# blockHash(atHeight:) The hash of the block at the given height, in internal byte order. ``` func blockHash(atHeight height: Int) async throws -> Data ``` ## Parameters `height` Height above genesis (0 = genesis). ## Return Value 32 bytes of hash data in internal (kernel) byte order. ## Discussion> Throws: ``doc://BitcoinKernel/documentation/BitcoinKernel/BlockSourceError/notFound`` if the source has no block at > that height, or a transport-level ``doc://BitcoinKernel/documentation/BitcoinKernel/BlockSourceError`` otherwise. > Warning: Returns whichever block the source currently thinks is at > that height. During a reorg, the answer can change. The sync engine > uses this method exactly once per run — at the fork-point check on > resume — and walks by hash everywhere else. Don’t use it for chain > identity in application code either.