Knowledge base
We've referenced “the blockchain” throughout this series without properly explaining it. Now that you've built up a solid foundation, it's time to open the hood and look at what's actually happening when Bitcoin moves from one person to another.
A blockchain is a continuously growing, shared record of every Bitcoin transaction ever made, copied across thousands of independent computers around the world, with each new batch of transactions cryptographically linked to the one before it.
Every Bitcoin transaction is digitally signed using the sender's private key - mathematically proving ownership without ever revealing the key itself. Anyone on the network can check that signature against the sender's public key and confirm it's genuine, without needing to trust the sender's word for it. We cover this properly in our dedicated article on public and private key pairings.
Rather than processing one transaction at a time, the network groups pending transactions together into a “block,” roughly every ten minutes. Producing a valid block requires solving a genuinely difficult computational puzzle - a process called mining, which we cover fully in our dedicated article on Bitcoin mining. For now, the key point is this: creating a new block takes real, verifiable effort, and that effort is exactly what makes the whole system trustworthy.
Each new block contains a cryptographic fingerprint, a “hash”, of the block immediately before it. That single design choice has a powerful effect: if anyone tried to alter a transaction buried deep in the blockchain's history, it would change that block's fingerprint, breaking the link to every block that came after it. The tampering would be immediately, mathematically obvious to anyone checking. This is why the blockchain is often described as tamper-evident, rather than simply “secure” in a vague sense.
This is the part that solves the problem covered in Bitcoin's original purpose: how do thousands of independent computers, with no central authority, agree on a single, shared version of events? Each participant (“node”) independently checks every new block against Bitcoin's rules - valid signatures, no attempts to spend the same coins twice, and so on. Where competing versions of the ledger briefly exist, the network follows a straightforward rule: the version representing the greatest total verified effort is treated as the genuine history. Over time, this causes the entire network to converge on one shared, agreed record, without anyone needing to be in charge of enforcing it.
Altering Bitcoin's history isn't just difficult in a general sense - it would require redoing an enormous amount of verified computational effort, faster than the rest of the honest network combined, for every single block being changed. At Bitcoin's current scale, this is considered practically impossible with existing technology. Who actually controls Bitcoin looks at this from a different angle, covering decentralisation and network security in more depth.
This article deliberately kept a few important pieces at arm's length, since each deserves proper treatment of its own: the mining process itself, Bitcoin's supply schedule, and how a wallet actually manages your own keys day to day. We pick each of these up as this Tier continues.
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In our next article, What Is Bitcoin Mining, and Why Does It Matter?, we'll properly explain the process this article touched on only briefly.
