Subtleties in the Definition of IND-CCA: When and How Should Challenge-Decryption be Disallowed?
Mihir Bellare, Dennis Hofheinz, Eike Kiltz · 2009
The definition of IND-CCA disallows an adversary from querying the challenge ciphertext to its decryption oracle. We point out that there are several ways to formalize this. We show that, surprisingly, for public-key encryption the resulting notions are not all equivalent. We then consider the same question for key-encapsulation mechanisms (KEMs) and show that in this case the four notions are all equivalent. Our discoveries are another manifestation of the subtleties that make the study of cryptography so attractive and are important towards achieving the definitional clarity and unity required for firm foundations.