Blockchain Papers

Philipp Locher, Rolf Haenni

No abstract is available for this record.

Maria Fueyo, Javier Herranz

The problem of revocation in anonymous authentication systems is subtle and has motivated a lot of work. One of the preferable solutions consists in maintaining either a whitelist LWof non-revoked users or a blacklist LBof revoked users, and then requiring users to additionally prove, when authenticating themselves, that they are in LW(membership proof) or that they are not in LB(non-membership proof). Of course, these additional proofs must not break the anonymity properties of the system, so they must be zero-knowledge proofs, revealing nothing about the identity of the users. In this paper, we focus on the RSA-based setting, and we consider the case of non-membership proofs to blacklists L = LB. The existing solutions for this setting rely on the use of universal dynamic accumulators; the underlying zero-knowledge proofs are bit complicated, and thus their efficiency; although being independent from the size of the blacklist L, seems to be improvable. Peng and Bao already tried to propose simpler and more efficient zero-knowledge proofs for this setting, but we prove in this paper that their protocol is not secure. We fix the problem by designing a new protocol, and formally proving its security properties. We then compare the efficiency of the new zero-knowledge non-membership protocol with that of the protocol, when they are integrated with anonymous authentication systems based on RSA (notably, the IBM product Idemix for anonymous credentials). We discuss for which values of the size k of the blacklist L, one protocol is preferable to the other one, and we propose different ways to combine and implement the two protocols.

Singh Toor Gurbakshish

The evolution of the traditional electricity infrastructure into smart grids promises more reliable and efficient power management, more energy aware consumers and inclusion of renewable sources for power generation. These fruitful promises are attracting initiatives by various nations all over the globe in various fields of academia. However, this evolution relies on the advances in the information technologies and communication technologies and thus is inevitably prone to various risks and threats. Even though many solutions have been proposed in the recent literature to overcome the security threats in smart grid networks, many issues still need to be addressed to make smart grids a reliable and efficient innovation. In this thesis, we first introduce the background, network architecture, security threats and the security requirements of smart grid networks. Our work focuses on the security aspects of Neighborhood Area Network (NAN) subsystems of smart grid. We present some of the prominent threats and attacks, specific to this subsystem, which violate the specific security goals requisite for its reliable operation. The proposed solutions and countermeasures for these security issues presented in the recent literature have been deeply reviewed to identify the promising solutions with respect to the specific security goals. Then we propose an improved VI dynamic key refreshment strategy for mesh security in the NAN and an authentication scheme based on software defined network (SDN) using dynamic one-way accumulators. The proposed dynamic key refreshment scheme can protect the mesh network system based on IEEE 802.11s standard from DoS attacks during the key refreshment whereby the intruder could launch the attack using the information from previous key refreshment cycle as proposed in the original key refreshment scheme. The use of simple hash based operation makes the scheme cost effective for the resource limited network devices. The proposed scheme also adds an enhancement to the sub-protocol of the original key refreshment scheme for enhanced security and reliability. The proposed SDN based authentication scheme employs one-way dynamic accumulators combined with zero-knowledge proofs for easy and cost efficient authentication process. The availability of the cross authentication among different NAN devices enables us to replicate the mesh network architecture. Using SDN as the backbone of the scheme helps us accommodate the advances of the upcoming wireless technologies where we can update the changes in the scheme conveniently. Our analysis shows that the proposed schemes can achieve the requisite authentication while withstanding multiple attacks and the balance between security and system performance is also achieved.

Kibin Lee, Joshua I. James, Tekachew Gobena Ejeta, Hyoung Rae Kim

Cryptocurrency, and its underlying technologies, has been gaining popularity for transaction management beyond financial transactions. Transaction information is maintained in the block-chain, which can be used to audit the integrity of the transaction. The focus on this paper is the potential availability of block-chain technology of other transactional uses. Block-chain is one of the most stable open ledgers that preserves transaction information, and is difficult to forge. Since the information stored in block-chain is not related to personally identify information, it has the characteristics of anonymity. Also, the block-chain allows for transparent transaction verification since all information in the block-chain is open to the public. These characteristics are the same as the requirements for a voting system. That is, strong robustness, anonymity, and transparency. In this paper, we propose an electronic voting system as an application of block-chain, and describe block-chain based voting at a national level through examples.

Judyta Przyłuska-Schmitt

No abstract is available for this record.

Shravanthi, R Pooja, Bhagya Shree J

Bitcoin is a crypto currency with several advantages over approaches. Transactions are confirmed and stored by a peer-to-peer network in a blockchain. Therefore, all transactions are public and soon solutions where designed to increase privacy in Bitcoin. Many come with downsides, like requiring a trusted third-party or requiring modifications to Bitcoin. In this paper, we compare these approaches according to several criteria. Based on survey, coin Join emerges as the best approach for anonymizing Bitcoins today.

