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.

Bug bounties are a helpful system for identifying vulnerabilities in smart contracts. Many web3 applications use bug bounties today. However, hackers might worry about not getting paid or receiving a reduced bounty if the bounty is not exchanged atomically with a bug report. This concern could lead hackers to exploit the identified bug instead of reporting it. Furthermore, other efforts related to bug bounties require modification of existing smart contracts. To solve this, we propose a practical bug bounty platform for smart contracts, ContractSafeguard, that utilizes Intel SGX to ensure the existence of bugs and atomic exchange of bounty and bug reports. Bug hunters send transactions to smart contracts hosted within the Enclave and glitch them. Test code within the Enclave generates a bug report that includes proof of the bug’s existence and a method that triggers the bug. In other words, within the Enclave, our mechanism detects and proves that the smart contract under test has entered an abnormal state. Our on-chain bug bounty contract then verifies the proof within the bug report and pays the bounty to the bug hunter atomically. Since the proof is produced within the Enclave, generating false proofs is impossible, and the on-chain contract can trust the report. Additionally, the developer can use existing smart contracts without modification since we have a runtime of smart contracts in the Enclave. We design ContractSafeguard for Ethereum smart contracts.

As transactions of monetary value on the public blockchain become more widespread, there is a growing demand that the transactions be verified as legitimate. As a result, it has become mandatory for crypto asset exchanges to perform identity verification. On the other hand, Bitcoin and other public blockchains are easy to start transactions with, and the anonymity of the transactions is a significant value for users. To achieve these conflicting requirements at a certain level, we developed a method to manage information generated from a user’s biometric information with smart contract and link transactions on the blockchain to real people. This mechanism makes it relatively easy for the user to claim their identity while the operator can control the degree of identity verification.

A Mnemonic phrase is a set of secret words that correspond one-to-one with the secret key of a user's crypto wallet. If this phrase is lost, the user cannot access the cryptocurrencies. Thus, the user wants to back up the phrase with multiple storage, but the risk of leakage increases. To solve this dilemma, this paper proposes a leakable mnemonic phrase system in which the phrase is encrypted with a secret key generated from the user's biometric information. Since the leakable mnemonic phrase can be decrypted only with the user's biometric information, it can be backed up to various external storage and devices to solve the contradictory problem of keeping the contents of the phrase secret while preventing its loss. In this paper, we also show the results of evaluating the performance of this system by implementing it on an actual biometric authentication device. The encryption/decryption processing time is fast (200 ms), and the system successfully decrypted the mnemonic phrase six months after enrollment.

Non-Fungible Token(NFT) issued on the blockchain made adding scarcity to digital data possible. However, as NFT transactions soared, illegal use of other people's content increased. To solve this situation, this paper proposes the Proof of Origin concept, which adds authenticity to NFTs by utilizing a technology that generates the cryptographic keys required for digital signatures directly from biometric information and describes the system architecture, data structure, and implementation of this concept. Using biometric information to link NFTs to actual persons makes it possible to provide trustworthiness and high-added value to data. At the same time, the creator does not need to manage secret keys and can safely and efficiently claim the originality of the data.

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.