What is single-slot finality? A new, faster way to finalize on the Ethereum network.

Finalizing an Ethereum block currently takes around 15 minutes. However, there are ways to improve the consensus mechanism so that blocks can be finalized much faster. One potential improvement is called single-slot finality (SSF), where a block is both proposed and finalized in the same slot, reducing the time to finality significantly.

In Ethereum’s proof-of-stake system, finality means that once a block is confirmed, it cannot be altered or removed from the blockchain unless at least 33% of the total staked ETH is lost. This concept ensures strong economic security, as the cost of changing the blockchain becomes prohibitively expensive for any rational actor to attempt.

The current 15-minute time frame for finality is considered too long by many users. Waiting for this period is inconvenient for decentralized applications and exchanges that require high transaction throughput. Furthermore, this delay creates a window for attacks, such as short reorgs, which could enable malicious actors to censor blocks or extract Miner Extractable Value (MEV). Moreover, the complexity of the finalization process increases the risk of security vulnerabilities. By reducing the time to finality to a single slot, many of these issues can be addressed.

Trade-offs Between Decentralization, Processing Power, and Time to Finality

Finality in Ethereum isn’t instantaneous because it requires at least 2/3 of validators to vote for a block, a process called “attestation.” Validators must process these votes before confirming a block. Speeding up this process demands more computing resources from each node. The more validators on the network, the more attestations need to be processed, further increasing the computational burden.

Thus, there is a balance to be struck between computational power, the number of validators (decentralization), and the time required to finalize a block. Ethereum’s current design balances these factors by setting a stake minimum of 32 ETH, which limits the number of validators and ensures that blocks can finalize in approximately 15 minutes. To speed up finality, either the number of validators must decrease, or each node must use more powerful hardware. However, new methods of processing attestations more efficiently are being explored, which could allow for finality within a single slot.

How SSF Could Work

Recent advancements, such as improvements in BLS signature aggregation, have shown that processing a large number of validators in a single slot is more feasible than originally thought. For example, nodes may soon be able to handle millions of validators without overwhelming computational resources. In a system with one million validators, and each validator attesting twice per slot, the hardware could handle 125,000 signature verifications per second. This capability could make single-slot finality a reality.

Additionally, the concept of supercommittees—smaller groups of randomly selected validators—could allow only a subset of validators to vote on a block in each slot, making it easier to finalize blocks faster. While this raises questions about potential attacks on these smaller groups, the community is exploring ways to set the cost of an attack high enough to deter such behavior.

In the current consensus model, the fork-choice rule determines which chain to follow in case of multiple chain options. In an SSF system, blocks would finalize in the same slot they are proposed, leaving no space for the fork-choice rule to operate. In cases where finality is not achieved (due to an insufficient number of attestations), the fork-choice rule would still be needed to resolve disputes. Maintaining some flexibility here could help protect the network against validator downtime or other issues.

Challenges to Implementing SSF

Achieving single-slot finality is not without its challenges. One major issue is scaling signature aggregation without overloading the peer-to-peer network. Additional layers of aggregation could help, but they also add complexity and latency. Another challenge is determining how to handle cases where the number of active validators exceeds the capacity for efficient processing in a single slot.

One possible solution is to consolidate stakes, allowing operators managing multiple validators to merge their resources, reducing the overall number of messages. Alternatively, placing a cap on the number of validators or the amount of staked ETH might be necessary, though this would introduce its own challenges in terms of fairness and participation.

Single-slot finality represents an exciting possibility for Ethereum’s future, promising much faster transaction finalization. While there are trade-offs to consider, such as computing power and network complexity, recent advancements suggest that achieving SSF is becoming more realistic. By finding a balance between decentralization, computational efficiency, and speed, Ethereum could significantly enhance its performance, creating a more user-friendly and secure network.