Rollup

The rollup model comes from Ethereum. Users transact on a separate execution layer, often an EVM, and the operator periodically posts to the base chain a summary of the resulting state plus the data needed to rebuild it. A zero-knowledge rollup adds a cryptographic proof that the summary is correct. An optimistic rollup instead lets anyone challenge a wrong summary within a delay.

Bitcoin cannot verify such proofs in its script, which is the whole difficulty. Citrea posts its data as inscriptions and relies on a BitVM bridge, where a fraudulent withdrawal can be contested through a challenge game. The approach is promising and still young. Its security rests on the bridge’s honest participants, on data availability and on tooling that remains experimental.