Currently, cloud services are widely used to manage data. Hence, protecting their security is extremely important. 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, allowing only authorized viewers to decrypt it. However, ABE requires a key generator—an organization that generates users’ public and private keys with sufficient authority to decrypt ciphertexts.

To solve this security problem, distributed ABE (DABE) methods without decryptable institutions have been proposed. DABE also faces a problem where increasing the number of ORs in the decryption condition enlarges the ciphertext size, posing a major challenge for practical use. In this study, we propose a DABE method with reduced ciphertext size and processing load without requiring an authority to decrypt ciphertexts. Based on DABE, we adopt a multi-value attribute algorithm with attributes defined as sets with restrictions on decryption conditions.

Security evaluations show that the proposed method achieves equivalent security strength to existing DABE schemes. Furthermore, the ciphertext size increase was greatly reduced—a major improvement over existing methods. The proposed method also achieved better decryption performance under certain conditions. This research contributes to advancing ABE by realizing a highly practical implementation combining efficiency and robust security.