Imperial College London

Alexei Zamyatin

Faculty of EngineeringDepartment of Computing

Research Postgraduate



alexei.zamyatin17 Website




358ACE ExtensionSouth Kensington Campus





Publication Type

9 results found

Zamyatin A, Harz D, Lind J, Panayiotou P, Gervais A, Knottenbelt Wet al., 2019, XCLAIM: trustless, interoperable, cryptocurrency-backed assets, 40th IEEE Symposium on Security and Privacy (IEEE S&P 2019), Publisher: IEEE, Pages: 193-210, ISSN: 2375-1207

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

Conference paper

Werner S, Pritz P, Zamyatin A, Knottenbelt Wet 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.

Conference paper

Zamyatin A, 2018, Poster Abstract: Multisignatures for cryptocurrency-backed tokens, 2nd International Workshop on Cryptocurrencies and Blockchain Technology - CBT'18

Conference paper

Stewart I, Illie D, Zamyatin A, Werner S, Torshizi M, Knottenbelt Wet 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.

Journal article

Zamyatin A, Stifter N, Schindler P, Weippl E, Knottenbelt Wet 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.

Journal article

Stifter N, Judmayer A, Schindler P, Zamyatin A, Weippl Eet al., 2018, Agreement with Satoshi – on the formalization of Nakamoto consensus

The term Nakamoto consensus is generally used to refer to Bitcoin’s novel consensus mechanism, by which agreement on its underlying transaction ledger is reached. It is argued that this agreement protocol represents the core innovation behind Bitcoin, because it promises to facilitate the decentralization of trusted third parties. Specifically, Nakamoto consensus seeks to enable mutually distrusting entities with weak pseudonymous identities to reach eventual agreement while the set of participants may change over time. When the Bitcoin white paper was published in late 2008, it lacked a formal analysis of the protocol and the guarantees it claimed to provide. Itwould take the scientific community several years before first steps towards such a formalization of the Bitcoin protocol and Nakamoto consensus were presented. However, since then the number of works addressing this topic has grown substantially, providing many new and valuable insights. Herein, we present a coherent picture of advancements towards the formalization of Nakamoto consensus, as well as a contextualization in respect to previous research on the agreement problem and fault tolerant distributed computing. Thereby, we outline how Bitcoin’s consensus mechanism sets itself apart from previous approaches and where it can provide new impulses and directions to the scientific community. Understanding the core properties and characteristics of Nakamoto consensus is of key importance, not only for assessing the security and reliability of variousblockchain systems that are based on the fundamentals of this scheme, but also for designing future systems that aim to fulfill comparable goals.

Working paper

Zamyatin A, Wolter K, Werner S, Mulligan CEA, Harrison PG, Knottenbelt WJet 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.

Conference paper

Judmayer A, Zamyatin A, Stifter N, Voyiatzis AG, Weippl Eet al., 2017, Merged Mining: Curse or Cure?, 1st International Workshop on Cryptocurrencies and Blockchain Technology (CBT'17), Pages: 316-316

Merged mining refers to the concept of mining more than one cryptocurrency without necessitating additional proof-of-work effort. Although merged mining has been adopted by a number of cryptocurrencies already, to this date little is known about the effects and implications. We shed light on this topic area by performing a comprehensive analysis of merged mining in practice. As part of this analysis, we present a block attribution scheme for mining pools to assist in the evaluation of mining centralization. Our findings disclose that mining pools in merge-mined cryptocurrencies have operated at the edge of, and even beyond, the security guarantees offered by the underlying Nakamoto consensus for extended periods. We discuss the implications and security considerations for these cryptocurrencies and the mining ecosystem as a whole, and link our findings to the intended effects of merged mining.

Conference paper

Judmayer A, Zamyatin A, Stifter N, Weippl Eet al., 2017, Bitcoin - Cryptocurrencies and Alternative Applications, ERCIM News, Vol: 110, Pages: 10-11, ISSN: 0926-4981

Journal article

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