Category: Cryptography & Network Security Objective Questions

Cryptography Multiple Choice Questions on “The Data Encryption Standard (DES) and It’s Strength”.

1. DES follows
a) Hash Algorithm
b) Caesars Cipher
c) Feistel Cipher Structure
d) SP Networks

Answer: c
Clarification: DES follows Feistel Cipher Structure.

2. The DES Algorithm Cipher System consists of ____________rounds (iterations) each with a round key
a) 12
b) 18
c) 9
d) 16

Answer: d
Clarification: The DES Algorithm Cipher System consists of 16 rounds (iterations) each with a round key.

3. The DES algorithm has a key length of
a) 128 Bits
b) 32 Bits
c) 64 Bits
d) 16 Bits

Answer: c
Clarification: DES encrypts blocks of 64 bits using a 64 bit key.

4. 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) False

Answer: b
Clarification: 56 bits are used, the rest 8 bits are parity bits.

5. 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, 32

Answer: a
Clarification: The round key is 48 bits. The input is 32 bits.

6. 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 ones

Answer: a
Clarification: 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.

7. The Initial Permutation table/matrix is of size
a) 16×8
b) 12×8
c) 8×8
d) 4×8

Answer: c
Clarification: There are 64 bits to permute and this requires a 8×8 matrix.

8. The number of unique substitution boxes in DES after the 48 bit XOR operation are
a) 8
b) 4
c) 6
d) 12

Answer: a
Clarification: The substitution consists of a set of 8 S-boxes, each of which accepts 6 bits as input and produces 4 bits as output.

9. In the DES algorithm the 64 bit key input is shortened to 56 bits by ignoring every 4th bit.
a) True
b) False

Answer: b
Clarification: Every 8th bit is ignored to shorten the key length.

Cryptography Interview Questions experienced on “Principles of PRNG”

1. Which among the following cannot be used in the case of stream ciphers?
a) TRNGs
b) PRNGs
c) PRFs
d) All of the mentioned

Answer: a
Clarification: TRNGs are not practical for stream ciphers.

2. A stream cipher key is usually of size ___ or 128 bits.
a) 56
b) 64
c) 54
d) 48

Answer: c
Clarification: A stream cipher key is usually of size 54 or 128 bits.

3. Which among the following has the lowest bitrate ?
a) PRF
b) TRNG
c) PRNG
d) All have the same bitrate

Answer: b
Clarification: TRNGs are the slowest. The mechanism used to generated True Random numbers may not be able to generate bits at a rate sufficient to keep up with the application requiring the random bits.

4. RC4 was built on the sole purpose of generating random bits i.e. to act as a PRNG.
a) True
b) False

Answer: a
Clarification: The statement is true. RC4 is a purpose built algorithm.

5. Maurer’s Universal Statistical Test is –
a) a test to determine whether the number of runs of ones and zeros of various lengths is as expected
b) a test to detect if a sequence can be significantly compressed without loss of information
c) a test to determine whether the number of ones and zeros in a sequence is approx the same as in a truely random o/p
d) a test for the check of evenness in the 1s and 0s

Answer: b
Clarification: Maurer’s Universal Statistical Test is a test to detect if a sequence can be significantly compressed without loss of information.

6. Runs Test is
a) a test to determine whether the number of runs of ones and zeros of various lengths is as expected
b) a test to detect if a sequence can be significantly compressed without loss of information
c) a test to determine whether the number of ones and zeros in a sequence is approx the same as in a truely random o/p
d) a test for the check of evenness in the 1s and 0s

Answer: a
Clarification: Runs Test is a test to determine whether the number of runs of ones and zeros of various lengths is as expected.

7. Frequency Test is
a) a test to determine whether the number of runs of ones and zeros of various lengths is as expected
b) a test to detect if a sequence can be significantly compressed without loss of information
c) a test for the check of evenness in the 1s and 0s
d) a test to determine whether the number of ones and zeros in a sequence is approx the same as in a truely random o/p

Answer: d
Clarification: Frequency Test is a test to determine whether the number of ones and zeros in a sequence is approx the same as in a truely random o/p.

8. We should not be able to determine the seed from any previous knowledge of the bits sequence. This is the property of backward unpredictability.
a) True
b) False

Answer: a
Clarification: The statement is true.

9. Forward Unpredictability is a property when the seed is unknown, and the next output bit is the sequence should be unpredictable.
a) True
b) False

Answer: a
Clarification: The statement is true.