Ilaria Chillotti, Nicolas Gama, Mariya Georgieva, Malika Izabachène

No abstract is available for this record.

Tomaso Aste

No abstract is available for this record.

Benoît Libert, San Ling, Khoa Nguyen, Huaxiong Wang

Abstract An accumulator is a function that hashes a set of inputs into a short, constant-size string while preserving the ability to efficiently prove the inclusion of a specific input element in the hashed set. It has proved useful in the design of numerous privacy-enhancing protocols, in order to handle revocation or simply prove set membership. In the lattice setting, currently known instantiations of the primitive are based on Merkle trees, which do not interact well with zero-knowledge proofs. In order to efficiently prove the membership of some element in a zero-knowledge manner, the prover has to demonstrate knowledge of a hash chain without revealing it, which is not known to be efficiently possible under well-studied hardness assumptions. In this paper, we provide an efficient method of proving such statements using involved extensions of Stern’s protocol. Under the Small Integer Solution assumption, we provide zero-knowledge arguments showing possession of a hash chain. As an application, we describe new lattice-based group and ring signatures in the random oracle model. In particular, we obtain: (i) the first lattice-based ring signatures with logarithmic size in the cardinality of the ring and (ii) the first lattice-based group signature that does not require any GPV trapdoor and thus allows for a much more efficient choice of parameters.

Dimaz Ankaa Wijaya, Joseph K. Liu, Ron Steinfeld, Shi-Feng Sun · 5 authors

No abstract is available for this record.

Paolo Tasca, Shaowen Liu

No abstract is available for this record.

Philipp Winter, Roya Ensafi, Karsten Loesing, Nick Feamster

Being a volunteer-run, distributed anonymity network, Tor is vulnerable to Sybil attacks. Little is known about real-world Sybils in the Tor network, and we lack practical tools and methods to expose Sybil attacks. In this work, we develop sybilhunter, the first system for detecting Sybil relays based on their appearance, such as configuration; and behavior, such as uptime sequences. We used sybilhunter's diverse analysis techniques to analyze nine years of archived Tor network data, providing us with new insights into the operation of real-world attackers. Our findings include diverse Sybils, ranging from botnets, to academic research, and relays that hijack Bitcoin transactions. Our work shows that existing Sybil defenses do not apply to Tor, it delivers insights into real-world attacks, and provides practical tools to uncover and characterize Sybils, making the network safer for its users.

Alin Tomescu, Srinivas Devadas

© 2017 IEEE. We present Catena, an efficiently-verifiable Bitcoinwitnessing scheme. Catena enables any number of thin clients, such as mobile phones, to efficiently agree on a log of application-specific statements managed by an adversarial server. Catenaimplements a log as an OP-RETURN transaction chain andprevents forks in the log by leveraging Bitcoin's security againstdouble spends. Specifically, if a log server wants to equivocate ithas to double spend a Bitcoin transaction output. Thus, Catenalogs are as hard to fork as the Bitcoin blockchain: an adversarywithout a large fraction of the network's computational powercannot fork Bitcoin and thus cannot fork a Catena log either. However, different from previous Bitcoin-based work, Catenadecreases the bandwidth requirements of log auditors from 90GB to only tens of megabytes. More precisely, our clients onlyneed to download all Bitcoin block headers (currently less than35 MB) and a small, 600-byte proof for each statement in a block. We implement Catena in Java using the bitcoinj library and use itto extend CONIKS, a recent key transparency scheme, to witnessits public-key directory in the Bitcoin blockchain where it can beefficiently verified by auditors. We show that Catena can securemany systems today, such as public-key directories, Tor directoryservers and software transparency schemes.

Svein Ølnes

The new technology Bitcoin has got a lot of attention since it was presented in late 2008 and implemented early 2009. However, the main attention has been to the currency and not so much the underlying blockchain technology. This paper argues that we need to look beyond the currency and investigate the potential use of the blockchain technology to enable smarter governments by utilizing the secure, distributed, open, and inexpensive database technology. The technology is discussed in the perspective of an information infrastructure to investigate its full potential. After a literature review of Bitcoin publications, with a special emphasis on eGovernment literature, the paper presents a relevant use case highlighting the innovation potential of the new technology. The literature review shows that Bitcoin is absent from the e-Government literature. The use case presented shows that Bitcoin could be a promising technology for validating many types of persistent documents in public sector.

Benny Applebaum, Pavel Raykov

No abstract is available for this record.

