International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Mohammad Zaheri

Publications

Year
Venue
Title
2021
PKC
On Selective-Opening Security of Deterministic Primitives
Adam O'Neill Mohammad Zaheri
Classically, selective-opening attack (SOA) has been studied for \emph{randomized} primitives, like randomized encryption schemes and commitments. The study of SOA for deterministic primitives, which presents some unique challenges, was initiated by Bellare \emph{et al.} (PKC 2015), who showed negative results. Subsequently, Hoang \emph{et al.} (ASIACRYPT 2016) showed positive results in the non-programmable random oracle model. Here we show the first positive results for SOA security of deterministic primitives in the \emph{standard} (RO devoid) model. Our results are: \begin{itemize} \item Any $2t$-wise independent hash function is SOA secure for an unbounded number of ``$t$-correlated'' messages, meaning any group of up to $t$ messages are arbitrarily correlated. \item A construction of a deterministic encryption scheme with analogous security, combining a regular lossy trapdoor function with a $2t$-wise independent hash function. \item The one-more-RSA problem of Bellare \emph{et al.} (J.~Cryptology 2003), which can be seen as a form of SOA, is hard under the $\Phi$-Hiding Assumption with large enough encryption exponent. \end{itemize} Somewhat surprisingly, the last result yields the first proof of RSA-based Chaum's blind signature scheme (CRYPTO 1982) based on a ``standard'' computational assumption. Notably, it avoids the impossibility result of Pass (STOC 2011) because lossiness of RSA endows the scheme with non-unique signatures.
2020
PKC
Toward RSA-OAEP Without Random Oracles 📺
Nairen Cao Adam O’Neill Mohammad Zaheri
We show new partial and full instantiation results under chosen-ciphertext security for the widely implemented and standardized RSA-OAEP encryption scheme of Bellare and Rogaway (EUROCRYPT 1994) and two variants. Prior work on such instantiations either showed negative results or settled for “passive” security notions like IND-CPA. More precisely, recall that RSA-OAEP adds redundancy and randomness to a message before composing two rounds of an underlying Feistel transform, whose round functions are modeled as random oracles (ROs), with RSA. Our main results are: Either of the two oracles (while still modeling the other as a RO) can be instantiated in RSA-OAEP under IND-CCA2 using mild standard-model assumptions on the round functions and generalizations of algebraic properties of RSA shown by Barthe, Pointcheval, and Báguelin (CCS 2012). The algebraic properties are only shown to hold at practical parameters for small encryption exponent ( $$e=3$$ ), but we argue they have value for larger e as well. Both oracles can be instantiated simultaneously for two variants of RSA-OAEP, called “ t -clear” and “ s -clear” RSA-OAEP. For this we use extractability-style assumptions in the sense of Canetti and Dakdouk (TCC 2010) on the round functions, as well as novel yet plausible “XOR-type” assumptions on RSA. While admittedly strong, such assumptions may nevertheless be necessary at this point to make positive progress. In particular, our full instantiations evade impossibility results of Shoup (J. Cryptology 2002), Kiltz and Pietrzak (EUROCRYPT 2009), and Bitansky et al. (STOC 2014). Moreover, our results for s -clear RSA-OAEP yield the most efficient RSA-based encryption scheme proven IND-CCA2 in the standard model (using bold assumptions on cryptographic hashing) to date.
2016
ASIACRYPT