## TL;DR

Cryptographic hash functions map arbitrary data to fixed-size digests (fingerprints). Key properties: preimage resistance (cannot reverse), second preimage resistance (cannot find different input with same hash), collision resistance (cannot find any two inputs with same hash). SHA-256 (32 bytes) is the current standard; SHA-3 (Keccak) is the next generation.

## Core Explanation

SHA-1 (broken, 2017 collision demonstrated). MD5 (completely broken). SHA-2 family: SHA-224, SHA-256, SHA-384, SHA-512 (all considered secure). SHA-3 is not a replacement but an alternative with different internal structure (sponge construction). Password hashing uses specialized functions: bcrypt (adaptive cost), scrypt (memory-hard), Argon2 (winner of PHC, 2015 — memory-hard + side-channel resistant).

## Further Reading

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