How can we make blockchains secure at scale? We suggest a data availability and validity scheme that make sharding efficient in terms of the number of validators and validating resources. We first describe the Polkadot data availability and validity scheme and consider its applicability to other sharded systems (e.g. ETH2.0). In Polkadot we tie an erasure coding data availability scheme with consensus, where we can not finalise an unavailable block. Moreover, reports of unavailability or invalid…