Symmetric encryption & modes

Symmetric encryption uses one shared key to both encrypt and decrypt. This module builds the intuition with a repeating-key XOR cipher, then shows the single most instructive failure in applied cryptography — ECB mode leaking the structure of the plaintext — and what an authenticated mode (AEAD) fixes.

You build the cipher, then the detector that reveals the ECB flaw, then reason about modes on the interview rung.