# Advanced Encryption Algorithm and Block Ciphers operations MCQs

1. AES uses a ____________ bit block size and a key size of __________ bits..

A. 128; 128 or 256.

B. 64; 128 or 192.

C. 256; 128, 192, or 256.

D. 128; 128, 192, or 256.

Answer= 128; 128, 192, or 256

2. Like DES, AES also uses Feistel Structure..

A. TRUE.

B. FALSE.

C. Nothing can be said.

D. None of the mentioned.

3. Which one of the following is not a cryptographic algorithm- JUPITER, Blowfish, RC6, Rijndael and Serpent?.

A. JUPITER.

B. Blowfish.

C. Serpent.

D. Rijndael.

4. Which algorithm among- MARS, Blowfish, RC6, Rijndael and Serpent -was chosen as the AES algorithm?.

A. MARS.

B. Blowfish.

C. RC6.

D. Rijndael.

5. How many rounds does the AES-192 perform?.

A. 10.

B. 12.

C. 14.

D. 16.

6. How many rounds does the AES-256 perform?.

A. 10.

B. 12.

C. 14.

D. 16.

7. What is the expanded key size of AES-192?.

A. 44 words.

B. 60 words.

C. 52 words.

D. 36 words.

8. The 4×4 byte matrices in the AES algorithm are called.

A. States.

B. Words.

C. Transitions.

D. Permutations.

9. In AES the 4×4 bytes matrix key is transformed into a keys of size __________.

A. 32 words.

B. 64 words.

C. 54 words.

D. 44 words.

10. For the AES-128 algorithm there are __________ similar rounds and _________ round is different..

A. 2 pair of 5 similar rounds ; every alternate.

B. 9 ; the last.

C. 8 ; the first and last.

D. 10 ; no.

11. Which of the 4 operations are false for each round in the AES algorithm i) Substitute Bytes ii) Shift Columns iii) Mix Rows iv) XOR Round Key.

A. i) only.

B. ii) iii) and iv).

C. ii) and iii).

D. only iv).

12. There is an addition of round key before the start of the AES round algorithms..

A. TRUE.

B. FALSE.

C. Nothing can be said.

D. None of the mentioned.

13. The multiplicative inverse of 0x95 in AES where m(x)=x8+x4+x3+x+1 is.

A. 0x8F.

B. 0xF8.

C. 0x8A.

D. 0xA8.

14. The S-box value for byte stored in cell (6,D).

A. 0x3C.

B. 0x7F.

C. 0xFD.

D. 0x4A.

15. The S-box value for byte stored in cell (B,3).

A. 0x3C.

B. 0xB3.

C. 0x4F.

D. 0x90.

16. The S-box value for byte stored in cell (3,3).

A. 0xC3.

B. 0x3C.

C. 0x44.

D. 0x9B.

17. The inverse s-box permutation follows, b'_i = b_(i+2) XOR b(i+5) XOR b_(i+7) XOR d_i ; Here d_i is.

A. d_i is the ith bit of a byte 'd' whose hex value is 0x15.

B. d_i is the ith bit of a byte 'd' whose hex value is 0x05.

C. d_i is the ith bit of a byte 'd' whose hex value is 0x25.

D. d_i is the ith bit of a byte 'd' whose hex value is 0x51.

Answer= d_i is the ith bit of a byte 'd' whose hex value is 0x05

18. The Inverse S-box value for byte stored in cell (3,3).

A. 0xC3.

B. 0x66.

C. 0x1F.

D. 0x9B.

19. The Inverse S-box value for byte stored in cell (6,3).

A. 0x00.

B. 0x11.

C. 0x01.

D. 0x04.

20. The Inverse S-box value for byte stored in cell (D,2).

A. 0x5F.

B. 0x2D.

C. 0x7F.

D. 0x5D.

21. How many computation rounds does the simplified AES consists of?.

A. 5.

B. 2.

C. 8.

D. 10.

22. For the case of Mixed Columns and Inverse Mixed Columns, is it true that b(x) = a-1(x)mod(x4 + 1); where a(x) = {03}x3 +  {01}x2 +  {01}x +  {02} and b(x) = {0B}x3 +  {0D}x2 +  {09}x +  {0E}.

