A blockchain is a decentralized, distributed digital ledger that records transactions across a network of computers. Once data is recorded, it cannot be altered without changing all subsequent blocks — making the record tamper-evident and highly secure.
Blockchain was originally developed as the underlying technology for Bitcoin (2008) but is now applied across many industries.
Each block in a blockchain contains three key elements:
| Component | Description |
|---|---|
| Data | The transaction or information being recorded |
| Hash | A unique cryptographic fingerprint of the block's content |
| Previous Hash | The hash of the block before it, linking blocks into a chain |
If any data in a block is changed, its hash changes — which breaks the link to the next block, immediately signaling tampering.
No single entity controls the blockchain. The ledger is copied and maintained across thousands of nodes (computers) worldwide. This eliminates a single point of failure.
Once a transaction is recorded and confirmed, it cannot be changed or deleted. This is enforced by cryptographic hashing — altering one block invalidates all blocks that follow it.
All participants in a public blockchain can view the entire transaction history, promoting trust and accountability.
Data is secured through cryptographic hashing and consensus mechanisms, making unauthorized changes computationally infeasible.
Before a new block is added to the chain, all nodes must agree that the transaction is valid. This agreement process is called a consensus mechanism.
| Mechanism | How it Works |
|---|---|
| Proof of Work (PoW) | Nodes (miners) compete to solve a complex mathematical puzzle; the winner adds the next block. Used by Bitcoin. |
| Proof of Stake (PoS) | Validators are chosen based on the amount of cryptocurrency they "stake" (lock up) as collateral. More energy-efficient than PoW. |
A smart contract is a self-executing program stored on a blockchain that automatically enforces and executes the terms of an agreement when predefined conditions are met — without requiring a human intermediary.
Example: An insurance smart contract automatically pays a claim when flight-delay data confirms a delay of more than 3 hours.
| Sector | Application |
|---|---|
| Finance | Cryptocurrency (Bitcoin, Ethereum), cross-border payments |
| Supply Chain | Tracking goods from manufacturer to consumer with full transparency |
| Healthcare | Secure sharing of patient records between hospitals |
| Voting | Tamper-proof digital voting systems |
| Education | Verifiable digital certificates and degrees |
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Disadvantages: