Random-oracle uninstantiability from indistinguishability obfuscation

Abstract

Assuming the existence of indistinguishability obfuscation (iO), we show that a number of prominent transformations in the randomoracle model are uninstantiable in the standard model. We start by showing that the Encrypt-with-Hash transform of Bellare, Boldyreva and O’Neill (CRYPTO 2007) for converting randomized public-key encryption schemes to deterministic ones is not instantiable in the standard model. To this end, we build on the recent work of Brzuska, Farshim and Mittelbach (CRYPTO 2014) and rely on the existence of iO for Turing machines or for circuits to derive two flavors of uninstantiability. The techniques that we use to establish this result are flexible and lend themselves to a number of other transformations such as the classical Fujisaki–Okamoto transform (CRYPTO 1998) and transformations akin to those by Bellare and Keelveedhi (CRYPTO 2011) and Douceur et al. (ICDCS 2002) for obtaining KDM-secure encryption and de-duplication schemes respectively. Our results call for a re-assessment of scheme design in the random-oracle model and highlight the need for new transforms that do not suffer from iO-based attacks. Keywords: Random oracle, uninstantiability, indistinguishability obfuscation, deterministic encryption, UCE, Fujisaki–Okamoto transform, KDM security, message-locked encryption.