Tal Moran

Receipt-free universally-verifiable voting with everlasting privacy

We present the first universally verifiable voting scheme that can be based on a general assumption (existence of a non-interactive commitment scheme). Our scheme is also the first receipt-free scheme to give “everlasting privacy” for votes: even a computationally unbounded party does not gain any information about individual votes (other than what can be inferred from the final tally). Our voting protocols are designed to be used in a “traditional” setting, in which voters cast their ballots in a private polling booth (which we model as an untappable channel between the voter and the tallying authority). Following in the footsteps of Chaum and Neff [7,16], our protocol ensures that the integrity of an election cannot be compromised even if the computers running it are all corrupt (although ballot secrecy may be violated in this case). We give a generic voting protocol which we prove to be secure in the Universal Composability model, given that the underlying commitment is universally composable. We also propose a concrete implementation, based on the hardness of discrete log, that is slightly more efficient (and can be used in practice).

Split-ballot voting: Everlasting privacy with distributed trust

In this article, we propose a new voting protocol with several desirable security properties. The voting stage of the protocol can be performed by humans without computers; it provides every voter with the means to verify that all the votes were counted correctly (universal verifiability) while preserving ballot secrecy. The protocol has “everlasting privacy”: Even a computationally unbounded adversary gains no information about specific votes from observing the protocols output. Unlike previous protocols with these properties, this protocol distributes trust between two authorities: a single corrupt authority will not cause voter privacy to be breached. Finally, the protocol is receipt-free: A voter cannot prove how she voted even if she wants to do so. We formally prove the security of the protocol in the universal composability framework, based on number-theoretic assumptions.

How to Use Bitcoin to Play Decentralized Poker

Back and Bentov (arXiv 2014) and Andrychowicz et al. (Security and Privacy 2014) introduced techniques to perform secure multiparty computations on Bitcoin. Among other things, these works constructed lottery protocols that ensure that any party that aborts after learning the outcome pays a monetary penalty to all other parties. Following this, Andrychowicz et al. (Bitcoin Workshop 2014) and concurrently Bentov and Kumaresan (Crypto 2014) extended the solution to arbitrary secure function evaluation while guaranteeing fairness in the following sense: any party that aborts after learning the output pays a monetary penalty to all parties that did not learn the output. Andrychowicz et al. (Bitcoin Workshop 2014) also suggested extending to scenarios where parties receive a payoff according to the output of a secure function evaluation, and outlined a 2-party protocol for the same that in addition satisfies the notion of fairness described above. In this work, we formalize, generalize, and construct multiparty protocols for the primitive suggested by Andrychowicz et al. We call this primitive secure cash distribution with penalties. Our formulation of secure cash distribution with penalties poses it as a multistage reactive functionality (i.e., more general than secure function evaluation) that provides a way to securely implement smart contracts in a decentralized setting, and consequently suffices to capture a wide variety of stateful computations involving data and/or money, such as decentralized auctions, market, and games such as poker, etc. Our protocol realizing secure cash distribution with penalties works in a hybrid model where parties have access to a claim-or-refund transaction functionality FCR}* which can be efficiently realized in (a variant of) Bitcoin, and is otherwise independent of the Bitcoin ecosystem. We emphasize that our protocol is dropout-tolerant in the sense that any party that drops out during the protocol is forced to pay a monetary penalty to all other parties. Our formalization and construction generalize both secure computation with penalties of Bentov and Kumaresan (Crypto 2014), and secure lottery with penalties of Andrychowicz et al. (Security and Privacy 2014).

Non-interactive Timestamping in the Bounded Storage Model

A timestamping scheme is non-interactive if a stamper can stamp a document without communicating with any other player. The only communication done is at validation time. Non-Interactive timestamping has many advantages, such as information theoretic privacy and enhanced robustness. Unfortunately, no such scheme exists against polynomial time adversaries that have unbounded storage at their disposal.

Shuffle-Sum: Coercion-Resistant Verifiable Tallying for STV Voting

There are many advantages to voting schemes in which voters rank all candidates in order, rather than just choosing their favorite. However, these schemes inherently suffer from a coercion problem when there are many candidates, because a coercer can demand a certain permutation from a voter and then check whether that permutation appears during tallying. Recently developed cryptographic voting protocols allow anyone to audit an election (universal verifiability), but existing systems are either not applicable to ranked voting at all, or reveal enough information about the ballots to make voter coercion possible. We solve this problem for the popular single transferable vote (STV) ranked voting system, by constructing an algorithm for the verifiable tallying of encrypted votes. Our construction improves upon existing work because it extends to multiple-seat STV and reveals less information than other schemes. The protocol is based on verifiable shuffling of homomorphic encryptions, a well-studied primitive in the voting arena. Our protocol is efficient enough to be practical, even for a large election.

