Hybrid Rollup

This architecture uses an optimistic execution model for speed, where transactions are processed and published to L1 immediately. A zero-knowledge validity proof (e.g., a zk-SNARK) is generated and submitted later to finalize state. This separates the fast path (optimistic) from the secure, final settlement path (ZK).

• Primary Benefit: Achieves near-instant confirmation for users while inheriting the strong cryptographic security of ZK proofs for finality.
• Trade-off: Requires a challenge period for fraud proofs (like optimistic rollups) if the ZK proof fails, or a separate proving system.

In this model, transaction execution and state transition validity are secured by zero-knowledge proofs, providing immediate cryptographic finality. However, transaction data is posted off-chain or to a separate data availability layer, relying on an optimistic or fraud-proof-based system to ensure data is retrievable.

• Primary Benefit: Dramatically reduces L1 data publishing costs (the main expense for ZK rollups) while maintaining ZK-level security for execution.
• Example Scenario: A rollup that posts only state roots and proofs to Ethereum, but places transaction data on a celestia or eigenlayer-based DA layer.

Hybrid designs often split core rollup functions across different layers. A common pattern is using one chain for settlement and dispute resolution (Store of Value, SoV) and another for data availability (DA).

• Settlement Layer: Typically a high-security L1 like Ethereum, which handles final proof verification and serves as the trust anchor.
• DA Layer: A specialized, cost-optimized chain (e.g., Celestia, Avail, EigenDA) that provides cheap, scalable data publishing. This is the modular blockchain paradigm applied to rollup design.

Hybrid rollups exist on a spectrum between pure optimistic and pure ZK designs, allowing developers to fine-tune for specific variables:

• Time to Finality: Optimistic for speed, ZK proofs for instant cryptographic finality.
• Cost Structure: ZK proofs are computationally expensive to generate but cheap to verify; optimistic models have lower overhead but higher L1 data costs.
• Security Assumptions: Balances economic security (fraud proofs & bonds) with cryptographic security (validity proofs).
• EVM Compatibility: Often easier with optimistic execution, but ZK provers are catching up.

The hybrid model is foundational for sovereign rollups. A sovereign rollup uses a separate chain (like Celestia) purely for data availability and publishes only data and state roots to it. It handles its own settlement and dispute resolution, making it "hybrid" by divorcing execution and DA from traditional settlement.

• Key Innovation: Enables a rollup to be its own settlement layer, choosing when and how to bridge to other ecosystems.
• This architecture maximizes flexibility and minimizes vendor lock-in, representing a next-step evolution from hybrid L2s.

A hybrid rollup's security is defined by its two-stage proving system. Transaction execution is initially assumed valid under an optimistic framework, with a challenge period (e.g., 7 days) for fraud proofs. Concurrently or periodically, a ZK-SNARK or ZK-STARK is generated to provide cryptographic finality, compressing the state transition. This creates a security gradient from economic security (optimistic) to cryptographic security (ZK) over time.

The model explicitly transitions trust assumptions:

• Short-term: Relies on at least one honest actor to submit a fraud proof during the challenge window. This is an economic honesty assumption.
• Long-term: After the ZK proof is verified on the base layer (e.g., Ethereum), security reduces to the cryptographic soundness of the ZK system and the base layer's security. This provides cryptographic finality, eliminating the need for further trust. This hybrid approach offers faster soft confirmation for users, with hard finality deferred until ZK proof submission.

Like all rollups, hybrid models require guaranteed data availability for security. Transaction data must be posted to a secure, available layer (typically the base chain's calldata or a dedicated data availability committee). This is non-negotiable for the optimistic phase, as verifiers need the data to construct fraud proofs. The choice of DA layer directly impacts security, cost, and censorship resistance. A failure in DA halts fraud proofs, forcing reliance solely on the slower ZK finality path.

A critical safety mechanism is the user's ability to exit the rollup if the sequencer is malicious or unresponsive. This relies on:

• Inclusion proofs: Users must prove their state membership from data published to the DA layer.
• Challenge period: In optimistic mode, exits are delayed until the fraud proof window passes.
• ZK finality proof: Once a ZK proof is verified, exits can be immediate based on the proven state. The design and timeliness of these exit games are a primary security audit focus.

Most rollups, including hybrids, use a single sequencer to order transactions, creating a centralization point. Risks include:

• Censorship: The sequencer can refuse to include transactions.
• MEV extraction: The sequencer can manipulate transaction order for profit.
• Liveness failure: If the sole sequencer goes offline. Security relies on the force-inclusion mechanism allowing users to submit transactions directly to the base layer contract, albeit with higher cost and latency. Decentralizing the sequencer set is a key security upgrade path.

The system depends on two prover roles:

1. Fraud Provers (Watchers): Economically incentivized to monitor and challenge invalid state transitions. Their absence extends the trust assumption.
2. ZK Provers: Must be incentivized to generate costly ZK proofs regularly. Prover centralization or failure delays cryptographic finality. Censorship of fraud proofs or ZK proofs by the sequencer or a dominant prover is a potential attack vector, mitigated by permissionless proof submission and slashing conditions for provers.