Paper 2007/023

Efficient Hybrid Encryption from ID-Based Encryption

Masayuki Abe, Yang Cui, Hideki Imai, and Eike Kiltz

Abstract

This paper deals with generic transformations from ID-based key encapsulation mechanisms (IBKEM) to hybrid public-key encryption (PKE). The best generic transformation known until now is by Boneh and Katz and requires roughly 704-bit overhead in the ciphertext. We present two new such generic transformations that are applicable to partitioned IBKEMs. A partitioned IBKEM is an IBKEM that provides some extra structure. Such IBKEMs are quite natural and in fact nearly all known IBKEMs have this additional property. Our first transformation yields chosen-ciphertext secure PKE schemes from selective-ID secure partitioned IBKEMs with a 256-bit overhead in ciphertext size plus one extra exponentiation in encryption/decryption. As the central tool a Chameleon Hash function is used to map the identities. The second transformation transforms adaptive-ID secure partitioned IBKEMs into chosen-ciphertext secure PKE schemes with no additional overhead. Applying our transformations to existing IBKEMs we propose a number of novel PKE schemes with different trade-offs. In some concrete instantiations the Chameleon Hash can be made implicit which results in improved efficiency by eliminating the additional exponentiation. Since our transformations preserve the public verifiability property of the IBE schemes it is possible to extend our results to build threshold hybrid PKE schemes. We show an analogue generic transformation in the threshold setting and present a concrete scheme which results in the most efficient threshold PKE scheme in the standard model.