What makes a hash function 'cryptographic'?

Cryptographic hash functions have specific security properties: pre-image resistance (can't find input from hash), second pre-image resistance (can't find different input with same hash), and collision resistance (can't find any two inputs with same hash). Regular hash functions (like for hash tables) don't need these properties.

Is MD5 still safe to use?

MD5 is cryptographically broken - collisions can be generated quickly. Never use it for security (passwords, signatures, certificates). It's acceptable only for non-security checksums like detecting accidental corruption, but SHA-256 is preferred even for that.

What is a hash collision?

A collision occurs when two different inputs produce the same hash output. Since hash outputs are fixed-size and inputs are unlimited, collisions must exist (pigeonhole principle). Cryptographic hashes make finding collisions computationally infeasible - but MD5 and SHA-1 have been broken.

Cryptographic Hash Functions Explained: SHA-256, MD5 & More

What is a Hash Function?

A hash function takes input data of any size and produces a fixed-size output (the "hash" or "digest").

Key Properties

Deterministic: Same input always produces same output
Fast: Quick to compute for any input size
One-way: Cannot reverse hash to get input
Collision-resistant: Hard to find two inputs with same hash
Avalanche effect: Small input change = completely different hash

Common Algorithms

Algorithm	Output Size	Status

MD5	128 bits	❌ Broken

SHA-1	160 bits	❌ Broken

SHA-256	256 bits	✅ Secure

SHA-384	384 bits	✅ Secure

SHA-512	512 bits	✅ Secure

SHA-3	Variable	✅ Secure

BLAKE3	Variable	✅ Secure, Fast

Example: SHA-256

Input: "Hello" SHA-256: 185f8db32271fe25f561a6fc938b2e264306ec304eda518007d1764826381969 Input: "hello" (just lowercase) SHA-256: 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824

Completely different output!

Use Cases

Use Case	Recommended

Password storage	bcrypt, Argon2 (NOT raw SHA)

File integrity	SHA-256

Digital signatures	SHA-256 or SHA-3

Hash tables	Non-crypto (MurmurHash)

Checksums	SHA-256 or BLAKE3

Git commits	SHA-1 (legacy) → SHA-256