05_Symmetric — Symmetric encryption — one key, shared

Level: 201 · working knowledge

One key, held by both sides, used to encrypt and to decrypt. This is the fast, boring, entirely solved part of cryptography — AES is unbroken after twenty-five years — and it is also where the most spectacular misuse happens, because a block cipher on its own encrypts exactly sixteen bytes and every real message needs a mode.

The mode is not a detail. It is the whole security property. The same AES key, in ECB, leaks the shape of your data; in CBC with a predictable IV, leaks more; in CTR with a repeated nonce, hands over the plaintext outright.

The pages

#	Page	The question it answers	Status
1	AES has no modes — you pick one	AES encrypts 16 bytes — so what encrypts a file?	planned
2	ECB leaks the picture	Why can you still see the picture through the encryption?	planned

Every page above is a plan until its row says written; see ROADMAP.md.

The through-line

Never choose a mode; choose an AEAD. The modern answer to this whole chapter is one recommendation — AES-GCM or ChaCha20-Poly1305 — and the chapter exists so you know what it is protecting you from.