Bitcoin Blockchain

The Bitcoin blockchain is a chronological chain of blocks. Each block contains a set of transactions and a reference to the block that came before it. This reference is a cryptographic hash. Changing any earlier block would change its hash, which would break the chain of references that follows. Nodes reject any version of history that does not satisfy this structure and the network’s consensus rules.

New blocks are added roughly every ten minutes. Miners compete to produce a block that meets the current difficulty target. Once a valid block is found, it is broadcast to the network. Other nodes verify it and, if it is valid, add it to their local copy of the chain. The longest valid chain is treated as the authoritative history. Transactions become progressively more difficult to reverse as additional blocks are built on top of them.

The blockchain is public. Anyone can download it and verify the full history independently. It does not require permission to read. Writing to it requires a valid transaction that spends previously unspent outputs, signed with the corresponding private key. The combination of cryptographic linking, proof-of-work, and independent verification by nodes is what makes the ledger difficult to alter after the fact.