State Relays

A light client relay runs a simplified, resource-efficient version of a blockchain's consensus client to verify and relay block headers. It proves state by validating cryptographic Merkle proofs (like Merkle-Patricia Trie proofs) against a trusted block header. This is the foundational model for many trust-minimized bridges.

• How it works: The relay submits compact block headers to the destination chain. Users or contracts can then prove the inclusion of specific transactions or state via Merkle proofs.
• Example: A bridge that relays Ethereum block headers to a sidechain, allowing the sidechain to verify that funds were locked in a specific Ethereum contract.

This model introduces a challenge period (e.g., 7 days) after a state root is relayed. During this window, any watcher can submit fraud proofs to challenge incorrect state transitions. If unchallenged, the state is accepted as valid.

• Security Model: Relies on the presence of at least one honest verifier in the network to submit a challenge.
• Trade-off: Provides strong security guarantees with longer finality times for cross-chain messages.
• Primary Use: Commonly used by optimistic rollups to relay their state roots back to a Layer 1 like Ethereum.

These relays use zero-knowledge proofs (ZK-SNARKs, ZK-STARKs) to cryptographically prove the validity of a source chain's state transition. A zk-proof relay submits a succinct proof that verifies the entire block's correctness, including all transactions and the resulting state root.

• Key Advantage: Provides near-instant cryptographic finality without a challenge period.
• Components: Involves a prover generating the proof and a verifier contract on the destination chain to check it.
• Example: A zkBridge that uses a ZK proof to verify the state of a Cosmos app-chain on Ethereum.

In this design, the validators or sequencers of the source chain directly participate in attesting to its state for the destination chain. This often involves a multi-signature scheme or a threshold signature scheme where a supermajority of source chain validators sign a message containing the latest state root.

• Trust Assumption: Security is derived directly from the security of the source chain's consensus mechanism.
• Implementation: Can be implemented via a smart contract on the source chain that aggregates validator signatures, which are then relayed.
• Use Case: Many Cosmos IBC connections function this way, where the consensus state of one chain is tracked by another.

A practical example of a light client relay within a single ecosystem. After The Merge, Ethereum consensus is managed by the Beacon Chain. Light clients (like those in wallets or other chains) sync by downloading and verifying sync committees.

• Sync Committees: A randomly selected group of ~512 validators who sign block headers for a period (~27 hours).
• Verification: A light client needs only a trusted sync committee public key to verify all signatures for that period, enabling efficient header verification.
• Cross-Chain Role: This mechanism is used by Layer 2s and other chains to build lightweight, trust-minimized relays for Ethereum's state.

A state relay's security is defined by its trust model. Light client relays inherit the security of the source chain's consensus, requiring trust in its validator set. Threshold signature schemes (TSS) or multi-party computation (MPC) relays introduce a new trusted committee, creating a centralization risk if the committee is small or colludes. The economic security of the relay is often less than the value it secures, creating a mismatch.

Relays must have continuous, uncensored access to the source chain's block headers. Key risks include:

• Network-level attacks: Blocking the relay's node from the peer-to-peer network.
• Data withholding: A malicious majority of source chain validators could withhold block data, preventing state proof generation.
• Timeliness guarantees: Relays must prove the latest state; delays can lead to stale price or liquidity oracle data being used on the destination chain.

The relay's on-chain verification contract is a high-value attack target. Vulnerabilities can arise from:

• Incorrect light client verification logic: Flaws in verifying Merkle proofs, signature aggregation, or consensus finality rules.
• Upgrade mechanisms: Malicious or compromised relay operator keys could upgrade the contract to a malicious version.
• Replay attacks: Improper handling of nonces or timestamps could allow old, valid state proofs to be reused maliciously.

Attackers can exploit the cost structure of relays. A spam attack with invalid state proofs can burden the destination chain with verification gas costs, potentially making the relay economically non-viable. Furthermore, if relay operation is not sufficiently incentivized, operators may go offline, causing liveness failures. This can freeze cross-chain applications that depend on fresh state updates.

A state relay's security is ultimately backed by the source chain. Therefore, it is vulnerable to consensus-level attacks on the source chain, such as long-range attacks, nothing-at-stake problems, or catastrophic consensus failures. A relay verifying Ethereum proof-of-stake checkpoints, for example, is only as secure as Ethereum's >33% honest validator assumption. A successful 51% attack on the source chain can forge fraudulent state proofs.

Relying on a single relay creates a single point of failure. Robust systems employ multiple independent relays (e.g., IBC's requirement for multiple light clients) or fraud proofs where a single honest actor can challenge incorrect state. Escape hatches or pause mechanisms are critical for governance to intervene in case of a verified compromise, though they reintroduce a degree of centralization.

A Merkle proof (or Merkle-Patricia proof) is a cryptographic proof that a specific piece of data, like a transaction or account balance, is part of a larger dataset (a Merkle tree) with a known root hash. State relays use these proofs to convince a destination chain that a piece of state on a source chain is valid and finalized.

• Core Mechanism: Enables efficient and secure verification of data inclusion.
• Application: A relay proves an asset lock event on Chain A to Chain B by providing a Merkle proof to Chain A's state root.

Optimistic verification is a security model where state updates are assumed to be correct but can be challenged during a dispute period. This is used by some state relay designs (like Optimistic Bridges) to reduce on-chain verification costs. A fraud-proof mechanism allows anyone to submit cryptographic proof that a relayed state is invalid, rolling it back if successful.

• Trade-off: Lower gas costs vs. longer finality times due to challenge windows.
• Example: Across Protocol and Nomad use optimistic verification models.

Canonical bridging refers to the native, official bridge for moving assets to and from a specific blockchain's ecosystem, often secured by that chain's validator set (e.g., the Ethereum Beacon Chain for ETH staking). State relays are a fundamental component of canonical bridges, as they are responsible for communicating the state (like token lock events) between the two chains in a trust-minimized way.

• Key Trait: Deep integration with the core protocol's security.
• Example: The canonical bridge between Ethereum and its Layer 2 rollups.