Hash Function Demo

A hash function is a one-way mathematical function that converts input data of any size into a fixed-size output called a hash or digest.

Key properties:

• Deterministic: Same input always produces same output
• Fast: Quick to compute
• One-way: Cannot reverse the hash to get original input
• Avalanche effect: Small change in input causes large change in output
• Collision resistant: Hard to find two inputs with same output

MD5 and SHA-1 are considered cryptographically broken because researchers have demonstrated practical collision attacks - creating two different inputs that produce the same hash.

Real-world impacts:

• MD5: The Flame malware (2012) used MD5 collisions to forge a Microsoft certificate
• SHA-1: The SHAttered attack (2017) generated two different PDFs with identical SHA-1 hashes

Current status:

• MD5: Should not be used for any security purpose
• SHA-1: Being phased out, major browsers no longer trust SHA-1 certificates
• Use SHA-256 or SHA-512 for all new applications

The avalanche effect means that changing even a single bit in the input should change approximately half of the bits in the output hash.

Why it matters: This property prevents attackers from finding patterns or relationships between similar inputs, making the hash function more secure.

Example: Changing "hello" to "Hello" (one character) completely transforms the SHA-256 hash - approximately 50% of the bits flip.

Collision resistance means it should be computationally infeasible to find two different inputs that produce the same hash output.

Birthday paradox: Due to the birthday paradox, you'd expect to find a collision after approximately 2^(n/2) attempts for an n-bit hash.

Security levels:

• MD5 (128-bit): ~2^64 attempts - achievable
• SHA-1 (160-bit): ~2^80 attempts - demonstrated
• SHA-256 (256-bit): ~2^128 attempts - infeasible
• SHA-512 (512-bit): ~2^256 attempts - far beyond current capability

• Password Storage: Use bcrypt, Argon2, or PBKDF2 (NOT plain SHA-256!)
• File Integrity: SHA-256 or SHA-512
• Digital Signatures: SHA-256 or SHA-512
• Checksums (non-security): CRC32 or MD5 (faster, but not secure)
• Blockchain: SHA-256 (Bitcoin) or other specialized functions
• Legacy Systems: May still require MD5/SHA-1, but migrate when possible

• Digital Signatures: Hash the message before signing (faster than signing entire message)
• Password Storage: Store hash of password, not plaintext
• File Integrity: Verify downloads by comparing computed hash with published hash
• Git: Uses hashes to identify commits and detect changes
• Blockchain: Bitcoin uses double SHA-256 for mining and transactions
• Data Deduplication: Identify duplicate files by comparing hashes

Common misconceptions about hash functions:

• ❌ Encryption: Hashes are one-way and cannot be decrypted
• ❌ Password Protection: Plain hashing is not enough - need salting and key stretching
• ❌ Message Authentication: Need HMAC or digital signatures, not just hashing
• ❌ Brute-force Protection: Weak passwords can still be cracked via rainbow tables