Making Code Voting Secure against Insider Threats using Unconditionally Secure MIX Schemes and Human PSMT Protocols

Code voting was introduced by Chaum as a solution for using a possibly infected-by-malware device to cast a vote in an electronic voting application. Chaum's work on code voting assumed voting codes are physically delivered to voters using the mail system, implicitly requiring to trust the mail system. This is not necessarily a valid assumption to make - especially if the mail system cannot be trusted. When conspiring with the recipient of the cast ballots, privacy is broken. It is clear to the public that when it comes to privacy, computers and "secure" communication over the Internet cannot fully be trusted. This emphasizes the importance of using: (1) Unconditional security for secure network communication. (2) Reduce reliance on untrusted computers. In this paper we explore how to remove the mail system trust assumption in code voting. We use PSMT protocols (SCN 2012) where with the help of visual aids, humans can carry out $\mod 10$ addition correctly with a 99\% degree of accuracy. We introduce an unconditionally secure MIX based on the combinatorics of set systems. Given that end users of our proposed voting scheme construction are humans we \emph{cannot use} classical Secure Multi Party Computation protocols. Our solutions are for both single and multi-seat elections achieving: \begin{enumerate}[i)] \item An anonymous and perfectly secure communication network secure against a $t$-bounded passive adversary used to deliver voting, \item The end step of the protocol can be handled by a human to evade the threat of malware. \end{enumerate} We do not focus on active adversaries

DRE-ip : A Verifiable E-Voting Scheme without Tallying Authorities

Nearly all verifiable e-voting schemes require trustworthy authorities to perform the tallying operations. An exception is the DRE-i system which removes this requirement by pre-computing all encrypted ballots before the election using random factors that will later cancel out and allow the public to verify the tally after the election. While the removal of tallying authorities significantly simplifies election management, the pre-computation of ballots necessitates secure ballot storage, as leakage of precomputed ballots endangers voter privacy. In this paper, we address this problem and propose DRE-ip (DRE-i with enhanced privacy). Adopting a different design strategy, DRE-ip is able to encrypt ballots in real time in such a way that the election tally can be publicly verified without decrypting the cast ballots. As a result, DRE-ip achieves end-to-end verifiability without tallying authorities, similar to DRE-i, but with a significantly stronger guarantee on voter privacy. In the event that the voting machine is fully compromised, the assurance on tallying integrity remains intact and the information leakage is limited to the minimum: only the partial tally at the time of compromise is leaked

Universally Convertible Directed Signatures

Many variants of Chaum and van Antwerpen's undeniable signatures have been proposed to achieve specific properties desired in real-world applications of cryptography. Among them, directed signatures were introduced by Lim and Lee in 1993. Directed signatures differ from the well-known confirmer signatures in that the signer has the simultaneous abilities to confirm, deny and individually convert a signature. The universal conversion of these signatures has remained an open problem since their introduction in 1993. This paper provides a positive answer to this quest by showing a very efficient design for universally convertible directed signatures (UCDS) both in terms of computational complexity and signature size. Our construction relies on the so-called xyz-trick applicable to bilinear map groups. We define proper security notions for UCDS schemes and show that our construction is secure, in the random oracle model, under computational assumptions close to the CDH and DDH assumptions. Finally, we introduce and realize traceable universally convertible directed signatures where a master tracing key allows to link signatures to their direction

Probable Innocence Revisited

International audienceOften we wish to ensure that the identity of the user performing a certain action is maintained secret. This property is called anonymity. Examples of situations in which we may wish to provide anonymity include: publishing on the web, retrieving information from the web, sending a message, etc. Many protocols have been designed for this purpose, for example, Crowds [15], Onion Routing [23], the Free Haven [7], Web MIX [1] and Freenet [4]

Information Leakage Games

We consider a game-theoretic setting to model the interplay between attacker and defender in the context of information flow, and to reason about their optimal strategies. In contrast with standard game theory, in our games the utility of a mixed strategy is a convex function of the distribution on the defender's pure actions, rather than the expected value of their utilities. Nevertheless, the important properties of game theory, notably the existence of a Nash equilibrium, still hold for our (zero-sum) leakage games, and we provide algorithms to compute the corresponding optimal strategies. As typical in (simultaneous) game theory, the optimal strategy is usually mixed, i.e., probabilistic, for both the attacker and the defender. From the point of view of information flow, this was to be expected in the case of the defender, since it is well known that randomization at the level of the system design may help to reduce information leaks. Regarding the attacker, however, this seems the first work (w.r.t. the literature in information flow) proving formally that in certain cases the optimal attack strategy is necessarily probabilistic

