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Efficient IdentityBased Encryption with Hierarchical KeyInsulation from HIBE
Authors K. Emura, A. Takayasu, and Y. WatanabeAbstract
Hierarchical keyinsulated identitybased encryption (HKIBE) is identitybased encryption (IBE) that allows users to update their secret keys to achieve (hierarchical) keyexposure resilience, which is an important notion in practice. However, existing HKIBE constructions have limitations in efficiency: sizes of ciphertexts and secret keys depend on the hierarchical depth. In this paper, we first triumph over the barrier by proposing simple but effective design methodologies to construct efficient HKIBE schemes. First, we show a generic construction from any hierarchical IBE (HIBE) scheme that satisfies a special requirement, called MSK evaluatability introduced by Emura et al. (ePrint, 2020). It provides several new and efficient instantiations since most pairingbased HIBE schemes satisfy the requirement. It is worth noting that it preserves all parameters' sizes of the underlying HIBE scheme, and hence we obtain several efficient HKIBE schemes under the klinear assumption in the standard model. Since MSK evaluatability is dedicated to pairingbased HIBE schemes, the first construction restricts pairingbased instantiations. To realize efficient instantiation from various assumptions, we next propose a generic construction of an HKIBE scheme from any plain HIBE scheme. It is based on Hanaoka et al.'s HKIBE scheme (Asiacrypt 2005), and does not need any special properties. Therefore, we obtain new efficient instantiations from various assumptions other than pairingoriented ones. Though the sizes of secret keys and ciphertexts are larger than those of the first construction, it is more efficient than Hanaoka et al.'s scheme in the sense of the sizes of master public/secret keys.