A. TRUE.

B. False. The expression for a(x) is wrong..

C. False. The expression for b(x) is wrong..

D. False. Both a(x) and b(x) are faulty..

23. For an inputs key of size 128 bits constituting of all zeros, what is w(7) ?.

A. {62 63 63 63}.

B. {62 62 62 62}.

C. {00 00 00 00}.

D. {63 63 63 62}.

24. On comparing AES with DES, which of the following functions from DES does not have an equivalent AES function?.

A. f function.

B. permutation p.

C. swapping of halves.

D. xor of subkey with function f.

25. On perform the Mix Columns transformation for the sence of bytes "77 89 AB CD" we get output.

A. {01 55 EE 4A}.

B. {0A 44 EF 4A}.

C. {08 55 FF 3A}.

D. {09 44 DD 4A}.

26. On perform the Mix Columns transformation for the sence of bytes "67 89 AB CD" we get output.

A. {08 55 FF 18}.

B. {28 45 EF 08}.

C. {28 45 FF 18}.

D. {25 35 EF 08}.

27. Is the following matrix the inverse matrix of the matrix used in the mix columns step?x3 + 1      xx            x3 + 1.

A. Yes.

B. No.

C. Can't say.

D. Insufficient Information.

28. For the cipher text 0000 0111 0011 1000 and Key 0110 1111 0110 1011, apply the Simplified AES to obtain the plaintext. The plain text is.

