Independent Delegation: Resolving Key Escrow Problem in HIBE and HIBS

Abstract

A HIBE scheme with independent delegation are free from Key Escrow Problem that is inherent in HIBE, given the root PKG is unconditionally trusted. We propose a new technique - Identifier Discrimination for composing private keys for entities in hierarchy. With the technique, we construct a HIBE scheme under Decisional Bilinear Diffie-Helleman (DBDH) assumption in standard model with independent delegation, in which the privilege of generating private keys for each individual entity is delegated by the root PKG to any of its ancestors through authorization, that we call Authorization Delegation. Moreover, basing on Naor transformation of an identity-based signature (IBS) out of an IBE, we build a new hierarchical IBS (HIBS) scheme from our HIBE scheme. Being unable to generate a private key for any of its descendants, an entity cannot sign messages on behalf of any of its descendants, which guarantees that authenticity and non-repudiation properties are achieved in HIBS setting.