Merged Consensus

The execution layer (e.g., rollups) processes transactions and computes state changes independently from the consensus layer. This separation allows for:

• High-throughput transaction processing using optimized virtual machines (VMs).
• Specialized execution environments (e.g., EVM, SVM, Cairo VM).
• The ability to batch and compress transaction data before submitting proofs to the consensus layer.

A robust, decentralized base layer (like Ethereum) provides ultimate settlement and consensus. Its primary roles are:

• Finality: Providing cryptographic guarantees that state transitions are irreversible.
• Data Availability: Publishing and guaranteeing access to the transaction data from execution layers.
• Dispute Resolution: Hosting fraud-proof or validity-proof verification mechanisms to ensure execution correctness.

A critical security mechanism where light nodes randomly sample small pieces of published data to probabilistically verify its availability without downloading the entire dataset. This enables secure scaling by allowing nodes with limited resources to trustlessly confirm that data is published and accessible for fraud proofs.

Cryptographic systems that bridge the execution and consensus layers, ensuring state correctness.

• Validity Proofs (ZK Proofs): Succinct cryptographic proofs (e.g., zk-SNARKs, zk-STARKs) that verify the correctness of a batch of transactions. Settlement is immediate upon proof verification.
• Fraud Proofs: A challenge-response system where verifiers can dispute invalid state transitions during a dispute window, rolling back incorrect changes.

Merged consensus enables a modular stack where different components can be swapped or upgraded independently. This creates an ecosystem where:

• Multiple execution layers (rollups) settle to a shared consensus layer.
• Innovations in one module (e.g., a new VM) don't require changes to the underlying consensus.
• Inter-blockchain communication becomes more standardized via the shared settlement layer.

Ethereum's roadmap exemplifies merged consensus. Ethereum L1 acts as the consensus and data availability layer. Rollups (like Arbitrum, Optimism, zkSync) act as execution layers. They:

• Process thousands of transactions off-chain.
• Post compressed data and a proof (validity or fraud) to Ethereum.
• Inherit Ethereum's security for final settlement, creating a secure scaling solution.

All Polygon PoS chains (now validiums) and zkEVM Layer 2s inherit security from a single, robust Ethereum Layer 1. This creates a shared state across the ecosystem, enabling:

• Atomic composability: Seamless cross-chain transactions.
• Trust-minimized bridging: Assets move securely without new trust assumptions.
• Collective security: No individual chain needs to bootstrap its own validator set.

By separating the consensus layer (Polygon's Proof-of-Stake network) from the execution layer (multiple zkEVM chains), the system can add new execution environments without congesting the base layer. This enables:

• Linear scaling: Throughput increases with each new chain.
• Specialized chains: Chains can be optimized for specific use cases (e.g., gaming, DeFi).
• No shared gas market: Activity on one chain does not increase fees on others.

The zkEVM chains within the architecture offer varying degrees of EVM equivalence, allowing developers to deploy existing Ethereum smart contracts with minimal changes. This provides:

• Developer familiarity: Use standard tools like Solidity, Hardhat, and MetaMask.
• Code portability: Migrate dApps without costly rewrites.
• Ecosystem access: Tap into Ethereum's vast liquidity and user base.

Merged Consensus leverages validium and volition models, where transaction data is posted off-chain but secured by zero-knowledge proofs. This offers a spectrum of choices:

• High-throughput validiums: Data availability managed by a Data Availability Committee (DAC) or EigenDA.
• Enhanced security volitions: Users can opt for Ethereum-caliber data availability for critical assets.
• Cost efficiency: Reduces L1 gas costs by ~90% compared to pure rollups.

The Interoperability Layer and shared bridge architecture abstract away chain boundaries for users. Benefits include:

• Single deposit address: Fund any chain from a unified bridge.
• Near-instant finality: Cross-chain transfers settle in seconds, not minutes.
• Unified ecosystem: Liquidity is not siloed, creating a seamless network effect.

The architecture is inherently modular, allowing components to be upgraded or replaced independently. This ensures:

• Protocol agility: New zk-proof systems or DA solutions can be integrated.
• Sovereign execution: Chains maintain control over their execution logic and governance.
• Sustainable evolution: The network avoids monolithic bottlenecks and can adapt to new cryptographic primitives.

The core security guarantee of merged consensus is that the primary chain's (L1) finality is the ultimate source of truth. The secondary chain (e.g., an L2) cannot revert a block that has been finalized on the L1. This creates a cryptoeconomic security inheritance model, where the secondary chain's security is a subset of the L1's. The key assumption is that the L1's consensus (e.g., Ethereum's proof-of-stake) is secure and live.

For fraud proofs or validity proofs to be verifiable, the transaction data (calldata or blobs) must be available on the L1. This is a critical trust assumption: users must trust that this data is published and cannot be withheld. Systems relying on Data Availability Committees (DACs) or optional data availability introduce an additional trust layer, as users must trust the committee's liveness and honesty to reconstruct the chain state.

Most merged consensus systems (like Optimistic Rollups) have a privileged sequencer responsible for ordering transactions. This creates a centralization vector:

• Censorship Risk: The sequencer can delay or exclude transactions.
• Liveness Risk: If the sole sequencer fails, the chain halts until a recovery mechanism (like a forced L1 inclusion) is triggered.
• MEV Extraction: The sequencer has privileged access to transaction order, enabling maximal extractable value (MEV) capture. Decentralizing the sequencer set is a major security challenge.

In Optimistic Rollups, a challenge period (typically 7 days) is a fundamental security parameter. During this window, any honest validator can submit a fraud proof to invalidate an incorrect state root. Users must trust that at least one honest, watchful validator exists. For ZK-Rollups, this trust assumption is removed, as validity is proven cryptographically, allowing for near-instant withdrawals. The security of withdrawals depends entirely on the correct functioning of the bridge smart contract on the L1.

The smart contracts governing the bridge and proof verification on the L1 are often upgradeable via a multisig or DAO. This introduces a trust assumption in the governance mechanism. If the upgrade keys are compromised or act maliciously, they could steal all bridged funds or alter security parameters. The security model degrades to the social consensus of the governing body, which may be a small group of developers or token holders.

Merged consensus enables secure cross-chain messaging (e.g., from L2 to L1). The security of these messages relies on the integrity of the state proofs posted to the L1. For ZK-Rollups, this is a validity proof (ZK-SNARK/STARK). For Optimistic Rollups, it's the assumption that no valid fraud proof was submitted during the challenge window. A failure in this verification breaks the atomicity and trustlessness of cross-chain applications.