Blocks in a blockchain

A block is the fundamental unit of data in a blockchain. Think of it as a container that securely stores a batch of information (usually transactions) and links it cryptographically to the previous block, forming a chain.

1. Block Structure (Typical Blockchain Example)

Although details vary between blockchain implementations (e.g., Bitcoin, Ethereum, Hyperledger), a block usually has these main components:

a) Block Header

The block header contains metadata that uniquely identifies the block and ensures security. It usually includes:

• Block Number (Height): Position of the block in the chain.
  Example: Block #100 follows Block #99.
• Previous Block Hash: A cryptographic hash of the previous block’s header.
  Ensures immutability — if Block #99 changes, its hash changes, making Block #100 invalid.
• Merkle Root Hash: A hash of all transactions inside the block, organized in a Merkle Tree.
  Allows efficient verification of individual transactions.
• Timestamp: The exact time the block was created.
  Example: 2025-08-17 14:35:27 UTC
• Nonce: A random number used in Proof-of-Work consensus to satisfy difficulty requirements.
• Difficulty Target: Defines how hard it is to mine a block.
• Version: Indicates the blockchain software version.

Bitcoin Example:

• Previous Block Hash = 0x000000...0 (none since it’s the first block)
• Merkle Root = Hash of the only transaction (50 BTC reward to Satoshi)
• Nonce = 2083236893
• Timestamp = 2009-01-03 18:15:05 UTC

b) Block Body

The block body contains the actual data/transactions:

• Transaction Counter → Number of transactions in this block.
• Transactions → List of validated transactions, typically including:
• Sender Address (Public Key)
• Receiver Address (Public Key)
• Amount
• Digital Signature (to prove authenticity)
• Smart Contract Code/Data (in platforms like Ethereum)

Ethereum Example:

• Transactions (value transfers, contract deployments, smart contract function calls)
• Gas usage and gas limits
• State changes in the Ethereum Virtual Machine (EVM)

Example: If Alice sends 1 BTC to Bob, and Charlie sends 2 BTC to Diana, both transactions are stored in the block body.

Block #100:
Header:
- Previous Hash: 0000abc...xyz123
- Timestamp: 2025-08-17 14:35:27 UTC
- Nonce: 734562
- Merkle Root: a34f2c9...ef6721

Body:
Transactions:
1. Alice -> Bob (1 BTC)
2. Charlie -> Diana (2 BTC)
3. Eve -> Frank (0.5 BTC)
  

Here:
• The previous hash links Block #100 to Block #99.
• The Merkle root summarizes all transactions.
• The nonce proves that miners solved the cryptographic puzzle.

Bitcoin Example

• Block size limit: ~1 MB
• Average transactions per block: 2,000–3,000
• Block time: ~10 minutes
• Genesis Block (Block #0): Created by Satoshi Nakamoto in 2009

Summary:

• A block = header + body
• The header secures the block with cryptographic hashes
• The body stores actual transaction data
• Blocks are linked via hashes, forming a tamper-proof chain
• Any modification in a block invalidates all following blocks

2. Cryptographic Linking Between Blocks

• Each block has a hash of its header.
• The next block includes this hash in its Previous Block Hash field.
• This creates a tamper-proof chain: if anyone alters a past block, its hash changes, breaking the chain.

Example:

• Block #100 has hash 0000a9d3...
• Block #101 references that hash in its header.
• If Block #100 changes, Block #101’s reference becomes invalid → consensus rejects it.

3. Example Walkthrough: Adding a Block (Bitcoin-like Chain)

1. Transactions Created → Alice sends 1 BTC to Bob, signed with her private key.
2. Transactions Verified → Nodes validate Alice’s balance, signature, and format.
3. Block Formation → Transactions grouped into a block with:
   • Previous Block Hash
   • Merkle Root of all transactions
   • Timestamp, nonce, difficulty
4. Proof of Work (Mining) → Miners try nonce values until the block’s hash meets difficulty (e.g., starts with 000000).
5. Block Added → Block is appended to the chain.
6. Network Agreement → Other nodes validate the block before accepting it.