Management of secret keys for digital signatures is one of the most critical issues in decentralized applications. Since there is no administrator, losing a secret key can result in losing all assets or rights. To address this problem, fuzzy extractors and fuzzy signatures, which generate private keys directly from biometric information, have been considered in addition to conventional biometric authentication. However, these methods using biometric secret keys do not support group signatures. Therefore, it is not applicable to use cases that require consensus building by a specific community (group), such as DAO and DeFi.In this paper, we propose a new scheme for converting existing fuzzy signatures to group signatures using zero-knowledge proofs to address this problem. More precisely, we first define an anonymizable signature that is a generalization of a group signature and then convert a fuzzy signature into an anonymizable signature using an ordinary (classical) zero-knowledge proof. In addition, the signature data size is optimized to a constant size using zk-SNARK. Our implementation experiments show that our schemes achieve practical signature generation and verification times and signature sizes even for a group of up to 100,000 people. This paper’s results can be used to prevent the loss of secret keys and enable flexible DApps use cases.

With the spread of cloud computing, information leakage from the cloud is recognized as one of the significant issues. Searchable symmetric encryption has been expected to be a core technology that fundamentally solves this problem. On the other hand, the technology has the security advantage of protecting data in the cloud, instead of the users being responsible for strictly controlling their secret keys. Computer environments and IT literacy vary from user to user, therefore it is important to note that users may be exposed to the risk of attackers gaining unauthorized access to their secret keys. This background underlines the importance of user's key management. In this paper, we propose a method that secures and simplifies this process by encrypting and storing the secret key of SSE using a fuzzy extractor. This method not only enhances the security of SSE but also offers significant benefits for end users, such as the ability to operate in multiple computer environments, thereby providing a more flexible and user-friendly experience. We implemented the proposed method on a laptop computer running Windows 10 and a smartphone running Android OS, which are the users' typical execution environments. As a result, we confirmed that the proposal performs practical application.

In recent years, the zero-knowledge succinct non-interactive argument of knowledge (zk-SNARK) have drawn significant attention as privacy-enhancing technologies in various domains. A post-quantum designated verifier type zk-SNARK for Boolean circuits was proposed by Gennaro et al. in ACM CCS '18. However, this scheme does not include arithmetic circuits and they claim that a construction of post zk-SNARKs for arithmetic circuits as open problem. In this paper we give answers to this problem. Our first proposal is based on the data structure used in Pinocchio, a previous study, and can be easily implemented using the existing Pinocchio-based systems. In our second proposal, which also employs QAPs, the zero-knowledge proof comprises three learning with errors (LWE) ciphertexts, and the size of the proof is smaller compared with that of the first proposal. Our second proposal is also more efficient than the first one or all other known post-quantum zk-SNARKs.

In this paper, we propose an encrypted k–anonymization scheme (EAS) to k–anonymize an encrypted database using a domain generalized hierarchy while maintaining the encryption state. Preparation of the domain generalized hierarchy is optional; the proposed EAS can generate domain generalized hierarchies using a Huffman code tree from a database encrypted with searchable encryption. As a result, the user can delegate k–anonymization processing to a third party organization such as the cloud while retaining the confidentiality of the database without preparing a generalized hierarchy. In addition, third-party organizations that are entrusted also have the advantage to eliminate possible of misconduct such as information leakage. In a standard computer experiment, we performed a generalization process, which is the major procedure for our EAS. The generalization process takes around 168 seconds only to achieve k–anonymity with k = 3 on 1,000,000 records consisting of 4 attributes. As a consequence, this high-speed performance means our EAS is applicable to not only batch processing but also real-time processing.

In recent years, Bitcoin, Ethereum and other cryptocurrencies have attracted a great deal of attention from the whole industry including the financial as a new settlement system. Transaction information of these cryptocurrencies is stored in a distribution ledger called Blockchain on the P2P network through processing such as PoW. Meanwhile, since PoW requires a large amount of computer resources, researches on private / consortium type blockchain that do not need PoW. In this paper, we propose a decentralized netting protocol using a consortium type block chain that has the channel function. On a system that implements the proposed protocol, netting settlement can be performed on P2P hiding information of the sender and receiver name of transaction, amount of money, calculation butt of netting, and without setting up a specific central organization such as a central server.

Cryptography is now popularized and is widely used anywhere for many aims such as data confidentiality and integrity. The key of cryptography has a lifetime, thus a key update issue is well-known to be one of hard problems in practice. According to the National Institute of Standards and Technology SP 800-57, the lifetime of the cryptographic key and the lifetime of encrypted data are strictly limited. In other words, the encrypted data is required to be periodically re-encrypted. In this paper, we point out that this key update issue is now crucial in network storage and propose a key update mechanism efficiently reducing the communication and computation cost of re-encryption.