What a blockchain is: blocks, hashing, immutability

A blockchain is a shared record of transactions, organised into blocks that are chained together in order. Its power comes from how that chain is built.

Blocks

Transactions are grouped into a block. Each block contains a batch of transactions, a timestamp, and a reference to the block before it. Linked one after another, the blocks form a chronological chain back to the genesis block.

Hashing

A hash function takes any input and produces a fixed-length string of characters — a kind of digital fingerprint. Two properties make it useful:

• The same input always gives the same hash, but even a tiny change to the input produces a completely different hash.
• You cannot run it backwards to recover the input from the hash.

Each block includes the hash of the previous block. Bitcoin uses the SHA-256 function for this.

Immutability

Because every block embeds the previous block's hash, the blocks are cryptographically linked. If someone alters a transaction in an old block, that block's hash changes, which breaks the link to the next block, whose hash then no longer matches, and so on down the chain.

To rewrite history, an attacker would have to redo the work for that block and every block after it, faster than the rest of the network is adding new ones. On a large network that is practically impossible. This is what people mean when they call a blockchain immutable — not that data physically cannot change, but that changing it is prohibitively expensive and immediately detectable.

Distributed copies

The ledger is not stored in one place. Thousands of computers each keep a full copy and check new blocks against the rules. There is no central server to hack, bribe or shut down — which is the whole point.