A "Free" Relay Attack Taking Advantage of The Lack of Full-RBF In Core

Peter Todd raises concerns regarding the proposed "weak block" mechanism, highlighting a significant vulnerability that could be exploited by attackers. The flaw lies in the potential for an attacker to partition all miners by creating transaction-relay asymmetries, such as identical fee rates and weights but different transaction IDs. This tactic would allow an attacker to broadcast a transaction that appears attractive, leading it to be placed at the top of the mempool. Subsequently, this could result in the generation of weak blocks up to 4MB for each miner partition, which are then relayed across the network. The primary issue with this scenario is the wasteful expenditure of 4 MB of block-relay bandwidth per affected miner, exponentially increased by the number of miners targeted.

Furthermore, Todd points out that the transaction vector responsible for the mempool partition may have been admitted with a sub-minimal fee rate, merely to gain entry into the mempool. Compounding the problem, if the attacker possesses sufficient hashrate capabilities to mine a block including the problematic transaction vector, the miners subjected to this denial-of-service (DoS) attack would see no financial gain from the weak block reward mechanism. This outcome holds true unless the reward system is external to the Bitcoin blockchain, a concept met with skepticism concerning the security model of Bitcoin.

Todd critically assesses the "weak block" proposal's claim to offer a decentralized, DoS-resistant mechanism, deeming such assertions as misleading and inaccurate within the community context. He underscores the need for quantitative evaluations to ascertain that the proposal does not inadvertently amplify the risk of DoS attacks.

In a broader perspective, Todd acknowledges the significance of addressing the challenge of rewarding miners' income, especially to incentivize solo mining and enhance the initial financial liquidity incentives that have historically driven miners to form pools since Bitcoin's inception. However, he expresses doubt regarding the viability of the "weak block" approach as a solution to these issues.