Block Ciphers & Modes Checkpoint

Worked scenarios: trace state, diagnose mistakes, and apply the module contract.

Get every question correct to pass this checkpoint. You can review feedback, revise your answers, and submit again.

1. For state[r+4*c], what flat index holds row 3, column 2? Answer in decimal.

2. Forward ShiftRows writes new row 1, column 0 from which old position? Select one.

Select one answer.

Row 1, column 3 Row 1, column 0 Row 1, column 1 Row 0, column 1

3. In AES field arithmetic, xtime(0xae) produces which two-digit hexadecimal byte?

4. In AES-128 key expansion, which words apply RotWord, SubWord, and Rcon before the recurrence XOR? Select one.

Select one answer.

Every word after the original key Only the last word of each round key Only w[4]; later round keys copy it Every fourth new word: i divisible by 4

5. w[0]=00010203 and transformed temp=d6ab76fe. Which value is w[4]? Select one.

Select one answer.

d6aa74fd d6ab76fe d7ab76fe d2af72fa

6. Which forward transformation is omitted in AES-128 round 10? Select one.

Select one answer.

SubBytes ShiftRows AddRoundKey MixColumns

7. Using unmodified forward-expanded keys, which order is used in an interior decryption round? Select one.

Select one answer.

InvSubBytes, InvMixColumns, AddRoundKey, InvShiftRows AddRoundKey, InvMixColumns, InvShiftRows, InvSubBytes InvShiftRows, InvSubBytes, InvMixColumns, AddRoundKey InvShiftRows, InvSubBytes, AddRoundKey, InvMixColumns

8. How many PKCS#7 padding bytes are added to a 16-byte message for a 16-byte block size?

9. After CBC decryption, a suffix ends in 02 03. Is checking only that the final byte is between 1 and 16 sufficient? Select one.

Select one answer.

No; every message must end with 10 No; every one of the indicated padding bytes must match Yes; a final 03 authenticates the message Yes; all positive final bytes are valid padding

10. A 17-byte CTR message begins at counter 7 under a fixed key and nonce. Which second message start immediately reuses a counter block? Select one.

Select one answer.

Counter 9 Counter 10 Counter 11 Counter 8

11. Two CTR ciphertexts use the same keystream S: C=P XOR S and D=Q XOR S. What is C XOR D? Select one.

Select one answer.

The secret AES key A value independent of both plaintexts P XOR Q P XOR Q XOR S

12. A known next-block plaintext byte is 0x61 and the target is 0x7a. What two-digit hex delta should be XORed into the corresponding previous ciphertext byte?

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