SHA-256 Algorithm

What is SHA-256?

SHA-256 (Secure Hash Algorithm 256-bit) is part of the SHA-2 family designed by the NSA and published by NIST in 2001. It produces a 256-bit (32-byte) hash value, displayed as 64 hexadecimal characters. SHA-256 is currently the most widely used secure hash function in the world.

How SHA-256 Works

SHA-256 processes messages using a sophisticated compression function:

1. Preprocessing: Message padded and divided into 512-bit blocks
2. State Initialization: Eight 32-bit working variables initialized with fractional parts of square roots of first 8 primes
3. Message Schedule: 64 words generated from each 512-bit block using bitwise operations
4. Compression Function: 64 rounds of operations using six logical functions (Ch, Maj, Σ0, Σ1, σ0, σ1)
5. Final Hash: 256-bit output from eight 32-bit state variables

Key Properties

• Output Size: 256 bits (64 hex characters)
• Block Size: 512 bits
• Rounds: 64 rounds per block
• Word Size: 32 bits
• Security Level: 128 bits (collision resistance)
• Internal State: Eight 32-bit words

Real-World Applications

Blockchain and Cryptocurrency

SHA-256 is the backbone of Bitcoin and many other cryptocurrencies. It is used for:

• Block hashing and proof-of-work
• Transaction verification
• Wallet address generation
• Merkle tree construction

Digital Certificates

SSL/TLS certificates use SHA-256 for:

• Certificate signing
• Certificate revocation lists
• OCSP responses
• Code signing

File Integrity

SHA-256 is perfect for verifying file integrity because:

• Any change, however small, produces a completely different hash
• Computationally infeasible to find collisions
• Widely supported across all platforms
• Fast enough for large files

Security Analysis

SHA-256 provides strong security guarantees:

• Preimage Resistance: Finding input from hash requires ~2^256 operations
• Second Preimage Resistance: Finding different input with same hash requires ~2^256 operations
• Collision Resistance: Finding two inputs with same hash requires ~2^128 operations

No practical attacks exist against SHA-256. The best theoretical attack reduces security from 128 bits to approximately 126 bits, still computationally infeasible.

SHA-256 vs Other Algorithms

Compared to deprecated algorithms:

• vs MD5: SHA-256 is 2^64 times more collision-resistant
• vs SHA-1: SHA-256 provides 2^48 times more security
• vs SHA-512: Similar security level, but SHA-512 uses 64-bit words