Paper 2021/539

More Efficient Adaptively Secure Revocable Hierarchical Identity-based Encryption with Compact Ciphertexts: Achieving Shorter Keys and Tighter Reductions

Atsushi Takayasu

Abstract

Revocable hierarchical identity-based encryption (RHIBE) is a variant of the standard hierarchical identity-based encryption (HIBE) satisfying the key revocation functionality. Recently, the first adaptively secure RHIBE scheme with compact ciphertexts was proposed by Emura et al. by sacrificing the efficiency of the schemes for achieving adaptive security so that the secret keys are much larger than Seo and Emura's selectively secure scheme with compact ciphertexts. In this paper, we propose a more efficient adaptively secure RHIBE scheme with compact ciphertexts. Our scheme has much shorter secret keys and key updates than Emura et al.'s scheme. Moreover, our scheme has much shorter key updates than Seo and Emura's selectively secure scheme. Emura et al. proved the adaptive security of their scheme by reducing the security of the underlying HIBE schemes to that of their proposed RHIBE scheme, where the adaptive security of the HIBE scheme is inherently proven through the dual system encryption methodology. In contrast, we prove the adaptive security of the proposed RHIBE scheme directly through the dual system encryption methodology. Furthermore, our security proof achieves a tighter reduction than that of Emura et al.