Can Two Walk Together: Privacy Enhancing Methods and Preventing Tracking of Users

We present a new concern when collecting data from individuals that arises from the attempt to mitigate privacy leakage in multiple reporting: tracking of users participating in the data collection via the mechanisms added to provide privacy. We present several definitions for untrackable mechanisms, inspired by the differential privacy framework. Specifically, we define the trackable parameter as the log of the maximum ratio between the probability that a set of reports originated from a single user and the probability that the same set of reports originated from two users (with the same private value). We explore the implications of this new definition. We show how differentially private and untrackable mechanisms can be combined to achieve a bound for the problem of detecting when a certain user changed their private value. Examining Google's deployed solution for everlasting privacy, we show that RAPPOR (Erlingsson et al. ACM CCS, 2014) is trackable in our framework for the parameters presented in their paper. We analyze a variant of randomized response for collecting statistics of single bits, Bitwise Everlasting Privacy, that achieves good accuracy and everlasting privacy, while only being reasonably untrackable, specifically grows linearly in the number of reports. For collecting statistics about data from larger domains (for histograms and heavy hitters) we present a mechanism that prevents tracking for a limited number of responses. We also present the concept of Mechanism Chaining, using the output of one mechanism as the input of another, in the scope of Differential Privacy, and show that the chaining of an $\varepsilon_1$-LDP mechanism with an $\varepsilon_2$-LDP mechanism is $\ln\frac{e^{\varepsilon_1+\varepsilon_2}+1}{e^{\varepsilon_1}+e^{\varepsilon_2}}$-LDP and that this bound is tight.Comment: 45 pages, 4 figures. To appear on FORC 202

SoK: Secure E-voting with Everlasting Privacy

In this work, we systematically analyze all e-voting protocols designed to provide everlasting privacy. Our main focus is to illustrate their relations and to identify the research problems which have or have not been solved in this area

SoK: Secure E-Voting with Everlasting Privacy

Vote privacy is a fundamental right, which needs to be protected not only during an election, or for a limited time afterwards, but for the foreseeable future. Numerous electronic voting (e-voting) protocols have been proposed to address this challenge, striving for everlasting privacy. This property guarantees that even computationally unbounded adversaries cannot break privacy of past elections. The broad interest in secure e-voting with everlasting privacy has spawned a large variety of protocols over the last three decades. These protocols differ in many aspects, in particular the precise security properties they aim for, the threat scenarios they consider, and the privacy-preserving techniques they employ. Unfortunately, these differences are often opaque, making analysis and comparison cumbersome. In order to overcome this non-transparent state of affairs, we systematically analyze all e-voting protocols designed to provide everlasting privacy. First, we illustrate the relations and dependencies between all these different protocols. Next, we analyze in depth which protocols do provide secure and efficient approaches to e-voting with everlasting privacy under realistic assumptions, and which ones do not. Eventually, based on our extensive and detailed treatment, we identify which research problems in this field have already been solved, and which ones are still open. Altogether, our work offers a well-founded reference point for conducting research on secure e-voting with everlasting privacy as well as for future-proofing privacy in real-world electronic elections

Security models for everlasting privacy

We propose security models for everlasting privacy, a property that protects the content of the votes cast in electronic elections against future and powerful adversaries. Initially everlasting privacy was treated synonymously with information theoretic privacy and did not take advantage of the information available to the adversary and his behavior during or after the election. More recent works provided variations of the concept, limiting the view of the future adversary to publicly available data. We consider an adversary that potentially has insider access to private election data as well. We formally express our adversarial model in game based definitions build on top of a generic voting scheme. This allows us to define a stronger version of everlasting privacy and contrast the two main proposals to achieve it, namely perfectly hiding commitment schemes and anonymous channels

NetVote: A strict-coercion resistance re-voting based internet voting scheme with linear filtering

This paper is an extended of: Querejeta-Azurmendi, I.; Hernández Encinas, L.; Arroyo Guardeño, D.; Hernandez-Ardieta, J.L. An internet voting proposal towards improving usability and coercion resistance. Proceedings of the International Joint Conference: 12th International Conference on Computational Intelligence in Security for Information Systems (CISIS 2019) and 10th International Conference on EUropean Transnational Education (ICEUTE 2019), Seville, Spain, 13-15 May 2019.This paper proposes NetVote, an internet voting protocol where usability and ease in deployment are a priority. We introduce the notion of strict coercion resistance, to distinguish between vote-buying and coercion resistance. We propose a protocol with ballot secrecy, practical everlasting privacy, verifiability and strict coercion resistance in the re-voting setting. Coercion is mitigated via a random dummy vote padding strategy to hide voting patterns and make re-voting deniable. This allows us to build a filtering phase with linear complexity, based on zero knowledge proofs to ensure correctness while maintaining privacy of the process. Voting tokens are formed by anonymous credentials and pseudorandom identifiers, achieving practical everlasting privacy, where even if dealing with a future computationally unbounded adversary, vote intention is still hidden. It is not assumed for voters to own cryptographic keys prior to the election, nor store cryptographic material during the election. This property allows voters not only to vote multiple times, but also from different devices each time, granting the voter a vote-from-anywhere experience. This paper builds on top of the paper published in CISIS'19. In this version, we modify the filtering. Moreover, we formally define the padding technique, which allows us to perform the linear filtering scheme. Similarly we provide more details on the protocol itself and include a section of the security analysis, where we include the formal definitions of strict coercion resistance and a game based definition of practical everlasting privacy. Finally, we prove that NetVote satisfies them all.This research has been partially supported by Ministerio de Economía, Industria y Competitividad (MINECO), Agencia Estatal de Investigación (AEI), and European Regional Development Fund (ERDF, EU), through project COPCIS, grant number TIN2017-84844-C2-1-R, and by Comunidad de Madrid (Spain) through project CYNAMON, grant number P2018/TCS-4566-CM, co-funded along with ERDF

