190 results found
Gudgeon L, Werner S, Perez Hernandez D, et al., 2020, DeFi protocols for loanable funds: interest rates, liquidity and market efficiency, 2nd ACM Conference on Advances in Financial Technologies (AFT 2020), Publisher: ACM
We coin the term Protocols for Loanable Funds (PLFs)to refer to pro-tocols which establish distributed ledger-based markets for loanable funds. PLFs are emerging as one of the main applications within De-centralized Finance (DeFi), and use smart contract code to facilitate the intermediation of loanable funds. In doing so, these protocols allow agents to borrow and save programmatically. Within these protocols, interest rate mechanisms seek to equilibrate the supply and demand for funds. In this paper, we review the methodologies used to set interest rates on three prominent DeFi PLFs, namely Compound, Aave and dYdX. We provide an empirical examination of how these interest rate rules have behaved since their inception in response to differing degrees of liquidity. We then investigate the market efficiency and inter-connectedness between multiple protocols, examining first whether Uncovered Interest Parity holds within a particular protocol and second whether the interest rates for a particular token market show dependence across protocols,developing a Vector Error Correction Model for the dynamics.
Marchenko Y, Knottenbelt WJ, Wolter K, 2020, EthExplorer: A Tool for Forensic Analysis of the Ethereum Blockchain, Pages: 100-117, ISSN: 0302-9743
© 2020, Springer Nature Switzerland AG. This paper presents EthExplorer, a graph-based tool for analysing the Ethereum blockchain. EthExplorer has been designed for the assessment of Ethereum transactions, which represent diverse and complex activities in a large-scale distributed system. EthExplorer shows Ethereum addresses as nodes and transactions as directed arcs between addresses. The graph is annotated in several ways: arcs are scaled according to the amount of Ether they carry and the nodes are colour encoded to indicate types of addresses, such as exchanges, miners or mining pools. Ether transfer transactions and smart contracts are distinguished by line styles. EthExplorer can be used to trace the flow of Ether between addresses. For a given address all its output or input transactions with the corresponding receiver or sender addresses can be found. The set of considered addresses can be increased by adding selected addresses to the set of analysed addresses.
Werner SM, Ilie DI, Stewart I, et al., 2020, Unstable Throughput: When the Difficulty Algorithm Breaks., CoRR, Vol: abs/2006.03044
Gudgeon L, Werner SM, Pérez D, et al., 2020, DeFi Protocols for Loanable Funds: Interest Rates, Liquidity and Market Efficiency., CoRR, Vol: abs/2006.13922
Ilie DI, Knottenbelt WJ, Stewart I, 2020, Committing to quantum resistance, better: a speed - and - risk - configurable defence for bitcoin against a fast quantum computing attack., Publisher: Cryptology ePrint Archive
In light of the emerging threat of powerful quantum computers appearing in the near future, we investigate the potential attacks onBitcoin available to a quantum-capable adversary. In particular, we illustrate how Shor’s quantum algorithm can be used to forge ECDSA basedsignatures, allowing attackers to hijack transactions. We then proposea simple commit–delay–reveal protocol, which allows users to securelymove their funds from non-quantum-resistant outputs to those adheringto a quantum-resistant digital signature scheme. In a previous paper we presented a similar scheme with a long fixed delay. Here we improveon our previous work, by allowing each user to choose their preferreddelay – long for a low risk of attack, or short if a higher risk is acceptableto that user. As before, our scheme requires modifications to the Bitcoinprotocol, but once again these can be implemented as a soft fork.
