Block Header
A block header is the compact summary miners hash to prove work and link each cryptocurrency block to the chain.
Definition
A block header is the small data section at the start of a cryptocurrency block. In proof-of-work mining, miners repeatedly hash the block header to search for a result that satisfies the network’s mining difficulty.
The header acts like a fingerprint for the block. It summarizes key information without storing every transaction directly.
How It Works
When a miner creates a candidate block, it gathers valid transactions and builds a block header from several fields. In Bitcoin, those fields include the version, previous block hash, Merkle root, timestamp, difficulty target, and nonce.
The previous block hash links the new block to the block before it. If an older block changes, its hash changes, which also changes later block headers that point back to it.
The Merkle root summarizes all transactions in the block as one compact hash. If any transaction changes, the Merkle root changes, and the block header hash changes too.
Mining is the process of hashing this header many times. Miners change the nonce and sometimes other adjustable data to create new header variations. Each variation produces a different hash. To be valid, the hash must be below the network’s current target. Because hashes are unpredictable, miners compete by making many attempts with large amounts of hash rate.
Once a miner finds a valid header hash, it broadcasts the full block. Other nodes can hash the header once, check that it meets the target, and validate the rest of the block.
Why It Matters
The block header makes proof-of-work efficient to verify. Creating a valid header can require trillions of attempts, but checking one valid result is fast.
It also protects the history of the blockchain. Since each header includes the previous block’s hash, changing an old transaction would force an attacker to redo the proof of work for that block and later blocks.
For miners, the block header is the main object being tested. Mining hardware does not search through entire blocks directly; it searches through block header variations until one produces a valid proof-of-work hash.