A block header is used to identify a particular block on an entire blockchain and is hashed repeatedly to create proof of work for mining rewards. A blockchain consists of a series of various blocks that are used to store information related to transactions that occur on a blockchain network. Each of the blocks contains a unique header, and each such block is identified by its block header hash individually.
Block headers are commonly used in Bitcoin developer documentation, and help to record tasks quickly and relatively easily. Entire blockchains can be stored in a simple database or as a flat-file. When considering blockchains as a whole, it helps to picture them as a vertical stack.
The blocks get layered—one on top of the other, with the first block being the foundation—and they grow in height until the end of the blockchain is reached and the sequence is complete. The first block in the chain is also known as the "Genesis Block." The layers and deep history of each sequence is one of the things that makes Bitcoin so secure.
As a part of a standard mining exercise, a block header is hashed repeatedly by miners by altering the nonce value. Through this exercise, they attempt to create a proof of work, which helps miners get rewarded for their contributions to keep the blockchain system running smoothly and efficiently.
The block header contains three sets of block metadata. It is an 80-byte long string, and it is comprised of the 4-byte long Bitcoin version number, 32-byte previous block hash, 32-byte long Merkle root, 4-byte long timestamp of the block, 4-byte long difficulty target for the block, and the 4-byte long nonce used by miners.
Block Header Components
Each of these components is vital to creating an accurate and reliable header. The primary identifier of each individual block is the cryptographic hash it contains. It is essentially a digital fingerprint, and it is created by hashing the block header through the applicable algorithm twice.
The Bitcoin version number is useful in keeping track of changes and updates throughout the protocol.