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2,611 results · page 101 of 109
Aggelos Kiayias, Thomas Zacharias, Bingsheng Zhang
No abstract is available for this record.
Nicolas T. Courtois, Pinar Emirdag, Zhouyixing Wang
In this paper we study the question of centralisation in bitcoin digital currency. In theory bitcoin has been designed to be a totally decentralized distributed system. Satoshi Nakamoto has very clearly postulated that each node should be collecting recent transactions and trying to create new blocks (Satoshi08). In bitcoin transactions are aggregated in block in order to authenticate them and form an official ledger and history of bitcoin transactions. In practice as soon as expensive ASIC bitcoin miners have replaced general-purpose hardware, production of bitcoins and the validation of transactions has concentrated in the hands of a smaller group of people. Then at some moment in early 2012 an important decision was taken: the Stratum protocol was designed (Palatinus12) which took a deliberate decision to move the power of selecting which transactions are included in blocks from miners to pool managers. The growing difficulty of mining and large standard deviation in this process (Rosenfeld13; CourtoisBahack14) made that majority of miners naturally shifted to pooled mining. At this moment bitcoin ceased being a decentralized democratic system. In this paper we survey the question of a 51% attacks and show that there is a large variety of plausible attack scenarios. In particular we study one particularly subversive attack scenario which depends on non-trivial internal details of the bitcoin hashing process. How does it compare with the current mining practices? We have study the Stratum protocol in four popular real-life mining configurations. Our analysis shows that pools could very easily cheat the majority of people. However the most subversive versions of the attack are NOT facilitated and could potentially be detected.
Liam Morris
Cash in the real world allows for parties to exchange currency without the need to go through some sort of central authority. One person, Alice, can simply hand cash over to another person, Bob. In this transaction the only two people that have knowledge of this exchange are Alice and Bob. Until recently there was no electronic equivalent to this exchange. In 1982 David Chaum proposed a system of anonymous electronic cash based on blind signatures, and in 1990 founded DigiCash as an electronic cash company. There were a few banks that implemented electronic cash systems, but these banks and DigiCash ultimately went bankrupt in 1997 and 1998 despite the enthusiasm surrounding anonymous electronic cash. Between 1998 and 2008 there were no successful implementations of electronic cash that offer a decentralized, anonymous, and untraceable system.\nIn 2008 a paper was published by Satoshi Nakamoto on the cryptocurrency known as Bitcoin. A cryptocurrency is a form of electronic cash backed by mathematical and cryptographic constructs, unlike traditional currency which was historically backed by gold or silver. Cryptocurrencies have seen rising popularity in recent years due to their decentralized, distributed, peer-to-peer protocols. Part of this rising popularity is also attributable to the supposed anonymity of these protocols; however, due to the public transaction history required for these protocols and the fact that transactions are pseudonymous and not purely anonymous, this supposed anonymity does not exist. While the systems may achieve the goal of decentralized currency it does not achieve the goal of untraceability. In this thesis we analyze the technical implementations of Bitcoin and other cryptocurrencies to determine the level of anonymity provided by these protocols. We also analyze proposed improvements for their feasibility.
Pierre Noizat
No abstract is available for this record.
Matthias Tarasiewicz, Andrew Newman
No abstract is available for this record.
Foteini Baldimtsi, Aggelos Kiayias, Thomas Zacharias, Bingsheng Zhang
We introduce a new class of protocols called Proofs of Work or Knowledge (PoWorKs). In a PoWorK, a prover can convince a verifier that she has either performed work or that she possesses knowledge of a witness to a public statement without the verifier being able to distinguish which of the two has taken place. We formalize PoWorK in terms of three basic properties, completeness, f-soundness and indistinguishabil-ity (where f is a function that determines the tightness of the proof of work aspect) and present a construction that transforms 3-move HVZK protocols into 3-move public-coin PoWorKs. To formalize the work aspect in a PoWorK protocol we define cryptographic puzzles that adhere to certain uniformity conditions, which may also be of independent interest. We instantiate our puzzles in the random oracle (RO) model as well as via constructing “dense ” versions of suitably hard one-way functions. We then showcase PoWorK protocols by presenting two applications. We first show how non-interactive PoWorKs can be used to reduce spam email by forcing users sending an e-mail to either prove to the mail server they are approved contacts of the recipient or to perform computational work. As opposed to previous approaches [DN92, DGN03] that applied proofs of work to this problem, our proposal of using PoWorKs is privacy-preserving as it hides the list of the receiver’s approved contacts from the mail server. Our second application for PoWorK relates to zero-knowledge protocols. We show that PoWorK protocols imply straight-line quasi-polynomial simulatable arguments of knowledge; by applying this result to our construction we obtain an efficient straight-line concurrent 3-move statistically quasi-polynomial simulatable argument of knowledge, improving the round complexity of the previously known four-move protocols, [Pas03].
