Author: Citrea Team
Translator: DeepTech TechFlow
Today, we introduce Citrea, the first rollup to enhance the capacity of the Bitcoin block space using zero-knowledge technology.
The scalability of Bitcoin has long relied on moving security and demand off-chain and outside of Bitcoin. Citrea aims to extend the entire Bitcoin using zero-knowledge proofs, ensuring on-chain verifiability and data availability within the Bitcoin network. This approach makes Citrea the first extension solution to support more complex applications without compromising Bitcoin's security and changing its consensus rules.
The problem of creating new block space
Bitcoin possesses the most secure, decentralized, and censorship-resistant block space. Extension proposals that create new block space cannot inherit any of these properties of Bitcoin. These proposals or implementations merely provide alternative block space; they do not extend the Bitcoin blockchain. Building sidechains with alternative block space will compete with Bitcoin in the long term and expose users' funds to risks through insecure bridging and network constructions.
Strong demand for Bitcoin block space
Due to its security, decentralization, and resistance to censorship, there is a strong and urgent demand for all on-chain activities related to Bitcoin. Over the past few months, we have observed an exponential increase in interest in Bitcoin block space, reflected in transaction fees. Whether 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, but there is a trade-off: due to high fees and necessary block size limits, some transactions will inevitably be excluded. It is crucial that Bitcoin expands 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 growing inclusivity demands of the Bitcoin block space.
Existing scalability proposals do not extend Bitcoin
Bitcoin has long faced the challenge of handling more transactions and supporting a wider range of applications without compromising its security or core principles. It has proven to be a multifaceted problem that is difficult to comprehensively solve. Some projects attempt to increase Bitcoin's payment throughput, such as the Lightning Network, while others focus on expanding the functionality of BTC assets, such as sidechains. The first scaling proposal failed to bring programmability to Bitcoin, and the latter missed a key point: effectively utilizing the Bitcoin blockchain itself.
In the current landscape, the intrinsic demand for the Bitcoin blockchain has been redirected to separate consensus protocols and sidechains. This approach comes with a trade-off—sacrificing Bitcoin's security and incentive alignment in exchange for lower fees and more BTC use cases. Therefore, Bitcoin-centric sidechains face challenges with extension solutions that do not meaningfully serve its long-term health and incentives. While these solutions have good intentions, they do not operate on the Bitcoin blockchain, do not contribute to Bitcoin's security, and ultimately cannot extend the Bitcoin blockchain as needed.
Once again, the demand for Bitcoin has shifted away from Bitcoin to sidechains.
Extending the Bitcoin block space
Truly extending Bitcoin requires establishing a symbiotic relationship between Bitcoin and extension solutions. Extension solutions must actively and efficiently utilize the Bitcoin block space to expand demand and increase its expressiveness and throughput without changing its protocol. Extending the Bitcoin 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 extend Bitcoin blocks is through sharding with on-chain verifiability and data execution using zero-knowledge or fraud proofs. The only way to easily verify Bitcoin is by utilizing zero-knowledge or fraud proofs. Since fraud proofs require writing a large amount of data on Bitcoin, ZK proofs are the most efficient way to verify execution validity within the block size limit of Bitcoin.
ZK Rollup creates a shard for execution and processes multiple transactions in that shard in batches, using concise mathematical proofs and minimal basic data to verify them on-chain to reconstruct the state. This approach allows ZK Rollup to efficiently and verifiably utilize the underlying block space in the most efficient way, extending L1 with its complete security.
Enter Citrea: Secure extension of Bitcoin
Citrea is the first rollup on Bitcoin that achieves greater expressiveness and enhances the utility of the Bitcoin block space without leaving Bitcoin and ensures that Bitcoin network acts 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 L2 verification within Bitcoin. Citrea's mission is to propel Bitcoin into the next stage, becoming the foundation of the world's financial infrastructure, where developers can build anything on Bitcoin.
Citrea batch processes thousands of transactions, processes them in zkVM, and produces a concise validity proof asserting the correctness of execution and output data. For the first time in Bitcoin's history, Citrea's validity proof is recorded and locally verified on the Bitcoin blockchain. Citrea embeds a native ZK proof validator smart contract on Bitcoin L1 within BitVM. Unlike monolithic sidechains, Citrea creates a modular world for Bitcoin through its execution shards, keeping settlement and data availability on-chain, within Bitcoin.
Modularity of Bitcoin
Citrea creates stable fee income for miners through data availability, extends BTC to Bitcoin in a trustworthy manner through validity proofs, and allows developers to build anything on its EVM-equivalent execution shards.
Extending Bitcoin's security
Citrea is the only truly Bitcoin-settled execution layer on Bitcoin. Every transaction on Citrea is fully secured 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 it meets the equivalent data availability, resistance to censorship, and resistance to reorganization guarantees as Bitcoin.
Bitcoin as the foundation of applications: Citrea's mission is to build a programmable liquidity layer on the most secure and decentralized blockchain, Bitcoin. We believe that Bitcoin block space must be efficiently utilized for various financial activities, such as trustless BTC purchases, leveraging BTC, or borrowing BTC. Most existing meta-protocols attempting to provide these functionalities are trustful and inefficient. Citrea is the most efficient and secure platform for building applications on Bitcoin.
Bitcoin settlement and minimal trust bidirectional peg: In the history of Bitcoin, general L2 settlement has been achieved on Bitcoin for the first time, resulting in the first general trust-minimized bidirectional peg. Prior to Citrea, second-layer solutions relied on the honest majority assumption of multi-signatures, 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, meaning that Citrea's ZK proofs are optimistically verified on the Bitcoin network. In the future, a ZK proof validator opcode will enable a fully trustless bidirectional anchoring mechanism.
EVM equivalence: Building with Citrea's fully EVM-equivalent construction allows all EVM developers to easily build on Bitcoin. Citrea has released a Type 2 zkEVM that is fully equivalent to EVM, built using RISC Zero. Citrea is not limited to a single VM in its design and, thanks to its modular architecture, can adopt new VMs in the future, such as the WASM VM.
What’s Next?
Internal Devnet Enabled
We are running the Citrea Devnet, using synthetic native BTC for critical infrastructure integration and testing. The trust-minimized bidirectional peg with BitVM is currently under active development.
Revealing the First Trust-Minimized Bidirectional Anchoring
Stay tuned for Citrea's trust-minimized anchoring design. We will announce our bridging design in the coming weeks and open-source our codebase!
Public Testnet
We are rapidly progressing. We have been working hard to build a robust testnet for the first rollup on Bitcoin. We are excited to share more information in the coming months.
Building with Citrea
Our developer contact form is now open. Citrea has been incubated by Chainway Labs for over a year, with sufficient funding.
Making Bitcoin the Foundation of the World's Finance
Citrea's vision is to build scalable infrastructure, propelling Bitcoin into its next stage as the foundation of the world's finance. To achieve this vision, we believe it is necessary to enhance the Bitcoin block space to include more transactions and provide more expressiveness and functionality without changing its consensus rules.
With its security, decentralization, and resistance to censorship, Bitcoin is fully capable of becoming the foundation of the world's finance. Citrea is built on these strengths, making Bitcoin the foundational 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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