Ilie DI, Karantias K, Knottenbelt WJ, 2020, Bitcoin Crypto - Bounties for Quantum Capable Adversaries., IACR Cryptol. ePrint Arch., Vol: 2020, Pages: 186-186
Zamyatin A, Avarikioti Z, Perez D, et al., 2020, TxChain: efficient cyptocurrency light clients via contingent transaction aggregation., Publisher: Cryptology ePrint Archive
Cryptocurrency light- or simplified payment verification (SPV) clientsallow nodes with limited resources to efficiently verify execution of payments.Instead of downloading the entire blockchain, only block headers and selectedtransactions are stored. Still, the storage and bandwidth cost, linear in blockchainsize, remain non-negligible, especially for smart contracts and mobile devices: asof April 2020, these amount to 50 MB in Bitcoin and 5 GB in Ethereum.Recently, two improved sublinear light clients were proposed: to validate theblockchain, NIPoPoWs and FlyClient only download a polylogarithmic numberof block headers, sampled at random. The actual verification of payments, however, remains costly: for each verified transaction, the corresponding block musttoo be downloaded. This yields NIPoPoWs and FlyClient only effective underlow transaction volumes.We present TXCHAIN, a novel mechanism to maintain efficiency of light clientseven under high transaction volumes. Specifically, we introduce the concept ofcontingent transaction aggregation, where proving inclusion of a single contingent transaction implicitly proves that n other transactions exist in the blockchain.To verify n payments, TXCHAIN requires a only single transaction in the best(n ≤ c), and dnc + logc(n)e transactions in the worst case (n > c). We deployTXCHAIN on Bitcoin without consensus changes and implement a soft fork forEthereum. To demonstrate effectiveness in the cross-chain setting, we implementTXCHAIN as a smart contract on Ethereum to efficiently verify Bitcoin payments.
Zamyatin A, Al-Bassam M, Zindros D, et al., 2019, SoK: communication across distributed ledgers., Publisher: Cryptology ePrint Archive
Communication across distributed systems, each running its own consensus, is a problem previously studied under the assumption of trust across systems. With the appearance of distributed ledgers or blockchains, numerous protocols have emerged, which attempt to achieve trustless communication between distrusting ledgers and participants. Cross-chain communication thereby plays a fundamental role in cryptocurrency exchanges, sharding, bootstrapping and extension of distributed ledgers. Unfortunately, existing proposals are designed ad-hoc for specific use-cases, making it hard to gain confidence on their correctness and to use them as building blocks for new systems.
Harz D, Gudgeon L, Gervais A, et al., 2019, Balance: dynamic adjustment of cryptocurrency deposits, 2019 ACM SIGSAC Conference on Computer & Communications Security (CCS '19), Publisher: ACM
In cryptoeconomic protocols, nancial deposits are fundamental totheir security. Protocol designers and their agents face a trade-owhen choosing the deposit size. While substantial deposits might in-crease the protocol security, for example by minimising the impactof adversarial behaviour or risks of currency uctuations, locked-up capital incurs opportunity costs for agents. Moreover, someprotocols require over-collateralization in anticipation of futureevents and malicious intentions of agents. We presentBalance,an application-agnostic system that reduces over-collateralizationwithout compromising protocol security. InBalance, maliciousagents receive no additional utility for cheating once their depositsare reduced. At the same time, honest and rational agents increasetheir utilities for behaving honestly as their opportunity costs forthe locked-up deposits are reduced.Balanceis a round-basedmechanism in which agents need tocontinuouslyperform desiredactions. Rather than treating agents’ incentives and behaviour asancillary, we explicitly model agents’ utility, proving the conditionsfor incentive compatibility.Balanceimproves social welfare givena distribution of honest, rational, and malicious agents. Further,we integrateBalancewith a cross-chain interoperability protocol,XCLAIM, reducing deposits by 10% while maintaining the sameutility for behaving honestly. Our implementation allows any num-ber of agents to be maintained for at most 55,287 gas (≈USD 0.07)to update the agents’ scores, and at a cost of 54,948 gas (≈USD0.07) to update the assignment of agents to layers.
