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In the first article we saw that Bitcoin is a giant ledger shared between thousands of computers. How is this ledger maintained without anyone being able to cheat? That’s what we’ll cover here, no jargon.
The blockchain: a ledger broken into pages
A ledger is a list of transactions. Bitcoin does exactly that, but instead of adding transactions one by one, it groups them into blocks, roughly every 10 minutes.
Each block contains:
- a list of recent transactions,
- a cryptographic link to the previous block (a mathematical “fingerprint” of the block before),
- a proof of work (more on that).
Hence the name: block-chain, a chain of blocks. Each block references the previous one. Modifying a past block would change its fingerprint, which would invalidate all blocks after it. That’s what makes the ledger tamper-resistant: the further back you go, the more locked-in it is.
Nodes: who keeps the book?
The book isn’t hosted on a central server. It’s copied identically onto tens of thousands of computers called nodes, distributed around the world. Anyone can run a node from home — it’s open-source software.
When you send a transaction, you announce it to the network. Nodes pass it along to each other. A valid transaction eventually gets included in a block, and from then on, everyone has it in their copy of the book.
Mining: who decides what goes in the next block?
Here’s the tricky part. If anyone can write to the book, what stops a clever person from writing “Alice sends me 1000 bitcoins” without Alice’s consent?
First, cryptographic signatures (we’ll get there). But mostly: the right to add the next block has to be earned.
Every 10 minutes or so, specialized computers called miners compete. The challenge: find a number that, combined with the transactions to be included, produces a mathematical fingerprint starting with a certain number of zeros. There’s no shortcut — you have to try billions of trillions of combinations. That’s Proof of Work.
Whoever finds it first wins:
- the right to add their block to the chain,
- a reward in bitcoins (3.125 BTC as of 2026, after the 2024 halving),
- the fees paid by the included transactions.
This competition costs electricity. That’s on purpose: it makes rewriting history (redoing the whole chain faster than the entire network) economically absurd.
Why so much energy? Because that’s precisely what secures the system. The “expense” proves real work was invested. If someone wanted to forge history tomorrow, they’d need to spend as much energy as the entire network combined — currently out of reach even for nation-states.
Your accounts: they’re your keys
On Bitcoin, you don’t have an account in the banking sense. You have cryptographic key pairs:
- A public key, which acts as an address. You can share it. It’s like a bank account number.
- A private key, which signs transactions. This is your absolute secret. Anyone who has it can spend your bitcoins. Anyone who loses it loses their bitcoins forever.
When you send 0.1 BTC to a friend, your wallet uses your private key to produce a cryptographic signature proving that you (and only you) authorize this transaction. Nodes verify the signature and accept — or not — the transaction in the next block.
That’s the logic replacing the bank: no one is needed to validate that you own those bitcoins — the mathematical signature proves it.
An analogy to tie it together
Imagine a shared notebook in a coffee shop:
- Anyone can read the notebook (transactions are public).
- To write a transaction, you have to sign with your personal stamp (your private key).
- Every 10 minutes, the regulars hold a sudoku contest. The winner gets to tear out a fresh page, copy recent transactions onto it, and add it to the notebook. As a reward, they get a free coffee (the block reward).
- For a page to be valid, it has to reference the previous page at the top. Modifying an old page breaks the whole chain.
It’s deliberately simplified, but the essence is there.
What you don’t need to understand
Good news: to use Bitcoin, you need none of this. Like the internet — you send emails without knowing TCP/IP. You install a wallet, get an address, use it. The rest is transparent.
But knowing the basics protects you: it helps you spot scams (“send us your private key to recover your funds” — never, ever), and make informed decisions about where to store your keys.
Next → Why does Bitcoin have value?
This article is not investment advice.