QingChun ShenTu, Jianping Yu

The Bitcoin system is an anonymous, decentralized crypto-currency. There are some deanonymizating techniques to cluster Bitcoin addresses and to map them to users' identifications in the two research directions of Analysis of Transaction Chain (ATC) and Analysis of Bitcoin Protocol and Network (ABPN). Nowadays, there are also some anonymization methods such as coin-mixing and transaction remote release (TRR) to cover the relationship between Bitcoin address and the user. This paper studies anonymization and de-anonymization technologies and proposes some directions for further research.

QingChun ShenTu, Jianping Yu

To strengthen the anonymity of Bitcoin, several centralized coin-mixing providers (mixers) such as BitcoinFog.com, BitLaundry.com, and Blockchain.info assist users to mix Bitcoins through CoinJoin transactions with multiple inputs and multiple outputs to uncover the relationship between them. However, these mixers know the output address of each user, such that they cannot provide true anonymity. This paper proposes a centralized coin-mixing algorithm based on an elliptic curve blind signature scheme (denoted as Blind-Mixing) that obstructs mixers from linking an input address with an output address. Comparisons among three blind signature based algorithms, Blind-Mixing, BlindCoin, and RSA Coin-Mixing, are conducted. It is determined that BlindCoin may be deanonymized because of its use of a public log. In RSA Coin-Mixing, a user's Bitcoins may be falsely claimed by another. In addition, the blind signature scheme of Blind-Mixing executes 10.5 times faster than that of RSA Coin-Mixing.

QingChun ShenTu, Jianping Yu

The anonymity of the Bitcoin system has some shortcomings. Analysis of Transaction Chain (ATC) and Analysis of Bitcoin Protocol and Network (ABPN) are two important methods of deanonymizing bitcoin transactions. Nowadays, there are some anonymization methods to combat ATC but there has been little research into ways to counter ABPN. This paper proposes a new anonymization technology called Transaction Remote Release (TRR). Inspired by The Onion Router (TOR), TRR is able to render several typical attacking methods of ABPN ineffective. Furthermore, the performance of encryption and decryption of TRR is good and the growth rate of the cipher is very limited. Hence, TRR is suited for practical applications.

Edward Lui

Enormous amounts of data are collected by hospitals, social networking systems, government agencies, and other organizations. There are huge social benefits in analyzing this data, but we must protect the privacy of the individuals in the data. The current standard definition of data privacy is differential privacy [22, 19]. In this thesis, we introduce new definitions of data privacy that can be better than differential privacy in certain ways. We first argue that differential privacy might not be strong enough in social network settings. We then introduce a zero-knowledge based definition of privacy called zero-knowledge privacy, which is strictly stronger than differential privacy and is particularly attractive when modeling privacy in social networks. Both differential privacy and zero-knowledge privacy provide strong privacy guarantees. However, for certain tasks, mechanisms satisfying these privacy definitions have to add a lot of "noise", thus lowering the utility of the released data. Thus, we introduce a new definition of privacy called crowd-blending privacy that strictly relaxes the notion of differential privacy. We demonstrate crowd-blending private mechanisms for histograms and for releasing synthetic data points, achieving strictly better utility than what is possible using differentially private mechanisms. Differential privacy guarantees the same level of privacy protection for all individuals. However, we demonstrate that some individuals may need more privacy than others. Thus, we introduce a generalization of differential privacy called tai- lored differential privacy, where an individual's privacy parameter is "tailored" for the individual based on the individual's data and the data set. We focus on a natural instance of tailored differential privacy, which we call outlier privacy: an individual's privacy parameter is determined by how much of an "outlier " the individual is. In this thesis, we also study the problem of strategy-proof voting, which is plagued by impossibility results. We take a bounded-rationality approach to this problem and consider a setting where voters have "coarse" beliefs (a notion that has gained popularity in the behavioral economics literature). In particular, we construct good voting rules that satisfy a notion of strategy-proofness with respect to coarse i.i.d. beliefs, thus circumventing the existing impossibility results.

Gurchetan S. Grewal, Mark Ryan, Liqun Chen, Michael R. Clarkson

Du-Vote is a new remote electronic voting protocol that eliminates the often-required assumption that voters trust general-purpose computers. Trust is distributed in Du-Vote between a simple hardware token issued to the voter, the voter's computer, and a server run by election authorities. Verifiability is guaranteed with high probability even if all these machines are controlled by the adversary, and privacy is guaranteed as long as at least either the voter's computer, or the server and the hardware token, are not controlled by the adversary. The design of the Du-Vote protocol is presented in this paper. A new non-interactive zero-knowledge proof is employed to verify the server's computations. Du-Vote is a step towards tackling the problem of internet voting on user machines that are likely to have malware. We anticipate that the methods of Du-Vote can be used in other applications to find ways of achieving malware tolerance, that is, ways of securely using platforms that are known or suspected to have malware.

