35 + Interview Questions in Block Ciphers And The Data Encryption Standard in Cryptograph & Network Security Page 1 InterviewSolution

1.	Assume input 10-bit key, K: 0010010111 for the SDES algorithm. What is K1?(a) 00101111(b) 01011011(c) 01101000(d) 10100111I had been asked this question by my school principal while I was bunking the class.My question comes from The Simplified Data Encryption Standard (SDES) topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct ANSWER is (a) 00101111 For explanation I WOULD say: The PERMUTED key P10 = 1000010111. Input to P8: 0000101111 and K1 is 00101111.

Discussion

2.	In the DES algorithm, although the key size is 64 bits only 48bits are used for the encryption procedure, the rest are parity bits.(a) True(b) FalseI got this question in final exam.This interesting question is from The Data Encryption Standard (DES) and It’s Strength in section Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The CORRECT ANSWER is (b) False Best explanation: 56 bits are USED, the rest 8 bits are parity bits.

Discussion

3.	Assume input 10-bit key, K: 0010010111 for the SDES algorithm. What is K2?(a) 11101010(b) 11011011(c) 01101000(d) 10101111The question was posed to me in an internship interview.Asked question is from The Simplified Data Encryption Standard (SDES) topic in chapter Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Right choice is (a) 11101010 To EXPLAIN: The permuted KEY P10 = 0000101111. Input to P8: 0010011101 and K2 is 11101010.

Discussion

4.	In SDES, Encryption algorithm can bewritten as a composition of functions:(a) IP-1 o fK2 o fK1 o SW o IP(b) True(c) FalseThis question was addressed to me in exam.I'm obligated to ask this question of The Simplified Data Encryption Standard (SDES) in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Right option is (B) True To elaborate: The SDES ALGORITHM FOLLOWS the order – IP-1 o fK2 o SW o fK1 o IP.

Discussion

5.	Using Differential Crypt-analysis, the minimum computations required to decipher the DES algorithm is(a) 2^56(b) 2^43(c) 2^55(d) 2^47I got this question during an online exam.Question is from The Data Encryption Standard (DES) and It’s Strength topic in division Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The CORRECT choice is (d) 2^47 Best explanation: DIFFERENTIAL Crypt-analysis requires only 2^47 COMPUTATIONS to DECIPHER the DES algorithm.

Discussion

6.	A preferable cryptographic algorithm should have a good avalanche effect.(a) True(b) FalseThe question was asked by my college professor while I was bunking the class.The question is from The Data Encryption Standard (DES) and It’s Strength in chapter Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Right option is (a) TRUE Easy explanation: Thus STATEMENT is true as a change in ONE bit of the PLAINTEXT or one bit of the key should produce a change in many bits of the ciphertext. This is REFERRED to as the avalanche effect.

Discussion

7.	The Plaintext for the Ciphertext 10100101, given that the key is 0010010111 is(a) 01100111(b) 00110110(c) 01001000(d) 01001100This question was posed to me by my college director while I was bunking the class.This question is from The Simplified Data Encryption Standard (SDES) in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Correct choice is (b) 00110110 The explanation is: Perform the SDES Decryption ALGORITHM and COMPUTE the CIPHER text.

Discussion

8.	The Plaintext for the Ciphertext 11110000, given that the key is 0000000000 is(a) 01100111(b) 00000000(c) 01001000(d) 01101100I got this question in examination.This intriguing question originated from The Simplified Data Encryption Standard (SDES) topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» RIGHT CHOICE is (b) 00000000 For explanation I WOULD say: Perform the SDES Decryption algorithm and compute the cipher text.

Discussion

9.	During decryption, we use the Inverse Initial Permutation (IP-1) before the IP.(a) True(b) FalseThe question was asked in unit test.Query is from The Data Encryption Standard (DES) and It’s Strength topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct CHOICE is (a) True Easy EXPLANATION: IP-1 is the first STEP and the last step is IP during DECRYPTION.

Discussion

10.	The Plaintext for the Ciphertext 00100010, given that the key is 1111111111 is(a) 01100111(b) 00001010(c) 01001000(d) 01001100I had been asked this question during an interview for a job.My query is from The Simplified Data Encryption Standard (SDES) in section Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Correct OPTION is (d) 01001100 The BEST explanation: Perform the SDES DECRYPTION algorithm and COMPUTE the CIPHER text.

