Solution: Bitcoin Staking

The two problems can be solved at once by integrating BTC staking into rollups.

BTC staking is a shared security protocol proposed by Babylon Labs. The protocol is designed to secure any system that requires consensus. Forkless rollup is a variant of the protocol to secure a rollup by using Bitcoin as collateral.

For each rollup block, the protocol adds a set of finality signatures backed by staked BTC. The rollup consensus is modified to include these finality votes. A block can only be finalized when it has received enough finality votes.

If the protocol votes for two different blocks at the same height, the staked BTC will be slashed. Therefore, as long as users only accept transactions that have received sufficient finality votes, a double-spend attack will not succeed.

This approach guarantees slashable safety against double-spending attacks and also enables fast second-level finality, unlocking high-value DeFi applications.