Enhanced Chosen-Ciphertext Security and Applications

We introduce and study a new notion of

enhanced chosen-ciphertext security

(ECCA) for public-key encryption. Loosely speaking, in the ECCA security experiment, the decryption oracle provided to the adversary is augmented to return not only the output of the decryption algorithm on a queried ciphertext but also of a

randomness-recovery

algorithm associated to the scheme. Our results mainly concern the case where the randomness-recovery algorithm is efficient.

We provide constructions of ECCA-secure encryption from adaptive trapdoor functions as defined by Kiltz

et al.

(EUROCRYPT 2010), resulting in ECCA encryption from standard number-theoretic assumptions. We then give two applications of ECCA-secure encryption: (1) We use it as a unifying concept in showing equivalence of adaptive trapdoor functions and tag-based adaptive trapdoor functions, resolving an open question of Kiltz

et al

. (2) We show that ECCA-secure encryption can be used to securely realize an approach to public-key encryption with non-interactive opening (PKENO) originally suggested by Damgård and Thorbek (EUROCRYPT 2007), resulting in new and practical PKENO schemes quite different from those in prior work.

Our results demonstrate that ECCA security is of both practical and theoretical interest.