This article introduces Citrea, the first ZK Rollup that enhances Bitcoin’s block space capacity through zero-knowledge technology. Its design aims to allow developers to build all applications on the Bitcoin chain. This article is sourced from Citrea’s article “Introducing Citrea: Bitcoin’s First ZK Rollup” and compiled and translated by Shenzhen Tide.
Table of Contents:
1. The Problem of Creating New Block Space
2. Enter Citrea: Expanding Bitcoin’s Security
3. Expanding Bitcoin’s Security
4. What’s Next?
5. Internal Devnet Activated
6. Public Testnet
7. Building with Citrea
Bitcoin’s scalability has long relied on moving security and demand off-chain and outside of Bitcoin through various solutions. Citrea aims to scale the entire Bitcoin network through zero-knowledge proofs, ensuring on-chain verifiability and data availability within the Bitcoin network. This approach makes Citrea the first scaling solution that supports more complex applications without compromising Bitcoin’s security or changing its consensus rules.
Bitcoin possesses the most secure, decentralized, and censorship-resistant block space. Existing proposals for creating new block space extensions do not inherit any of these properties of Bitcoin. These proposals or implementations only provide alternative block spaces; they do not scale the Bitcoin blockchain. Building with sidechains that have alternative block spaces will compete with Bitcoin in the long term and expose users’ funds to risks through insecure bridging and network structures.
The demand for Bitcoin block space is high due to its security, decentralization, and censorship resistance, and people have a strong urgency for all on-chain needs in Bitcoin. In the past few months, we have observed exponential growth in interest in Bitcoin block space, reflected in fees. Whether it’s for payments or script transactions, people want to join the block space and are willing to pay for it. Ensuring sustainable participation in the Bitcoin network is crucial for its long-term health and security budget, but there is a trade-off: due to high costs and necessary block size limitations, some transactions will inevitably be excluded. It is essential for Bitcoin to scale to include more complex transactions without changing its core principles.
This growing interest highlights a significant problem with existing scalability proposals: they fail to meet the inclusive demands of Bitcoin block space’s continuous growth.
Existing scalability proposals have not scaled Bitcoin. Bitcoin has long faced the challenge of handling more transactions and supporting broader applications without compromising its security or core principles. The multifaceted nature of this problem has proven difficult to solve comprehensively. Some projects attempt to scale Bitcoin’s payment throughput, such as the Lightning Network, while others focus on scaling the functionality of BTC assets, such as sidechains. The first scalability proposal failed to bring programmability to Bitcoin, and the latter missed a crucial point: actively and efficiently utilizing the Bitcoin blockchain itself.
In the current landscape, the inherent demand for Bitcoin block space has been redirected to separate consensus protocols and sidechains. This approach brings a trade-off – sacrificing Bitcoin’s security and consistency with its incentives in exchange for lower costs and more BTC use cases. As a result, sidechain-centric Bitcoin faces the dilemma of scaling solutions that cannot meaningfully serve its long-term health and incentives. While these solutions have good intentions, they do not run on the Bitcoin blockchain, do not contribute to Bitcoin’s security, and ultimately cannot scale the Bitcoin blockchain as needed.
It is worth emphasizing that the demand for Bitcoin that has shifted to sidechains is no longer the demand for Bitcoin itself.
To truly scale Bitcoin, a symbiotic relationship must be established between Bitcoin and scaling solutions. Scaling solutions must actively and efficiently utilize Bitcoin’s block space to accommodate demand and increase its expressiveness and throughput without changing its protocol. Scaling Bitcoin’s block space is the only way to transact securely with Bitcoin. No other alternative solution can fully inherit Bitcoin’s security.
The most secure and incentive-compatible way to scale Bitcoin’s blocks is through sharding execution across chain-verifiable and data-on assertions. Implementing Bitcoin’s easily verifiable execution relies on zero-knowledge or fraud proofs. Since fraud proofs require writing a significant amount of data on Bitcoin, ZK proofs are the most efficient way to verify execution validity within the block size limits of Bitcoin.
ZK Rollup creates a shard of execution and processes multiple transactions in batches on that shard, while using concise mathematical proofs and minimal underlying data to verify them on-chain for state reconstruction. This approach allows ZK Rollup to efficiently utilize the underlying block space in the most verifiable way, extending L1 with its full security.
Citrea is the first rollup on Bitcoin that achieves greater expressiveness and utility of Bitcoin’s block space without leaving Bitcoin. Citrea keeps the demand for Bitcoin within its network and ensures that the Bitcoin network serves as the data availability and settlement layer for Citrea transactions. Citrea does not require any changes to Bitcoin’s consensus.
Citrea represents an important milestone for Bitcoin: it is the only Bitcoin-based execution layer, the first ZK proof verification, and the first general-purpose L2 validation within Bitcoin. Citrea’s mission is to propel Bitcoin into the next stage as the foundation of the world’s financial infrastructure, where developers can build everything on Bitcoin.
Citrea batch processes thousands of transactions, processes them in zkVM, and generates a concise validity proof that asserts the correctness of execution and output data. In Bitcoin’s history, Citrea’s validity proof is burned and locally verified on the Bitcoin blockchain for the first time. Citrea integrates a native ZK proof validator smart contract on Bitcoin L1 in BitVM. Unlike monolithic sidechains, Citrea creates a modular world for Bitcoin through its shard of execution, keeping settlement and data availability on-chain, on Bitcoin.
Citrea creates stable fee income for miners through data availability and extends BTC off Bitcoin in a trust-minimized manner through validity proofs, allowing developers to build anything on its EVM-equivalent execution shard.
Citrea is the only execution layer that settles on Bitcoin. Every transaction that occurs on Citrea is fully guaranteed by zero-knowledge proofs and verified by Bitcoin. Citrea’s execution environment is trustless relative to Bitcoin and open to all participants in the Bitcoin network.
Therefore, Citrea ensures that it meets the data availability, censorship resistance, and anti-reorganization guarantees equivalent to Bitcoin.
Bitcoin as the cornerstone of applications: Citrea’s mission is to build a programmable liquidity layer on top of the most secure and decentralized blockchain, Bitcoin. We believe that Bitcoin block space must be efficiently utilized to enable various financial activities, such as trustless purchase of BTC, utilization of BTC, or borrowing against BTC. Most existing meta-protocols that attempt to provide these functionalities are trust-based and inefficient. Citrea is the most efficient and secure platform for building applications on Bitcoin.
Bitcoin settlement and minimal trust two-way peg: In Bitcoin’s history, universal L2 settlement is carried out on Bitcoin for the first time, enabling the first universal trust-minimized two-way peg. Before Citrea, second-layer solutions relied on the honest majority assumption of multisignature, such as open or closed federations. With Citrea, as long as one network participant is honest, validity proofs can be verified in Bitcoin using the BitVM paradigm. BitVM is based on fraud proofs, which means Citrea’s ZK proofs are optimistically verified on the Bitcoin network. In the future, a ZK proof validator opcode will enable a fully trustless two-way anchoring mechanism.
EVM equivalence: Building with Citrea’s fully EVM-equivalent allows all EVM developers to easily build on Bitcoin. Citrea introduces a fully EVM-equivalent Type 2 zkEVM built with RISC Zero. Citrea is not limited to a single VM in its design and benefits from its modular architecture, allowing for future adoption of new VMs such as the WASM VM.
We are running the Citrea Devnet for internal use, integrating and testing critical infrastructure with synthetic native BTC. The trust-minimized two-way peg with BitVM is under active development.
Stay tuned for the unveiling of the trust-minimized two-way anchoring design of Citrea. We will announce our bridging design in the coming weeks and open-source our code repositories!
We are rapidly progressing and have been working tirelessly to build a robust testnet for the first rollup on Bitcoin. We look forward to sharing more information in the coming months. Please follow our social media and blog.
Our developer contact form is now open. Citrea has been incubated by Chainway Labs for over a year and is well-funded. If you are interested in collaborating with us or deploying on Citrea, please reach out to us. Check out our documentation to learn about Citrea’s technology.
Making Bitcoin the foundation of the world’s finance: Citrea’s vision is to build scalable infrastructure that propels Bitcoin into its next stage as the foundation of the world’s finance. To achieve this vision, we believe that Bitcoin’s block space must be enhanced to include more transactions and provide more expressiveness and functionality while preserving its consensus rules.
Bitcoin, with its security, decentralization, and resistance to censorship, is fully capable of becoming the foundation of the world’s finance. Citrea is built upon these strengths and makes Bitcoin the underlying layer for dynamic ecosystems such as DeFi, gaming, NFTs, and more. As Citrea’s modular world grows larger and more advanced, it will only become more powerful.
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