Discussion

11.	The S-Box is used to provide confusion, as it is dependent on the unknown key.(a) True(b) FalseThe question was posed to me at a job interview.I'm obligated to ask this question of Block Cipher Systems topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The CORRECT CHOICE is (a) True For EXPLANATION I would say: The S-Box is USED to PROVIDE confusion, as it is dependent on the unknown key. The P-Box is fixed, and there is no confusion due to it, but it provides diffusion.

Discussion

12.	Super-Encipherment using two affine transformations results in another affine transformation.(a) True(b) FalseI got this question in an online interview.The question is from Block Cipher Systems in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct CHOICE is (a) True For EXPLANATION I would say: f(g(m))=A_1 g(m)+c_1 f(g(m))=A_1 (A_2 m+c_2)+c_1 f(g(m))=A_1 A_2 m+A_1 c_2+c_1 f(x)=A_3 m+c_3 where A_3=A_1 A_2 c_3=A_1 c_2+c_1 This results in another affine TRANSFORMATION, and does not improve the security.

Discussion

13.	The Ciphertext for the Plaintext 01110010, given that the keys K1 is 10100100 and K2 is 01000011 is(a) 01110111(b) 10010110(c) 01010110(d) 01000101I got this question in homework.The query is from The Simplified Data Encryption Standard (SDES) topic in chapter Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The CORRECT choice is (a) 01110111 To explain: Perform the SDES ALGORITHM and compute the CIPHER text.

Discussion

14.	Assume input 10-bit key, K: 1010000010 for the SDES algorithm. What is K1?(a) 10100100(b) 01011011(c) 01101000(d) 10100111I got this question in class test.I need to ask this question from The Simplified Data Encryption Standard (SDES) topic in section Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Right option is (a) 10100100 Best explanation: The PERMUTED KEY P10 = 1000001100. INPUT to P8: 0000111000 and K1 is 10100100.

Discussion

15.	In the DES algorithm the 64 bit key input is shortened to 56 bits by ignoring every 4th bit.(a) True(b) FalseI had been asked this question during an interview for a job.Question is taken from The Data Encryption Standard (DES) and It’s Strength topic in section Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct option is (b) False Best explanation: Every 8th BIT is IGNORED to SHORTEN the KEY length.

Discussion

16.	What is the size of the key in the SDES algorithm?(a) 24 bits(b) 16 bits(c) 20 bits(d) 10 bitsThis question was posed to me in an online interview.My question comes from The Simplified Data Encryption Standard (SDES) topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Correct OPTION is (d) 10 bits For explanation I WOULD say: The SIZE of the key in the SDES algorithm is 10 bits.

Discussion

17.	In triple DES, the key size is ___ and meet in the middle attack takes ___ tests to break the key.(a) 2192 ,2112(b) 2184,2111(c) 2168,2111(d) 2168,2112This question was addressed to me in an online interview.My question is taken from The Data Encryption Standard (DES) and It’s Strength in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Correct answer is (d) 2168,2112 To explain: The KEY size is 2168 and MEET in the middle attack TAKES 2112 tests to BREAK.

Discussion

18.	In affine block cipher systems if f(m)=Am + t, what is f(m1+m2) ?(a) f(m1) + f(m2) + t(b) f(m1) + f(m2) + 2t(c) f(m1) + t(d) f(m1) + f(m2)This question was addressed to me in an interview for job.The origin of the question is Block Cipher Systems in section Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Correct answer is (a) f(M1) + f(M2) + t The explanation: In generalf(∑(i=1 to n) m_i = ∑(i=1 to n) f(m_i) + tδ_n) where δ_n=0 if n is ODD and 1 if n is EVEN.

Discussion

19.	The number of tests required to break the DES algorithm are(a) 2.8×10^14(b) 4.2×10^9(c) 1.84×10^19(d) 7.2×10^16I got this question by my school teacher while I was bunking the class.Question is from The Data Encryption Standard (DES) and It’s Strength topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The CORRECT OPTION is (d) 7.2×10^16 Easy EXPLANATION: There are 2^56 KEYS =7.2×10^16.

