## Cryptology ePrint Archive: Report 2015/648

Adaptive Proofs of Knowledge in the Random Oracle Model

David Bernhard and Marc Fischlin and Bogdan Warinschi

Abstract: We formalise the notion of adaptive proofs of knowledge in the random oracle model, where the extractor has to recover witnesses for multiple, possibly adaptively chosen statements and proofs. We also discuss extensions to simulation soundness, as typically required for the encrypt-then-prove'' construction of strongly secure encryption from IND-CPA schemes. Utilizing our model we show three results:

(2) The encrypt-then-prove'' construction with a simulation-sound adaptive proof yields CCA security. This appears to be a folklore'' result but which has never been proven in the random oracle model. As a corollary, we obtain a new class of CCA-secure encryption schemes.

(3) We show that the Fiat-Shamir transformed Schnorr protocol is _not_ adaptively secure and discuss the implications of this limitation.

Our result not only separates adaptive proofs from proofs of knowledge, but also gives a strong hint why Signed ElGamal as the most prominent encrypt-then-prove example has not been proven CCA-secure without making further assumptions.

Category / Keywords: foundations / proofs of knowledge, sigma protocols, schnorr, fiat-shamir, metareduction

Original Publication (with major differences): IACR-PKC-2015