Bitcoin Lacks True ZK Rollups Due to Architectural Constraints
Bitcoin enthusiasts often encounter terms like “zk rollups,” but the reality is complex. Ethereum’s zk rollups utilize zero-knowledge proofs for trustless verification, whereas Bitcoin faces architectural constraints that hinder the implementation of true zk rollups.
Misleading claims about Bitcoin’s layer-2 capabilities and trust assumptions can confuse users. The Bitcoin builder community is addressing these issues.
Ethereum zk rollups use the Ethereum mainnet (layer-1) to verify cryptographic proofs on-chain. Bitcoin’s 4MB block size and limited scripting capabilities cannot support these computationally intensive verifiers. Alexei Zamyatin, a core contributor to BitVM, states that a zk rollup verifier “simply won’t fit into a block.”
Projects like BitVM adopt a different strategy by combining SNARK-based compression with optimistic verification. Instead of verifying proofs on-chain, BitVM divides a program into smaller, sequentially verifiable chunks, resembling optimistic rollups like Arbitrum rather than Ethereum’s zk rollups.
“The zk part stops at the compression stage,” notes Zamyatin. “Everything else is an optimistic fraud-proof system.”
The conversation about Bitcoin L2s extends beyond rollups. Janusz Grzegorz, founder of Bitcoin Layers, has discussed the evolving definition of Bitcoin L2s. Initially, projects were evaluated based solely on their claim of being a “Bitcoin L2,” excluding meaningful activities such as BTC-backed tokens on Ethereum L2s.
Grzegorz advocates for focusing on trust assumptions instead of semantics, suggesting that bridges underpinning these systems should be evaluated regardless of their marketing as an L2. Examples include protocols like Rootstock and tBTC on Arbitrum.
Grzegorz, known as “Janusz,” has also identified projects like Merlin and BitcoinOS as sources of confusion. These projects often market themselves as zk rollups on Bitcoin without the necessary infrastructure for security guarantees typical of genuine rollups. He advises users to examine projects for tangible progress, such as distributed or optimistic bridges, instead of relying on buzzwords.
Zamyatin emphasizes the importance of education and transparency. By concentrating on trust assumptions, clear definitions, and thorough analyses, the Bitcoin community can better grasp feasible innovations. Currently, zk rollups on Bitcoin are misrepresented, but initiatives like BitVM show potential for scaling within Bitcoin's specific constraints.
To achieve true zk rollups, Bitcoin requires a soft fork to enable a native zk verifier. Several new opcodes are under consideration in light of a recent push to clarify Bitcoin’s future roadmap.
An open Wiki-style forum allows stakeholders to express preferences and justify their views. Zamyatin’s team at Build on Bitcoin (BOB) plans to contribute soon.
“This effort collects rationales in one place — instead of debating on Twitter or scattered threads, you can see reasoning for each stance,” said Zamyatin. “It facilitates evaluating proposals, summarizing pros and cons, and having meaningful discussions.”