Olivier Blazy, Céline Chevalier, Damien Vergnaud
No abstract is available for this record.
Syed Taha Ali, Dylan Clarke, Patrick McCorry
No abstract is available for this record.
Syed Taha Ali, Patrick McCorry, Peter Hyun-Jeen Lee, Feng Hao
No abstract is available for this record.
Duane Wilson, Giuseppe Ateniese
PGP is built upon a Distributed Web of Trust in which the trustworthiness of a user is established by others who can vouch through a digital signature for that particular identity. Preventing its wholesale adoption are a number of inherent weaknesses to include (but not limited to) the following: 1) Trust Relationships are built on a subjective honor system, 2) Only first degree relationships can be fully trusted, 3) Levels of trust are difficult to quantify with actual values, and 4) Issues with the Web of Trust itself (Certification and Endorsement). Although the security that PGP provides is proven to be reliable, it has largely failed to garner large scale adoption. In this paper, we propose several novel contributions to address the aforementioned issues with PGP and associated Web of Trust. To address the subjectivity of the Web of Trust, we provide a new certificate format based on Bitcoin which allows a user to verify a PGP certificate using Bitcoin identity-verification transactions - forming first degree trust relationships that are tied to actual values (i.e., number of Bitcoins transferred during transaction). Secondly, we present the design of a novel Distributed PGP key server that leverages the Bitcoin transaction blockchain to store and retrieve Bitcoin-Based PGP certificates. Lastly, we provide a web prototype application that demonstrates several of these capabilities in an actual environment.
Sarah Meiklejohn, Claudio Orlandi
No abstract is available for this record.
Jordi Herrera‐Joancomartí
No abstract is available for this record.
Luke Valenta, Brendan Rowan
No abstract is available for this record.
Florian Tschorsch, Björn Scheuermann
Besides attracting a billion dollar economy, Bitcoin revolutionized the field of digital currencies and influenced many adjacent areas. This also induced significant scientific interest. In this survey, we unroll and structure the manyfold results and research directions. We start by introducing the Bitcoin protocol and its building blocks. From there we continue to explore the design space by discussing existing contributions and results. In the process, we deduce the fundamental structures and insights at the core of the Bitcoin protocol and its applications. As we show and discuss, many key ideas are likewise applicable in various other fields, so that their impact reaches far beyond Bitcoin itself.
Ahmed E. Kosba, Andrew Miller, Elaine Shi, Zikai Alex Wen · 5 authors
Emerging smart contract systems over decentralized cryptocurrencies allow mutually distrustful parties to transact safely without trusted third parties. In the event of contractual breaches or aborts, the decentralized blockchain ensures that honest parties obtain commensurate compensation. Existing systems, however, lack transactional privacy. All transactions, including flow of money between pseudonyms and amount transacted, are exposed on the blockchain. We present Hawk, a decentralized smart contract system that does not store financial transactions in the clear on the blockchain, thus retaining transactional privacy from the public's view. A Hawk programmer can write a private smart contract in an intuitive manner without having to implement cryptography, and our compiler automatically generates an efficient cryptographic protocol where contractual parties interact with the blockchain, using cryptographic primitives such as zero-knowledge proofs. To formally define and reason about the security of our protocols, we are the first to formalize the blockchain model of cryptography. The formal modeling is of independent interest. We advocate the community to adopt such a formal model when designing applications atop decentralized blockchains.
