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With the surge in cloud storage popularity, more individuals are choosing to store large amounts of data on remote cloud service providers (CSPs) to save local storage resources. However, users' primary worries revolve around maintaining data integrity and authenticity. Consequently, several cloud auditing methods have emerged to address these concerns. Many of these approaches rely on traditional public-key cryptography systems or are grounded in identity-based cryptography systems or certificateless cryptography systems. However, they are vulnerable to the increased costs linked with certificate management, key escrow, or the significant expenses of establishing a secure channel, respectively. To counter these limitations, Li et al. introduced a certificate-based cloud auditing protocol (LZ22), notable for its minimal tag generation overhead. Nonetheless, this protocol exhibits certain security vulnerabilities. In this paper, we devise a counterfeiting technique that allows the CSP to produce a counterfeit data block with an identical tag to the original one. Our counterfeiting method boasts a 100% success rate ∀ data block and operates with exceptional efficiency. The counterfeiting process for a single block of 10 kB, 50 kB, and 100 kB takes a maximum of 0.08 s, 0.51 s, and 1.04 s, respectively. By substituting the exponential component of homomorphic verifiable tags (HVTs) with non-public random elements, we formulate a secure certificate-based cloud auditing protocol. In comparison to the LZ22 protocol, the average tag generation overhead of our proposed protocol is reduced by 6.80%, 13.78%, and 8.66% for data sizes of 10 kB, 50 kB, and 100 kB, respectively. However, the auditing overhead of our proposed protocol shows an increase. The average overhead rises by 3.05%, 0.17%, and 0.45% over the LZ22 protocol's overhead for data sizes of 10 kB, 50 kB, and 100 kB, correspondingly. [ABSTRACT FROM AUTHOR]

The security of multimedia content and personal privacy for big data has triggered widespread concern in the society. Fully homomorphic encryption (FHE), which can homomorphically compute arbitrary functions on the encrypted data without knowing the secret key, is valuable in protecting user's data security. However, most of the FHE schemes only take single-bit of ciphertext as the input, which makes the evaluation process complicated. In EUROCRYPT'2015, Ducas and Micciancio proposed an FHE scheme FHEW with the plaintext space , and gave an assumption of extending the plaintext space to . In this paper, we optimize the decryption algorithm of FHEW in bootstrapping, and propose an FHE scheme with large plaintext space . Firstly, we optimize the rounding function of the decryption algorithm in FHEW to the msdExtract algorithm, which can homomorphically extract the most significant digit of the plaintext. Secondly, we design the msdExtract algorithm by employing the homomorphic accumulator, and present the process of general bootstrapping. Finally, based on the msdExtract algorithm, we extend the plaintext space of our scheme to , comparing to in FHEW. The security of our scheme is based on the basic LWE scheme and FHEW. What's more, our scheme can perform the evaluation more conveniently with large plaintext space, and can be applied to more scenarios. [ABSTRACT FROM AUTHOR]