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Four Practical Attacks for "Optimistic Mixing for Exit-Polls" (2003)

by Douglas Wikström
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A Verifiable Secret Shuffle of Homomorphic Encryptions

by Jens Groth , 2003
"... We show how to prove in honest verifier zero-knowledge the correctness of a shuffle of homomorphic encryptions (or homomorphic commitments.) A shuffle consists in a rearrangement of the input ciphertexts and a reencryption of them so that the permutation is not revealed. Our scheme ..."
Abstract - Cited by 80 (7 self) - Add to MetaCart
We show how to prove in honest verifier zero-knowledge the correctness of a shuffle of homomorphic encryptions (or homomorphic commitments.) A shuffle consists in a rearrangement of the input ciphertexts and a reencryption of them so that the permutation is not revealed. Our scheme
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...huffles. Subsequent papers on mix-nets [6, 48, 28, 22, 31, 15, 29, 42, 30, 46, 48, 52] have tried in many ways to guarantee correctness of a shuffle, most of which have been partially or fully broken =-=[3, 39, 53, 49]-=-. Remaining are suggestions [15, 48, 28, 52], of which the first three have various drawbacks. Desmedt and Kurosawa [15] require that at most a small fraction of the mix-servers is corrupt. Peng et al...

A universally composable mix-net.

by Douglas Wikström - TCC 2004. LNCS, , 2004
"... Abstract. A mix-net is a cryptographic protocol executed by a set of mix-servers that provides anonymity for a group of senders. The main application is electronic voting. Numerous mix-net constructions and stand-alone definitions of security are proposed in the literature, but only partial proofs ..."
Abstract - Cited by 30 (5 self) - Add to MetaCart
Abstract. A mix-net is a cryptographic protocol executed by a set of mix-servers that provides anonymity for a group of senders. The main application is electronic voting. Numerous mix-net constructions and stand-alone definitions of security are proposed in the literature, but only partial proofs of security are given for most constructions and no construction has been proved secure with regards to any kind of composition. We define an ideal mix-net in the universally composable security framework of Canetti
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...g [24,25,26] focusing on efficiency. Furukawa and Sako [15], and Neff [33] respectively have recently found efficient proofs of a correct shuffle, but these proposals have incomplete or flawed analysis. Groth [23] builds on Neff’s ideas to form an abstract protocol for any homomorphic cryptosystem. Desmedt and Kurosawa [10] describe an attack on a protocol by Jakobsson [24]. Similarly Mitomo and Kurosawa [32] exhibit a weakness in another protocol by Jakobsson [25]. Pfitzmann has given some general attacks on mix-nets [40,39], and Michels and Horster give additional attacks in [31]. Wikstrom [46] gives several attacks for a protocol by Golle et al. [22]. He also gives attacks for the protocols by Jakobsson [25] and Jakobsson and Juels [27]. Abe [2] has independently found related attacks. Canetti [6] and independently Pfitzmann and Waidner [41], proposed security frameworks for reactive processes. We use the former framework. Both frameworks has composition theorems, and are based on older definitional work. The initial ideal-model based definitional approach for secure function evaluation is informally proposed by Goldreich, Micali, and Wigderson in [18]. The first formalizations app...

Efficient Anonymity-Preserving Data Collection

by Justin Brickell, Vitaly Shmatikov , 2006
"... The output of a data mining algorithm is only as good as its inputs, and individuals are often unwilling to provide accurate data about sensitive topics such as medical history and personal finance. Individuals may be willing to share their data, but only if they are assured that it will be used in ..."
Abstract - Cited by 21 (1 self) - Add to MetaCart
The output of a data mining algorithm is only as good as its inputs, and individuals are often unwilling to provide accurate data about sensitive topics such as medical history and personal finance. Individuals may be willing to share their data, but only if they are assured that it will be used in an aggregate study and that it cannot be linked back to them. Protocols for anonymity-preserving data collection provide this assurance, in the absence of trusted parties, by allowing a set of mutually distrustful respondents to anonymously contribute data to an untrusted data miner. To effectively provide anonymity, a data collection protocol must be collusion resistant, which means that even if all dishonest respondents collude with a dishonest data miner in an attempt to learn the associations between honest respondents and their responses, they will be unable to do so. To achieve collusion resistance, previously proposed protocols for anonymity-preserving data collection have quadratically many communication rounds in the number of respondents, and employ (sometimes incorrectly) complicated cryptographic techniques such as zero-knowledge proofs. We describe a new protocol for anonymity-preserving, collusion resistant data collection. Our protocol has linearly many communication rounds, and achieves collusion resistance without relying on zero-knowledge proofs. This makes it especially suitable for data mining scenarios with a large number of respondents.

