Generic Methods to Achieve Tighter Security Reductions for a Category of IBE Schemes

We show that Katz-Wang’s duplicating key and ciphertext technique can be extended to a generic method that can be used in a certain category of Identity-Based Encryption (IBE) schemes for the purposes of improving their security reductions. We further develop two refined approaches by adapting the randomness reuse technique in the Katz-Wang technique: one is public key duplication, and the other is master key duplication. Compared to the Katz-Wang technique, our two refined approaches do not only improve the performances of the resulting IBE schemes but also enable a reduction algorithm to deal with decryption queries correctly and therefore can achieve chosen ciphertext security. As case studies, we apply these two approaches to modify the BonehFranklin IBE scheme and the Boneh-Boyen IBE scheme, respectively. Both of the modifications improve the tightness of security reductions, compared to the original schemes, with a reasonably low cost.