On July 25, BOB (Build on Bitcoin) announced a partnership with zero-knowledge proof infrastructure RISCZero, becoming the first Rollup project in the industry to use zero-knowledge proofs as fraud proofs through the integration of the hybrid architecture framework Kailua launched by RISC Zero. This will significantly save time and costs for existing setups, allowing withdrawal times to be reduced from the usual 7 days to 1 hour. This is a key step for BOB in achieving Rollup Phase 1 and Phase 2 on L2beat.
It is reported that Kailua is the OP stack extension built by RISC Zero using its verifiable computing layer Boundless Network, which is a hybrid architecture that can upgrade Optimistic Rollup to Hybrid ZK Rollup (i.e., Hybrid Rollup), combining the advantages of Optimistic Rollups and ZK Rollup, aiming for efficient blockchain settlement and dispute resolution. It quickly processes transactions by default using Optimistic Rollups and enables ZK proofs when disputes arise or quick settlements are needed, attempting to balance high throughput, low cost, and high security. In simple terms, it integrates the capabilities of ZK Rollup advantages into existing Optimistic Rollups.
One of the most important features is that it can shorten the settlement period from the original 7-day challenge period to as short as 1 hour, and for Optimistic Rollup networks integrated with Kailua, there is no need to worry about the ongoing generation costs of ZK proofs as its implementation is demand-driven.
BOB is the first chain to use zero-knowledge proofs (ZK). Compared to the slow withdrawal speed and expensive fraud prevention mechanisms of Optimistic Rollup and the high operational costs of ZK Rollup, this new approach combines the advantages of both.
BOB perfectly combines efficiency (lower proposal submission costs compared to effective Rollups) and security (proving the correctness of blocks in the event of fraud). Furthermore, ZK proofs are much cheaper than on-chain binary games on current Optimism or Ink platforms (less than $100, while the latter is around $1 million).
Three major advantages:
Anyone can challenge the proposals of the BOB sequencer with a margin as low as 0.5 ETH, without requiring hundreds of ETH like non-zero-knowledge fraud proofs.
In the event of a challenge, the settlement of fraud proofs on Ethereum can be completed with just one transaction, allowing finality within minutes, while non-zero-knowledge versions may take hours or days and involve multiple transactions.
In the long run, the withdrawal time of BOB 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 shortened to a few hours once the system stabilizes.
How it works:
Kailua introduces its independently developed new error-proof game design, providing top-notch security for Rollup transaction ordering.
BOB will use zero-knowledge proofs (ZK) for fraud proofs, instead of multiple transactions in the Optimism binary game.
Zero-knowledge proofs allow BOB to streamline the error-proof mechanism into a single transaction on Ethereum, instead of multiple transactions in the binary game.
Specific process:
For each checkpoint, the BOB sequencer (run by Conduit) will propose a new canonical chain for a specific height.
Anyone can use the Kailua binary file to propose another proposal and dispute the canonical chain. Proposing requires a margin deposit in ETH on Layer 1. The initial setting is 0.5 ETH, allowing anyone to easily challenge and significantly lowering the threshold for permissionless verification.
When conflicting proposals exist at the same height, challengers or validators (run by Conduit) will generate a zero-knowledge proof (using the Boundless algorithm) to resolve the issue with the canonical chain. Since BOB's construction depends on the order of transaction encoding on Layer 1, only one valid chain can exist. The margin of the incorrect proposer or challenger will be forfeited.
Alexei Zamyatin, co-founder of BOB and founding member of the BitVM Alliance, stated: "With the launch of a fully functional proof system, BOB L2 will have complete Ethereum security. This means that BOB will remain at the L2 layer of Phase 0, while most other L2 layers will move to other categories on L2Beat. Phase 1 Rollup solely depends on the launch of a security council, which will be announced in the near future. Phase 2 Rollup only 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 RISC Zero zkVM. Currently, the market choices for Rollup solutions are either Optimistic Rollup or ZK Rollup. Due to low costs and high throughput, Optimistic Rollup dominates the current market but requires a 7-day challenge window, which delays finality and limits interoperability. ZK Rollup can provide fast finality, but generating ZK proofs for each block incurs substantial additional costs, with high throughput Rollups potentially costing millions of dollars or more.
Now the third approach is online on BOB, supported by the Kailua architecture from RISC Zero, which combines the advantages of both methods: Optimistic ZK Rollup has both the low operational costs of the Optimistic Rollup system and the fast finality of ZK Rollup.