BOB has launched a major upgrade that makes it the first blockchain to leverage zero-knowledge (ZK) proofs in two novel ways, introducing a new rollup architecture known as a Hybrid ZK Rollup.
The upgrade, built using OP Kailua—a new extension of the OP Stack open-source framework—was developed by RISC Zero with ZK proof generation provided by the Boundless network. BOB now supports ZK fraud proofs for transaction validation and enables validity proofs on demand, combining the strengths of optimistic and validity rollups in a single system.
Introducing Hybrid ZK Proofs
BOB is now the first rollup to implement ZK fraud proofs. Instead of relying on the traditional multi-step fraud resolution process used by optimistic rollups, BOB compresses fraud resolution into a single transaction using a ZK proof. This reduces cost and complexity while maintaining high throughput.
BOB is also the first rollup to offer validity proofs on demand, allowing users to finalize withdrawals to Ethereum instantly. When immediate finality is not needed, BOB defaults to optimistic execution with low transaction costs.
“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.” - Alexei Zamyatin, co-founder of BOB
Four Benefits of OP Kailua
The OP Kailua system enables four key benefits for BOB:
- Accessibility: BOB is the only optimistic rollup that allows anyone to submit a fraud proof. While other rollups may require millions in capital, BOB’s system only requires 0.5 ETH in collateral and less than $50 in proof and onchain submission costs.
- Cost Efficiency: ZK fraud proofs settle disputes in a single transaction on Ethereum. This drastically reduces settlement time compared to non-ZK systems, which require multiple transactions over several hours or days.
- Instant Withdrawals on Demand: Users who want to avoid the standard challenge period can pay to submit a validity proof and finalize their withdrawal immediately. If speed is not a priority, the system defaults to inexpensive optimistic transactions.
- Speed: BOB’s withdrawal times will initially be set to four days but may be reduced to a few hours once the system proves stable.
Built by RISC Zero, Boundless, and Conduit
OP Kailua was developed by RISC Zero and powered by the Boundless network. Introduced in 2024, Boundless is designed to scale ZK compute across chains. It allows computations to be executed by a single node and verified trustlessly by others without re-execution.
To bring OP Kailua to production, Boundless partnered with Conduit, a leading rollup-as-a-service provider that operates the BOB sequencer.
A New Fraud Proof Model
BOB also introduces a new fraud proof game design that replaces the traditional bisection game used by optimistic rollups. For each checkpoint, the BOB sequencer proposes a canonical chain. If a challenger disputes the chain, they can submit an alternative proposal using the Kailua binary and a 0.5 ETH bond on Ethereum.
Boundless is then used to generate a ZK proof that invalidates the incorrect proposal. Only one chain can be valid due to BOB’s design, which relies on the L1 ordering of encoded transactions. The party with the invalid proposal loses their bond, creating a low-cost, permissionless, and secure validation process.
Learn more at BOB’s blog.
