Quisquis: A new design for anonymous cryptocurrencies

Despite their usage of pseudonyms rather than persistent identifiers, most existing cryptocurrencies do not provide users with any meaningful levels of privacy. This has prompted the creation of privacy-enhanced cryptocurrencies such as Monero and Zcash, which are specifically designed to counteract the tracking analysis possible in currencies like Bitcoin. These cryptocurrencies, however, also suffer from some drawbacks: in both Monero and Zcash, the set of potential unspent coins is always growing, which means users cannot store a concise representation of the blockchain. Additionally, Zcash requires a common reference string and the fact that addresses are reused multiple times in Monero has led to attacks to its anonymity. In this paper we propose a new design for anonymous cryptocurrencies, Quisquis, that achieves provably secure notions of anonymity. Quisquis stores a relatively small amount of data, does not require trusted setup, and in Quisquis each address appears on the blockchain at most twice: once when it is generated as output of a transaction, and once when it is spent as input to a transaction. Our result is achieved by combining a DDH-based tool (that we call updatable keys) with efficient zero-knowledge arguments

SoK:A Systematic Study of Anonymity in Cryptocurrencies

Blockchain and cryptocurrencies have been widely deployed and used in our daily life. Although there are numerous works in the literature surveying technical challenges and security issues in blockchains, very few works focused on the anonymity guarantees provided in cryptocurrencies. In this work, we conduct a systematic survey on anonymity in cryptocurrencies with a clear categorization for the different tiers of anonymity offered in the various cryptocurrencies as well as their known weaknesses and vulnerabilities. We also study the techniques that have been used to achieve each tier of anonymity. Finally, we asses the current techniques, and present a forecast for the technological trends in this fiel

Kleptography and steganography in blockchains

Despite its vast proliferation, the blockchain technology is still evolving, and witnesses continuous technical innovations to address its numerous unresolved issues. An example of these issues is the excessive electrical power consumed by some consensus protocols. Besides, although various media reports have highlighted the existence of objectionable content in blockchains, this topic has not received sufficient research. Hence, this work investigates the threat and deterrence of arbitrary-content insertion in public blockchains, which poses a legal, moral, and technical challenge. In particular, the overall aim of this work is to thoroughly study the risk of manipulating the implementation of randomized cryptographic primitives in public blockchains to mount kleptographic attacks, establish steganographic communication, and store arbitrary content. As part of our study, we present three new kleptographic attacks on two of the most commonly used digital signatures: ring signature and ECDSA. We also demonstrate our kleptographic attacks on two real cryptocurrencies: Bytecoin and Monero. Moreover, we illustrate the plausibility of hijacking public blockchains to establish steganographic channels. Particularly, we design, implement, and evaluate the first blockchain-based broadcast communication tool on top of a real-world cryptocurrency. Furthermore, we explain the detrimental consequences of kleptography and steganography on the users and the future of the blockchain technology. Namely, we show that kleptography can be used to surreptitiously steal the users' secret signing keys, which are the most valuable and guarded secret in public blockchains. After losing their keys, users of cryptocurrencies will inevitably lose their funds. In addition, we clarify that steganography can be used to establish subliminal communication and secretly store arbitrary content in public blockchains, which turns them into cheap cyberlockers. Consequently, the participation in such blockchains, which are known to store unethical content, can be criminalized, hindering the future adoption of blockchains. After discussing the adverse effects of kleptographic and steganographic attacks on blockchains, we survey all of the existing techniques that can defend against these attacks. Finally, due to the shortcomings of the available techniques, we propose four countermeasures that ensure kleptography and steganography-resistant public blockchains. Our countermeasures include two new cryptographic primitives and a generic steganographyresistant blockchain framework (SRBF). This framework presents a universal solution that deters steganography and practically achieves the right to be forgotten (RtbF) in blockchains, which represents a regulatory challenge for current immutable blockchains

On Defeating Graph Analysis of Anonymous Transactions

In a ring-signature-based anonymous cryptocurrency, signers of a transaction are hidden among a set of potential signers, called a ring, whose size is much smaller than the number of all users. The ring-membership relations specified by the sets of transactions thus induce bipartite transaction graphs, whose distribution is in turn induced by the ring sampler underlying the cryptocurrency. Since efficient graph analysis could be performed on transaction graphs to potentially deanonymise signers, it is crucial to understand the resistance of (the transaction graphs induced by) a ring sampler against graph analysis. Of particular interest is the class of partitioning ring samplers. Although previous works showed that they provide almost optimal local anonymity, their resistance against global, e.g. graph-based, attacks were unclear. In this work, we analyse transaction graphs induced by partitioning ring samplers. Specifically, we show (partly analytically and partly empirically) that, somewhat surprisingly, by setting the ring size to be at least logarithmic in the number of users, a graph-analysing adversary is no better than the one that performs random guessing in deanonymisation up to constant factor of 2

A Deep-dive into Cryptojacking Malware: From an Empirical Analysis to a Detection Method for Computationally Weak Devices

