ECB and CBC Modes

AES encrypts 16 bytes at a time. To encrypt longer messages, use a mode of operation.

ECB (Electronic Codebook): simplest. Encrypt each block independently.

ciphertext = AES(P0) || AES(P1) || AES(P2) || ...

Don't use ECB. Identical plaintext blocks produce identical ciphertext blocks → patterns leak. The famous Linux penguin image encrypted in ECB looks like... a penguin.

CBC (Cipher Block Chaining): each block XOR'd with the previous ciphertext before encrypting:

C0 = AES(P0 XOR IV)
C1 = AES(P1 XOR C0)
C2 = AES(P2 XOR C1)
...

The IV (Initialization Vector) must be unpredictable (random). Same plaintext + different IV = different ciphertext. CBC IV is sent alongside ciphertext (no need to keep secret).

CBC's downsides:

• Sequential encrypt (can't parallelize)
• Padding required for short messages (PKCS#7)
• Padding oracle attacks if not authenticated (use HMAC or AEAD)

For new code: use AEAD modes (GCM, ChaCha20-Poly1305). They authenticate AND encrypt in one operation.