The Problem of Censorship
The Problem of Censorship in Blockchain Technologies
Censorship-resistance is a crucial property in decentralized cryptoeconomic systems, enabling the secure operation of various financial protocols. However, ensuring that transactions remain censorship-proof is a complex challenge. While significant work has been done in cryptoeconomics to prevent block reversion and ensure the continuous creation of new blocks, less attention has been focused on preventing the censorship of transactions that users want to include in the blockchain.
The Threat Model
To address the problem of censorship, we need to consider the economic model under which we are operating. Who are the censors, how much can they do, and how much does it cost them? We will split this into two cases: the first where the censors are not powerful enough to independently block transactions, and the second where they are powerful enough.
Case 1: Censors Not Powerful Enough
In the first case, the censors are not powerful enough to independently block transactions. This entails the censors having less than 33% of all validator positions, in which case they can certainly restrict transactions from their own blocks, but those transactions would simply make it into the next block that does not censor them, and that block would still get its requisite 67% signatures from the other nodes.
Case 2: Censors Powerful Enough
In the second case, the censors are powerful enough. In the Bitcoin case, we can think of the top five mining firms and data centers colluding, and in the Tendermint case, a group of very large stakeholders. This may seem like a silly scenario to worry about, but it is essential to consider.
Making Censorship Costly
The first way to discourage censorship is to make it unprofitable or at least expensive. Notably, proof of work actually fails this property: censorship is profitable, since if you censor a block, you can take all of its transactions for yourself, and in the long run, take its block reward.
Timelock Consensus
One approach to make censorship costly is to use timelock puzzles. A timelock puzzle is a kind of encryption where a piece of data takes a particular amount of time to decrypt and cannot be sped up via parallelization. The typical approach to timelock puzzles is using modular exponentiation.
Event-Based Denial-of-Service Attack Vector
In the context of Ethereum, the halting problem opens up a particular denial-of-service attack vector. If a censor wishes to block transactions that have an undesirable effect, that effect could appear after running for millions of computational steps, and so the censor would need to process every transaction and discard the ones that they want censored.
Introducing Events
To overcome this denial-of-service vulnerability, we need to introduce an upcoming Ethereum 1.1 feature: events. Events are a feature that allows a contract to create a kind of delayed message that is only played at some prespecified block in the future.
Covert Latent Message Scheme
To expand the functionality of this scheme, we can also add another protocol feature: create a specialized address where messages sent to that address are played as transactions. The messages would contain the transaction data in some form, after a few hundred blocks trigger events to combine the data together, and the data would then have to be immediately played as a regular transaction.
Conclusion
In conclusion, censorship-resistance is a crucial property in decentralized cryptoeconomic systems, and ensuring that transactions remain censorship-proof is a complex challenge. By considering the economic model under which we are operating and introducing features such as timelock puzzles and events, we can make censorship costly and difficult to achieve. However, it is essential to note that censorship can still be a significant threat, and it is crucial to continue working towards developing more robust and censorship-resistant protocols.
Final Thoughts
The problem of censorship in blockchain technologies is a complex and multifaceted challenge. While significant progress has been made in developing censorship-resistant protocols, there is still much work to be done. As the use of blockchain technologies continues to grow and mature, it is essential to prioritize the development of protocols that are resistant to censorship and can ensure the secure operation of various financial protocols. By doing so, we can create a more secure and trustworthy digital economy.
Source: https://blog.ethereum.org/en/2015/06/06/the-problem-of-censorship