Harz D, Gudgeon L, Gervais A, et al., 2019, Balance: dynamic adjustment of cryptocurrency deposits., Publisher: Cryptology ePrint Archive
Financial deposits are fundamental to the security of cryptoeconomic protocols as they serve as insurance against potential misbehaviour of agents. However, protocol designers and their agents face a trade-off when choosing the deposit size. While substantial deposits might increase the protocol security, for example by minimising the impact of adversarial behaviour or risks of currency fluctuations, locked-up capital incurs opportunity costs. Moreover, some protocols require over-collateralization in anticipation of future events and malicious intentions of agents. We present Balance, an application-agnostic system that reduces over-collateralization without compromising protocol security. In Balance, malicious agents receive no additional utility for cheating once their deposits are reduced. At the same time, honest and rational agents increase their utilities for behaving honestly as their opportunity costs for the locked-up deposits are reduced. Balance is a round-based mechanism in which agents need to continuously perform desired actions. Rather than treating agents' incentives and behaviour as ancillary, we explicitly model agents' utility, proving the conditions for incentive compatibility. Balance improves social welfare given a distribution of honest, rational, and malicious agents. Further, we integrate Balance with a cross-chain interoperability protocol, XCLAIM, reducing deposits by 10% while maintaining the same utility for behaving honestly. Our implementation allows any number of agents to be maintained for at most 55,287 gas (ca. USD 0.07) to update all agents' scores, and at a cost of 54,948 gas (ca. USD 0.07) to update the assignment of all agents to layers.
Wu H, Knottenbelt W, Wolter K, 2019, An efficient application partitioning algorithm in mobile environments, IEEE Transactions on Parallel and Distributed Systems, Vol: 30, Pages: 1464-1480, ISSN: 1045-9219
Application partitioning that splits the executions into local and remote parts, plays a critical role in high-performance mobile offloading systems. Mobile devices can obtain the most benefit from Mobile Cloud Computing (MCC) or Mobile Edge Computing (MEC) through optimal partitioning. Due to unstable resources at the wireless network (network disconnection, bandwidth fluctuation, network latency, etc.) and at the service nodes (different speeds of mobile devices and cloud/edge servers, memory, etc.), static partitioning solutions with fixed bandwidth and speed assumptions are unsuitable for offloading systems. In this paper, we study how to dynamically partition a given application into local and remote parts effectively, while keeping the total cost as small as possible. For general tasks (i.e., arbitrary topological consumption graphs), we propose a Min-Cost Offloading Partitioning (MCOP) algorithm that aims at finding the optimal partitioning plan (determine which portions of the application to run on mobile devices and which portions on cloud/edge servers) under different cost models and mobile environments. Simulation results show that the MCOP algorithm provides a stable method with low time complexity which significantly reduces execution time and energy consumption by optimally distributing tasks between mobile devices and servers, besides it well adapts to mobile environmental changes.
Koutsouri A, Poli F, Alfieri E, et al., 2019, Balancing cryptoassets and gold: a weighted-risk-contribution index for the alternative asset space, 1st International Conference on Mathematical Research for Blockchain Economy, Publisher: Springer Verlag, ISSN: 0302-9743
Bitcoin is foremost amongst the emerging asset class knownas cryptoassets. Two noteworthy characteristics of the returns of non-stablecoin cryptoassets are their high volatility, which brings with it ahigh level of risk, and their high intraclass correlation, which limits thebenefits that can be had by diversifying across multiple cryptoassets. Yetcryptoassets exhibit no correlation with gold, a highly-liquid yet scarceasset which has proved to function as a safe haven during crises affectingtraditional financial systems. As exemplified by Shannon’s Demon, a lackof correlation between assets opens the door to principled risk controlthrough so-called volatility harvesting involving periodic rebalancing.In this paper we propose an index which combines a basket of five cryp-toassets with an investment in gold in a way that aims to improve therisk profile of the resulting portfolio while preserving its independencefrom mainstream financial asset classes such as stocks, bonds and fiatcurrencies. We generalise the theory of Equal Risk Contribution to allowfor weighting according to a desired level of contribution to volatility. Wefind a crypto–gold weighting based on Weighted Risk Contribution to behistorically more effective in terms of Sharpe Ratio than several alterna-tive asset allocation strategies including Shannon’s Demon. Within thecrypto-basket, whose constituents are selected and rebalanced monthly,we find an Equal Weighting scheme to be more effective in terms of thesame metric than a market capitalisation weighting.