Discussion

20.	If the block size is ‘s’, how many affine transformations are possible ?(a) 2^s (2^s-1)(2^s-1)(2^s-1^2)………(2^s-1^(s-1))(b) 2^s (2^s-1)(2^s-2)(2^s-2^2)………(2^s-2^(s-2))(c) 2^ss (2^s-1)(2^s-2)(2^s-2^2)………(2^s-2^(s-1))(d) 2^s (2^s-1)(2^s-2)(2^s-2^2)………(2^s-2^(s-3))This question was posed to me in final exam.My doubt stems from Block Cipher Systems in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» CORRECT choice is (c) 2^ss (2^s-1)(2^s-2)(2^s-2^2)………(2^s-2^(s-1)) To EXPLAIN: 2^s (2^s-1)(2^s-2)(2^s-2^2)………(2^s-2^(s-1)) is the MAXIMUM number of affine transformations possible for a BLOCK size ‘s’ matrix.

Discussion

21.	The Plaintext for the Ciphertext 00001111, given that the key is 1111111111 is(a) 01100111(b) 00001010(c) 11111111(d) 01101101The question was asked in homework.I would like to ask this question from The Simplified Data Encryption Standard (SDES) topic in section Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct option is (C) 11111111 Explanation: Perform the SDES Decryption ALGORITHM and COMPUTE the cipher text.

Discussion

22.	The number of tests required to break the Double DES algorithm are(a) 2112(b) 2111(c) 2128(d) 2119I got this question in homework.I need to ask this question from The Data Encryption Standard (DES) and It’s Strength topic in chapter Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Right OPTION is (b) 2111 Explanation: For DOUBLE DES key is 2112 bits, should require 2111 TESTS to BREAK.

Discussion

23.	Using Linear Crypt-analysis, the minimum computations required to decipher the DES algorithm is(a) 2^48(b) 2^43(c) 2^56(d) 2^64The question was posed to me in an online quiz.This intriguing question comes from The Data Encryption Standard (DES) and It’s Strength in section Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» CORRECT choice is (B) 2^43 Explanation: Linear Crypt-analysis requires only 2^43 computations to DECIPHER the DES algorithm.

Discussion

24.	The Initial Permutation table/matrix is of size(a) 16×8(b) 12×8(c) 8×8(d) 4×8This question was addressed to me in an online quiz.This interesting question is from The Data Encryption Standard (DES) and It’s Strength topic in division Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The CORRECT answer is (C) 8×8 To explain: There are 64 BITS to permute and this REQUIRES a 8×8 matrix.

Discussion

25.	In the DES algorithm the round key is __________ bit and the Round Input is ____________bits.(a) 48, 32(b) 64,32(c) 56, 24(d) 32, 32I have been asked this question in examination.Question is from The Data Encryption Standard (DES) and It’s Strength in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct OPTION is (a) 48, 32 The best I can EXPLAIN: The ROUND key is 48 bits. The INPUT is 32 bits.

Discussion

26.	The Ciphertext for the Plaintext 11010101, given that the key is 0111010001 is(a) 00010001(b) 10110010(c) 11010010(d) 01110011This question was addressed to me during an internship interview.Origin of the question is The Simplified Data Encryption Standard (SDES) topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» CORRECT choice is (d) 01110011 Explanation: Perform the SDES Encryption ALGORITHM and COMPUTE the cipher TEXT.

Discussion

27.	The DES algorithm has a key length of(a) 128 Bits(b) 32 Bits(c) 64 Bits(d) 16 BitsThe question was posed to me in my homework.My question comes from The Data Encryption Standard (DES) and It’s Strength in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» RIGHT OPTION is (c) 64 Bits To explain: DES encrypts BLOCKS of 64 bits using a 64 BIT key.

Discussion

28.	DES follows(a) Hash Algorithm(b) Caesars Cipher(c) Feistel Cipher Structure(d) SP NetworksThe question was asked by my school teacher while I was bunking the class.This key question is from The Data Encryption Standard (DES) and It’s Strength in section Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct ANSWER is (c) FEISTEL Cipher Structure To ELABORATE: DES follows Feistel Cipher Structure.