ROYALE: A Framework for Universally Composable Card Games with Financial Rewards and Penalties Enforcement

While many tailor made card game protocols are known, the vast majority of those suffer from three main issues: lack of mechanisms for distributing financial rewards and punishing cheaters, lack of composability guarantees and little flexibility, focusing on the specific game of poker. Even though folklore holds that poker protocols can be used to play any card game, this conjecture remains unproven and, in fact, does not hold for a number of protocols (including recent results). We both tackle the problem of constructing protocols for general card games and initiate a treatment of such protocols in the Universal Composability (UC) framework, introducing an ideal functionality that captures general card games constructed from a set of core card operations. Based on this formalism, we introduce Royale, the first UC-secure general card games which supports financial rewards/penalties enforcement. We remark that Royale also yields the first UC-secure poker protocol. Interestingly, Royale performs better than most previous works (that do not have composability guarantees), which we highlight through a detailed concrete complexity analysis and benchmarks from a prototype implementation

One-Time Delegation of Unlinkable Signing Rights and Its Application

Delegation of signing rights can be useful to promote effective resource sharing and smooth cooperation among participants in distributed systems, and in many situations, we often need restricted delegation such as one-timeness and unlinkability rather than simple full delegation. Particularly, one-timesness cannot be achieved just by deploying cryptographic measures, and one needs to resort to some form of tamper-proofness or the assistance from external cloud servers for ``key-disabling\u27\u27. In this work, we extend the latter such that a delegatee can sign a message without the delegator\u27s involvement with the assumption that there exists at least one honest cloud server with secure erasure to achieve one-timeness. In this setting, if the delegator just shares their signing key between the delegatee and cloud servers, it may be problematic. It is because in the worst case, the delegator cannot know whether or not a signing key theft occurred because the signatures generated illegally are indistinguishable from the ones generated legally. To solve this, first we propose an efficient one-time delegation scheme of Okamoto-Schnorr signing. Further we combine the basic delegation scheme with anonymous credentials such that the delegator can detect the signing key theft even if one-time delegation is broken while also achieving unlinkability for both the delegator and cloud servers. Further we show its application to an e-cash scheme, which can prevent double-spending

Compact E-Cash and Simulatable VRFs Revisited

Abstract. Efficient non-interactive zero-knowledge proofs are a powerful tool for solving many cryptographic problems. We apply the recent Groth-Sahai (GS) proof system for pairing product equations (Eurocrypt 2008) to two related cryptographic problems: compact e-cash (Eurocrypt 2005) and simulatable verifiable random functions (CRYPTO 2007). We present the first efficient compact e-cash scheme that does not rely on a random oracle. To this end we construct efficient GS proofs for signature possession, pseudo randomness and set membership. The GS proofs for pseudorandom functions give rise to a much cleaner and substantially faster construction of simulatable verifiable random functions (sVRF) under a weaker number theoretic assumption. We obtain the first efficient fully simulatable sVRF with a polynomial sized output domain (in the security parameter).

The Security of the FDH Variant of Chaum’s Undeniable Signature Scheme

In this paper, a new kind of adversarial goal called forge-and-impersonate in undeniable signature schemes is introduced. Note that forgeability does not necessarily imply impersonation ability. The security of the full-domain hash (FDH) variant of Chaum's undeniable signature scheme is then classified according to three dimensions, the goal of adversaries, the attacks, and the zero-knowledg (ZK) level of confirmation and disavowal protocols. Each security is then related to some well-known computational problem. In particular, the security of the FDH variant of Chaum's scheme with noninteractive zero-knowledge (NIZK) protocol confirmation and disavowal protocols is proven to be equivalent to the computational Diffie-Hellman (CDH) problem, as opposed to the gap Diffie-Hellman (GDH) problem as claimed by Okamoto and Pointcheval