Bin Lian, Gongliang Chen, Maode Ma, Jianhua Li
In a periodic K-times anonymous authentication system, user can anonymously show credential at most K times in one time period. In the next time period, user can automatically get another K-times authentication permission. If a user tries to show credential beyond K times in one time period, anyone can identify the dishonest user (the violator). But identifying violators is not enough for some systems, where it is also desirable to revoke violators' credentials for preventing them from abusing the anonymous property again. However, the problem of revoking credential without trusted third party has not been solved efficiently and practically. To solve it, we present an efficient scheme with efficient revocation of violator's credential. In fact, our method also solves an interesting problem-leaking information in a statistic zero-knowledge way, so our solution to the revocation problem outperforms all prior solutions. For achieving it, we use the special zero-knowledge proof with special information leak for revoking the violator's credential, but it can still be proven to be perfect statistic zero knowledge for guaranteeing the honest user's anonymity. Comparing with existing schemes, our scheme is efficient, and moreover, our method of revoking violator's credential is more practical with the least additional costs.
Iddo Bentov, Charles Lee, Alex Mizrahi, Meni Rosenfeld
We propose a new protocol for a cryptocurrency, that builds upon the Bitcoin protocol by combining its Proof of Work component with a Proof of Stake type of system. Our Proof of Activity protocol offers good security against possibly practical attacks on Bitcoin, and has a relatively low penalty in terms of network communication and storage space.
George Bissias, A. Pinar Ozisik, Brian Neil Levine, Marc Liberatore
A fundamental limitation of Bitcoin and its variants is that the movement of coin between addresses can be observed by examining the public block chain. This record enables adversaries to link addresses to individuals, and to identify multiple addresses as belonging to a single participant. Users can try to hide this information by mixing, where a participant exchanges the funds in an address coin-for-coin with another participant and address. In this paper, we describe the weaknesses of extant mixing protocols, and analyze their vulnerability to Sybil-based denial-of-service and inference attacks. As a solution, we propose Xim, a two-party mixing protocol that is compatible with Bitcoin and related virtual currencies. It is the first decentralized protocol to simultaneously address Sybil attackers, denial-of-service attacks, and timing-based inference attacks. Xim is a multi-round protocol with tunably high success rates. It includes a decentralized system for anonymously finding mix partners based on ads placed in the block chain. No outside party can confirm or find evidence of participants that pair up. We show that Xim's design increases attacker costs linearly with the total number of participants, and that its probabilistic approach to mixing mitigates Sybil-based denial-of-service attack effects. We evaluate protocol delays based on our measurements of the Bitcoin network.
Danushka Jayasinghe, Konstantinos Markantonakis, Keith Mayes
Fair-exchange and anonymity are two important attributes in e-commerce. It is much more difficult to expect fairness in e-commerce transactions using Bit coin due to anonymity and transaction irreversibility. Genuine consumers and merchants who would like to make and receive payments using Bit coin may be reluctant to do so due to this uncertainty. The proposed protocol guarantees strong-fairness while preserving anonymity of the consumer and the merchant, using Bit coin as a payment method which addresses the aforementioned concern. The involvement of the trusted third party (TTP) is kept to a minimum, which makes the protocol optimistic and the exchanged product is not revealed to TTP. It achieves dispute resolution within the protocol run without any intervention of an external judge. Finally we show how the protocol can be easily adapted to use other digital cash systems designed using public ledgers such as Zerocoin/Zerocash.
Alex Biryukov, Ivan Pustogarov
Bitcoin is a decentralized P2P digital currency in which coins are generated by a distributed set of miners and transaction are broadcasted via a peer-to-peer network. While Bitcoin provides some level of anonymity (or rather pseudonymity) by encouraging the users to have any number of random-looking Bitcoin addresses, recent research shows that this level of anonymity is rather low. This encourages users to connect to the Bitcoin network through anonymizers like Tor and motivates development of default Tor functionality for popular mobile SPV clients. In this paper we show that combining Tor and Bitcoin creates an attack vector for the deterministic and stealthy man-in-the-middle attacks. A low-resource attacker can gain full control of information flows between all users who chose to use Bitcoin over Tor. In particular the attacker can link together user's transactions regardless of pseudonyms used, control which Bitcoin blocks and transactions are relayed to the user and can \ delay or discard user's transactions and blocks. In collusion with a powerful miner double-spending attacks become possible and a totally virtual Bitcoin reality can be created for such set of users. Moreover, we show how an attacker can fingerprint users and then recognize them and learn their IP address when they decide to connect to the Bitcoin network directly.
