  Blockchain relay schemes enable cross-chain state proofs without requiring
trusted intermediaries. This is achieved by applying the source blockchain's
consensus validation protocol on the target blockchain. Existing chain relays
allow for the validation of blocks created using the Proof of Work (PoW)
protocol. Since PoW entails high energy consumption, limited throughput, and no
guaranteed finality, Proof of Stake (PoS) blockchain protocols are increasingly
popular for addressing these shortcomings. We propose Verilay, the first chain
relay scheme that enables validating PoS protocols that produce finalized
blocks, for example, Ethereum 2.0, Cosmos, and Polkadot. The concept does not
require changes to the source blockchain protocols or validator operations.
Signatures of block proposers are validated by a dedicated relay smart contract
on the target blockchain. In contrast to basic PoW chain relays, Verilay
requires only a subset of block headers to be submitted in order to maintain
full verifiability. This yields enhanced scalability. We provide a prototypical
implementation that facilitates the validation of Ethereum 2.0 beacon chain
headers within the Ethereum Virtual Machine (EVM). Our evaluation proves the
applicability to Ethereum 1.0's mainnet and confirms that only a fraction of
transaction costs are required compared to PoW chain relay updates.
