Blocks, hashes, and blockchains

A block is a batch of transactions sealed together; a hash is a digital fingerprint of data; a blockchain links each block to the previous block’s hash so changing old history breaks the chain of fingerprints.

This is why “editing yesterday’s payment quietly” is hard: the fingerprints after it would no longer match, and other copies of the chain would disagree.

A hash function takes any data and produces a short, fixed-length fingerprint. The same data always gives the same fingerprint, and changing even one letter gives a completely different one. You cannot rebuild the data from the fingerprint, but you can check whether data matches it.

A network gathers recent transactions into a batch, called a block, and computes the block’s hash. The next block includes that hash at its top before it is sealed. So each block carries the fingerprint of the one before it, all the way back to the start. If someone edits an old block, its fingerprint changes, the next block no longer points to it, and every later link breaks too.

Real example: on the XRP Ledger, blocks are called ledger versions. Each one records its own hash and the hash of the parent ledger it was built on, back to the earliest ledger. Any server can recompute those hashes and spot a tampered copy right away. Source: Consensus Protocol — Ledger History.