Polling with physical envelopes: a rigorous analysis of a human-centric protocol

We propose simple, realistic protocols for polling that allow the responder to plausibly repudiate his response, while at the same time allow accurate statistical analysis of poll results. The protocols use simple physical objects (envelopes or scratch-off cards) and can be performed without the aid of computers. One of the main innovations of this work is the use of techniques from theoretical cryptography to rigorously prove the security of a realistic, physical protocol. We show that, given a few properties of physical envelopes, the protocols are unconditionally secure in the universal composability framework.

On complete primitives for fairness

For secure two-party and multi-party computation with abort, classification of which primitives are complete has been extensively studied in the literature. However, for fair secure computation, where (roughly speaking) either all parties learn the output or none do, the question of complete primitives has remained largely unstudied. In this work, we initiate a rigorous study of completeness for primitives that allow fair computation. We show the following results: No “short” primitive is complete for fairness. In surprising contrast to other notions of security for secure two-party computation, we show that for fair secure computation, no primitive of size O(logk) is complete, where k is a security parameter. This is the case even if we can enforce parallelism in calls to the primitives (i.e., the adversary does not get output from any primitive in a parallel call until it sends input to all of them). This negative result holds regardless of any computational assumptions. A fairness hierarchy. We clarify the fairness landscape further by exhibiting the existence of a “fairness hierarchy”. We show that for every “short” l=O(logk), no protocol making (serial) access to any l-bit primitive can be used to construct even a (l+1)-bit simultaneous broadcast. Positive results. To complement the negative results, we exhibit a k-bit primitive that is complete for two-party fair secure computation. We show how to generalize this result to the multi-party setting. Fairness combiners. We also introduce the question of constructing a protocol for fair secure computation from primitives that may be faulty. We show that this is possible when a majority of the instances are honest. On the flip side, we show that this result is tight: no functionality is complete for fairness if half (or more) of the instances can be malicious.

Attacking paper-based e2e voting systems

In this paper, we develop methods for constructing vote-buying/coercion attacks on end-to-end voting systems, and describe vote-buying/coercion attacks on three proposed end-to-end voting systems: Punchscan, Pret-a-voter, and ThreeBallot. We also demonstrate a different attack on Punchscan, which could permit corrupt election officials to change votes without detection in some cases. Additionally, we consider some generic attacks on end-to-end voting systems.

A mix-net from any CCA2 secure cryptosystem

We construct a provably secure mix-net from any CCA2 secure cryptosystem. The mix-net is secure against active adversaries that statically corrupt less than λ out of k mix-servers, where λ is a threshold parameter, and it is robust provided that at most min (λ−1,k−λ) mix-servers are corrupted. The main component of our construction is a mix-net that outputs the correct result if all mix-servers behaved honestly, and aborts with probability 1−O(H−(t−1)) otherwise (without disclosing anything about the inputs), where t is an auxiliary security parameter and H is the number of honest parties. The running time of this protocol for long messages is roughly 3tc, where c is the running time of Chaums mix-net (1981).

Non-interactive Timestamping in the Bounded-Storage Model

A timestamping scheme is non-interactive if a stamper can stamp a document without communicating with any other player. The only communication done is at validation time. Non-Interactive timestamping has many advantages, such as information theoretic privacy and enhanced robustness. Non-Interactive timestamping, however, is not possible against polynomial-time adversaries that have unbounded storage at their disposal. As a result, no non-interactive timestamping schemes were constructed up to date.In this paper we show that non-interactive timestamping is possible in the bounded-storage model, i.e., if the adversary has bounded storage, and a long random string is broadcast to all players. To the best of our knowledge, this is the first example of a cryptographic task that is possible in the bounded-storage model but is impossible in the “standard cryptographic setting,” even when assuming “standard” cryptographic assumptions.We give an explicit construction that is secure against all bounded storage adversaries and a significantly more efficient construction secure against all bounded storage adversaries that run in polynomial time.