Cryptojacking is an act of using a victim\u27s computation power without his/her consent. Unauthorized mining costs extra electricity consumption and decreases the victim host\u27s computational efficiency dramatically. In this thesis, we perform an extensive research on cryptojacking malware from every aspects. First, we present a systematic overview of cryptojacking malware based on the information obtained from the combination of academic research papers, two large cryptojacking datasets of samples, and numerous major attack instances. Second, we created a dataset of 6269 websites containing cryptomining scripts in their source codes to characterize the in-browser cryptomining ecosystem by differentiating permissioned and permissionless cryptomining samples. Third, we introduce an accurate and efficient IoT cryptojacking detection mechanism based on network traffic features that achieves an accuracy of 99%. Finally, we believe this thesis will greatly expand the scope of research and facilitate other novel solutions in the cryptojacking domain

On Privacy Preserving Blockchains and zk-SNARKs

Viimastel aastatel on krüptoraha ja plokiahela tehnoloogia leidnud suurt tähelepanu nii kaubanduslikust kui ka teaduslikust vaatenurgast. Krüptoraha kujutab endast digitaalseid münte, mis kasutades krüptograafilisi vahendeid võimaldab turvalisi tehinguid võrdvõrkudes. Bitcoin on kõige tuntum krüptoraha, mis võimaldab otsetehinguid kasutajate pseudonüümide vahel ilma, et oleks vaja kolmandaid osapooli. Paraku kui kasutaja pseudonüüm on seotud tema identiteediga, on kõik tema tehingud jälgitavad ning kaob privaatsus.Selle lahendamiseks on välja pakutud erinevaid privaatsust säilitavaid krüptorahasi, mis kasutavad anonüümsete tehingute saavutamiseks krüptograafilisi tööriistu. Zerocash on üks populaarseimatest privaatsetest krüptorahadest, mis kasutab iga tehingu allika, sihtkoha ja väärtuse varjamiseks nullteadmustõestust.Antud töö koosneb kahest peamisest osast.Esimeses osas kirjeldame, pärast lühikest ülevaadet mõnest privaatsest krüptorahast (Bitcoin, Monero ja Zerocoin), Zerocashi konstruktsiooni ja anname intuitsiivse seletuse selle tööpõhimõttele. Me tutvustame kasutuselevõetud primitiive ja arutleme iga primitiivi rolli üle mündi konstruktsioonis. Erilist tähelepanu pöörame kompaktsetele nullteadmustõestusetele (zk-SNARKidele), millel on peamine roll Zerocashis.Kuna nullteadmustõestus on niivõrd olulisel kohal Zerocashis (ja teistes privaatsetes rakendustes) siis töö teises osas pakume välja uue variatsiooni Grothi 2016. aasta zk-SNARKile, mis on seni kõige tõhusam.Erinevalt Grothi konstruktsioonist, meie variatsioonis ei ole võimalik tõestusi modifitseerida.Muudatused mõjutavad nullteadmustõestuse tõhusust vaid minimaalselt ning meie konstruktsioon on kiirem kui Grothi ja Malleri 2017. nullteadmustõestus, mis samuti välistab muudetavuse.During last few years, along with blockchain technology, cryptocurrencies have found huge attention from both commercial and scientific perspectives. Cryptocurrencies are digital coins which use cryptographic tools to allow secure peer-to-peer monetary transactions. Bitcoin is the most well-known cryptocurrency that allows direct payments between pseudonyms without any third party. If a user's pseudonym is linked to her identity, all her transactions will be traceable, which will violate her privacy. To address this, various privacy-preserving cryptocurrencies have been proposed that use different cryptographic tools to achieve anonymous transactions. Zerocash is one of the most popular ones that uses zero-knowledge proofs to hide the source, destination and value of each transaction. This thesis consists of two main parts. In the first part, after a short overview of some cryptocurrencies (precisely Bitcoin, Monero and Zerocoin), we will explain the construction of Zerocash cryptocurrency and discuss the intuition behind the construction. More precisely, we will introduce the deployed primitives and will discuss the role of each primitive in the construction of the coin. In particular, we explain zero-knowledge Succinct Non-Interactive Arguments of Knowledge (a.k.a. zk-SNARKs) that play the main role in achieving strong privacy in Zerocash. Due to the importance of zk-SNARKs in privacy-preserving applications, in the second part of the thesis, we will present a new variation of Groth's 2016 zk-SNARK that currently is the most efficient pairing-based scheme. The main difference between the proposed variation and the original one is that unlike the original version, new variation guarantees non-malleability of generated proofs. Our analysis shows that the proposed changes have minimal effects on the efficiency of the original scheme and particularly it outperforms Groth and Maller's 2017 zk-SNARK that also guarantees non-malleability of proofs

Vulnerability of blockchain technologies to quantum attacks

Quantum computation represents a threat to many cryptographic protocols in operation today. It has been esti- mated that by 2035, there will exist a quantum computer capable of breaking the vital cryptographic scheme RSA2048. Blockchain technologies rely on cryptographic protocols for many of their essential sub-routines. Some of these protocols, but not all, are open to quantum attacks. Here we analyze the major blockchain-based cryp- tocurrencies deployed today—including Bitcoin, Ethereum, Litecoin and ZCash, and determine their risk exposure to quantum attacks. We finish with a comparative analysis of the studied cryptocurrencies and their underlying blockchain technologies and their relative levels of vulnerability to quantum attacks