Werner S, Pritz P, Zamyatin A, et al., 2019, Uncle traps: harvesting rewards in a queue-based ethereum Mining Pool, 12th EAI International Conference on Performance Evaluation Methodologies and Tools, Publisher: ACM, Pages: 127-134
Mining pools in Proof-of-Work cryptocurrencies allow miners topool their computational resources as a means of reducing payoutvariance. In Ethereum,uncle blocksare valid Proof-of-Work solu-tions which do not become the head of the blockchain, yet yieldrewards if later referenced by main chain blocks. Mining pool opera-tors are faced with the non-trivial task of fairly distributing rewardsfor both block types among pool participants.Inspired by empirical observations, we formally reconstruct aSybil attack exploiting the uncle block distribution policy in a queue-based mining pool. To ensure fairness of the queue-based payoutscheme, we propose a mitigation. We examine the effectiveness ofthe attack strategy under the current and the proposed policy via adiscrete-event simulation. Our findings show that the observed attackcan indeed be obviated by altering the current reward scheme.
Seakhoa-King S, Balaji P, Alvarez NT, et al., 2019, Revenue-Driven Scheduling in Drone Delivery Networks with Time-sensitive Service Level Agreements, 12th EAI International Conference on Performance Evaluation Methodologies and Tools (VALUETOOLS), Publisher: ASSOC COMPUTING MACHINERY, Pages: 183-186
Werner SM, Pritz PJ, Zamyatin A, et al., 2019, Uncle Traps: Harvesting Rewards in a Queue-based Ethereum Mining Pool., IACR Cryptol. ePrint Arch., Vol: 2019, Pages: 70-70
Zamyatin A, Harz D, Lind J, et al., 2018, XCLAIM: decentralized, interoperable, cryptocurrency-backed assets, 40th IEEE Symposium on Security and Privacy (IEEE S&P 2019), Publisher: IEEE
Building trustless cross-blockchain trading protocols is challenging. Centralized exchanges thus remain the preferred route to execute transfers across blockchains. However, these services require trust and therefore undermine the very nature of the blockchains on which they operate. To overcome this,several decentralized exchanges have recently emerged which offer support for atomic cross-chain swaps (ACCS). ACCS enable the trustless exchange of cryptocurrencies across blockchains,and are the only known mechanism to do so. However, ACCS suffer significant limitations; they are slow, inefficient and costly,meaning that they are rarely used in practice.We present XCLAIM: the first generic framework for achieving trustless and efficient cross-chain exchanges using cryptocurrency-backed assets(CBAs). XCLAIM offers protocols for issuing,transferring, swapping and redeeming CBAs securely in anon-interactive manner on existing blockchains. We instanti-ate XCLAIM between Bitcoin and Ethereum and evaluate our implementation; it costs less than USD 0.50 to issue an arbi-trary amount of Bitcoin-backed tokens on Ethereum. We show XCLAIMis not only faster, but also significantly cheaper than atomic cross-chain swaps. Finally, XCLAIMis compatible with the majority of existing blockchains without modification, and enables several novel cryptocurrency applications, such as cross-chain payment channels and efficient multi-party swaps
Stewart I, Illie D, Zamyatin A, et al., 2018, Committing to Quantum Resistance: A Slow Defence for Bitcoin against a Fast Quantum Computing Attack, Royal Society Open Science, ISSN: 2054-5703
Quantum computers are expected to have a dramatic impact on numerous fields, due to their anticipated ability to solve classes of mathematical problems much more efficiently than their classical counterparts. This particularly applies to domains involving integer factorisation and discrete logarithms, such as public key cryptography. In this paper we consider the threats a quantum-capable adversary could impose on Bitcoin, which currently uses the Elliptic Curve Digital Signature Algorithm (ECDSA) to sign transactions. We then propose a simple but slow commit-delay-reveal protocol, which allows users to securely move their funds from old (non-quantum-resistant) outputs to those adhering to a quantum-resistant digital signature scheme. The transition protocol functions even if ECDSA has already been compromised. While our scheme requires modifications to the Bitcoin protocol, these can be implemented as a soft fork.
