Validator-Based Bridge

The core security model relies on a multi-signature (multisig) or federated committee of known or permissioned entities. These validators observe events on the source chain, reach consensus, and authorize the release of assets on the destination chain. Users must trust that a majority of this set will not collude to steal funds. Examples include Multichain's MPC network and Polygon's PoS Bridge guardians.

Transactions are typically faster and cheaper than fully cryptoeconomic bridges. Since validators use off-chain consensus, they do not need to pay for gas to run complex on-chain verification (like light clients). Finality is achieved once the predefined threshold of validator signatures is collected, often in seconds. This makes them suitable for high-frequency, low-value transfers.

This architecture creates central points of failure:

• Censorship Risk: The validator set can choose to censor specific transactions or addresses.
• Collusion Risk: If a majority of validators are compromised or act maliciously, user funds can be stolen.
• Operational Risk: Reliance on the uptime and correctness of a few entities. The Nomad Bridge hack exemplified the risk of a bug in a trusted updater contract.

There are two primary implementations:

• Lock-and-Mint (Non-Custodial): Assets are locked in a smart contract on the source chain, and a wrapped representation is minted on the destination. Validators control the minting function.
• Liquidity Pool (Custodial): Assets are deposited into a pool managed by the bridge operator. Users receive assets from this pool on the other side. This model, used by Celer cBridge, can be more capital efficient but involves direct custody.

Validator bridges dominate where speed and user experience are paramount and where the trust in the operator is established. They are common for:

• Bridging to and from high-throughput sidechains (e.g., Polygon, BNB Chain).
• Enterprise and institutional transfers where counterparties are known. The trade-off is accepting extrinsic trust (in the validators) versus the intrinsic cryptographic trust of the underlying blockchains.

Many projects are evolving their validator sets to reduce centralization:

• Increasing the number and diversity of validators.
• Implementing slashing mechanisms or bonds to penalize malice.
• Using TSS (Threshold Signature Schemes) for improved key security.
• Transitioning towards optimistic or light client verification models over time, as seen with Polygon's zkBridge efforts.

Multichain utilized a SMPC (Secure Multi-Party Computation) network of nodes to manage cross-chain routers. These nodes collectively managed the threshold signatures required to authorize asset minting on destination chains.

• Security Model: Relied on a federated MPC model where a predefined set of nodes held shares of the signing key.
• Historical Context: Once a major player, it demonstrated both the scalability and the custodial risks inherent in validator/multisig models when node control is concentrated.

Validator-based bridges introduce specific security assumptions distinct from trust-minimized bridges like rollups or light client bridges.

• Trust Assumption: Users must trust the honesty and security of the validator set.
• Attack Vectors: Vulnerable to validator collusion or compromise, which could lead to fraudulent minting.
• Mitigations: Implementations use slashing, fraud proofs, or TEEs to increase security, but the fundamental trust model remains.

The security of a validator-based bridge is anchored in its multi-signature (multisig) consensus model. A committee of known or permissioned validators must collectively attest to and sign off on the validity of a cross-chain transaction. This creates a trust assumption, as users must rely on the honesty of the majority of these validators, rather than the underlying blockchains' native security.

A typical validator-based bridge operates with a lock-and-mint or burn-and-mint mechanism on connected chains. Key components include:

• Wrapped Assets: On the destination chain, a token contract mints a synthetic representation (e.g., wBTC) of the locked asset.
• Validator Set: The off-chain committee monitors events and signs attestations.
• Relayer Network: Often separate from validators, relayers submit signed proofs to the destination chain to trigger the mint or unlock.

These bridges prioritize transaction finality speed and cost-efficiency over decentralization. Because they do not need to wait for the full economic finality of the source chain (like proof-of-work confirmations), they can offer fast transfers. However, this comes with the trusted third-party risk—if the validator set is compromised, user funds can be stolen. This is a fundamental trade-off compared to trust-minimized bridges.

Many major bridges in production use a validator-based model due to its operational simplicity.

• Multichain (formerly Anyswap): Utilized a Federation of nodes with a threshold signature scheme.
• Polygon PoS Bridge: Relies on a set of Heimdall validators to checkpoint state from Ethereum to Polygon.
• Wormhole (Guardian Network): Employs a set of 19 Guardian nodes run by major organizations to observe and attest to messages.

The centralized trust model introduces specific vulnerabilities:

• Validator Collusion: A majority of the committee can steal all locked assets.
• Private Key Compromise: Theft of validator keys can lead to fraudulent attestations.
• Governance Attacks: If the validator set is upgraded via a governance token, the vote itself can be attacked to appoint malicious validators. Historical bridge hacks, like the Wormhole ($325M) and Nomad ($190M) exploits, often exploited flaws in the validator or relayer logic.

Validator-based bridges differ fundamentally from other models:

• vs. Native Bridges: Native bridges (e.g., Arbitrum's L1<>L2 bridge) are secured by the base layer's validators via fraud or validity proofs, requiring no external trust.
• vs. Light Client Bridges: Light client bridges (e.g., IBC, some rollup bridges) verify block headers using the cryptographic security of the source chain, aiming for trust-minimization. Validator bridges outsource this verification to an external committee.