Hashing

Problem

You have a 5 GB file. Later you want to know whether even one byte changed without comparing every byte against another copy.

Solution — Cryptographic Hash 

A hash function produces a fixed-size fingerprint:
5 GB file
   ↓
SHA-256
   ↓
A84F...92C1
Change the input and the resulting hash will, with overwhelming probability, differ.
Core idea: hashing creates a compact fingerprint of data.