Rootstock

RSK (Rootstock) is a Bitcoin sidechain launched in 2018 that enables smart contract functionality while using Bitcoin as its native asset, addressing Bitcoin’s limited programmability without requiring a new speculative token. Built as an evolution of QixCoin and Ethereum concepts, RSK provides a Turing-complete virtual machine (RVM) that’s highly compatible with Ethereum’s tools and dApps, offering faster transaction confirmations (around 30 seconds) while maintaining Bitcoin’s security through SHA-256D merged mining with over 40% of Bitcoin’s hash rate as of 2019. The platform uses a two-way peg system where Bitcoins become “Smart Bitcoins” (RBTC) on the RSK blockchain and can be converted back without additional costs beyond standard fees, with no new currency issuance since all RBTC originates from actual Bitcoin. RSK currently operates with a federated peg and includes protection against selfish mining through the DECOR+ protocol, while the community has planned future upgrades including storage rent, parallel transaction processing, block propagation optimizations, transaction compression protocols, support for additional VMs, and a hybrid federation/drivechain-based peg system, all documented as RSK Improvement Proposals (RSKIPs) in their GitHub repository, with RSK Labs serving as the primary development company since 2015. ...

We present Union, a trust-minimized bridge protocol that enables secure transfer of BTC between Bitcoin and Rootstock. The growing ecosystem of blockchain systems built around Bitcoin has created a pressing need for secure and efficient bridges to transfer BTC between networks while preserving Bitcoin’s security guarantees. Union employs a multi-party variant of BitVMX, an optimistic proving system on Bitcoin, to create a bridge that operates securely under the assumption that at least one participant remains honest. This 1-of-n honest approach is strikingly different from the conventional honest-majority assumption adopted by practically all federated systems. The protocol introduces several innovations: a packet-based architecture that allows security bonds to be reused for multiple bridge operations, improving capital efficiency; a system of enablers to manage functionaries participation and to enforce penalties; a flexible light client framework adaptable to various blockchain architectures; and an efficient stop watch mechanism to optimize time-lock management. Union is a practical and scalable solution for Bitcoin interoperability that maintains strong security guarantees and minimizes trust assumptions. ...