The tl;dr - Build on Bitcoin

Build on Bitcoin (BOB) is a protocol that looks to be the bridge between Bitcoin security and Ethereum liquidity. It’s getting a lot of headlines right now due to a recent $10m fundraising round and having over $100m value locked in their BOB Fusion smart contract (less than a week after opening deposits), a contract where users can deposit Ethereum assets and earn “points” prior to its mainnet launch.

When looking at BOB’s docs, you’ll see that it’s an OP Stack rollup settling on Ethereum. So, what’s its relationship with Bitcoin?

The tl;dr

BOB is an Ethereum rollup that leverages the OP stack and an EVM-compatible virtual machine. They are launching with the BOB SDK, which will enable developers on BOB to build applications that will add utility to BTC the asset. They are actively researching how they can inherit more security from Bitcoin miners and full nodes.

This iterative approach of launching an Ethereum rollup, with the intention of migrating the some of its architecture to support a Bitcoin L2 in the future, is different to many of the sidechains building on Bitcoin today. While things like BitVM seem promising, the technology to build trust minimized validating bridges, for rollups on Bitcoin, is still under development.

This is why BOB is choosing to launch on Ethereum, and continually add features that see the protocol derive more security from Bitcoin miners and full nodes. Related to Bitcoin, their two primary research areas are 1) Optimine: a way for Bitcoin miners to merge-mine the BOB sequencer and add Proof-of-Work verification to BOB blocks and 2) a BitVM bridge program that would provide a trust-minimized two-way peg for Bitcoin assets on the BOB chain.

Let’s dive into the tech stack and roadmap for BOB.

Tech stack

EVM based

BOB is EVM-based and looks to tap into the developer resources and tooling that have been built up over the years in Ethereum. This is certainly a strategic decision, because when looking at the Electric Capital developer report, you see a number of metrics that show that onchain applications are initially deployed on the EVM. This means that BOB can easily port over applications to its own execution environment, and also tap into Ethereum’s developer resources.

BOB SDK

Bitcoin Wallet React Hooks

The BOB developer SDK enables developers to add Bitcoin functionality to Ethereum applications. For example, Metamask could use the sats-wagmi Bitcoin Wallet React Hooks to allow it to send Bitcoin, create inscriptions, and more.

This means that users can stay on the applications they’re accustomed to and experience new Bitcoin features.

Bitcoin Light Client

BOB also leverages a Bitcoin light client. This light client verifies past transactions on the Bitcoin main chain, and also allows a smart contract on BOB to verify Bitcoin block headers. This gives BOB a view into Bitcoin state, and will be useful for future bridge programs.

Current research

BOB is launching on mainnet this month (April 2024). And while the BOB SDK is the primary connection to Bitcoin at the application level, the team is dedicating resources to a number of research initiatives that will see the network derive more security from Bitcoin miners and full nodes.

Optimine

Optimine is a novel solution that sees blocks produced by BOB’s sequencer be verified by Bitcoin miners. Leveraging merge-mining, a group of Bitcoin miners would delegate a percentage of their resources (hashrate) to verify that BOB’s blocks were produced correctly. In the event the sequencer misbehaves, block building and proposing would simply fall back to the miners participating in merge-mining BOB.

BOB blocks cannot be executed without this check from Bitcoin miners merge-mining BOB. This technology would see BOB derive security from a portion of the Bitcoin miner set.

BitVM

BitVM enables Turing-complete offchain computation that can be verified on Bitcoin. Currently, BitVM is being researched as a mechanism to secure Bitcoin bridges in a more trust-minimized fashion, and also verify SNARKs containing rollup state transitions.

BOB is currently researching how it can leverage BitVM to secure its Bitcoin bridge. In this setup, BOB would need to run a Bitcoin light client (see above) and a BitVM bridge program would also need to execute an Ethereum light client to verify BOB’s L1 smart contract.

The bridge would operate under a 1-of-N trust assumption, meaning that the Bitcoin bridge is secure as long as only one of the verifiers is honest. The initial BitVM concept involved a federated set of verifiers, but the recent BitVM2 model would support permissionless verification.

BOB co-founder, Alexei Zamyatin, drafted a write up on BitVM supporting Bitcoin sidechain bridges. He is also referenced on the BitVM website as a research collaborator.

Users pay gas in wrapped BTC

BOB will enable users to pay gas fees (fees to BOB’s sequencer) in a wrapped BTC token. From a user perspective, this means that users who deposit (or acquire) BTC into BOB and leverage Bitcoin-centric applications on the network, will never have to interact with a token that is not a wrapped form of BTC.

Hybrid Bitcoin and Ethereum rollup

BOB plans on becoming a hybrid L2 between both Ethereum and Bitcoin. In their proposed long-term design, Ethereum validators would verify proofs (or continue to leverage a fraud proof mechanism) submitted to BOB’s L1 smart contract to ensure execution was done correctly. On the Bitcoin side, Bitcoin full nodes participating in their BitVM bridge program would run and verify an Ethereum light client to ensure that execution was done correctly, and additionally participate in securing the two-way peg for Bitcoin assets.

This would see the L2 have a trustless bridge contract on Ethereum, and a trust-minimized bridge with Bitcoin. Bridges from both chains would be under the best security models available to them. This is why they could be considered a “Hybrid L2”.

It is my understanding that in this model, BOB would still use Ethereum for data availability. In relation to Bitcoin, we view Ethereum as the second most secure DA layer for L2s. It’ll be interesting to see if they plan to use Ethereum DA long-term, switch to Bitcoin DA, or use another solution.

Open research questions

I have a few open questions around the work BOB is doing, and how they plan to further integrate with Bitcoin:

• When looking at the BOB roadmap, they state that they want to leverage BitVM to support “BTC-staked” sequencers. This idea has been mentioned a few times on Twitter, and it’ll be interesting to learn more about their prospective design. I’m also curious how this will integrate with Optimine.

• I’m also curious about the long-term solution for data availability for the BOB chain. Since it is hoping to be a hybrid L2 between Bitcoin and Ethereum, we question whether it needs to use either chain for data availability. An interesting research area is leveraging an alternative DA solution for higher performance and throughput, and checkpointing it against Bitcoin and Ethereum for additional security measures.

To finish

BOB is an Ethereum rollup that is planning on integrating additional protocols to strengthen the security it derives from Bitcoin miners and full nodes. Its long-term roadmap sees it become a Hybrid L2, inheriting security from both Ethereum and Bitcoin. I’m looking forward to their mainnet launch and seeing the added functionality BOB provides for Bitcoin the asset. And I’m especially excited about the research they’ll be doing on Bitcoin-native security.

If you’re interested in building on BOB, join their community channels here. You can also join the Bitcoin Layers community chat here.