Identity-Based Encryption Tightly Secure Under Chosen-Ciphertext Attacks
We propose the first identity-based encryption (IBE) scheme that is (almost) tightly secure against chosen-ciphertext attacks. Our scheme is efficient, in the sense that its ciphertext overhead is only seven group elements, three group elements more than that of the state-of-the-art passively (almost) tightly secure IBE scheme. Our scheme is secure in a multi-challenge setting, i.e., in face of an arbitrary number of challenge ciphertexts. The security of our scheme is based upon the standard symmetric external Diffie-Hellman assumption in pairing-friendly groups, but we also consider (less efficient) generalizations under weaker assumptions.
Chosen Ciphertext Secure Encryption over Semi-smooth Subgroup
In this paper we propose two public key encryption schemes over the semi-smooth subgroup introduced by Groth05. Both the schemes are proved secure against chosen ciphertext attacks under the factoring assumption. Since the domain of exponents is much smaller, both our schemes are significantly more efficient than Hofheiz-Kiltz 2009 encryption.