Distributed attribute-based encryption with small ciphertext

Abstract

Currently, cloud services are widely used to manage data. Hence protecting their security is extremely im-portant. This requires meticulous control of access rights for viewers, especially for confidential corporate data in the cloud. Attribute-based encryption (ABE) is a next-generation cryptographic technique that can efficiently control access rights by associating decryption conditions based on the attributes of a given viewer with a ciphertext to allow only viewers who satisfy the decryption conditions. However, ABE requires a key generator, which is an organization that generates users public and private keys with sufficient authority to decrypt the ciphertext. To solve this security problem, distributed ABE (DABE) methods without decrypt-able institutions have been presented.DABE also involves a problem that the more ORs in the decryption condition, the larger the ciphertext size becomes, which is a major challenge for practical use. In this study, we propose a DABE method with reduced ciphertext size and processing load without the need for an au-thority that can decrypt ciphertext. To this end, based on DABE, we adopt a multi-value attribute algorithm with attributes as a set with restrictions on decryption conditions. The results of a security evaluation are presented to show that the security strength of the proposed method is equivalent to that of the existing DABE  scheme. Furthermore, the increase in ciphertext size was greatly reduced, which was a major issue for the existing DABE.We also confirmed that the proposed method performed better in terms of decryption time under certain conditions. This research contributes to the development of ABE by realizing a highly practical implementation with both efficient processing and strong capabilities.