London – Thursday 24 July 2025 – BOB (“Build on Bitcoin”) has become the first-ever blockchain to leverage Zero-Knowledge (ZK) proofs as fraud proofs, combining the speed of ZK rollups with the low cost and efficiency of optimistic rollups.
This upgrade, powered by OP Kailua, a new extension of the OP Stack open-source framework for building Ethereum Layer 2 networks, was built by RISC Zero with ZK proof generation by the Boundless network. It brings BOB very close to achieving Rollup Stage 1 and Stage 2 status on L2Beat, a blockchain analytics platform that objectively tracks the security, decentralization, and maturity of layer-2 networks.
Traditionally, rollups have followed two separate models: optimistic rollups, which assume transactions are valid but require a 7-day challenge window, and ZK rollups, which verify transactions instantly via zero-knowledge proofs but are expensive to operate. OP Kailua introduces a third Hybrid ZK option by applying ZK proofs to the fraud resolution process in optimistic rollups, merging low operating costs with fast finality, dramatically increasing scalability, security, and decentralization in layer-2 design.
Alexei Zamyatin, co-founder of BOB, said: “The launch of our fully functioning proof system brings full Ethereum-level security to the BOB L2, a major milestone on our path to decentralization. With this upgrade, BOB becomes the first Hybrid ZK rollup and progresses to Stage 0 on L2Beat, while most rollups in the space will remain in the ‘Others’ category due to the absence of a working proof system. This marks a new standard of trust and transparency for our users. The final step to achieve Stage 1 status is the launch of our security council, which is forthcoming. From there, reaching Stage 2 will involve opening up proposals to anyone. This hybrid rollup model reflects our hybrid vision: combining the strengths of multiple systems to unlock the full potential of Bitcoin DeFi.”
This novel, hybrid approach to fraud proof enables three primary benefits:
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Accessibility: Anyone will be able to challenge proposals by BOB’s sequencer with as little as 0.5 ETH in collateral, compared to the hundreds of ETH needed in a non-ZK fraud proof.
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Efficiency: The fraud-proof settlement in case of a challenge is settled within a single transaction on Ethereum, and therefore finalized within a few minutes, compared to multiple hours or days, which would require many transactions in the non-ZK version.
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Scalability: BOB will be able to reduce withdrawal times drastically in the long term since the on-chain fraud resolution is fast and cheap. This will start with four days but will be reduced to a few hours when the system is proven stable.
Jacob Everly, RISC Zero product lead, commented: “Optimistic rollups have driven adoption thanks to their obvious benefits; low costs and high throughput, but the 7-day challenge window has hindered true scalability. OP Kailua changes that by using ZK fraud proofs. It brings the speed and finality of ZK to an optimistic framework; a breakthrough that combines the best of both worlds and sets the stage for the next evolution of DeFi.”
OP Kailua’s fraud-proof game design provides best-in-class security guarantees for sequencing rollup transactions. Beyond cost and speed, ZK fraud proofs allow BOB to reduce the fraud mechanisms to a single transaction on Ethereum, rather than multiple transactions in the bisection games conventionally used on optimistic rollups.
RISC Zero introduced Boundless in 2024. It is designed to scale zero-knowledge proof (ZKP) compute across chains. The protocol aims to make ZK-powered compute accessible, scalable, and trustless. Boundless enables computations to be performed by a single node and efficiently verified by all others without re-execution. Boundless has collaborated with Conduit, one of the most trusted RaaS (rollup as a service) providers, to test and bring OP Kailua to production.
Andrew Huang, Founder of Conduit, said: “This is exactly what we envisioned when we worked with RISC Zero to build the OP Kailua integration. With ZK fraud proofs running inside our managed OP Stack environment, BOB gets Ethereum-grade security and withdrawal times measured in hours, not days. Our partnership with RISC Zero enables us to integrate this capability into any rollup seamlessly. This is the frontier of rollup design.”
BOB, RISC Zero / Boundless, Conduit, and L2BEAT are hosting a live stream to discuss Hybrid ZK innovation on Thursday, 31 July 2025.
