Block header

A block header is a data structure that contains metadata about a particular block in the chain. It includes important information such as the block's timestamp, transaction data, a reference to the previous block, and a unique cryptographic hash of the block itself. The block header is used to verify the authenticity and integrity of the block and the transactions it contains. Because each block's header includes a reference to the previous block's hash, altering any historical block would change its hash and invalidate every subsequent block in the chain — this is the fundamental mechanism that makes blockchains tamper-resistant.

In Bitcoin's proof-of-work system, the block header is the specific data that miners hash repeatedly in the search for a valid solution. Miners adjust a field in the header called the nonce — a 32-bit number — trillions of times per second, computing the SHA-256 hash of the header with each iteration until they find a hash value that meets the network's current difficulty target. When a valid hash is found, the miner broadcasts the completed block to the network, and other nodes can verify the solution almost instantly by hashing the same header themselves and confirming the result meets the target.

Block headers also play an important role in lightweight client verification, sometimes called Simplified Payment Verification (SPV). Rather than downloading and storing the entire blockchain, lightweight clients — such as mobile wallets — download only block headers, which are much smaller in size. By verifying the chain of block headers and using Merkle proofs to confirm that a specific transaction is included in a block, SPV clients can confirm transaction validity without storing all transaction data, making participation in the Bitcoin network practical on resource-constrained devices.