Public Evidence from Secret Ballots

Elections seem simple---aren't they just counting? But they have a unique, challenging combination of security and privacy requirements. The stakes are high; the context is adversarial; the electorate needs to be convinced that the results are correct; and the secrecy of the ballot must be ensured. And they have practical constraints: time is of the essence, and voting systems need to be affordable and maintainable, and usable by voters, election officials, and pollworkers. It is thus not surprising that voting is a rich research area spanning theory, applied cryptography, practical systems analysis, usable security, and statistics. Election integrity involves two key concepts: convincing evidence that outcomes are correct and privacy, which amounts to convincing assurance that there is no evidence about how any given person voted. These are obviously in tension. We examine how current systems walk this tightrope.Comment: To appear in E-Vote-Id '1

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

Universally Verifiable Poll-Site Voting Schemes Providing Everlasting Privacy

Computer based voting brings up huge challenges for technology. On the one hand an electronic voting system has to be transparent enough to allow verification of its correct functioning; on the other hand, it must ensure that these verification procedures do not allow an attacker to violate voter privacy. Both requirements can be addressed by providing cryptographically secured voting receipts. Each voter cast his or her vote in encoded form and receives a copy of the recorded ballot as receipt. The voters can use these receipts to verify that their vote is contained in the input of the tally. Furthermore, the encoded votes are publicly processed, which allows voters and observers to check that the election outcome has been determined correctly. However, to provide a private and free election, no voter should be able to prove to someone else for whom he or she voted. This must not only be prevented during the election, but also afterwards for an indefinite period of time. Especially with respect to everlasting privacy this is not ensured by most verifiable voting systems. If the receipt contains, for instance, the voting decision encrypted using some public key cryptography, an attacker can determine the candidates selected as soon as the underlying computational problem has been solved for the key length chosen. In this work we provide a summary of privacy weaknesses that may arise in verifiable electronic poll-site voting systems, and we identify and solve open issues. More precisely, we concentrate on the following three questions: (1) How can we show correct anonymization of votes in an efficient and privacy preserving manner using a generic approach? (2) How can we introduce everlasting privacy to mixing and homomorphic tallying based voting schemes? (3) How can we reduce the amount of trust voters have to put in authorities regarding privacy? In electronic voting so-called reencryption mix-nets are used to anonymize votes. These mix-nets shuffles votes in a universally verifiable manner, i.e., they publish some audit information allowing voters and observers to verify that the votes came out as they went in. In practice, mostly generic verification procedures are used to show correctness of this process. However, many of them do not provide an adequate level of privacy. To address (1), we investigate several proposals and introduce a new protocol that combines existing approaches but improves them with respect to privacy and efficiency. Another drawback of mixing based voting schemes is that all implementations provide computational privacy only. We address (2) by presenting a mix-net that uses a homomorphic and unconditionally hiding commitment scheme to encode the votes and audit data, implying everlasting privacy. The correctness of the anonymization process is guaranteed with overwhelming probability, even if all authorities collaborate. An implication of our result is that many current voting systems that use mix-nets can be upgraded to everlasting privacy. Subsequently, we show that this protocol can be applied to Prêt à Voter and Split-Ballot imposing only minor changes to current implementations. The same approach is used to introduce everlasting privacy to homomorphic tallying based schemes. The votes are encoded with an unconditionally hiding commitment scheme, they are homomorphically tallied in public, and the result is decoded afterwards. To show that our solution can be applied to poll-site voting, we describe how the Scratch & Vote voting system can be improved using our tallying protocol. Again only minor changes to the classical scheme are necessary. To address (3), the approach of non-personalized receipts is analyzed. If the receipts handed out to the voters do not contain a link to their vote cast, they do not have to put their trust in authorities keeping this association secret. We introduce an electronic ballot box that generates non-personalized receipts using a process that is similar to the anonymization procedure carried out by mix-nets. The correctness of the receipt generation is universally verifiable. Furthermore, our approach improves on existing solutions with respect to correctness and privacy. Finally, we compare all voting systems that are improved in this work, highlight their advantages and disadvantages, and conclude with key issues for future work