How blocks link into a chain
Each block carries the fingerprint of the one before it. Break one, and you break every block after - see it happen in the builder below.
What's going on
A block is a small bundle: some transactions, a timestamp, its own hash, and - crucially - the hash of the previous block. That last piece is what turns a pile of blocks into a chain.
Because every block contains the fingerprint of the block before it, the blocks are physically dependent on each other's history. Block 2 points at Block 1's fingerprint. Block 3 points at Block 2's. If an attacker edits Block 1, its fingerprint changes - and now Block 2's stored "previous hash" is wrong, which changes Block 2's own hash, which breaks Block 3, and so on down the line.
This is why blockchains are described as tamper-evident. You can't quietly change old history; the damage cascades forward and everyone can see the chain no longer agrees with itself. The interactive chain below lets you tamper with a block and watch the break spread in red.
Provenance - proving where something came from - relies entirely on this. A supply chain recorded this way means a buyer can trust the product's full history, because rewriting any past step would visibly break the chain.
Key terms
- Block
- A bundle of transactions plus a timestamp, its own hash, and the previous block's hash.
- Previous hash
- Each block stores the fingerprint of the block before it - the link in the chain.
- Tamper-evident
- You can't change old data without visibly breaking everything after it.