Biblio

In the 2010s, there has been significant interest in developing methods, such as searchable encryption for exact matching and order-preserving/-revealing encryption for range search, to perform search on encrypted data. However, the symmetric searchable encryption method has been steadily used not only in databases but also in full-text search engine because of its quick performance and high security against intruders and system administrators. Contrarily, order-preserving/-revealing encryption is rarely employed in practice: almost all related schemes suffer from inference attacks, and some schemes are secure but impractical because they require exponential storage size or communication complexity. In this study, we define the new security models based on order-revealing encryption (ORE) for performing range search, and explain that previous techniques are not satisfied with our weak security model. We present two generic constructions of ORE using the searchable encryption method. Our constructions offer practical performance such as the storage size of O(nb) and computation complexity of O(n2), where the plaintext space is a set of n-bit binaries and b denotes the block size of the ciphertext generated via searchable encryption. The first construction gives the comparison result to the server, and the security considers a weak security model. The second construction hides the comparison result from the server, and only the secret-key owner can recover it.

Blockchain technology is attracting attention as an innovative system for decentralized payments in fields such as financial area. On the other hand, in a decentralized environment, management of a secret key used for user authentication and digital signature becomes a big issue because if a user loses his/her secret key, he/she will also lose assets on the blockchain. This paper describes the secret key management issues in blockchain systems and proposes a solution using a biometrics-based digital signature scheme. In our proposed system, a secret key to be used for digital signature is generated from the user's biometric information each time and immediately deleted from the memory after using it. Therefore, our blockchain system has the advantage that there is no need for storage for storing secret keys throughout the system. As a result, the user does not have a risk of losing the key management devices and can prevent attacks from malware that steals the secret key.