Citrea Overview: Bitcoin’s First ZK Rollup

Source: Citrea Team; Compiled by: Deng Tong, Golden Finance

Today we introduce Citrea, the first Rollup to enhance the Bitcoin block space function through zero-knowledge technology.

Bitcoin’s scalability landscape has long relied on solutions that move security and demand off-chain, outside of Bitcoin. Citrea will change this reality by enabling Bitcoin scaling through zero-knowledge proofs, ensuring on-chain verifiability and data availability within the Bitcoin network. This approach makes Citrea the first scalable solution that enables more complex applications without compromising Bitcoin’s security and changing its consensus rules.

The problem of creating new block space

Bitcoin has the most secure, decentralized and censorship-resistant block space. Scaling proposals that create new blockspaces cannot inherit any of these properties of Bitcoin. These proposals or implementations merely provide alternative block spaces; they do not extend the Bitcoin blockchain. In the long term, building sidechains with alternative block spaces would compete with Bitcoin and risk user funds through insecure bridge and network construction.

Bitcoin block space is in high demand

Due to its security, decentralization and censorship resistance, people demand Bitcoin for all their on-chain needs. Over the past few months, we have observed an exponential increase in interest in the Bitcoin block space, which is reflected in the fee rate. Whether it’s payments or inscription transactions, people want to be included in the block space and are willing to pay for it. Ensuring sustainable participation in the Bitcoin network is critical to its long-term robustness, but comes with a trade-off: certain transactions will inevitably be excluded due to high fees and necessary block size limits. Bitcoin must expand to include more complex transactions without changing its core principles.

This growing interest highlights a significant problem with existing scalability proposals: they fail to address the growing demand for inclusion in Bitcoin’s block space.

Existing scalability proposals fail to scale Bitcoin

Bitcoin has long faced challenges More transactions and the challenge of enabling a wider range of applications without compromising its security or core principles. This multifaceted problem has proven difficult to fully resolve. Some attempts aim to increase Bitcoin’s payment throughput, such as the Lightning Network, while others focus on extending the functionality of BTC assets, such as sidechains. While the first extension proposal failed to achieve programmability in Bitcoin, the latter missed a key point: active and efficient utilization of the Bitcoin blockchain itself.

In the current situation, the inherent need for the Bitcoin blockchain is redirected to independent consensus protocols, namely sidechains. This approach creates a trade-off - in order to lower fees and provide more use cases for BTC, Bitcoin's security is sacrificed and its incentives are inconsistent. As a result, Bitcoin, dominated by sidechains, suffers from scaling solutions that fail to serve its long-term health and incentives in any meaningful way. While these solutions are well-intentioned, they do not run on the Bitcoin blockchain, do not contribute to Bitcoin’s security, and ultimately cannot scale to the extent that the Bitcoin blockchain requires.

Again, shifting demand for Bitcoin to sidechains is not the same as demand for Bitcoin.

Expanding Bitcoin block space

True scaling of Bitcoin requires the establishment of Bitcoin and scaling solutions symbiotic relationship between them. Scaling solutions must aggressively and efficiently utilize Bitcoin’s block space to meet demand and improve its performance and throughput without changing its protocol. Expanding Bitcoin’s block space is the only way to transact with the security of Bitcoin. No other alternative can inherit the full security of Bitcoin.

Sharded execution of on-chain verifiability and data is the most secure and incentive-aligned way to scale Bitcoin blocks. The only way to achieve Bitcoin's verifiability is to utilize zero-knowledge or fraud proofs. Since fraud proofs require large amounts of data to be written into Bitcoin, ZK proofs are the most efficient way to verify the validity of Bitcoin's internal execution within the block size limit.

ZK Rollups create an execution shard and batch multiple transactions on that shard while validating them on-chain using concise mathematical proofs and the minimum necessary data to reconstruct the state. This approach enables ZK Rollups to utilize the underlying block space in the most efficient way and extend L1 with its complete security.

Enter Citrea: Bitcoin Security at Scale

Citrea is the first Rollup to run on Bitcoin, increasing the utility of the Bitcoin block space. Citrea keeps demand for Bitcoin within its network and ensures the Bitcoin network serves as the data availability and settlement layer for Citrea transactions. Citrea does not require consensus changes to Bitcoin.

