BOB has announced the integration of the RISC Zero into the Kailua architecture, making it the first Rollup project to use ZK Proof as a fraud proof.

2025-07-25 12:17

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BOB Introduces ZK Fraud Proof, Rollup Architecture Undergoes Paradigm Shift

On July 25, BOB (Build on Bitcoin) announced a partnership with the zero-knowledge proof infrastructure RISCZero. They will launch the industry's first Rollup project using zero-knowledge proof as a fraud proof through the integration of RISC Zero's hybrid architecture framework, Kailua. This integration will significantly reduce the time and cost of existing setups and can reduce withdrawal times from the usual 7 days to 1 hour. This is a crucial step for BOB in achieving Rollup Phase 1 and Phase 2 on L2beat.

Reportedly, Kailua is an OP stack extension built by RISC Zero on its verifiable computation layer Boundless Network. It is a hybrid architecture that can upgrade Optimistic Rollup to Hybrid ZK Rollup (Hybrid Rollup), combining the advantages of Optimistic Rollups and ZK Rollup to achieve efficient blockchain settlement and dispute resolution. It rapidly processes transactions by default using Optimistic Rollups and enables ZK proofs when disputes arise or quick settlements are needed, aiming to balance high throughput, low costs, and high security. In simple terms, it provides the ability to integrate existing Optimistic Rollups with the advantages of ZK Rollup.

Its most significant feature is its ability to shorten the settlement period from the original 7-day challenge period to as little as 1 hour, without the ongoing generation cost of ZK proofs for Optimistic Rollup networks integrated with Kailua because of its on-demand intervention implementation.

BOB is the first chain to use zero-knowledge proofs (ZK). In contrast to the slow withdrawal speed and expensive anti-fraud mechanisms of Optimistic Rollup, and the high operating costs of ZK Rollup, this new approach combines the advantages of both.

BOB perfectly combines efficiency (lower cost of submitting proposals compared to an Effective Rollup) and security (ability to prove block correctness in case of fraud). In addition, ZK proofs are much cheaper than on-chain binary games costs on current platforms like Optimism or Ink (less than $100 compared to about $1 million).

Three Key Advantages

Anyone can challenge a BOB sequencer proposal with a deposit as low as 0.5 ETH, without the need for hundreds of ETH as in a zk-rollup fraud proof.

In case of a challenge, fraud proof settlement on Ethereum requires only one on-chain transaction, thus finality can be achieved within minutes, compared to hours or days and multiple transactions in the zk-rollup version.

In the long run, BOB's withdrawal time can be significantly shortened as on-chain fraud resolution is fast and cost-effective. The initial estimated withdrawal time is 4 days, which can be reduced to a few hours once the system is stable.

How It Works

· Kailua introduces its proprietary and innovative fault-tolerant game design to provide top-notch security for Rollup transaction sequencing.

· BOB will use zero-knowledge (ZK) proofs for fraud proofs, rather than multiple transactions in Optimistic Rollup's game of binary fraud.

· Zero-knowledge proofs enable BOB to streamline the fault-tolerance mechanism into a single on-chain transaction on Ethereum, as opposed to multiple transactions in the binary fraud game.

Specific Process

· For each checkpoint, the BOB sequencer (operated by Conduit) will propose a new canonical chain for a specific height.

· Anyone can propose another proposal using the Kailua binary, challenging the canonical chain. Proposing a proposal requires a deposit in ETH on Layer1. The initial setting is 0.5 ETH, allowing anyone to easily challenge and significantly lowering the permissionless validation threshold.

· When conflicting proposals exist at the same height, a challenger or validator (operated by Conduit) will generate a zero-knowledge proof (using the Boundless algorithm) to resolve the canonical chain's issue. Since BOB's construction relies on the sequence of transactions encoded on Layer1, only one valid chain can exist. The deposit of the incorrect proposer or challenger will be slashed.

BOB co-founder and BitVM Alliance founding member Alexei Zamyatin stated: "With the introduction of a feature-complete proof system, BOB L2 will have full Ethereum security. This means that BOB will still be in the Layer 2 phase 0, while most other Layer 2s will be moving to other categories on L2Beat. The phase 1 Rollup depends only on the launch of a security council, which will be announced shortly. The phase 2 Rollup simply requires opening the proposal process to anyone."

The RISC Zero team believes that Kailua will upgrade Optimistic Rollup to use zero-knowledge proofs (ZK Fault Proofs) powered by the RISC Zero zkVM. Currently, the market's choice for Rollup solutions is either Optimistic Rollup or ZK Rollup. Due to its low cost and high throughput, Optimistic Rollup dominates the current market, but it requires a 7-day challenge window, which delays finality and limits interoperability. ZK Rollup can provide fast finality, but generating a ZK proof for each block incurs significant additional costs, and the cost of high-throughput Rollup could be in the millions of dollars or more.

Now a third way has been launched on BOB, supported by the solution provided by RISC Zero's Kailua architecture. It combines the advantages of both methods: Optimistic ZK Rollup has the low operating costs of an Optimistic Rollup system and the fast finality of ZK Rollup.

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