Block Chain - Hash Function in Blockchain:

1. Definition

A Hash Function is a cryptographic algorithm that takes an input (any size: text, file, transaction data) and produces a fixed-size output (called a hash or digest).

  • In blockchain, hash functions are used to secure data, link blocks, and ensure integrity.

  • Example: Bitcoin uses SHA-256 (Secure Hash Algorithm – 256 bit).

2. Properties of Hash Functions

  1. Deterministic – Same input → always the same output.

  2. Fixed Output Size – Output (hash) length is always the same, regardless of input size.

    • Example: SHA-256 always outputs 256 bits (64 hex characters).

  3. Pre-image Resistance – You can’t reverse a hash to find the original input.

  4. Collision Resistance – No two different inputs should produce the same hash.

  5. Avalanche Effect – Small input change → huge change in output hash.

3. How It Ensures Integrity and Immutability

  • Integrity: If even 1 character in a transaction changes, the hash completely changes → tampering is obvious.

  • Immutability: Since each block contains the previous block’s hash, changing one block requires recalculating all subsequent hashes → nearly impossible on large networks.

4. Example

Input: "Blockchain"
SHA-256 Output:

6250...7e47f3c (64-character hash)

Input: "blockchain" (lowercase "b")
SHA-256 Output:

ef78...93a2df1 (completely different hash)

This shows the avalanche effect.

5. Role of Hash in Blockchain

  • Block Linking: Each block stores the previous block’s hash → forms the “chain.”

  • Mining (Proof of Work): Miners find a hash with certain properties (like starting with a number of zeros).

  • Digital Signatures: Transactions are hashed and signed to ensure authenticity.

In short:
A Hash Function is the backbone of blockchain security. It guarantees that data is tamper-proof, verifiable, and immutable by turning information into unique digital fingerprints that cannot be reversed or duplicated.