Zamyatin A, Stifter N, Schindler P, et al., 2018, Flux: revisiting near blocks for proof-of-work blockchains, Cryptology ePrint Archive: Report 2018/415
The term near or weak blocks describes Bitcoin blocks whose PoW does not meet the required target difficulty to be considered valid under the regular consensus rules of the protocol. Near blocks are generally associated with protocol improvement proposals striving towards shorter transaction confirmation times. Existing proposals assume miners will act rationally based solely on intrinsic incentives arising from the adoption of these changes, such as earlier detection of blockchain forks.In this paper we present Flux, a protocol extension for proof-of-work blockchains that leverages on near blocks, a new block reward distribution mechanism, and an improved branch selection policy to incentivize honest participation of miners. Our protocol reduces mining variance, improves the responsiveness of the underlying blockchain in terms of transaction processing, and can be deployed without conflicting modifications to the underlying base protocol as a velvet fork. We perform an initial analysis of selfish mining which suggests Flux not only provides security guarantees similar to pure Nakamoto consensus, but potentially renders selfish mining strategies less profitable.
Wolter K, Knottenbelt W, 2018, Companion of the 2018 ACM/SPEC International Conference on Performance Engineering, ICPE 2018, Berlin, Germany, April 09-13, 2018, 2018 ACM/SPEC International Conference on Performance Engineering, ICPE, Publisher: ACM
Wolter K, Knottenbelt W, 2018, Proceedings of the 2018 ACM/SPEC International Conference on Performance Engineering, ICPE 2018, Berlin, Germany, April 09-13, 2018, 2018 ACM/SPEC International Conference on Performance Engineering, ICPE, Publisher: ACM
Zamyatin A, Stifter N, Judmayer A, et al., 2019, (Short Paper) A Wild Velvet Fork Appears! Inclusive Blockchain Protocol Changes in Practice, 5th Workshop on Bitcoin and Blockchain Research at Financial Cryptography and Data Security 2018
The loosely defined terms hard fork and soft fork have establishedthemselves as descriptors of different classes of upgrade mechanisms for the underlying consensus rules of (proof-of-work) blockchains. Recently, a novel approach termed velvet fork, which expands upon the concept of a soft fork, was outlined. Specifically, velvet forks intend to avoid the possibility of disagreement by a change of rules through rendering modifications to the protocol backward compatible and inclusive to legacy blocks.We present an overview and definitions of these different upgrade mechanisms and outline their relationships. Hereby, we expose examples where velvet forks or similar constructions are already actively employed in Bitcoin and other cryptocurrencies. Furthermore, we expand upon the concept of velvet forks by proposing possible applications and discuss potentially arising security implications.
Harz D, Knottenbelt W, 2018, Towards Safer Smart Contracts: A Survey of Languages and Verification Methods
With a market capitalisation of over USD 205 billion in just under ten years,public distributed ledgers have experienced significant adoption. Apart fromnovel consensus mechanisms, their success is also accountable to smartcontracts. These programs allow distrusting parties to enter agreements thatare executed autonomously. However, implementation issues in smart contractscaused severe losses to the users of such contracts. Significant efforts aretaken to improve their security by introducing new programming languages andadvance verification methods. We provide a survey of those efforts in twoparts. First, we introduce several smart contract languages focussing onsecurity features. To that end, we present an overview concerning paradigm,type, instruction set, semantics, and metering. Second, we examine verificationtools and methods for smart contract and distributed ledgers. Accordingly, weintroduce their verification approach, level of automation, coverage, andsupported languages. Last, we present future research directions includingformal semantics, verified compilers, and automated verification.
Zamyatin A, Harz D, Knottenbelt WJ, 2018, Issue, Trade, Redeem: Crossing Systems Bounds with Cryptocurrency-Backed Tokens., IACR Cryptology ePrint Archive, Vol: 2018, Pages: 643-643
Zamyatin A, Stifter N, Judmayer A, et al., 2018, A Wild Velvet Fork Appears! Inclusive Blockchain Protocol Changes in Practice - (Short Paper)., Publisher: Springer, Pages: 31-42
Pesu T, Kettunen J, Knottenbelt WJ, et al., 2017, Three-way optimisation of response time, subtask dispersion and energy consumption in split-merge systems, Pages: 244-251
© 2017 ACM. This paper investigates various ways in which the triple trade-off metrics between task response time, subtask dispersion and energy can be improved in split-merge queueing systems. Four ideas, namely dynamic subtask dispersion reduction, state-dependent service times, multiple redundant subtask service servers and restarting subtask service, are examined in the paper. It transpires that all four techniques can be used to improve the triple trade-off, while combinations of the techniques are not necessarily beneficial.
