BOB has become the first blockchain to leverage Zero-Knowledge (ZK) proofs in two new ways: 

  1. BOB is the first rollup to use ZK fraud proofs, reducing a many transaction flow to a single transaction. This combines the speed of validity rollups with the low cost and efficiency of optimistic rollups. 
  2. BOB is also the first rollup to support validity proofs on demand. When users want to withdraw quickly from BOB, a validity proof can be submitted to finalize withdrawals to Ethereum instantly.

We believe that Hybrid ZK proofs are the best and most efficient proving system in use by any rollup today.

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 also 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. 

Join a livestream to discuss the upgrade with BOB, RISC Zero, Boundless, Conduit and L2BEAT on Thursday 31 July: https://x.com/i/broadcasts/1lPJqMbWdbQJb 

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ZK Fraud Proofs on BOB

Traditionally, rollups have followed two separate models: optimistic rollups, which assume transactions are valid but require a 7-day challenge window, and validity 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. This model also gives the option to submit validity proofs on demand, allowing BOB to get the best of both worlds: cheap transactions and verification by default, and full validity for fast finality when required.

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 hybrid rollup model reflects our hybrid vision: combining the strengths of multiple systems to unlock the full potential of Bitcoin DeFi.”

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Four Key Benefits of OP Kailua 

This novel, hybrid approach to proofs enables four primary benefits: 

  • Accessibility: In comparison to all other optimistic rollups, BOB is the only one that actually allows anyone to submit a fraud proof. In Arbitrum, Base, Optimism, etc., fraud proofs can cost millions of dollars. With BOB, only 0.5 ETH collateral is needed, and the proof plus onchain submission costs less than $50.‍
  • Cost 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.‍
  • Instant withdrawals on demand: If someone wants to settle withdrawals immediately without a challenge period, it is possible to pay to submit a validity proof. This is not possible with normal optimistic models which must enforce the full 7 day period. However, if immediate withdrawal is not needed, then the Hybrid ZK system defaults to cheap optimistic transactions. Full ZK chains like Starknet, zkSync, and Scroll always need validity proofs even when speed is not as important, pushing up their operating costs.
  • Speed: 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 could 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.”

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Powered by RISC Zero, Boundless & Conduit

The OP Kailua upgrade on BOB was built by RISC Zero with ZK proof generation by the Boundless network. 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 proofs running inside our managed OP Stack environment, BOB gets Ethereum-grade security and withdrawal times measured in hours, not days. And our partnership with RISC Zero allows us to bring this capability to any rollup seamlessly. This is the frontier of rollup design.”

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Step by step fraud proof process

OP Kailua introduces its own novel fraud proof game design which provides best in class security guarantees for sequencing rollup transactions. Instead of using the usual Optimism bisection game for fraud proofs, BOB will use ZK fraud proofs. This allows the fraud proof mechanism to be reduced to a single transaction on Ethereum rather than the multiple transactions needed in the bisection game.

  1. For each checkpoint, the BOB sequencer (run by Conduit) will propose a new canonical chain for a particular height.
  2. Anyone can still make another proposal to dispute the canonical chain using the Kailua binary. To make a proposal requires a bond (in ETH) deposited on the L1. This is initially set to 0.5 ETH, making it relatively trivial for anyone to challenge and drastically lowering the barrier for permissionless validation vs most optimistic chains.
  3. When contradictory proposals exist for the same height either the challenger or the Validator will use Boundless to generate a ZK proof that invalidates the provably incorrect proposal. There can only be one valid chain due to the construction of BOB depending on the ordering of encoded transactions on the L1. The incorrect proposer or challenger will have their bond slashed.

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Live stream on 31 July

BOB, RISC Zero / Boundless, Conduit and L2BEAT are hosting a live stream to discuss Hybrid ZK innovation on Thursday 31 July 2025. Set a reminder here: https://x.com/i/broadcasts/1lPJqMbWdbQJb