Addressing new challenges in public-key cryptography

Abstract

Public-key cryptography, introduced by Diffie and Hellman in 1976, has found numerous applications in reality. After years of development, public-key cryptography has been well-studied and is gradually becoming mature. However, the emerging of several exciting technologies in computer science, while bringing convenience to our daily life, also imposes new challenges to current public-key cryptographic systems deployed in practical applications. In this thesis, we focus on addressing some new challenges in two well-known representatives of public-key cryptography, namely, public-key encryption and digital signature. In particular, we aim at designing an encryption scheme suitable for flexible and efficient data-sharing in the cloud as well as enhancing its resilience against side-channel attacks. In addition, we investigate the real-world applicability of signature scheme in blockchain-based cryptocurrencies and explore how to strengthen the signature component of blockchain-based cryptocurrencies to be quantum safe. More precisely, we present the following results: - We present a new variant of public-key encryption named as cross-system proxy re-encryption, which could make data-sharing in the cloud flexible and efficient. It allows one to transform ciphertext of a large class of attribute-based encryption schemes (a variant of public-key encryption that supports fine-grained control over the decryption ability) into ciphertext of any public-key encryption scheme. - We present a general framework for constructing attribute-based encryption schemes secure against side-channel attacks. - We present the best possible statistical attack for tracing payers' identity in transactions of privacy-preserving blockchain-based cryptocurrencies based on ring signature. We also fnd that if some natural conditions are satisfied, our attack will not compromise security of the cryptocurrency. In this way, we identify a safe mode to use public-key cryptographic schemes in a blockchain-based cryptocurrency. - We present the first lattice-based universal accumulator secure against quantum attacks. This primitive can be used to construct quantum safe dynamic group signature, a widely-used variant of digital signature that can be used to protect signers' identity.