Why Bitcoin Bridges Can’t Be Fully Trustless

Bitcoin's Script is Deliberately Limited. Its non-Turing-complete language cannot verify proofs from external chains like Ethereum or Solana. This creates a fundamental verification asymmetry that trustless bridges like Across or LayerZero on EVM chains rely on.

Solutions Impose a Trusted Third Party. Protocols like Bitcoin's Federated Peg (e.g., Wrapped BTC) or multi-signature custodian models (e.g., early RenVM) are the only viable architecture. The security collapses to the honesty of the federation or custodian.

Light Clients Are Theoretically Trustless. A bridge could use a Bitcoin SPV (Simplified Payment Verification) client to verify headers. However, the resource cost for a smart contract to validate Bitcoin's proof-of-work makes this prohibitively expensive on any destination chain.

Evidence: The total value locked in Bitcoin bridges exceeds $10B, yet the dominant model remains custodial or federated. Even advanced designs like tBTC v2 or the upcoming BitVM framework require optimistic assumptions or committees, not cryptographic finality.

No Native Smart Contracts prevent Bitcoin from verifying external state. A bridge's security depends entirely on its off-chain attestation mechanism, not Bitcoin's proof-of-work.

Multi-Party Custody models, like those in wBTC, introduce regulatory and counterparty risk. The trust shifts from cryptographic proofs to legal entities and KYC/AML procedures.

Light Client Relays are the theoretical ideal but face prohibitive cost and latency. Verifying Bitcoin headers on another chain requires constant, expensive data submission.

Evidence: The 2022 Ronin Bridge hack exploited a 5-of-9 multisig. This demonstrates how bridged security collapses to its weakest validator set, a fatal divergence from Bitcoin's trust model.

Drivechains rely on miner voting for security, which introduces a new trust assumption. The BIP-300/301 proposal delegates custody of locked BTC to a federation of miners, creating a soft-consensus checkpoint. This is more decentralized than a multi-sig but less secure than Bitcoin's proof-of-work.

Sidechains operate with independent security, like Liquid Network or Stacks. Their security budget is decoupled from Bitcoin's, making them vulnerable if their own validator set fails or is attacked. This is a classic scalability trilemma trade-off.

The trust spectrum is continuous. A federated bridge like wBTC is high-trust, a drivechain is medium-trust, and a Layer 2 like a rollup aims for low-trust. No bridge to an external system achieves Bitcoin's native trustlessness.

Evidence: The Liquid sidechain has a 15-of-15 multi-sig federation. While robust, this model has processed billions in value, proving that pragmatic, audited trust can be sufficient for specific use cases like fast settlements.

Bitcoin's Finality is Probabilistic. A bridge must decide when a Bitcoin transaction is 'final enough' to mint a wrapped asset on another chain. This creates a trusted assumption about chain reorganization depth, a risk that pure smart contract chains like Ethereum do not face with their single-block finality.

No Native Smart Contract Logic. Bitcoin's limited scripting language prevents on-chain verification of bridge operations. Solutions like tBTC v2 or Multichain rely on off-chain committees or federations to attest to deposits, introducing a trusted validator set that users must audit.

The Security Spectrum. Compare WBTC's centralized custodian model to Lightning Network's multi-hop payment channels. The former offers deep liquidity with high trust, the latter enables fast micropayments with lower trust but limited capacity. There is no universal solution.

Evidence: The 2022 $190M Wormhole bridge hack exploited a signature verification flaw in its off-chain guardians, not the underlying chains. This proves the security bottleneck for Bitcoin bridges is the external attestation layer, not Bitcoin itself.