blockHash

BlockHash is a cryptographic digest that uniquely identifies a block within a blockchain. It is produced by hashing the block header and serves as a tamper-evident identifier: altering any header field—such as the previous block reference, the Merkle root, the timestamp, or the nonce—produces a different hash. The block hash is included in the block’s metadata and is used by nodes to verify integrity and to link blocks in the chain.

Computation and representation: The block header contains fields such as version, previous block hash, Merkle root, timestamp, and a nonce (or other consensus-specific data). A cryptographic hash function outputs a fixed-length digest, typically shown as a hexadecimal string. In Bitcoin, the hash is the double SHA-256 of the serialized block header; in Ethereum, Keccak-256 is used for the header, and the resulting value serves as the block identifier. In proof-of-work systems, the block hash must satisfy a network-specific target, tying the hash to mining difficulty. In proof-of-stake or other consensus schemes, the block hash still serves as an identifier, computed according to protocol rules.

Role and implications: The block hash enables efficient block referencing, synchronization, and fork resolution. Nodes and light clients fetch blocks by their hashes and verify that a block is the correct successor to its predecessor. The hash acts as a fingerprint for the block header; any mismatch signals tampering or data corruption. While publicly visible, it does not by itself expose transaction data, which is summarized by the Merkle root in the header.