Analysis, improvement, and simplification of Prêt à Voter with Paillier encryption. In Electronic Voting Technology Workshop (EVT), 2008. Cited on page 63. Wissenschaftlicher Werdegang November 2010 - November 2013 Wissenschaftliche Mitarbeiterin und Do

by Zhe Xia, Steve A Schneider, James Heather, Jacques Traoré
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...orks is for mix servers to prove the shuffle. There have been some attempts to improve efficiency. Some repetitive robust mixnets are interesting, e.g. [31, 32, 27], but most of them have been broken =-=[19, 39, 1, 55]-=-. We advocate choosing one from [25, 41, 28] 2 . Schemes based on homomorphic encryption were first introduced by Benaloh [15, 10, 7]. To cast a vote, each voter generates an encrypted value of her de...

A Sender Verifiable Mix-Net and a New Proof of a Shuffle

by Douglas Wikström , 2005
"... We introduce the first El Gamal based mix-net in which each mix-server partially decrypts and permutes its input, i.e., no reencryption is necessary. An interesting property of the construction is that a sender can verify non-interactively that its message is processed correctly. We call this sende ..."
Abstract - Cited by 12 (2 self) - Add to MetaCart
We introduce the first El Gamal based mix-net in which each mix-server partially decrypts and permutes its input, i.e., no reencryption is necessary. An interesting property of the construction is that a sender can verify non-interactively that its message is processed correctly. We call this sender verifiability. The mix-net is provably UC-secure against static adversaries corrupting any minority of the mix-servers. The result holds under the decision Diffie-Hellman assumption, and assuming an ideal bulletin board and an ideal zero-knowledge proof of knowledge of a correct shuffle. Then we construct the first proof of a decryption-permutation shuffle, and show how this can be transformed into a zero-knowledge proof of knowledge in the UC-framework. The protocol is sound under the strong RSA-assumption and the discrete logarithm assumption. Our proof of a shuffle is not a variation of existing methods. It is based on a novel idea of independent interest, and we argue that it is at least as efficient as previous constructions.

Cryptanalysis of a universally verifiable efficient re-encryption mixnet

by Shahram Khazaei, Björn Terelius - In International conference on Electronic Voting Technology/Workshop on Trustworthy Elections , 2012
"... We study the heuristically secure mix-net proposed by Puiggalí and Guasch (EVOTE 2010). We present practical attacks on both correctness and privacy for some sets of parameters of the scheme. Although our attacks only allow us to replace a few inputs, or to break the privacy of a few voters, this sh ..."
Abstract - Cited by 7 (0 self) - Add to MetaCart
We study the heuristically secure mix-net proposed by Puiggalí and Guasch (EVOTE 2010). We present practical attacks on both correctness and privacy for some sets of parameters of the scheme. Although our attacks only allow us to replace a few inputs, or to break the privacy of a few voters, this shows that the scheme can not be proven secure. 1
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...nd Sako [7]. 1Many other works in the field aim to improve the efficiency of mix-nets, e.g., [11, 10, 8, 12, 13], but vulnerabilities have been found in most mix-nets not based on proofs of shuffles =-=[18, 15, 4, 22, 14]-=-. Puiggalí and Guasch [20] proposed a heuristically secure mix-net at EVOTE 2010 (called the Scytl mix-net in the rest of the paper) which combines ideas of Golle et al. [8] and Jakobsson et al. [13]....

