bitcoin & Beyond is an educational series from the team at x.com/therollupco”>The summary focused on a new and emerging class of builders in the bitcoin ecosystem. Through spaces, panels and interactive presentations, the goal is to provide deep technical knowledge on innovative projects of scale.
In this episode, the team discusses the creation of bitcoin's first ZK (zero-knowledge) rollup with x.com/0x_orkun”>Orkún of Citrea. Rollups are a new technology that promises to improve the utility of bitcoin, allowing for different scalability improvements while preserving the security of the bitcoin infrastructure.
“A rollup is a blockchain that uses another blockchain as a data availability layer,” Orkun says emphatically.
Many other elements are considered in cumulative design, but he believes they should not be part of the definition. “Where does the agreement, the bridge fit in? ZK or optimistic? The execution layer? It doesn't matter.”
The motivation behind Citrea
Citrea's motivation behind creating a zero-knowledge rollup on bitcoin comes from bitcoin's unparalleled security and censorship resistance. Despite these strengths, bitcoin has limitations in block size and scripting capabilities. Orkun noted: “What you can do with bitcoin beyond simple payments is extremely limited today. We want to do more using the security of the bitcoin blockspace.”
Overcoming bitcoin Limitations with Modularity
Citrea is attempting to address these limitations through modularity. By creating a rollup, developers can customize their stack to create various applications such as payment rollups, game rollups, and EVM rollups. This flexibility allows for different optimizations that could scale blockchains without requiring changes to their core protocol. The combination of different services becomes fertile ground for experimentation that was not possible before.
Security is paramount for bitcoin and any layer built on top of it. “Building a rollup is the only way to get that security, if you're not building channels like Lightning or Mercury, which are still limited by bitcoin functionality.”
Citrea's innovation is to use bitcoin as a data availability layer. Thanks to landmark changes like SegWit and Taproot, developers are discovering new ways to enroll data in bitcoin transactions. This makes it possible to use bitcoin as a data availability layer for rollups. “So you can post data to bitcoin, but that data can be arbitrary because it is never run on the blockchain,” Orkun explained.
Using bitcoin for data availability involves tradeoffs. While it ensures high security, it may not be suitable for high-speed, low-cost applications. “If you want full bitcoin security, you should use bitcoin as the data availability layer. However, for high-speed, low-cost applications, other layers such as Celestia may be more appropriate.”
The Clementine Bridge
To move bitcoins in and out of the system, Citrea has created Clementine, a BitVM-based two-way peg that optimistically verifies ZK proofs. This mechanism adds proof of bitcoin, reducing the need for frequent settlements and improving security. “We are simply writing these tests to bitcoin every hour. Other rollups can read the tests from there and run them based on that,” Orkun explained.
The evolution of btc bridges has seen a shift from federated and custodial threshold bridges to modern cryptoeconomic security bridges. Federated bridges rely on a majority consensus within a committee, while cryptoeconomic bridges like Stacks or tBTC use staked assets to ensure security. Orkun detailed: “In cryptoeconomic security, you still rely on a federation, but those people are actually betting on other assets. If they steal the money, then you can cut that asset.”
Clementine, however, goes one step further. It uses an optimistic approach inspired by BitVM to verify ZK tests, which is cost-effective and secure. This approach allows for the aggregation of tests, making the process efficient and scalable.
The core idea behind Clementine is to provide optimistic deals for ZK rollups. “We just added bitcoin's bitcoin proofs to settle less frequently because it can't be settled on every block. It will be expensive,” Orkun explained. By periodically recording data and adding tests, Clementine ensures that the state remains accurate and secure.
To achieve this, an operator will initially cover users' withdrawal requests out of pocket and then add the necessary proofs in a single submission to the network. If other operators suspect a crime has been committed, they can challenge the filing. Successful challenges result in the rogue operator losing their initial link and being removed from the network. If the operator's presentation is not questioned, he can claim the equivalent amount that he disbursed from the users' original deposits.
This setup introduces a minimized trust assumption in which only one participant needs to be honest to ensure security. “We call it minimized trust because now we have this 1 out of N assumption. As long as one person out of these N people is honest, then their money is safe,” Orkun emphasized. This is a significant improvement over traditional models that require majority consensus for security.
Future plans and impact on the ecosystem
Looking ahead, Citrea plans to introduce volition, a hybrid model that balances on-chain security with off-chain profitability. This allows applications to choose their data storage method based on their specific needs. Orkun also emphasized the importance of transaction fees for the long-term security of bitcoin, and Citrea's use of bitcoin as a data availability layer contributes to maintaining miner incentives and network security.
“So depending on your usage, if you want to deploy a gaming app now, you can use off-chain data. It's very cheap, very fast, but still has this bitcoin interoperability. If you want to build a stablecoin backed by bitcoin app, you can use on-chain data so your stablecoin is fully secured on-chain, fully secured by bitcoin. A little expensive, but you still get this interoperability between the gaming app and the stablecoin app.”
Combining the resilience of bitcoin with the flexibility of rollups could push the boundaries of what is possible with bitcoin. Verify citrea website to learn more about his work. Follow our bitcoin & Beyond series on x.com/therollupco”>therollup.co to learn more about the state of evolution of bitcoin scaling solutions.
