e-cash, anonymous credentials, smart contracts, and e-voting. We believe that our techniques can be used to develop further advanced signature schemes to be deployed in other application scenarios. For instance, in information security systems that perform critical operations such as distributed key generation, anonymization of medical data, and updating reliable routing information

ACTIVE-HASH-TABLE BASED PUBLIC AUDITING FOR SECURE CLOUD STORAGE

Public auditing scheme for secure cloud storage based on dynamic hash table, which is a new two-dimensional data structure located at a third-party auditor (TPA) to record the data property information for dynamic auditing. Differing form the existing works, the proposed scheme migrates the authorized information from the cloud services provider to the TPA and thereby significantly reduces the computational cost and communication overhead. Our scheme can also achieve higher updating efficiency than the state of the art schemes. In addition, we extend our scheme to support privacy preservation by combining the homomorphic authenticator based on the public key with the random masking generated by the TPA and achieve batch auditing by employing the aggregate BLS signature technique. We formally prove the security of the proposed scheme and evaluate the auditing performance by detailed experiments and comparisons with the existing ones. The results demonstrate that the proposed scheme can effectively achieve secure auditing for cloud storage and outperform the previous schemes’ in computation complexity, storage costs, and communication overhead

Lattice-Based Group Signatures: Achieving Full Dynamicity (and Deniability) with Ease

In this work, we provide the first lattice-based group signature that offers full dynamicity (i.e., users have the flexibility in joining and leaving the group), and thus, resolve a prominent open problem posed by previous works. Moreover, we achieve this non-trivial feat in a relatively simple manner. Starting with Libert et al.'s fully static construction (Eurocrypt 2016) - which is arguably the most efficient lattice-based group signature to date, we introduce simple-but-insightful tweaks that allow to upgrade it directly into the fully dynamic setting. More startlingly, our scheme even produces slightly shorter signatures than the former, thanks to an adaptation of a technique proposed by Ling et al. (PKC 2013), allowing to prove inequalities in zero-knowledge. Our design approach consists of upgrading Libert et al.'s static construction (EUROCRYPT 2016) - which is arguably the most efficient lattice-based group signature to date - into the fully dynamic setting. Somewhat surprisingly, our scheme produces slightly shorter signatures than the former, thanks to a new technique for proving inequality in zero-knowledge without relying on any inequality check. The scheme satisfies the strong security requirements of Bootle et al.'s model (ACNS 2016), under the Short Integer Solution (SIS) and the Learning With Errors (LWE) assumptions. Furthermore, we demonstrate how to equip the obtained group signature scheme with the deniability functionality in a simple way. This attractive functionality, put forward by Ishida et al. (CANS 2016), enables the tracing authority to provide an evidence that a given user is not the owner of a signature in question. In the process, we design a zero-knowledge protocol for proving that a given LWE ciphertext does not decrypt to a particular message

On Using Encryption Techniques to Enhance Sticky Policies Enforcement

How to enforce privacy policies to protect sensitive personal data has become an urgent research topic for security researchers, as very little has been done in this field apart from some ad hoc research efforts. The sticky policy paradigm, proposed by Karjoth, Schunter, and Waidner, provides very useful inspiration on how we can protect sensitive personal data, but the enforcement is very weak. In this paper we provide an overview of the state of the art in enforcing sticky policies, especially the concept of sticky policy enforcement using encryption techniques including Public-Key Encryption (PKE), Identity-Based Encryption (IBE), Attribute-Based Encryption (ABE), and Proxy Re-Encryption (PRE). We provide detailed comparison results on the (dis)advantages of these enforcement mechanisms. As a result of the analysis, we provide a general framework for enhancing sticky policy enforcement using Type-based PRE (TPRE), which is an extension of general PRE

Dynamic Provable Data Possession Protocols with Public Verifiability and Data Privacy

Cloud storage services have become accessible and used by everyone. Nevertheless, stored data are dependable on the behavior of the cloud servers, and losses and damages often occur. One solution is to regularly audit the cloud servers in order to check the integrity of the stored data. The Dynamic Provable Data Possession scheme with Public Verifiability and Data Privacy presented in ACISP'15 is a straightforward design of such solution. However, this scheme is threatened by several attacks. In this paper, we carefully recall the definition of this scheme as well as explain how its security is dramatically menaced. Moreover, we proposed two new constructions for Dynamic Provable Data Possession scheme with Public Verifiability and Data Privacy based on the scheme presented in ACISP'15, one using Index Hash Tables and one based on Merkle Hash Trees. We show that the two schemes are secure and privacy-preserving in the random oracle model.Comment: ISPEC 201