Rerandomizable RCCA Encryption

We give the first perfectly rerandomizable, Replayable-CCA (RCCA) secure encryption scheme, positively answering an open problem of Canetti et al. (CRYPTO 2003). Our encryption scheme, which we call the Double-strand Cramer-Shoup scheme, is a non-trivial extension of the popular Cramer-Shoup encryption. Its security is based on the standard DDH assumption. To justify our definitions, we define a powerful “Replayable Message Posting” functionality in the Universally Composable (UC) framework, and show that any encryption scheme that satisfies our definitions of rerandomizability and RCCA security is a UC-secure implementation of this functionality. Finally, we enhance the notion of rerandomizable RCCA security by adding a receiver-anonymity (or keyprivacy) requirement, and show that it results in a correspondingly enhanced UC functionality. We leave open the problem of constructing a scheme achieving this enhancement.