BOB on the OP Stack

The first deployment of BOB will be using the OP Stack, benefiting from infrastructure, marketing, and BD support. Eventually, The BOB OP-Stack deployment can be Superchain-compatible making it easy for users to move seamlessly between different Superchain rollups.

We are working on extending the functionality of the OP stack specifically for Bitcoin: (1) fees payable in tokenized BTC via OpenGSN and account abstraction to ensure easy on-ramp from Bitcoin, and (2) in the mid-term, support for Bitcoin as data availability layer. Where possible, we implement smart contracts directly on the EVM to ensure OP stack updates can be directly applied to BOB.

Why OP Stack?

There are several great options for rollups including building them from scratch or using zk rollups (Starkware, zkSync, Polygon, Scroll, ...) and optimistic rollups (Arbitrum, OP Stack, ...). BOB's key values are experimentation, real-world impact, and freedom of choice.

In our opinion, OP Stack aligns best with these values as it allows developers to quickly deploy new protocols without having to reimplement existing protocols in new programming languages. It also gives users choice as the Superchain concept allows users to freely move between OP Stack rollups. We have further considered Arbitrum, but opted against this since, under the hood, Arbitrum uses WASM and has more incompatibilities than OP Stack (albeit the team is working very hard to mitigate those). ZK rollups are too untested for our preference and require smart contract modifications (type 3 and 4 zkEVM rollups), are slow (type 1 and 2 zkEVM), or require complete rewrites of wallet software and integrations (Starkware, ...).

The way that BOB is built, it should be able to be deployed on any EVM rollup stack without major changes.

Adopting the OP Stack

BOB is adopting several changes compared to the standard OP Stack:

Governance

BOB will have its own governance to reach community decisions and enact changes.

If the BOB Rollup chooses to be comptabile with the Superchain, then parts of BOB will be subject to OP governance with the Superchain upgrade. An OP Security Council will be capable of enacting security upgrades of the OP chain and L1 and L2 bridge contracts.

Data Availability

A Data Availability (DA) layer stores the raw inputs to the state transition function for the execution layer. The OP Stack supports multiple DAs but sets Ethereum as the de facto DA. The OP Stack Superchain is considering a dedicated Plasma DA chain to address the rising cost and limited scalability of Ethereum as a DA layer.

BOB is using Ethereum on testnet, but we are investigating alternatives for mainnet launch:

• Bitcoin: The Celestia team has a specification for using Bitcoin as a DA layer that suits BOB needs and would ensure compatibility with Celestia deployments. Using Bitcoin as a DA layer would introduce complexity around wallets as users would need to provide BTC alongside their EVM transactions to pay for DA costs.
• Celestia: If Celestia were to add an option to pay for DA cost in (bridged) Bitcoin, Celestia might become a cheaper and decent alternative to using Ethereum or Bitcoin as a DA layer.

Sequencing

The OP Stack uses a single, centralized sequencer. BOB will use the same sequencer model for its launch but with the addition of merged mining, add functionality to verify the work of the sequencer by Bitcoin miners.

We are welcoming efforts to decentralize sequencers pushed by the OP Stack.

Execution

OP Stack uses a fork of go-ethereum called op-geth. BOB has no changes to op-geth for the testnet deployment.

Instead of changing the execution client, necessary features including proving Bitcoin state and paying transaction fees in bridged Bitcoin are added within the EVM as smart contracts. This has the advantage that:

1. The BOB OP Stack deployment stays up-to-date with improvements made by other teams improving the OP Stack without having to maintain a custom fork. In turn, this ensures that features available on other EVM chains and rollups are 1:1 applicable to BOB.
2. Technology built on top of the EVM within BOB like the merged mining security, paying in bridged Bitcoin for transaction fees, and the BTC light client can be deployed on any EVM chain. BOB is thus ready for a multi-chain and multi-rollup world.

Settlement

The OP Stack settles on Ethereum. Currently, the OP Stack is lacking fraud proofs which is a major downside of using OP Stack as users cannot prove fraudulent behavior by the sequencer.

However, OP Stack has a candidate for fraud proofs in testing and is actively working on increasing the efficiency of fraud proofs using ZK.

We are working towards an approach to add merged mining security to the rollup by making a PoW part of state transition. Consequently, a lack of PoW constitutes a fault by the sequencer. Thereby, Bitcoin miners validate the rollup to offset the trust in the sequencer (in both the centralized and decentralized sequencer case).