X Layer Hybrid Proof: Using ZK to Backstop the Security of an Optimistic Rollup

1. Why Hybrid

There are two mainstream proof models for rollups, each with clear tradeoffs.Optimistic Rollup with Interactive Fault Proofs. Transactions are assumed valid by default, and economic incentives drive anyone to open a dispute within the challenge window. The cost profile is attractive — there is no proving overhead during normal operation. The downsides are twofold. First, the 7-day challenge period delays withdrawal finality enough to meaningfully constrain bridge and liquidity application design and lock up capital. Second, dispute resolution relies on a multi-round on-chain bisection game; a single dispute can require dozens of on-chain interactions, with neither Gas cost nor settlement time bounded in any predictable way.ZK Validity Rollup. Validity proofs are generated for every block. State becomes final as soon as the proof is verified on L1. The tradeoff is the continuous proving cost during normal operation and a hard dependency on prover availability — if the proving pipeline falls behind, blocks cannot finalize on L1, and the entire chain experiences a liveness risk.Hybrid Proof is built on a simple observation: the overwhelming majority of transactions are never challenged. Letting the optimistic path handle day-to-day operation captures the low operating cost, while reserving zero-knowledge proofs as an arbitration mechanism — proving only the challenged state transitions, and resolving disputes definitively when they occur — captures the strong finality guarantee. Provers do not need to be online during normal operation, and the chain does not pay proving costs per block. At the same time, replacing the interactive bisection game with a single ZK verification compresses the Gas cost and time of dispute resolution by an order of magnitude.

2. Comparing the Three Proof Models

3. How Hybrid Proof Works

X Layer's Hybrid Proof flow is organized around two roles — Proposer and Challenger:1. Optimistic proposal. The Proposer follows the standard OP Stack pipeline and periodically posts L2 Output Roots to the dispute game factory contract on L1. No proof is generated at this stage; Gas cost and latency are identical to a conventional Optimistic Rollup.2. Challenge. During the challenge window, anyone can open a dispute game against an Output Root by posting the required bond. Once a challenge is opened, the system enters the proving window, awaiting self-attestation from the Proposer.3. ZK self-attestation. Within the proving window, the Proposer uses op-succinct and the SP1 zkVM to replay the disputed state transitions, produce the corresponding ZK proof, and submit it to L1. SP1 verifiably executes both the OP Stack derivation pipeline and EVM execution.4. One-shot settlement. The dispute game contract on L1 verifies the ZK proof. If verification succeeds, the Proposer wins and the Challenger's bond is slashed; if the Proposer fails to submit a valid proof within the proving window, the Challenger wins and the proposal is rolled back. The entire dispute is resolved in a single contract call — no multi-round game.Because dispute resolution is anchored in cryptographic verification rather than interactive game play, the challenge period can shrink from 7 days (needed to accommodate multi-round bisection) to just 1 day (sufficient buffer for proof generation and verification).

4. op-succinct and SP1

X Layer's Hybrid Proof is built on op-succinct, the open-source framework from the Succinct team. op-succinct treats the OP Stack state-transition function (including the derivation pipeline and EVM execution) as a program for the SP1 zkVM, so any segment of L2 history can be replayed deterministically inside SP1 and a corresponding validity proof produced.op-succinct supports two deployment modes. In Validity mode, the framework runs as a full ZK rollup, generating proofs for every block. In Fault Dispute Game mode, it preserves the optimistic execution path of OP Stack and only triggers SP1 proof generation when a fault dispute game is opened. X Layer adopts the latter as the foundation of Hybrid Proof.The proof system is SP1, Succinct's RISC-V zkVM. The OP Stack state-transition function written in Rust is compiled to RISC-V bytecode, executed deterministically inside the zkVM, and a STARK proof is produced and wrapped into a Groth16 proof for verification on Ethereum L1.To improve proving turnaround during the testnet phase, X Layer also enabled local GPU (CUDA) proving — when a dispute is opened, proofs can be generated on a self-hosted cluster, avoiding a hard dependency on external proving networks.

5. Implementation Details on X Layer

Activation. Hybrid Proof went live on X Layer testnet on May 6, 2026. Contract deployment is complete, and the fault dispute game has been switched to the op-succinct ZK arbitration backend. Mainnet rollout will be planned based on testnet operational data.Challenge period. The protocol challenge window has been adjusted from 7 days to 1 day, comfortably accommodating the engineering buffer for proof generation and verification while substantially improving the user withdrawal experience.Default execution path is unchanged. Block production, batch submission, and Output Root posting continue to use the standard OP Stack pipeline. The sequencer, batcher, and proposer behave exactly as before, with no extra proving cost imposed on normal operation. Proving compute is invoked only when a dispute is triggered.Security model. The dispute path requires no trusted third party — outcomes are determined entirely by the cryptographic verification of the SP1 proof on L1. Liveness is guaranteed by the optimistic path; the ultimate safety floor is anchored in the ZK proof system.

6. What This Means for Developers and Users

For users. The most visible change is that the wait time for L2-to-L1 withdrawal finality drops from 7 days to 1 day. Both native bridge withdrawals and the third-party fast-bridge experience should improve correspondingly. Day-to-day transaction fees and confirmation latency remain on the OP Stack default profile.For developers. Cross-chain applications, market-making strategies, and liquidity management tooling can be redesigned around 1-day finality, which meaningfully reduces capital lockup. The smart contract layer is fully backward compatible — contracts deployed on X Layer continue to run unchanged. RPC interfaces and node behavior are also unchanged.For bridge providers. Third-party "fast withdrawal" bridges typically price the challenge period as the dominant risk parameter. A shorter challenge period directly improves capital turnover and pricing flexibility within the same risk budget.

7. Summary

Source: okx/op-succinct (proof framework)