Paper 2003/149

Forward-Secure Hierarchical ID-Based Cryptography

Danfeng Yao and Anna Lysyanskaya

Abstract

We present a forward-secure hierarchical identity-based encryption (FHIBE) scheme, which is based on the hierarchical identity-based encryption (HIBE) scheme by Gentry and Silverberg. Canetti, Halevi and Katz presented a forward-secure public key encryption scheme based on HIBE scheme. They give the formal definition of Binary Encryption Tree (BET), which is a relaxed version of HIBE and is essential to their forward-secure encryption.We unify their idea with HIBE scheme, and present a forward-secure hierarchical identity-based encryption scheme. In the FHIBE scheme, secret keys of each entity on the hierarchy are updated at regular intervals throughout the lifetime of the system; furthermore, exposure of an entity's secret key corresponding to a given interval does not enable an adversary to break the ancestors of the entity for any prior time period. Entities can join in the hierarchy at any time and at any position, and are able to update their secret keys on their own once they are initialized by their parent entities. These features are important in the distributed settings. The forward-secure hierarchical identity-based encryption scheme can be generalized into a collusion resistant multiple hierarchical identity-based encryption (MHIBE) scheme, where a message can be encrypted under multiple identities of a user.