Zamyatin A, Wolter K, Werner S, et al., 2017, Swimming with fishes and sharks: beneath the surface of queue-based ethereum mining pools, 25th Annual Meeting of the IEEE International Symposium on Modelling, Analysis and Simulation of Computer and Telecommunication Systems, Publisher: IEEE
Cryptocurrency mining can be said to be the modernalchemy, involving as it does the transmutation of electricityinto digital gold. The goal of mining is to guess the solutionto a cryptographic puzzle, the difficulty of which is determinedby the network, and thence to win the block reward andtransaction fees. Because the return on solo mining has a veryhigh variance, miners band together to create so-called miningpools. These aggregate the power of several individual miners,and, by distributing the accumulated rewards according to somescheme, ensure a more predictable return for participants.In this paper we formulate a model of the dynamics of a queue-based reward distribution scheme in a popular Ethereum miningpool and develop a corresponding simulation. We show that theunderlying mechanism disadvantages miners with above-averagehash rates. We then consider two-miner scenarios and show howlarge miners may perform attacks to increase their profits at theexpense of other participants of the mining pool. The outcomes ofour analysis show the queue-based reward scheme is vulnerableto manipulation in its current implementation.
Mora SV, Knottenbelt WJ, 2017, Deep Learning for Domain-Specific Action Recognition in Tennis, 30th IEEE/CVF Conference on Computer Vision and Pattern Recognition Workshops (CVPRW), Publisher: IEEE, Pages: 170-178, ISSN: 2160-7508
Pesu T, Knottenbelt WJ, 2017, Optimising hidden stochastic PERT networks, 10th EAI International Conference on Performance Evaluation Methodologies and Tools, Publisher: EAI, Pages: 133-136
This paper introduces a technique for minimising subtask dispersion in hidden stochastic PERT networks. The technique improves on existing research in two ways. Firstly, it enables subtask dispersion reduction in DAG structures, whereas previous techniques have only been applicable to single-layer split-merge or fork-join systems. Secondly, the exact distributions of subtask processing times do not need to be known, so long as there is some means of generating samples. The technique is further extended to use a metric which trades off subtask dispersion and task response time.
Haughian G, Osman R, Knottenbelt WJ, 2016, Benchmarking replication in cassandra and MongoDB NoSQL datastores, 27th International Conference, DEXA 2016, Publisher: Springer, Pages: 152-166, ISSN: 0302-9743
The proliferation in Web 2.0 applications has increased the volume, velocity, and variety of data sources which have exceeded the limitations and expected use cases of traditional relational DBMSs. Cloud serving NoSQL data stores address these concerns and provide replication mechanisms to ensure fault tolerance, high availability, and improved scalability. In this paper, we empirically explore the impact of replication on the performance of Cassandra and MongoDB NoSQL datastores. We evaluate the impact of replication in comparison to non-replicated clusters of equal size hosted on a private cloud environment. Our benchmarking experiments are conducted for read and write heavy workloads subject to different access distributions and tunable consistency levels. Our results demonstrate that replication must be taken into consideration in empirical and modelling studies in order to achieve an accurate evaluation of the performance of these datastores.
This work presents a systemic top-down visualization of Bitcoin transaction activity to explore dynamically generated patterns of algorithmic behavior. Bitcoin dominates the cryptocurrency markets and presents researchers with a rich source of real-time transactional data. The pseudonymous yet public nature of the data presents opportunities for the discovery of human and algorithmic behavioral patterns of interest to many parties such as financial regulators, protocol designers, and security analysts. However, retaining visual fidelity to the underlying data to retain a fuller understanding of activity within the network remains challenging, particularly in real time. We expose an effective force-directed graph visualization employed in our large-scale data observation facility to accelerate this data exploration and derive useful insight among domain experts and the general public alike. The high-fidelity visualizations demonstrated in this article allowed for collaborative discovery of unexpected high frequency transaction patterns, including automated laundering operations, and the evolution of multiple distinct algorithmic denial of service attacks on the Bitcoin network.
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