Block Hash

Overview

A block hash is produced by applying the protocol’s hash function to a serialized block header or equivalent block object. Because a small change in the input produces a different digest, the hash acts as a compact identifier and integrity commitment.

Bitcoin nodes refer to a block by the double-SHA-256 hash of its 80-byte header. The header contains the previous block hash, Merkle root, timestamp, difficulty target encoding, version, and proof-of-work nonce. Ethereum uses several hashes or roots across execution and consensus objects, including execution block hashes and beacon block roots.

A block hash is intended to be unique in practice, but it does not by itself prove that the block is part of the canonical chain. Competing or stale blocks also have valid hashes. The block must be validated and connected to the selected chain.

Changing any committed transaction or header field changes the hash, which in turn breaks the link from later blocks. This property contributes to tamper evidence, but consensus and validation are still required to decide which history is accepted.

A block hash is not a human-readable summary and does not reveal the block’s transactions by itself. Its meaning comes from the protocol rules that define the header fields and the hashing process. Explorers, wallets, and APIs use it as the most precise reference to a particular block because block heights can point to different blocks during a reorganization.

Applications should store both block height and block hash when tracking payments. If the hash at a previously observed height changes, the transaction may have moved to another block or returned to the pending pool. In proof-of-work systems, the hash may also be compared with a difficulty target, while in other protocols it mainly serves as a cryptographic identifier.