Why Cross-Chain Finality Is Meaningless Without DA

Finality is not data availability. A chain's finality promise only ensures state cannot be reverted. It does not guarantee that the data needed to verify a cross-chain state transition is published and accessible. Bridges like LayerZero and Wormhole rely on external oracles/relayers for this data, creating a separate trust vector.

Fast finality creates a false sense of security. Chains like Solana and Avalanche offer sub-second finality, which is useless if the transaction's proof or calldata is withheld. The Celestia and EigenDA ecosystems treat data availability as a first-class primitive, exposing the vulnerability of systems that assume it.

The verification gap is the attack surface. Without accessible data, a destination chain cannot independently verify the validity of a cross-chain message. This is why optimistic rollups have long challenge periods—they assume data is available. Protocols like Across use optimistic verification, but still depend on a data publisher.

Evidence: The Nomad bridge hack exploited this exact gap. The attacker submitted fraudulent root updates; the system's 'finality' was intact, but the underlying merkle proof data was invalid. The failure was in data verification, not chain finality.

Header-based finality is a lie. Bridges like Stargate and LayerZero accept block headers as proof of state, but a header is just a commitment. Without the underlying transaction data, you cannot verify the validity of the state change it claims to represent. This creates a trust dependency on the source chain's liveness.

Proof-based systems require Data Availability. Protocols like Across and zkBridge models require the relayer to submit validity proofs. The critical dependency is that the verifier contract must have access to the historical data to check the proof. If that data is not available, the proof is unverifiable and the system halts.

The security floor is the DA layer. A cross-chain message's finality is only as strong as the weakest data availability guarantee in its path. An optimistic rollup bridge fails if its parent chain censors data; a zk-rollup bridge fails if its data is not posted. The bridge abstraction leaks the underlying chain's properties.

Evidence: The Celestia and EigenDA ecosystems explicitly architect for this, making verifiable DA a first-class primitive. Bridges built atop them, like Hyperlane, inherit a stronger security model where finality is defined by data publication, not just social consensus.

Finality is not data availability. A bridge like LayerZero can attest to a finalized source chain block, but this is worthless if the block's data is unavailable for verification. The receiving chain cannot execute the state transition, rendering the finality proof a hollow promise.

Light clients require full data. Optimistic systems like Across and ZK bridges like zkBridge depend on the ability to reconstruct the source chain's state. Without the underlying transaction data, their cryptographic proofs are built on unverifiable assumptions, creating systemic risk.

Modular chains expose the flaw. Rollups on Celestia or EigenDA separate execution from data publishing. A cross-chain message claiming finality from such a chain is meaningless if the sequencer withholds the critical batch data, a failure mode traditional monolithic chains obscure.

Evidence: The 2022 Nomad bridge hack exploited this exact gap. The protocol accepted optimistic finality messages on one chain without verifying the availability and validity of the corresponding data on the other, leading to a $190M loss.