In this paper, we identify a new form of attack, called the Balance attack, against proof-of-work blockchain systems. The novelty of this attack consists of delaying network communications between multiple subgroups of nodes with balanced mining power. Our theoretical analysis captures the precise tradeoff between the network delay and the mining power of the attacker needed to double spend in Ethereum with high probability. We quantify our probabilistic analysis with statistics taken from the R3 consortium, and show that a single machine needs 20 minutes to attack the consortium. Finally, we run an Ethereum private chain in a distributed system with similar settings as R3 to demonstrate the feasibility of the approach, and discuss the application of the Balance attack to Bitcoin. Our results clearly confirm that main proof-of-work blockchain protocols can be badly suited for consortium blockchains.
Ethereum proposed the Ghost protocol to cope with this issue @cite_37 . The idea is simply to account for the blocks proposed by correct miners in the multiple branches of the correct blockchain to select the main branch. As a result, growing a branch the fastest is not sufficient for an attacker of Ethereum to be able to double spend. Even though the propagation strategy of Ethereum differs from the pull strategy of Bitcoin, some network attacks against Bitcoin could affect Ethereum. In the Eclipse attack @cite_19 the attacker forces the victim to connect to 8 of its malicious identities. The Ethereum adaptation would require to forge @math more identities and force as many connections as the default number of clients is 25. @cite_9 proposed a BGP hijacking attack and showed that the number of Internet prefixes that need to be hijacked for the attack to succeed depends on the distribution of the mining power. BGP-hijacking typically requires the control of network operators but is independent from Bitcoin and could potentially be exploited to delay network messages and execute a Balance attack in Ethereum.
@cite_19 @cite_37 @cite_9
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As the most successful cryptocurrency to date, Bitcoin constitutes a target of choice for attackers. While many attack vectors have already been uncovered, one important vector has been left out though: attacking the currency via the Internet routing infrastructure itself. Indeed, by manipulating routing advertisements (BGP hijacks) or by naturally intercepting traffic, Autonomous Systems (ASes) can intercept and manipulate a large fraction of Bitcoin traffic. This paper presents the first taxonomy of routing attacks and their impact on Bitcoin, considering both small-scale attacks, targeting individual nodes, and large-scale attacks, targeting the network as a whole. While challenging, we show that two key properties make routing attacks practical: (i) the efficiency of routing manipulation; and (ii) the significant centralization of Bitcoin in terms of mining and routing. Specifically, we find that any network attacker can hijack few (<100) BGP prefixes to isolate 50 of the mining power---even when considering that mining pools are heavily multi-homed. We also show that on-path network attackers can considerably slow down block propagation by interfering with few key Bitcoin messages. We demonstrate the feasibility of each attack against the deployed Bitcoin software. We also quantify their effectiveness on the current Bitcoin topology using data collected from a Bitcoin supernode combined with BGP routing data. The potential damage to Bitcoin is worrying. By isolating parts of the network or delaying block propagation, attackers can cause a significant amount of mining power to be wasted, leading to revenue losses and enabling a wide range of exploits such as double spending. To prevent such effects in practice, we provide both short and long-term countermeasures, some of which can be deployed immediately. Scaling the transaction throughput of decentralized blockchain ledgers such as Bitcoin and Ethereum has been an ongoing challenge. Two-party duplex payment channels have been designed and used as building blocks to construct linked payment networks, which allow atomic and trust-free payments between parties without exhausting the resources of the blockchain. Once a payment channel, however, is depleted (e.g., because transactions were mostly unidirectional) the channel would need to be closed and re-funded to allow for new transactions. Users are envisioned to entertain multiple payment channels with different entities, and as such, instead of refunding a channel (which incurs costly on-chain transactions), a user should be able to leverage his existing channels to rebalance a poorly funded channel. To the best of our knowledge, we present the first solution that allows an arbitrary set of users in a payment channel network to securely rebalance their channels, according to the preferences of the channel owners. Except in the case of disputes (similar to conventional payment channels), our solution does not require on-chain transactions and therefore increases the scalability of existing blockchains. In our security analysis, we show that an honest participant cannot lose any of its funds while rebalancing. We finally provide a proof of concept implementation and evaluation for the Ethereum network. The gas mechanism in Ethereum charges the execution of every operation to ensure that smart contracts running in EVM (Ethereum Virtual Machine) will be eventually terminated. Failing to properly set the gas costs of EVM operations allows attackers to launch DoS attacks on Ethereum. Although Ethereum recently adjusted the gas costs of EVM operations to defend against known DoS attacks, it remains unknown whether the new setting is proper and how to configure it to defend against unknown DoS attacks. In this paper, we make the first step to address this challenging issue by first proposing an emulation-based framework to automatically measure the resource consumptions of EVM operations. The results reveal that Ethereum’s new setting is still not proper. Moreover, we obtain an insight that there may always exist exploitable under-priced operations if the cost is fixed. Hence, we propose a novel gas cost mechanism, which dynamically adjusts the costs of EVM operations according to the number of executions, to thwart DoS attacks. This method punishes the operations that are executed much more frequently than before and lead to high gas costs. To make our solution flexible and secure and avoid frequent update of Ethereum client, we design a special smart contract that collaborates with the updated EVM for dynamic parameter adjustment. Experimental results demonstrate that our method can effectively thwart both known and unknown DoS attacks with flexible parameter settings. Moreover, our method only introduces negligible additional gas consumption for benign users. Bitcoin is without a doubt the most successful cryptocurrency in circulation today, making it an extremely valuable target for attackers. Indeed, many studies have highlighted ways to compromise one or several Bitcoin nodes. In this paper, we take a different perspective and study the effect of large-scale network-level attacks such as the ones that may be launched by Autonomous Systems (ASes). We show that attacks that are commonly believed to be hard, such as isolating 50 of the mining power, are actually within the reach of anyone with access to a BGP-enabled network and hijacking less than 900 prefixes. Once on path, AS-level adversaries can then partition the Bitcoin network or delay block propagation significantly. The key factors that enable these attacks are the extreme centralization of Bitcoin, both from a routing and a mining perspective, along with the fact that Bitcoin messages are sent unencrypted, without integrity guarantees. We demonstrate the feasibility of large-scale attacks in practice against the deployed Bitcoin software and quantify their disruptive network-wide impact. The potential damage to Bitcoin is severe. By isolating a part of the network or delaying the propagation of blocks, network-level attackers can cause a significant amount of mining power to be wasted, leading to revenue losses and enabling a wide range of attacks such as double spending. We provide several suggestions on approaches to mitigate such attacks employing both short-term and long-term measures.