Unit 2: Modern Symmetric-key Encipherment - Practice Quiz

1 What are the possible coefficient values of polynomials in ?

2 Which operation is equivalent to polynomial addition in ?

3 How many elements are present in the finite field ?

4 What is the block size used by DES?

5 How many rounds are performed in standard DES encryption?

6 What type of structure is used by DES?

7 What is the main purpose of the S-boxes in DES?

8 What is the effective key length of DES?

9 Why is DES considered insecure for many modern applications?

10 Which cipher was introduced to improve security by applying DES multiple times?

11 What fixed block size does AES use?

12 Which key sizes are supported by AES?

13 How many rounds does AES-128 perform?

14 Which AES transformation replaces each byte using an S-box?

15 Which AES transformation cyclically shifts bytes within the rows of the state?

16 Which AES transformation combines each state column using arithmetic in ?

17 What operation is used by AddRoundKey to combine the state with a round key?

18 What is produced by the AES key expansion process?

19 Which AES transformation is omitted from the final encryption round?

20 Which block cipher mode can provide both encryption and authentication?

21 In defined by the irreducible polynomial , what is ?

22 In defined using , which polynomial is the multiplicative inverse of ?

23 What is the sum of the field elements 0x57 and 0x83 in ?

24 Why does the DES expansion permutation transform the 32-bit right half into 48 bits before the round function continues?

25 In one DES Feistel round, suppose the input halves are and , and the round function produces . What are the output halves?

26 Which sequence correctly describes how a DES round key is derived from the supplied 64-bit key?

27 Assuming all DES keys are equally likely, approximately how many keys must an exhaustive-search attacker test on average before finding the correct key?

28 An analyst studies many plaintext pairs having carefully selected XOR differences and observes the resulting ciphertext differences. Which attack is being applied to DES?

29 If DES encryption satisfies , what does the DES complementation property imply?

30 A system uses AES with a 192-bit key. How many encryption rounds are applied to each 128-bit block?

31 The bytes 00 01 02 ... 0F are loaded into an AES state in order. Which sequence forms the first row of the state?

32 During AES ShiftRows, a byte initially located in row 2, column 0 moves to which column?

33 What is the result of applying AES MixColumns to the column ?

34 Which AES transformation is its own inverse when the same round key is used?

35 Which sequence of transformations is used in the final AES encryption round?

36 How many 32-bit words are generated by the AES-128 key expansion, including the original key words?

37 For AES-128 key expansion, which operation is applied when generating a word whose index is a multiple of 4?

38 An image encrypted with a block cipher still reveals repeated visual patterns because identical plaintext blocks produce identical ciphertext blocks. Which mode was most likely used?

39 During CBC transmission, one bit of ciphertext block is accidentally changed. What is the effect after decryption?

40 Two messages are encrypted in CTR mode using the same key and nonce. If their ciphertexts are and , what information becomes directly available to an attacker?

41 In the AES field defined by , what is the product of the bytes 0x57 and 0x13?

42 Suppose the AES modulus were replaced by a reducible degree-8 polynomial over . Which consequence would most directly invalidate the algebraic construction of the AES S-box?

43 Under the AES polynomial , which byte is the multiplicative inverse of 0x53?

44 The AES xtime operation multiplies a byte by in . What is xtime(0x83)?

45 Why can DES decryption use the same Feistel round implementation as encryption?

46 Which statement precisely describes the role of the DES expansion permutation ?

47 A 6-bit input 101011 is supplied to DES S-box . Using the outer bits for the row and the inner four bits for the column, what is the 4-bit output?

48 Let under DES, and let an overbar denote bitwise complementation. Which relation follows from the DES complementation property?

49 Which comparison between the classic differential and linear cryptanalysis attacks on full 16-round DES is most accurate?

50 For a DES weak key, all 16 generated round subkeys are identical. Which property then holds for every plaintext ?

51 AES maps input bytes into the state by . After ShiftRows, where does byte move?

52 Applying the AES MixColumns transformation to the column [0xD4, 0xBF, 0x5D, 0x30] produces which output column?

53 A difference entering MixColumns has exactly one nonzero byte. Ignoring cancellation with other columns, how many nonzero output bytes must that column contain?

54 A full nonfinal AES encryption round applies SubBytes, ShiftRows, MixColumns, and AddRoundKey in that order. Which sequence directly inverts that round when using the same untransformed round key?

55 Why is the AES S-box transformation bijective over all 256 byte values?

56 For AES-128, which expression correctly generates the first word beyond the original four key words?

57 Which additional rule distinguishes the AES-256 key expansion from the AES-128 and AES-192 schedules?

58 In an AES equivalent inverse cipher implementation, how are the intermediate decryption round keys modified relative to the reversed encryption key schedule?

59 Two equal-length plaintexts are encrypted with AES-CTR using the same key and initial counter value. What can an observer compute directly from the ciphertexts?

60 Assuming independent uniformly distributed 128-bit block outputs, approximately what is the probability of at least one collision after processing blocks?