Discussion

29.	How many keys does the Triple DES algorithm use?(a) 2(b) 3(c) 2 or 3(d) 3 or 4I got this question in an interview for internship.The origin of the question is The Data Encryption Standard (DES) and It’s Strength topic in chapter Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» CORRECT option is (c) 2 or 3 To elaborate: For TRIPLE DES we can EITHER have 2 or 3 KEYS. Using two keys: c = Ek1(Dk2(Ek1(m))) Using three keys: c = Ek3(Ek2(Ek1(m))).

Discussion

30.	In the DES algorithm the Round Input is 32 bits, which is expanded to 48 bits via ____________(a) Scaling of the existing bits(b) Duplication of the existing bits(c) Addition of zeros(d) Addition of onesThe question was asked in an interview for internship.The origin of the question is The Data Encryption Standard (DES) and It’s Strength topic in chapter Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct choice is (a) Scaling of the existing bits For explanation I would SAY: The round key is 48 bits. The input is 32 bits. This input is first EXPANDED to 48 bits (permutation PLUS an expansion), that INVOLVES DUPLICATION of 16 of the bits.

Discussion

31.	The DES Algorithm Cipher System consists of ____________rounds (iterations) each with a round key(a) 12(b) 18(c) 9(d) 16This question was addressed to me during a job interview.My enquiry is from The Data Encryption Standard (DES) and It’s Strength topic in division Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The correct option is (d) 16 To explain: The DES Algorithm CIPHER SYSTEM consists of 16 ROUNDS (iterations) each with a round KEY.

Discussion

32.	Assume input 10-bit key, K: 1010000010 for the SDES algorithm. What is K2?(a) 10100111(b) 01000011(c) 00100100(d) 01011010I got this question in my homework.This intriguing question comes from The Simplified Data Encryption Standard (SDES) topic in division Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Correct answer is (B) 01000011 To ELABORATE: Input to P8: 0010000011 and K2 is 01000011.

Discussion

33.	The number of unique substitution boxes in DES after the 48 bit XOR operation are(a) 8(b) 4(c) 6(d) 12The question was posed to me by my college director while I was bunking the class.The above asked question is from The Data Encryption Standard (DES) and It’s Strength topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» The CORRECT CHOICE is (a) 8 Explanation: The substitution consists of a set of 8 S-boxes, each of which accepts 6 bits as input and PRODUCES 4 bits as output.

Discussion

34.	Confusion hides the relationship between the ciphertext and the plaintext.(a) True(b) FalseThe question was asked in homework.My enquiry is from Block Cipher Systems in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Correct answer is (B) False Best explanation: CONFUSION hides the relationship between the CIPHERTEXT and the key.

Discussion

35.	In affine block cipher systems if f(m)=Am + t, what is f(m1+m2+m3) ?(a) f(m1) + f(m2) + f(m3) + t(b) f(m1) + f(m2) + f(m3) +2t(c) f(m1) + f(m2) + f(m3)(d) 2(f(m1) + f(m2) + f(m3))The question was asked in quiz.My doubt is from Block Cipher Systems in division Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security
Answer» Correct answer is (c) f(m1) + f(m2) + f(m3) For explanation I WOULD SAY: In generalf(∑(i=1 to n) m_i =∑(i=1 to n) f(m_i ) + tδ_n) where δ_n=0 if n is odd and 1 if n is EVEN.

Discussion

Explore topic-wise InterviewSolutions in .

During decryption, we use the Inverse Initial Permutation (IP-1) before the IP.(a) True(b) FalseThe question was asked in unit test.Query is from The Data Encryption Standard (DES) and It’s Strength topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security

The S-Box is used to provide confusion, as it is dependent on the unknown key.(a) True(b) FalseThe question was posed to me at a job interview.I'm obligated to ask this question of Block Cipher Systems topic in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security

Super-Encipherment using two affine transformations results in another affine transformation.(a) True(b) FalseI got this question in an online interview.The question is from Block Cipher Systems in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security

The DES algorithm has a key length of(a) 128 Bits(b) 32 Bits(c) 64 Bits(d) 16 BitsThe question was posed to me in my homework.My question comes from The Data Encryption Standard (DES) and It’s Strength in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security

Confusion hides the relationship between the ciphertext and the plaintext.(a) True(b) FalseThe question was asked in homework.My enquiry is from Block Cipher Systems in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security