Ivan Pustogarov

This thesis is devoted to low-resource off-path deanonymisation techniques for two popular systems, Tor and Bitcoin. Tor is a software and an anonymity network which in order to confuse an observer encrypts and re-routes traffic over random pathways through several relays before it reaches the destination. Bitcoin is a distributed payment system in which payers and payees can hide their identities behind pseudonyms (public keys) of their choice. The estimated number of daily Tor users is 2,000,000 which makes it arguable the most used anonymity network. Bitcoin is the most popular cryptocurrency with market capitalization about 3.5 billion USD. In the first part of the thesis we study the Tor network. At the beginning we show how to remotely find out which Tor relays are connected. This effectively allows for an attacker to reduce Tor users' anonymity by ruling out impossible paths in the network. Later we analyze the security of Tor Hidden Services. We look at them from different attack perspectives and provide a systematic picture of what information can be obtained with very inexpensive means. We expose flaws both in the design and implementation of Tor Hidden Services that allow an attacker to measure the popularity of arbitrary hidden services, efficiently collect hidden service descriptors (and thus get a global picture of all hidden services in Tor), take down hidden services and deanonymize hidden services. In the second part we study Bitcoin anonymity. We describe a generic method to deanonymize a significant fraction of Bitcoin users and correlate their pseudonyms with their public IP addresses. We discover that using Bitcoin through Tor not only provides limited level of anonymity but also exposes the user to man-in-the middle attacks in which an attacker controls which Bitcoin blocks and transactions the user is aware of. We show how to fingerprint Bitcoin users by setting an "address cookie" on their computers. This can be used to correlate the same user across different sessions, even if he uses Tor, hidden-services or multiple proxies. Finally, we describe a new anonymous decentralized micropayments scheme in which clients do not pay services with electronic cash directly but submit proof of work shares which the services can resubmit to a crypto-currency mining pool. Services credit users with tickets that can later be used to purchases enhanced services.

Joseph Bonneau, Andrew Miller, Jeremy Clark, Arvind Narayanan · 6 authors

Bit coin has emerged as the most successful cryptographic currency in history. Within two years of its quiet launch in 2009, Bit coin grew to comprise billions of dollars of economic value despite only cursory analysis of the system's design. Since then a growing literature has identified hidden-but-important properties of the system, discovered attacks, proposed promising alternatives, and singled out difficult future challenges. Meanwhile a large and vibrant open-source community has proposed and deployed numerous modifications and extensions. We provide the first systematic exposition Bit coin and the many related crypto currencies or 'altcoins.' Drawing from a scattered body of knowledge, we identify three key components of Bit coin's design that can be decoupled. This enables a more insightful analysis of Bit coin's properties and future stability. We map the design space for numerous proposed modifications, providing comparative analyses for alternative consensus mechanisms, currency allocation mechanisms, computational puzzles, and key management tools. We survey anonymity issues in Bit coin and provide an evaluation framework for analyzing a variety of privacy-enhancing proposals. Finally we provide new insights on what we term disinter mediation protocols, which absolve the need for trusted intermediaries in an interesting set of applications. We identify three general disinter mediation strategies and provide a detailed comparison.

Béla Gipp, Norman Meuschke, André Gernandt

Trusted timestamping is a process for proving that certain information existed at a given point in time. This paper presents a trusted timestamping concept and its implementation in form of a web-based service that uses the decentralized Bitcoin block chain to store anonymous, tamper-proof timestamps for digital content. The service allows users to hash files, such as text, photos or videos, and store the created hashes in the Bitcoin block chain. Users can then retrieve and verify the timestamps that have been committed to the block chain. The non-commercial service enables anyone, e.g., researchers, authors, journalists, students, or artists, to prove that they were in possession of certain information at a given point in time. Common use cases include proving that a contract has been signed, a photo taken, a video recorded, or a task completed prior to a certain date. All procedures maintain complete privacy of the user's data.

Michael Fleder, Michael S. Kester, Sudeep Pillai

Bitcoins have recently become an increasingly popular cryptocurrency through which users trade electronically and more anonymously than via traditional electronic transfers. Bitcoin's design keeps all transactions in a public ledger. The sender and receiver for each transaction are identified only by cryptographic public-key ids. This leads to a common misconception that it inherently provides anonymous use. While Bitcoin's presumed anonymity offers new avenues for commerce, several recent studies raise user-privacy concerns. We explore the level of anonymity in the Bitcoin system. Our approach is two-fold: (i) We annotate the public transaction graph by linking bitcoin public keys to "real" people - either definitively or statistically. (ii) We run the annotated graph through our graph-analysis framework to find and summarize activity of both known and unknown users.