Despite Bitcoin’s success, the Bitcoin network faces key technical challenges such as:
High transaction fees
Slow transaction times
Limited programmability
Rollups offer a promising solution to these issues by batching non-standard transactions and publishing them to the Bitcoin blockchain, significantly reducing computational costs while maintaining Bitcoin’s security guarantees.
Layer 2 solutions that leverage Bitcoin for securely storing data and finalizing transactions can boost the overall transaction throughput and functional capabilities of Bitcoin. Rollups can enhance Bitcoin's scalability by batching non-standard transactions and publishing them to Bitcoin. They can also increase the capacity and usage of the network, providing more consistent revenue to miners, ensuring the long-term sustainability of the Bitcoin network.
Building a Bitcoin Rollup
Building a ZK rollup on Bitcoin demands a different perspective toward designing a proving architecture due to the limited nature of Bitcoin script. Citrea includes both blockspace and execution proving in the circuit to overcome some of these challenges. Together these ZK proofs enable Citrea to inherit many of Bitcoin’s security guarantees and censorship resistance. RISC Zero’s technology enabled Citrea to build a custom proving environment quickly and effectively, tailored specifically for Bitcoin and BitVM.
Powering Citrea’s Proving
With RISC Zero’s technology, Citrea is pioneering the first Bitcoin zkRollup. Citrea also launched an open-source BitVM Groth16 verifier, allowing developers to generate STARK proofs using RISC Zero, wrap them into Groth16 proofs, and create corresponding verifiers in the rv32i and BitVM instruction sets. This advancement was crucial for verifying proofs on Bitcoin.
By leveraging RISC Zero, Citrea is able to power a programmable ZK execution layer that enables developers to make Bitcoin the bedrock for diverse onchain apps and financial activities.
The RISC Zero STARK Prover is integral to Citrea’s operation, generating batch proofs by processing numerous Citrea blocks and outputting the state changes, initial state root, and latest state root. These proofs are then verified with bitVM enabling the verification of all Citrea blocks on the Bitcoin base layer. The RISC Zero’s zkVM allows Citrea to develop a custom ZK proving system swiftly, thanks to the native Rust interface and easy support for bitVM, overcoming the challenges posed by Bitcoin's limited script capabilities.
Instead of inscribing raw transaction data on Bitcoin, Citrea records the resulting state differences (state diffs) from transactions in a batch. These state diffs reflect only the changes to an account’s nonce, balance, storageRoot, or codeHash. Consequently, aggregating a large batch of transactions becomes more cost-effective, as the data cost is distributed among each transaction. RISC Zero’s technology ensures this process is efficient, secure, and scalable by providing a highly performant and robust zkVM which acts as the core foundation behind all Citrea batches.
The RISC Zero zkVM is also core to Citrea’s light client proofs. Using composition every batch proof can be recursed together into a singular root proof, which when verified can attest to the entire state of Citrea. Together these zkVM proof systems have enabled Citrea to build a trust minimized two way native ZK bridge between Bitcoin and Citrea enabling the network to tap into the deep Bitcoin ecosystem.
Conclusion
Citrea fully relies on Bitcoin for batch sequencing and liveness. Its distinctive proving mechanism scans every single Bitcoin block without skipping, proves them with a zkVM based type 2 zkEVM, and then verifies the proofs on Bitcoin using bitVM. This meticulously designed architecture, enabled by RISC Zero, inherits Bitcoin’s censorship resistance, data availability guarantees, and validity through BitVM.
As a result, Citrea stands out as one of the only scalability solutions that is strictly tied to Bitcoin, leveraging its unparalleled security to provide a scalable and efficient platform for onchain applications. By harnessing the power of verifiable offchain execution and batch proving, powered by RISC Zero, Citrea is paving the way for a more scalable, secure, and versatile Bitcoin ecosystem.
To dive deeper into Citrea’s batch proving design, check out their blog post:
