Unlike most blockchains, Celestia derives most of its functionality from stateless commitments to data rather than stateful transitions. This means that the protocol relies heavily on block validity rules. Notably, resource constrained light clients must be able to detect when a subset of these validity rules have not been followed in order to avoid making an honest majority assumption on the consensus network. This has a significant impact on their design. More information on how light clients can check the invalidity of a block can be foud in the Fraud Proofs spec.
Note Celestia relies on CometBFT (formerly tendermint) for consensus, meaning that it has single slot finality and is fork-free. Therefore, in order to ensure that an invalid block is never committed to, each validator must check that each block follows all validity rules before voting. If over two thirds of the voting power colludes to break a validity rule, then fraud proofs are created for light clients. After light clients verify fraud proofs, they halt.
Before any Celestia specific validation is performed, all CometBFT block validation rules must be followed.
Notably, this includes verifying data availability. Consensus nodes verify data availabily by simply downloading the entire block.
Note Light clients only sample a fraction of the block. More details on how sampling actually works can be found in the seminal "Fraud and Data Availability Proofs: Maximising Light Client Security and Scaling Blockchains with Dishonest Majorities" and in the
Celestia specifc validity rules can be categorized into two groups:
BlobTx transactions must be valid according to the BlobTx validity rules.
All remaining transactions must be decodable and pass all AnteHandler checks.
For a complete list of modules see state machine modules.
The data root must be calculated from a correctly constructed data square per the data square layout rules