What Is A Chain

The word blockchain contains two ideas. The block part — a batch of transactions — was covered yesterday. The chain part is what makes the whole system tamper-proof.

Each block contains the hash of the block that came before it. That hash is a mathematical fingerprint of the entire previous block — its transactions, its timestamp, its own previous hash. Any change to any previous block changes its hash, which means the next block’s record of that hash is now wrong, which invalidates that block, which invalidates every block after it.

Imagine a physical chain. Each link is connected to the one before it. To remove or alter a link in the middle, you can’t just swap it — you’d have to rebuild every link from that point forward. And in Bitcoin’s case, rebuilding those links requires an enormous amount of computational work.

Here’s why that matters practically.

Someone who wanted to change a Bitcoin transaction from five years ago — perhaps to send themselves coins they don’t own — would need to:

Remine the block containing that transaction with the altered data. Then remine every block that came after it. And do all of that faster than the entire global Bitcoin mining network is currently adding new honest blocks.

With Bitcoin’s current hash rate, the computational cost of rewriting even a single day of history would require more energy than most countries produce in a year.

The chain isn’t just a metaphor. It’s a mathematical structure that makes rewriting history progressively more expensive the further back you go — until it becomes, for all practical purposes, impossible.

Tomorrow: what is a digital signature — how Bitcoin proves you own what you’re spending.

— The Daily Bit