A mix-net from any cca2 secure cryptosystem

by Shahram Khazaei, Tal Moran - In ASIACRYPT , 2012
"... Abstract. We construct a provably secure mix-net from any CCA2 secure cryp-tosystem. 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 c ..."
Abstract - Cited by 5 (0 self) - Add to MetaCart
Abstract. We construct a provably secure mix-net from any CCA2 secure cryp-tosystem. 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 re-sult 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 Chaum’s mix-net (1981). 1
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...suggested a fix; this in turn, in addition to the schemes of Jakobsson and Juels [11], of Golle, Zong, Boneh, Jakobsson and Juels [7] were all shown to be vulnerable (to various attacks) by Wikström =-=[23]-=-. While a formal proof of security is not an iron-clad guarantee that no vulnerabilities will ever be found (proofs may have subtle errors, and assumptions may be shown to be wrong), they do significa...

Randomized Partial Checking Revisited

by Shahram Khazaei, Douglas Wikström , 2012
"... We study mix-nets with randomized partial checking (RPC) as proposed by Jakobsson, Juels, and Rivest (2002). RPC is a technique to verify the correctness of an execution both for Chaumian and homomorphic mix-nets. The idea is to relax the correctness and privacy requirements to achieve a more effici ..."
Abstract - Cited by 3 (0 self) - Add to MetaCart
We study mix-nets with randomized partial checking (RPC) as proposed by Jakobsson, Juels, and Rivest (2002). RPC is a technique to verify the correctness of an execution both for Chaumian and homomorphic mix-nets. The idea is to relax the correctness and privacy requirements to achieve a more efficient mix-net. We identify serious issues in the original description of mix-nets with RPC and show how to exploit these to break both correctness and privacy, both for Chaumian and homomorphic mix-nets. Our attacks are practical and applicable to real world mix-net implementations, e.g., the Civitas and the Scantegrity voting systems. 1
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...often be broken, and this seems to be particularly true for mix-nets. Historically the proposals of heuristically secure mix-nets [20, 11, 12, 14, 9] have been followed by discovery of security flaws =-=[22, 21, 5, 17, 24]-=-. A formal proof of security does not guarantee that no attack will ever be found (proofs can have subtle errors, assumptions can be wrong, and the adversarial model can be unrealistic), but it increa...

Designing and Attacking Anonymous Communication Systems

by George Danezis , 2004
"... This report contributes to the field of anonymous communications over widely deployed communication networks. It describes ..."
Abstract - Cited by 3 (1 self) - Add to MetaCart
This report contributes to the field of anonymous communications over widely deployed communication networks. It describes

Privacy and Verifiability in Voting Systems: Methods, Developments and Trends

by Hugo Jonker , Sjouke Mauw , Jun Pang , 2013
"... One of the most challenging aspects in computer-supported voting is to combine the apparently conflicting requirements of privacy and verifiability. On the one hand, privacy requires that a vote cannot be traced back from the result to a voter, while on the other hand, verifiability states that a vo ..."
Abstract - Cited by 2 (0 self) - Add to MetaCart
One of the most challenging aspects in computer-supported voting is to combine the apparently conflicting requirements of privacy and verifiability. On the one hand, privacy requires that a vote cannot be traced back from the result to a voter, while on the other hand, verifiability states that a voter can trace the effect of her vote on the result. This can be addressed using various privacy-enabling cryptographic primitives which also offer verifiability. As more and more refined voting systems were proposed, understanding of first privacy and later verifiability in voting increased, and notions of privacy as well as notions of verifiability in voting became increasingly more refined. This has culminated in a variety of verifiable systems that use cryptographic primitives to ensure specific kinds of privacy. However, the corresponding privacy and verifiability claims are not often verified independently. When they are investigated, claims have been invalidated sufficiently often to warrant a cautious approach to them. The multitude of notions, primitives and proposed solutions that claim to achieve both privacy and verifiability form an interesting but complex landscape. The purpose of this paper is to survey this landscape by providing an overview of the methods, developments and current trends regarding privacy and verifiability in voting systems.
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