Long-Hai Li, Chengqiang Huang, Shao-Feng Fu
In this paper, we devise a novel anonymous voting protocol based on bilinear pairings. This protocol can be used by a group of participants to vote for one or more resolutions under the decentralized condition (boardroom voting), at the meantime preserving the privacy of participants. More specifically, in the session key set-up phase, every participant just need to broadcast a single message containing an ephemeral public key and a zero-knowledge proof. In the commitment phase, participants should broadcast the commitment of their votes although the actual votes are not revealed. After that, in the voting phase, every participant should encrypts his or her vote by combining these public keys and his secret session key, and then broadcast it to others. Finally, the voting result can be recovered by multiplying all the votes together. The advantage of our protocol over others lies in the fact that once the session keys have been established, they can be used repeatedly over multiple commitment and voting phases. So it requires just two rounds of broadcasting in every voting process after the first one, rendering it pretty good in terms of round efficiency. More precisely, if a group need to vote for several resolutions in one meeting, our protocol would involve less computational load and bandwidth usage in every vote. In fact, it is better than all the other techniques known. Our protocol is provably secure under the Decision Bilinear Diffie-Hellman assumption and random oracle assumption.
Gheith A. Abandah, Khalid A. Darabkh, Tawfiq Ammari, Omar Qunsul
Electronic voting provides accuracy and efficiency to the electoral processes. World democracies would benefit from a secure e-voting system not only to improve voter participation and trust but also to prevent electoral fraud. However, current e-voting systems are complex and have security weaknesses. In this paper, we describe a secure e-voting system for national and local elections (S-Vote). This system satisfies the important requirements of an e-voting system through state-of-the-art technologies and secure processes. S-Vote relies on homomorphic cryptography, zero-knowledge proofs, biometrics, smartcards, open-source software, and secure computers for securely and efficiently implementing the system processes over the various stages of the electoral process, without relying on online network connections. We outline the main conclusions of the pilot implementations of S-Vote that tested the main technologies and processes used. We also explain how the used technologies and processes achieve the system requirement. In conclusion, we recommend adopting S-Vote for its security, flexibility, economic, and scalability features.
Andy Yee
Bitcoin is the first decentralised, peer-to-peer network that allows for the proof and transfer of ownership of virtual currencies without the need for a trusted third party. The purpose of this article is to address how we can capture Bitcoin’s potential benefits for the economy while addressing new regulatory challenges.
Ryan Henry
Zero-knowledge proofs provide a means for a prover to convince a verifier that some claim is true and nothing more. The ability to prove statements while conveying zero information beyond their veracity has profound implications for cryptography and, especially, for its applicability to privacy-enhancing technologies. Unfortunately, the most common zero-knowledge techniques in the literature suffer from poor scalability, which limits their usefulness in many otherwise promising applications. This dissertation addresses the problem of designing communication- and computation-efficient protocols for zero-knowledge proofs and arguments of propositions that comprise many "simple" predicates. In particular, we propose a new formal model in which to analyze batch zero-knowledge protocols and perform the first systematic study of systems for batch zero-knowledge proofs and arguments of knowledge. In the course of this study, we suggest a general construction for batch zero-knowledge proof systems and use it to realize several new protocols suitable for proving knowledge of and relationships among large batches of discrete logarithm (DL) representations in prime-order groups. Our new protocols improve on existing protocols in several ways; for example, among the new protocols is one with lower asymptotic computation cost than any other such system in the literature. We also tackle the problem of constructing batch proofs of partial knowledge, proposing new protocols to prove knowledge of a DL that is equal to at least k-out-of-n other DLs, at most k-out-of-n other DLs, or exactly k-out-of-n other DLs. These constructions are particularly interesting as they prove some propositions that appear difficult to prove using existing techniques, even when efficiency is not a primary consideration. We illustrate the applicability of our new techniques by using them to construct efficient protocols for anonymous blacklisting and reputation systems.