⛓️ Blockchain Fundamentals
What Is a Blockchain?
The ledger idea, blocks, and why the chain is hard to tamper with.
6 min read
A shared ledger
A blockchain is, at heart, a ledger - a running list of transactions. What makes it special is that the ledger is not kept by one bank or company. Thousands of independent computers each keep an identical copy and constantly check each other's work.
Because everyone holds the same copy and agrees on updates through a shared set of rules, no single participant can quietly change the record. Trust comes from math and replication rather than from one trusted institution.
Why 'blocks' and a 'chain'
Transactions are grouped into batches called blocks. Each new block includes a cryptographic fingerprint (a hash) of the block before it. That backward reference is what forms the chain.
If someone tried to edit an old transaction, that block's hash would change, which would break the reference stored in the next block, and the next, and so on. Tampering with one record invalidates everything after it - which is why the ledger is described as immutable.
Editing history isn't just against the rules - it's computationally impractical, because you'd have to redo every block that came after.
Who runs it?
The network is made of nodes - computers running the same open-source software. Anyone can join. Nodes relay transactions, validate them against the rules, and store the history.
Because there's no central server to shut down or bribe, the system keeps running as long as enough independent nodes participate.
Check your understanding
3 questions from this lesson, with the correct answer already marked.
1. What links one block to the previous one?
- A shared password
- A hash of the previous block
- A central database ID
- The miner's signature
Each block stores the hash of the block before it. That backward reference forms the chain and makes tampering detectable.
2. Why is a blockchain considered hard to tamper with?
- A single admin approves every edit
- Changing an old block breaks every block after it, across thousands of copies
- The data is encrypted so no one can read it
- Transactions are deleted after a day
Altering an old block changes its hash, which invalidates every following block - and you'd have to do that on a majority of the network's copies.
3. Who keeps the ledger?
- One central bank
- A government agency
- Many independent nodes, each with a full copy
- The wallet apps
The ledger is replicated across many independent nodes, which is what makes the system decentralized.