APRIL 3
The Chain
How Cryptographic Fingerprints Link Bitcoin’s Blocks Into One History
The Signal
Each Bitcoin block carries the digital fingerprint of the block before it, creating a chain that grows harder to rewrite with every new link.
The Reading
Yesterday, we learned that Bitcoin gathers transactions into blocks. Now we connect them. Each block has a small header containing the cryptographic hash of the previous block’s header. Think of a hash as a digital fingerprint.
The transactions are summarised into one fingerprint called a Merkle root, stored in the header. Change one transaction and the summary changes, so the block’s fingerprint changes too.
The next block was built using the old fingerprint. If someone alters the earlier block, its new fingerprint no longer matches the one recorded in the next block. The link breaks. To hide the change, every block after it must also be rebuilt.
Rebuilding is not simple editing. The proof of work for the changed block and every later block must be done again while honest miners continue adding blocks. Bitcoin nodes reject invalid blocks and follow the valid chain with the most accumulated work.
Each new block added above an older block gives it another confirmation. It adds more work an attacker would need to replace. Reversal is not mathematically impossible, but it becomes increasingly difficult and unlikely as the chain grows.
Bitcoin’s history is protected by linked fingerprints, independently checked rules and accumulated proof of work, not by somebody’s promise.
First Principles
Each block’s header stores the cryptographic hash of the previous block’s header.
Changing an earlier block changes its fingerprint and breaks the next link.
Every later block adds proof of work, making the history below it increasingly difficult to replace.
Today’s Challenge
Take two forks and slide their teeth together.
They fit because their shapes match.
Now replace one fork with a spoon. It no longer fits. The chain is broken.
The spoon represents a changed block. Its digital fingerprint no longer matches the fingerprint stored in the next block, so Bitcoin’s nodes will not accept it into the chain.
Tomorrow’s Signal
Why History Cannot Be Easily Changed