Blockchain

What is the Blockchain?

The blockchain is Bitcoin's public ledger — a continuously growing list of records (blocks) that are linked and secured using cryptography. Think of it as a book of accounts that everyone can read but no one can erase.

Structure of a Block

Each block contains:

• Block Header: Previous block hash, Merkle root, timestamp, nonce, difficulty target
• Transaction List: All transactions included in this block
• Coinbase Transaction: The miner's reward

Block #840000
├── Previous Hash: 0000000000000000...
├── Merkle Root: abc123...
├── Timestamp: 2024-04-20 00:09:01
├── Nonce: 3,918,273,816
└── Transactions: [3,821 transactions]

Why is it "Chain" of Blocks?

Each block contains the hash of the previous block. This creates a chain. If you change a transaction in block 100, its hash changes, which breaks block 101, then 102, then 103... You'd need to redo ALL the work for all subsequent blocks. This is what makes Bitcoin immutable.

Immutability

Once a transaction has 6+ confirmations (6 blocks built on top of it), it is considered irreversible. An attacker would need 51% of Bitcoin's hashrate to rewrite history — an astronomically expensive attack.

The Blockchain is NOT a Database

Unlike a database, the blockchain:

• Cannot delete records
• Cannot update existing data
• Is replicated identically on thousands of nodes
• Is write-once, append-only