Citrea represents an important milestone for Bitcoin: it is the only execution layer settled on Bitcoin, the first ZK proof verification, and the first universal L2 verification within Bitcoin. Citrea's mission is to drive Bitcoin to the next stage and become the foundation of the world's financial infrastructure, where developers can build everything on top of Bitcoin.

Citrea batches thousands of transactions, processes them through zkVM, and generates concise proofs of validity asserting the correctness of the execution and the correctness of the output data. For the first time in Bitcoin history, Citrea’s proof of validity is inscribed on the Bitcoin blockchain and verified locally. Citrea features a native ZK proof validator smart contract on Bitcoin L1, built on BitVM. Unlike a single sidechain, Citrea creates a modular Bitcoin world through its execution of sharding, keeping settlement and data availability on-chain, i.e. on Bitcoin.

Citrea: Modularity of Bitcoin

Citrea creates stable fee income for miners through data availability, Extends BTC beyond Bitcoin trustlessly through validity proofs and allows developers to build any application on top of its EVM equivalent execution shard.

Extending Bitcoin Security

Citrea is the only execution layer that actually settles on Bitcoin. Every transaction that occurs on Citrea is fully secured by zero-knowledge proofs and verified by Bitcoin. Citrea's execution environment is trustless to Bitcoin and accessible to all participants in the Bitcoin network.

Citrea therefore ensures that Bitcoin equivalent guarantees of data availability, censorship resistance and reorganization are met.

Bitcoin as an Application Cornerstone: Citrea’s mission is to build a programmable liquidity layer on Bitcoin, the most secure and decentralized blockchain. We believe that Bitcoin’s block space must be used to efficiently settle various financial activities, such as buying BTC, leveraging BTC, or lending BTC without trust. Most existing meta-protocols that attempt to provide these capabilities are trustless and inefficient. Citrea is the most efficient and secure platform for building applications on Bitcoin.

Bitcoin settlement and minimum trust two-way peg: For the first time in Bitcoin history, a universal L2 is settled on Bitcoin, achieving the first ever universal minimum trust Trust the two-way hook. Prior to Citrea, Layer 2 solutions relied on honest majority assumptions for multi-signatures, such as open or closed federations. Through Citrea, proofs of validity can be verified on Bitcoin as long as a network participant is honest. BitVM is based on fraud proofs, meaning Citrea’s ZK proofs are being optimistically verified on the Bitcoin network. In the future, a ZK proof validator opcode will enable a completely trustless two-way peg mechanism.

EVM Equivalence:Citrea was built with full EVM equivalence, making it easy for all EVM developers to build applications on Bitcoin. Citrea ships with a Type 2 zkEVM, which is exactly the same as the EVM, built using RISC Zero. By design, Citrea is not limited to a single virtual machine, and due to its modular architecture, new virtual machines, such as WASM virtual machines, can be adopted in the future.

What’s your next plan?

Internal Devnet Enabled

We are running the Citrea development network internally, using synthetic native BTC for critical infrastructure integration and testing. A minimal-trust two-way hook to BitVM is under active development.

Revealing the minimum trust two-way hook for the first time 

Please pay close attention to the reveal of Citrea's minimum trust two-way hook design. We will be announcing our bridge design and open sourcing our codebase in the coming weeks!

Public Testnet 

We are making rapid progress. We have been working hard to build the first Rollup robust testnet on Bitcoin. We're excited to share more information in the coming months. Please follow us on social media and blog.

Build with Citrea 

Our developer contact form is open today. Citrea has been incubated by Chainway Labs for over a year and is fully funded.

Let Bitcoin become the foundation of world finance

Citrea’s vision is to build scalable infrastructure to advance Bitcoin to its next stage and become the cornerstone of world finance. To realize this vision, we believe that Bitcoin’s block space must be enhanced to include more transactions and provide more expressiveness and functionality without changing its consensus rules.

With its security, decentralization and censorship resistance, Bitcoin is perfectly positioned to become the cornerstone of world finance. Recognizing this potential, Citrea relies on these advantages and makes Bitcoin the base layer of a dynamic DeFi, gaming, NFT and other ecosystems. Citrea will continue to grow larger and more advanced with its modular world anchored by Bitcoin.

We are very optimistic about our vision and committed to working towards it.

On Bitcoin, for Bitcoin.