On Slow and Fast Block Times

The Case for Fast Block Times

Blockchain security is often touted as a matter of time, with faster block times being seen as inherently less secure. However, this assumption is based on a flawed understanding of the underlying security models. In reality, the benefits of fast block times are multifaceted and far-reaching, providing more granularity of information, faster convergence on the correct fork, and improved predictability.

The Normal Case

In the normal case, there are no attackers, and all miners are altruistic. The question is, what is the probability that a transaction will be finalized after a certain number of seconds? The answer is that it approaches 100% after a few seconds, regardless of block time. However, if we relax the altruism assumption and introduce a small percentage of attackers, the probability of non-reversion decreases. But even in this case, faster block times still have an advantage.

Attackers

When we introduce attackers into the picture, the situation becomes more complex. The simplest mathematical model to use is the weighted random walk. We start off assuming that a transaction has been confirmed for k blocks, and that the attacker, who is also a miner, now tries to start a fork of the blockchain. The probability of the attacker winning such a game is given by a mathematical formula, which can be combined with a probability estimate for k using the Poisson distribution.

Economically Bounded Attackers

In this scenario, the attacker has a budget of $X, which they can spend on bribes, near-infinite instantaneous hashpower, or anything else. The question is, how high is the requisite X to revert a transaction after k seconds? The answer is that it is equivalent to the expected economic expenditure to revert the number of blocks that will have been produced on top of a transaction after k seconds.

The Benefits of Fast Block Times

Fast block times provide more granularity of information, which is essential for the system to converge on the correct fork. In the BFT security models, this granularity ensures that the system can more quickly converge on the correct fork over an incorrect fork. In an economic security model, this means that the system can more quickly give notification to users of when an acceptable security margin has been reached.

The Costs of Fast Block Times

However, fast block times also have their costs. Stale rates are perhaps the largest, and it is of course necessary to balance the two - a balance which will require ongoing research, and perhaps even novel approaches to solving centralization problems arising from networking lag.

A Recent Proposal: Bitcoin NG

A recent interesting proposal presented at the Scaling Bitcoin conference in Montreal is the idea of splitting blocks into two types: (i) infrequent "key blocks" which select the "leader" that creates the next blocks that contain transactions, and (ii) frequent "microblocks" which contain transactions. The theory is that we can get very fast blocks without the centralization risks by essentially electing a dictator only once every (on average) ten minutes, for those ten minutes, and allowing the dictator to produce blocks very quickly.

Conclusion

Fast block times are good because they provide more granularity of information. In the BFT security models, this granularity ensures that the system can more quickly converge on the correct fork over an incorrect fork, and in an economic security model this means that the system can more quickly give notification to users of when an acceptable security margin has been reached. However, fast block times do have their costs, and it is of course necessary to balance the two - a balance which will require ongoing research, and perhaps even novel approaches to solving centralization problems arising from networking lag.

Future Research Directions

Further research on fast block times and their implications is essential. This includes exploring new algorithms and techniques for balancing the trade-offs between security and performance, as well as investigating the potential benefits and drawbacks of different block time regimes. Additionally, the development of novel approaches to solving centralization problems arising from networking lag is crucial for ensuring the long-term viability of blockchain systems.

Final Thoughts

In conclusion, fast block times are not inherently less secure than slower block times. In fact, they provide a range of benefits, including improved predictability, faster convergence on the correct fork, and more granularity of information. However, they also have their costs, and it is essential to balance these trade-offs through ongoing research and development. As the blockchain ecosystem continues to evolve, it is likely that we will see a range of different block time regimes emerge, each with its own strengths and weaknesses. By understanding the implications of these different regimes, we can ensure that blockchain systems are secure, scalable, and reliable.

Source: https://blog.ethereum.org/en/2015/09/14/on-slow-and-fast-block-times