Chapter 03 · The Bitcoin breakthrough

The blockchain, dissected

7 min

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This chapter builds on the ones before it. You can read it now, but it will not count towards your progress until the earlier chapters are done.

Chapter

Block

# block 0001

A blockchain is a chain of blocks. Literally. The word says what it does. The real question isn't "what is it?": it's why it's so hard to falsify that no one seriously tries. That's where things get interesting.

A blockchain is a chain of blocks. There, chapter done.
Kidding.

# block 0002

Picture a giant ledger that records every transaction since the network came to life (time t = 0, the very first block, known as the "Genesis Block"):

a block = a page. Each page is filled with transactions.
a chain of blocks = a book whose pages are all linked together, forming a chain you can't break without breaking the whole thing.

# block 0003

Dissecting a block

A block has two parts: a header and a body.

The header holds the block's metadata:

Previous block hash (the fingerprint that links to the block before)
Merkle root (a single fingerprint that sums up every transaction in the block at once)
Timestamp (the block's birth date)
Nonce (a number used by miners, which we'll get to in the next chapter)

The body contains the list of transactions.

Fictional example: Block #100826

Header

Previous block hash9023dc01…3d43ba

Merkle root2290b31f…a9892e

Timestamp2024-01-14 09:17:42 UTC

Nonce748,291

Body

Transactions3,240 transactions

# block 0004

"But how are these pages linked?", "And this mathematical glue, what is it, concretely?"
Great questions. Let's clear that up right now.

The mathematical glue: hashing

To link blocks together, the Bitcoin protocol uses a hash function. The name sounds scary, but the idea behind it is dead simple.

Picture a meat grinder: you toss in any data you want (a word, an entire book, an image) and the machine spits out a digital fingerprint of fixed size.

Your turn, give it a shot!
Type whatever you want into the hashing machine and watch what happens.

SHA-256

You just ran a single SHA-256⁠(opens in a new tab) hash. Like a butcher's apprentice. Beautiful. Bitcoin does it twice in a row: it hashes the block header, then hashes the result. A historical design choice by Satoshi to strengthen resistance against certain mathematical attacks. We won't dwell on it here, but here's a link for the curious: Mastering Bitcoin, chapter 4⁠(opens in a new tab).

# block 0005

This double hash is applied to the block's header (and only to its header), producing a fingerprint called the "block hash". That fingerprint serves as an identifier - and you'll see in the that it has a lot more going on than just that. This hash is then carried into the header of the next block.

The result: the slightest change to a block immediately invalidates every block that follows. Rewriting the blockchain becomes extremely hard in practice.

Here's one block, on its own. Add another behind it. You'll see what links them.

# block 0006

The blockchain is therefore a ledger that's practically impossible to falsify, where each page is sealed by the next one. But a solid ledger isn't enough: we still need to decide who is allowed to write in it, and at what cost.

Because that nonce, the one you ran into in every block, isn't there by accident. It's the key to a worldwide race between miners, where computing power replaces trust, and where the hash you just discovered plays a central role.

On to the next chapter: .