Vincent Gramoli

Traditional blockchain systems offer a secure way of tracking the ownership of digital assets as long as the attacker does not control a large portion of the overall computational or mining power. They typically require participants to generate a proof-of-work before proposing a block at a given index of the chain. To choose one block among the candidate blocks at the same index, Nakamoto’s consensus, Ghost , and the original Ethereum’s consensus select, respectively, the longest branch, the heaviest subtree and the branch with the most difficult crypto-puzzles. This allows an attacker who can generate proofs-of-work faster than others to double spend by overwriting any given branch. In this article, we present a double spending attack, called the Balance attack, that simply needs to delay some messages. This result sheds new lights on an important, often implicit, assumption of the blockchain, synchrony , under which the transmission delay of any message should be within a known upper bound. We show that the attack succeeds with high probability on the protocols of the two largest blockchain systems in market capitalization, Bitcoin and Ethereum. To quantify the impact of our attack, we replicated the blockchain network run by 50 financial institutions and achieved double spending in less than 20 minutes. Finally, we demonstrate the success of the attack empirically by modifying the geth software and hijacking BGP in a controlled distributed system whose distribution of mining power is set to the distribution observed on the Ethereum main blockchain.

Byzantine fault tolerant consensus protocols are implemented with consecutive broadcasts but suffer from a low throughput at large geographical scale or planetary scale. A reason for this inefficiency is believed to be their all-to-all communication complexity, which led researchers to design new consensus protocols with more consecutive one-to-all broadcasts but cumulatively fewer messages. We show, through a step-by-step evaluation, ranging from LAN/WAN broadcast benchmarks to a state machine replication (SMR) application, that this intuition can be misleading. In particular, we identify two underestimated factors that can impact consensus performance much more at a large scale: (i) the good-put of the broadcast as the rate at which bits are delivered to the application and (ii) the hiccup or waiting time between consecutive broadcast phases. Finally, we show that a leaderless SMR with O(n⁴) complexity can outperform a leader-based SMR with O(n³) complexity by 20x. This work promotes a new family of byzantine consensus protocols exclusively based on all-to-all broadcasts that take into account these two factors. Our result promises to impact the design of blockchain systems that aim at performing well in WANs at a planetary scale.

With the recent advent of blockchains, we have witnessed a plethora of blockchain proposals. These proposals range from using work to using time, storage or stake in order to select blocks to be appended to the chain. As a drawback it makes it difficult for the application developer to choose the right blockchain to support their applications. In particular, the scalability and performance one can obtain from a specific blockchain is typically unknown. The claimed results are often obtained in isolation by the developers of the blockchain themselves. The experimental conditions corresponding to these results are generally missing and the lack of details make these results irreproducible. In this paper, we propose the most extensive evaluation of blockchain to date. First, we show how the experimental settings impact the performance of 6 state-of-the-art blockchains and argue for more detailed experiments. Second, and to cope with this limitation, we propose a unifying framework to evaluate blockchains on the same ground. The framework includes a suite of 5 realistic Decentralized Applications (DApps), helps deploy the blockchain nodes at different scales and evaluate their performance. Finally, we show that selecting a particular virtual machine or weakening guarantees can help handle computationally demanding workloads but that none of the tested blockchains can yet support the load of these realistic DApps.

Blockchain governance is paramount to lead securely a large group of users towards the same decisions without disputes about the legitimacy of a blockchain instance over another. As of today, there is no efficient way of protecting this governance against an oligarchy. This paper aims to offer a new dimension to the security of blockchains by proposing a solution known as proportional governance reconfiguration. This solution mitigates the formation of an oligarchy by (1) electing governors proportionally using a proportional multi-winner election protocol (2) reconfiguring the governance automatically and periodically. The proportional governance reconfiguration relies on a Solidity based implementation making it compatible and usable in many smart contract supported blockchains. We prove our solution solves the proportional governance problem and we evaluate our solution on two smart contract supporting blockchains Ethereum-PoA and Smart Redbelly Blockchain. Our results indicate that our proportional governance can elect 200 governors within 6–12 minutes when 1000 voters from 5 continents vote for 500 candidates.