How Does ‘Data Availability’ Relate to the Security of Both Sharding and Layer 2 Rollups?

Data availability is crucial because it ensures that all necessary transaction data is published to the Layer 1 chain. For rollups, this data is needed for users to reconstruct the off-chain state and for fraud-proof or validity-proof verification.

Without data availability, a malicious sequencer could hide the data for an invalid state, making it impossible to challenge. In sharding, data availability ensures all nodes can access the data from all shards to maintain the overall network state.