Paper 2021/745

Generic-Group Identity-Based Encryption: A Tight Impossibility Result

Gili Schul-Ganz and Gil Segev

Abstract

Following the pioneering work of Boneh and Franklin (CRYPTO '01), the challenge of constructing an identity-based encryption scheme based on the Diffie-Hellman assumption remained unresolved for more than 15 years. Evidence supporting this lack of success was provided by Papakonstantinou, Rackoff and Vahlis (ePrint '12), who ruled out the existence of generic-group identity-based encryption schemes supporting an identity space of sufficiently large polynomial size. Nevertheless, the breakthrough result of D{ö}ttling and Garg (CRYPTO '17) settled this long-standing challenge via a non-generic construction. We prove a tight impossibility result for generic-group identity-based encryption, ruling out the existence of any non-trivial construction: We show that any scheme whose public parameters include $n_{\sf pp}$ group elements may support at most $n_{\sf pp}$ identities. This threshold is trivially met by any generic-group public-key encryption scheme whose public keys consist of a single group element (e.g., ElGamal encryption). In the context of algebraic constructions, generic realizations are often both conceptually simpler and more efficient than non-generic ones. Thus, identifying exact thresholds for the limitations of generic groups is not only of theoretical significance but may in fact have practical implications when considering concrete security parameters.