Block Ciphers & Modes Checkpoint

AES structure, block cipher modes, padding, and misuse cases.

1. ECB mode is insecure because:

Equal plaintext blocks encrypt to equal ciphertext blocks It requires a nonce It uses too much memory It is slower than CBC

2. CBC encryption requires a unique, unpredictable:

IV Key schedule S-box Padding

3. CTR mode turns a block cipher into:

A stream cipher A hash A MAC A signature

4. In CBC decryption, plaintext block i is:

Dec(Ci) XOR C(i-1) Enc(Ci) XOR C(i-1) Dec(Ci) XOR Ki Ci XOR C(i-1)

5. PKCS#7 padding for a full block adds:

A full block of 0x10 bytes No padding A single 0x00 byte Random bytes

6. CBC is malleable because:

Flipping bits in C(i-1) flips bits in Pi AES is weak Padding is random IV is secret

7. CTR mode security requires the nonce/counter pair to be:

Unique for each key Secret All zeros Randomly padded

8. AES-128 uses how many rounds?

10 12 14 8

9. MixColumns provides primarily:

Diffusion Confusion Authentication Randomness

10. Which mode provides confidentiality but NOT integrity by itself?

CBC AEAD SIV GCM

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