UNION BRIDGE - Rootstock
THE CHALLENGE
Creating a secure, decentralized, and trust-minimized bridge for the Rootstock sidechain, to enable trust-minimized operations where Bitcoin unlocks only when a valid redemption occurs on another blockchain, without relying on a centralized custodian or requiring protocol changes to the Bitcoin protocol.
“With BitVMX and Fairgate, we achieved something previously thought impossible: a non-custodial, permissionless bridge that upholds Bitcoin’s core principles while enabling seamless cross-chain functionality staying true to Rootstock’s core objectives.”
Henrik Jondell (CTO, Rootstock)
THE TECHNOLOGY
BitVMX fraud proofs enforce correct peg-out validation. Bitcoin Taproot is used to execute long scripts with standard transactions, while zero-knowledge proofs verify transactions, blocks, and events. Cross-chain relays track state and submit peg-out requests. An optimistic execution model allows fast processing with secure dispute resolution.
Access Union Bridge whitepaper 📑here.
THE SOLUTION
BitVMX enables optimistic verification of off-chain computations, allowing Bitcoin to validate peg-out requests from another blockchain. Users lock BTC to mint a wrapped asset, and when redeeming, BitVMX verifies the burn transaction using an on-chain fraud-proof mechanism to resolve disputes. If valid, BTC is released; if fraudulent, the process halts, eliminating the need for trusted third parties.
Transaction graph structure for multi-party BitVMX showing the Locking, Kickoff, dispute, and Unlocking transactions
THE OUTCOME
A fully non-custodial and permissionless bridge mechanism that eliminates the need for trusted intermediaries. Fraud-proof mechanisms prevent invalid peg-outs while minimizing costs. The system enhances interoperability while maintaining decentralization, making Bitcoin bridges more secure and censorship-resistant.
UNION BRIDGE IS NOW ON TESTNET
In May 2026, Union Bridge reached its first public milestone: a live deployment on Rootstock Testnet. Accessible via Atlas (Rootstock's unified asset interface) this experimental release covers core peg-in and peg-out flows, marking the transition from architecture to running infrastructure.
The launch validates what Fairgate and Rootstock set out to build: a Bitcoin bridge that doesn't ask users to trust an operator, a committee, or a governance process. Dispute resolution and fraud-proof challenges, the full BitVMX security model will be introduced in Testnet V2, with a phased mainnet rollout planned for 2027.
Union Bridge is being built in the open. Core components are progressively open sourced, and this testing phase depends on community participation: real usage, real feedback, and real adversarial conditions.
UNION: A Trust Minimized Bridge for Bitcoin | Rootstock (RSK)
Deploy EVM compatible smart contracts on Rootstock while leveraging the security of the Bitcoin network
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