Consensus Verification

The correctness of the block itself must be verified to ensure that retrieved execution layer data originates from a valid blockchain state. Execution layer proofs confirm that specific data, such as transactions, events, or account states, are part of a given block. However, this does not guarantee that legitimate validators correctly produced and signed the block.

Ethereum

Ethereum's consensus mechanism relies on a sync committee system, which is integral to the protocol and ensures efficient block verification. While this approach benefits light clients by reducing synchronization overhead, it is a fundamental part of Ethereum's consensus design. Every 256 epochs (8192 slots), a new sync committee of 512 validators is selected to sign beacon chain headers. A stateless client must verify that the current sync committee is valid by proving the continuous transition of committee membership from a trusted checkpoint.

This requires:

• Sync Committee Proofs—Verifying that the validators signing the block are part of the active sync committee.

• Light Client Updates—Ensuring correct transitions between sync committees by checking the signatures of the previous committee.

• Finality Verification—Confirming that the beacon chain finalization process was executed correctly.

This process requires verifying the entire sequence of validator committee transitions from the checkpoint to the required block. If the client has not stored any additional verified information beyond the initial checkpoint, it must retrieve and validate the entire transition history.

Optimizations can be applied to improve efficiency:

• Caching Verified Transitions—The client can store previously verified committee transitions, reducing redundant verification steps when validating future blocks.

• Zero-Knowledge Proofs—Instead of directly verifying each committee transition, ZK-proofs can aggregate multiple transitions into a single proof, significantly improving efficiency (see later explanation).

While caching intermediate checkpoints optimizes verification, it introduces trade-offs. It increases storage requirements, making the client more resource-intensive, and prevents it from being fully stateless. Additionally, if the client relies on a newer checkpoint to reduce verification effort, it loses the ability to validate older blocks unless the entire chain of validator transitions starting at an older checkpoint is retrieved. The alternative of always retrieving and verifying the entire transition history requires significant computational overhead and transmission bandwidth.

To overcome these limitations, an advanced solution involves using zero-knowledge proofs (ZK-proofs). ZK-proofs enable efficient validation of multiple transitions in a single proof, eliminating the need to store intermediate checkpoints while maintaining full verification capabilities (see later explanation).

Layer-2 Rollups on Ethereum

For Ethereum-based Layer-2 solutions, an additional two-step verification process is required:

• Rollup Proof Validation—The first step involves verifying that the rollup proof correctly represents the rollup's state transition. Depending on the rollup type, this proof can be either a zero-knowledge proof (ZK-rollups) or a fraud-proof-based system (Optimistic rollups). The proof ensures that the rollup state transition was executed correctly and is included in an Ethereum Layer-1 transaction.

• Consensus Verification of the Rollup Proof—Once the rollup proof is verified, the next step is to ensure it was correctly published on Ethereum Layer-1. This requires verifying that the Ethereum block containing the rollup proof is valid by following the same sync committee transition verification process used for Layer-1 Ethereum blocks.

For optimistic rollups, finality depends on the fraud-proof challenge period defined by the rollup protocol. Transactions can only be considered final once the challenge window has passed without valid fraud-proof being submitted. In contrast, ZK-rollups achieve immediate finality upon proof submission, as the validity proof guarantees the correctness of the state transition without requiring a dispute period.

Other EVM-Compatible Layer-1 Chains

For EVM-compatible Layer-1 blockchains, execution layer validation follows the same principles as Ethereum. However, consensus verification differs based on the chain’s consensus mechanism. These chains may employ:

• Proof-of-Stake Validator Sets—A rotating set of validators signing blocks requiring validator set verification.

• Byzantine Fault Tolerance (BFT) Mechanisms—Checking quorum signatures to confirm block validity.

• Custom Consensus Models—Adapting verification logic to specific consensus rules defined by the blockchain.

Non-EVM Layer-1 Chains

For non-EVM Layer-1 blockchains, both execution and consensus verification mechanisms vary. Stateless clients must validate consensus using blockchain-specific proofs, which may involve:

• Threshold Signatures or Multi-Signature Schemes – Used in networks with committee-based finalization.

• Quorum-Based Verification – Ensuring that a majority of validators signed off on a block.

• Zero-Knowledge Proofs for Consensus – Some newer blockchains utilize zk-proofs to verify block production without disclosing validator details.

By generalizing the verification process across different consensus mechanisms, C4 can operate in diverse blockchain environments while maintaining a trustless and stateless architecture. Additionally, alternative approaches such as Directed Acyclic Graph (DAG) structures, Byzantine Fault Tolerance (BFT), Proof-of-Authority (PoA), and other consensus algorithms can be integrated, provided that a proof mechanism exists to verify the correctness of block production and validation.