Cryptography Multiple Choice Questions on “Elliptic Curve Arithmetic/Cryptography”.

1. What is the general equation for elliptic curve systems?
a) y³+b_1 xy+b_2 y=x³3+a_1 x²+a_2 x+a_3
b) y³+b_1 x+b_2 y=x²+a_1 x²+a_2 x+a_3
c) y²+b_1 xy+b_2 y=x³+a_1 x²+a_2
d) y²+b_1 xy+b_2 y=x³+a_1 x²+a_2 x+a_3

Answer: d
Clarification: The general equations for an elliptic curve system is y²+b_1 xy+b_2 y=x³+a_1 x²+a_2 x+a_3.

2. In Singular elliptic curve, the equation x^3+ax+b=0 does ____ roots.
a) does not have three distinct
b) has three distinct
c) has three unique
d) has three distinct unique

Answer: a
Clarification: In Singular elliptic curve, the equation x^3+ax+b=0 does not have three distinct roots.

3. How many real and imaginary roots does the equation y2=x3-1 have
a) 2 real, 1 imaginary
b) all real
c) all imaginary
d) 2 imaginary, 1 real

Answer: d
Clarification: On solving the equation we get 2 imaginary and 1 real root.

4. How many real and imaginary roots does the equation y2=x3-4x have
a) 2 real, 1 imaginary
b) all real
c) all imaginary
d) 2 imaginary, 1 real

Answer: b
Clarification: On solving the equation we get all real roots.

5. In the elliptic curve group defined by y2= x3- 17x + 16 over real numbers, what is P + Q if P = (0,-4) and Q = (1, 0)?
a) (15, -56)
b) (-23, -43)
c) (69, 26)
d) (12, -86)

Answer: a
Clarification: P=(x1, y1)= (0,-4)
Q=(x2, y2)= (1,0)
From the Addition formulae:
λ= (0-(-4)) / (1-0) = 4
x3= = 16 – 0 – 1 = 15 and
y3= 4(0 – 15) –(-4) = -56
Thus R=P + Q = (15, -56).

6. In the elliptic curve group defined by y2= x3- 17x + 16 over real numbers, what is 2P if P = (4, 3.464)?
a) (12.022, -39.362)
b) (32.022, 42.249)
c) (11.694, -43.723)
d) (43.022, 39.362)

Answer: a
Clarification: From the Doubling formulae:
λ = (3*(4)2+ (-17)) / 2*(3.464) = 31 / 6.928 = 4.475
x3= (4.475)2- 2(4) = 20.022 – 8 = 12.022 and
y3= -3.464 + 4.475(4 – 12.022) = – 3.464 – 35.898 = -39.362
Thus 2P = (12.022, -39.362).

7. “Elliptic curve cryptography follows the associative property.”
a) True
b) False

Answer: a
Clarification: ECC does follow associative property.

8. “In ECC, the inverse of point P =(x1, y1) is Q = (-x1, y1). “
a) True
b) False

Answer: b
Clarification: The inverse of point P =(x1, y1) is Q =(x1, -y1).

Cryptography Aptitude Test on “Symmetric Key Distribution”.

1. Larger networks would prefer a full decentralization.
a) True
b) False

Answer: b
Clarification: Full decentralization is not practical for larger networks as there would be too many connections.

2. Which of these is not a type of session key?
a) PIN-encrypting key
b) File-encrypting key
c) Session encrypting key
d) Data encrypting key

Answer: c
Clarification: Data, PIN and File are the different session keys.

3. Which session key is used for electronic funds transfer and point of sale applications?
a) Data-encrypting key
b) File-encrypting key
c) PIN-encrypting key
d) None of the mentioned

Answer: c
Clarification: PIN-encrypting key is the session key which is used for electronic funds transfer and point of sale applications.

4. Sometimes a simple tag is introduced along with the session key. This tag has 8 bits. Which of the following options is wrong?
a) One bit indicates whether the key is a session key or a master key
b) One bit indicates whether the key can be used for encryption
c) Three bit indicates whether the key can be used for decryption
d) Remaining bits are for future use

Answer: c
Clarification: One bit indicates whether the key can be used for decryption.

5. Hash Value = H = h(CV)
Key Input = Km XOR H
Ciphertext = E([Km XOR H],Ks)
What is CV here?
a) Cipher vector
b) Current vector
c) Control vector
d) None of the mentioned

Answer: c
Clarification: CV is known as Control Vector.

