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Grokking Cartesi Public Goods: Dave

Tech/Jan 25, 2024/EDUARDO TIÓ

What happens when there’s a disagreement in optimistic rollups like Cartesi Rollups? Cartesi has designed a novel way to leverage the base layer's security with a public good called Dave. It’s an improvement upon common designs for dispute resolution systems or fraud-proof protocols that need to balance safety, settlement delay, and costs while being permissionless — that is, allowing anyone to participate.

Let’s take a look at how Dave guarantees that everything that happens on an L2 can be trusted as long as we have a single honest node participating in the network.

What are fraud-proof protocols?

Rollups work by moving the bulk of the computation off-chain and then efficiently proving to the base layer the final result of executing said computation. In other words, all the work done by Cartesi Nodes in taking data on and off-chain needs to be verifiable.

One way to ensure this verifiability is to have a fraud-proof protocol. Typically, when a node posts information back to an L1 chain as a claim, there is a validation process whereby other nodes can challenge the claims until they’re finalized in the main chain.

During a specified timeframe, nodes can submit fraud proofs that contest the results of the information processed on the L2. This is why bridging between L1s and L2s today can take up to a week, depending on the L2’s fraud-proof protocol.

Current problems with permissionless fraud-proof protocols

There’s been a lot of experimentation in web3 with different designs for dispute resolution systems. Using the traditional approach, there are two ways of building a fraud-proof system with multiple participants.

The first is making players fight pairwise, one at a time. This way, all players are, at most, fighting a single dispute at a time. The second is allowing everyone to fight everyone at the same time.

However, when we make the validator set permissionless, these two fail. The first fails because of delay attacks, and the second fails because the attacker may exhaust the honest validators' resources. Let’s break these down.

One of the main cases where current designs fall short is the case of a delay attack. With enough funds, a malicious node could exploit the dispute resolution system to prevent users from bridging back to an L1 by continuously making challenges to their transactions. Each challenge would set back their withdrawal time by the amount of time established as the “challenge period.”

What’s even worse is that if there’s a large number of dishonest challengers, it becomes very costly for a single honest node to defend their claim on the main chain. The result is that disputes involving a significant number of parties become impractical and vulnerable to what’s known as Sybil attacks.

Dave vs Goliath

How do we prevent situations where dishonest network participants with enough money and clout can effectively thwart the operations of a single honest node?

Cartesi’s Research & Reference units are working on Dave, a game-changing dispute resolution algorithm where a single honest competitor’s delay times and amount of computing power needed for disputes grows logarithmically with the number of opponents.

This means that instead of having to spend more resources as more nodes challenge a claim, defending against every new challenge is less expensive for the claimer than the previous one. What this looks like in practice is that you'll never need more than a laptop to win (regardless of the number of attackers), but if there are a lot of attackers, you'll need to keep your laptop turned on for longer.

When Cartesi Nodes post vouchers or notices back on the main chain, there will still be a window for challenging the voucher. However, a single honest claimer will be able to defend itself in the event of a malicious challenge.

This new take on a fraud-proof protocol is based on the permissionless refereed tournaments algorithm proposed by Cartesi co-founders Diego Nehab and Augusto Teixeira. It allows anyone to validate a rollup and enforce the correct state on-chain. There’s no need to trust validators; you can verify any rollup yourself.

Do you think you grokked Dave? Test your knowledge by taking the Grokking Public Goods: Dave quiz in the Cartesi questboard and earn community points.

Learn more about Cartesi in general by reading the docs. You can also stay in the loop with everything happening in the Cartesi ecosystem by joining the community.

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