Symmetric Encryption

The shared-secret model, plaintext and ciphertext, what a cipher actually does, and the vocabulary for the rest of this subdomain.

What AES is, how an open NIST competition chose it in 2001, where you meet it every day in TLS, VPNs and disk encryption, how hardware acceleration made it nearly free, and how it has held up against decades of attack including quantum.

Fixed-size input blocks transformed by a key. DES and 3DES historically, Rijndael becoming AES in 2001, and why AES is the cipher you meet almost everywhere today.

Encrypting one bit at a time vs fixed-size blocks. ChaCha20 as the modern stream standard, RC4 as the cautionary tale, and when streams beat blocks in practice.

How a block cipher encrypts data longer than one block. ECB and its famous penguin, CBC, CTR turning a block cipher into a stream, and GCM combining encryption with integrity.

Making variable-length plaintext fit fixed-size blocks. PKCS#7 padding, why IVs must be unique, and the padding-oracle attacks that exploit careless implementations.

Encryption alone is not enough. AES-GCM and ChaCha20-Poly1305 combine confidentiality and integrity in one operation, defeating bit-flipping and chosen-ciphertext attacks.

Generation, storage, rotation, and destruction of symmetric keys. KEKs and DEKs, hardware security modules, key escrow, and how cloud KMS services handle the lifecycle.