Encryption

Problem

You send: password=secret123 across networks you don't control. Someone who can observe the traffic shouldn't be able to understand it.

Solution — Encryption

A key transforms readable plaintext into ciphertext. 
password=secret123
        ↓ key
8F3A91B7C2... Someone without the required key sees meaningless data.
Core idea: encryption protects the confidentiality of information.