A. 0110 1001 0111 0001.

B. 0110 1111 0110 1011.

C. 0010 1001 0110 1011.

D. 1111 0101 0111 1111.

29. What is the block size in the Simplified AES algorithm?.

A. 8 bits.

B. 40 bits.

C. 16 bits.

D. 36 bits.

30. What is the key size in the S-AES algorithm?.

A. 16 bits.

B. 32 bits.

C. 24 bits.

D. None of the mentioned.

31. S-AES and S-DES were both developed by the same person as an educational cryptography system to teach students.

A. TRUE.

B. FALSE.

C. Nothing can be said.

D. None of the mentioned.

32. Which of the following is a faulty S-AES step function?.

B. Byte substitution.

C. Shift rows.

D. Mix Columns.

33. Which mode of operation has the worst "error propagation" among the following?.

A. OFB.

B. CFB.

C. CBC.

D. ECB.

34. Which block mode limits the maximum throughput of the algorithm to the reciprocal of the time for one execution?.

A. OFB.

B. CTR.

C. CBC.

D. ECB.

35. Which mode requires the implementation of only the encryption algorithm?.

A. ECB.

B. CBC.

C. CTR.

D. OFB.

36. Which of the following modes of operation does not involve feedback?.

A. ECB.

B. CBC.

C. CTR.

D. OFB.

37. Which of the following is a natural candidates for stream ciphers?.

A. OFB.

B. CFB.

C. CBC.

D. ECB.

38. The XTS-AES mode was approved by NIST in.

A. 1999.

B. 2010.

C. 2006.

D. 2002.

39. The XTS-AES mode is based on the concept of tweakable block cipher..

A. TRUE.

B. FALSE.

C. Nothing can be said.

D. None of the mentioned.

40. The purpose of a 'tweak' in XTS-AES mode is to.

A. secure the public key.

B. provide security.

C. provide variability.

D. all of the mentioned.

41. A tweak is used in XTS-AES mode to provide a different output for the same input PT and same key..

A. TRUE.

B. FALSE.

C. Nothing can be said.

D. None of the mentioned.

42. XTS-AES mode of operation is a better version of.

A. OFB.

B. CFB.

C. CTR.

D. ECB.

43. What is the size of the XTS-AES key?.

A. 1024 bits.

B. 64 bits.

C. 512 bits.

D. 128 bits.

44. Which of the following represent the tweak?.

A. j.

B. i.

C. alpha.

D. alphaj.

45. Which of the following is true for the tweak in XTS-AES mode?.

A. they are non-negative integers.

B. they are of size 128 bits.

C. they are assigned consecutively.

D. all of the mentioned.

46. Which of the following is the correct encryption statement representation for the XTS-AES mode?.

A. E(K1,P) XOR T.

B. E(K2,T) XOR P.

C. E(K1,T XOR P) XOR P.

D. E(K1,P XOR T) XOR T.

Answer= E(K1,P XOR T) XOR T

47. The last two blocks of the XTS-AES mode are

B. encrypted/ decrypted using ciphertext-stealing.

D. padded and then swapped after encryption/ decryption.

48. How many modes of operation are there in in DES and AES?.

A. 4.

B. 3.

C. 2.

D. 5.

49. Which one of the following modes of operation in DES is used for operating short data?.

A. Cipher Feedback Mode (CFB).

B. Cipher Block chaining (CBC).

C. Electronic code book (ECB).

D. Output Feedback Modes (OFB).

50. Which of the following is false for ECB mode of operation i) The Plain text is broken into blocks of size 128 bytes ii) Blocks can be swapped, repeated, replaced without recipient noticing iii) Good for short data iv) Encryption of each block is done separately using a randomly generated key for each block.

A. i) only.

B. ii) and iii).

C. i) and iv).

D. i) ii) and iv).

51. Which of the following statements are true : i) In the CBC mode, the plaintext block is XORed with previous ciphertext block before encryption  ii) The CTR mode does not require an Initialization Vector  iii) The last block in the CBC mode uses an Initialization Vector  iv) In CBC mode repetitions in plaintext do not show up in ciphertext.

A. iii).

B. ii) and iv).

C. All the Statements are true.

D. i) ii) and iv).

52. There is a dependency on the previous 's' bits in every  stage in CFB mode. Here 's' can range from ___.

A. 8-16 bits.

B. 8-32 bits.

C. 4-16 bits.

D. 8-48 bits.

53. Which of the following can be classified under advantages and disadvantages of OFB mode?  i) Transmission errors  ii) A bit error in a ciphertext segment  iii) Cannot recover from lost ciphertext segments  iv) Ciphertext or segment loss.

54. In OFB Transmission errors do not propagate: only the current ciphertext is affected, since keys are generated "locally"..

A. TRUE.

B. FALSE.

C. Nothing can be said.

D. None of the mentioned.

55. Which of the following modes does not implement chaining or "dependency on previous stage computations"?.

A. CTR, ECB.

B. CTR, CFB.

C. CFB, OFB.

D. ECB, OFB.

56. The counter value in CTR modes repeats are a regular interval..

A. TRUE.

B. FALSE.

C. Nothing can be said.

D. None of the mentioned.

57. How many step function do Round 1 and 2 each have in S-AES?.

A. 4 and 3.

B. Both 4.

C. 1 and 4.

D. 3 and 4.

58. For a key 25D5 and PT input A479 what is the output we obtain after the "add round key" function?.

A. F34D.

B. 81AC.

C. 79DF.

D. 327D.

59. The output of the previous stion, on passing through "nibble substitution" gets us the output.

A. 3267.

B. 1344.

C. 64C0.

D. CA37.

60. The output of the previous stion on passing through the "shift row" step function gives us the output.

A. C046.

B. 0C64.

C. 64C0.

D. 640C.

61. The output of the previous stion on passing through the "mix columns" step function gives us the output.

A. 3252.

B. 3743.

C. 3425.

D. 3473.

62. The inverse transformation matrix times the forward transformation matrix equals the identity matrix..

A. TRUE.

B. FALSE.

C. Nothing can be said.

D. None of the mentioned.

63. Which of the following are true: i) In the AES key expansion algorithm, the function g() operates on w i+3  ii) Perform a one-byte right circular rotation on the argument 4-byte word  iii) The round constant follows the formula RC[j]  = RC[j ? 1].

A. i) ii).

B. ii) only.

C. i) only.

D. All.

64. How many round keys are generated in the AES algorithm?.

A. 11.

B. 10.

C. 8.

D. 12.