Symmetric Encryption: One Shared Key

Symmetric encryption uses the same key to lock and unlock. Modern standards such as AES do this extremely well and fast, so they protect files, disks and the bulk of web traffic.

The XOR operation shows the core idea. XOR-ing data with a key scrambles it; XOR-ing again with the same key restores it. A one-time pad (key as long as the message, random, used once) is unbreakable, but impractical, which is why real systems use AES with managed keys.

The hard part of symmetric encryption is sharing the key safely. That problem leads to public-key cryptography.

This code is a teaching toy. In real applications use a vetted library, never hand-written crypto.