The Hand-off Problem is a concept that explores the practical limitations of forced inclusion mechanisms designed to enhance censorship resistance in blockchain technologies, specifically in Ethereum-based systems. Init4, a research collective focusing on the development of next-generation Ethereum tools, has delved into this issue in a research note published on their blog on September 3, 2024. The note emphasizes that while discussing the nuances and gaps in the security models of deployed and proposed systems, it would be inaccurate to label these as vulnerabilities or previously unknown.
At the heart of cryptocurrencies and Ethereum, censorship resistance is a core value, aiming to ensure that on-chain transactions are available to anyone, with the rules of the chain applying equally to all users and uses. The research highlights the importance of engineering processes in testing values against reality to identify where and how they might break.
The note defines censorship as the intentional prevention of transactions from appearing in the canonical ordering (transaction exclusion). It considers orders fair or neutral when they depend solely on the economic outcome for the ordering system and unfair or censored when influenced by other information. For instance, refusing to include a transaction based on the sender’s identity, rather than the transaction’s fee, is deemed unacceptable. A transaction is considered censored if its inclusion is determined by non-economic information. If a transaction generates more observable profit for the ordering system than any included transaction but is not included, it is considered censored.
To combat censorship, forced inclusion mechanisms have been developed, aiming to ensure that a user can force the inclusion of their transaction, making censorship under the defined terms impossible. These mechanisms are particularly crucial for rollup systems, which often have centralized sequencing, making censorship by the sequencer easy. Rollups like Optimism and Arbitrum have implemented forced inclusion mechanisms, allowing users to ensure their transactions will be executed by the rollup after a time delay, irrespective of the sequencer’s actions. These transactions are theoretically as resistant to censorship as other Ethereum transactions.
Forced inclusion mechanisms introduce new constraints on valid orderings, rendering broad classes of orderings invalid according to protocol rules. They allow the user to specify a subset of the future ordering, with all valid orderings required to expand upon the forced subset. A similar mechanism has been proposed for Ethereum via EIP-7547, allowing block proposals to partially specify the contents of the next block, providing a mitigation against censorship under the assumption that block proposers have fewer incentives to censor than block builders.
However, the research note acknowledges that transaction confirmation is merely a means to an end. The note does not fully address the implications of this statement, but it suggests that while forced inclusion mechanisms aim to protect against censorship in transaction confirmation, the broader impact of censorship on the overall functionality and purpose of the blockchain technology must be considered. This might include issues such as the integrity of the blockchain, user trust, and the ability of the system to serve its intended functions without undue interference or manipulation.
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