6. Which is the correct representation for session key recovery for the plain text?
a) D ([Km XOR H], E([Km XOR H], Ks)
b) D([Km XOR H],Ks)
c) D ([Km XOR H], E([Km XOR H])
d) None of the mentioned

Answer: a
Clarification: The correct representation is D([Km XOR H], E([Km XOR H], Ks), to recover the session key.

7. Which of the following is required to find the session key?
i) Control Vector
ii) Master Key
iii) Encrypted session Key

a) i)
b) i) and ii)
c) i) and iii)
d) i) ii) and iii)

Answer: d
Clarification: We require all three to find the session key.

8. “Meet in the middle attack” and “man in the middle attack” are the same.
a) True
b) False

Answer: b
Clarification: Man is the middle attack is different from meet in the middle attack.

9. “Meet in the middle attack” is an attack
a) where the timing required for the attack via brute force is drastically reduced
b) where the adversary uses 2 or more machines to decrypt thus trying to reduce the time
c) where messages are intercepted and then either relayed or substituted with another message
d) where cryptanalysis takes lesser time than the brute force decryption

Answer: c
Clarification: “Meet in the middle attack” is an attack where messages are intercepted and then either relayed or substituted with another message.

Network Security Interview Questions experienced on “IP Addressing”.

1. Convert the following binary notation to dotted-decimal notation –
10000000 00001011 00000011 00011111
a) 128.11.5.32
b) 128.11.3.31
c) 127.11.3.32
d) 127.12.5.31

Answer: b
Clarification: Converting the binary value to their respective decimal values yields 128.11.3.31.

2. Convert the following binary notation to hexadecimal notation –
10000000 00001011 00000011 00011111
a) 0x 80 0B 03 1E
b) 0x 81 0B 04 1E
c) 0x 80 0C 03 1F
d) 0x 80 0B 03 1F

Answer: d
Clarification: Converting the binary value to their respective hex values yields 0x 80 0B 03 1F.

3. Convert the following binary notation to dotted-decimal notation –
10000001 00001011 00001011 11101111
a) 129.11.11.239
b) 128.11.12.231
c) 127.11.13.244
d) 129.12.1.231

Answer: a
Clarification: Converting the binary value to their respective decimal values yields 129.11.11.239.

4. Convert the following dotted-decimal notation to binary notation – 111.56.45.78
a) 01101111 00111000 00101101 01001110
b) 11101111 00111000 00101101 10001110
c) 10000000 00001011 00000011 00011111
d) 10000001 00001011 00001011 11101111

Answer: a
Clarification: Replacing each decimal number with its binary equivalent we get 01101111 00111000 00101101 01001110.

5. What is the error (if any) in the following representation – 111.56.045.78?
a) There should be no leading zeros
b) We cannot have more than 4 bytes in an IPv4 address
c) Each byte should be less than or equal to 255
d) No error

Answer: a
Clarification: There should be no leading zeros.

6. What is the error (if any) in the following representation – 221.34.7.8.20?
a) There should be no leading zeros
b) Each byte should be less than or equal to 255
c) We cannot have more than 4 bytes in an IPv4 address
d) No error

Answer: c
Clarification: We cannot have more than 4 bytes in an IPv4 address.

7. What is the error (if any) in the following representation – 75.45.301.14?
a) There should be no leading zeros
b) We cannot have more than 4 bytes in an IPv4 address
c) Each byte should be less than or equal to 255
d) No error

Answer: c
Clarification: None.

8. What is the error (if any) in the following representation – 11100010.23.14.67?
a) There should be no leading zeros
b) We cannot have more than 4 bytes in an IPv4 address
c) Each byte should be less than or equal to 255
d) None of the mentioned

Answer: d
Clarification: The error is that there is a mixture of binary and dotted-decimal notation.

9. The following IPv4 addresses in hexadecimal notation is – 10000001 00001011 00001011 11101111-
a) 0x810B0BEF
b) 0x810D0AFF
c) 0x810B0BFE
d) 0x810C0CEF

Answer: a
Clarification: We replace each group of 4 bits with its hexadecimal equivalent. Note that 0X (or 0x) is added at the beginning or the subscript 16 at the end. 0x810B0BEF or 810B0BEF_16.

10. Find the number of addresses in a range if the first address is 146.102.29.0 and last address is 146.102.32.255.
a) 1028
b) 1024
c) 578
d) 512

Answer: b
Clarification: Subtract the first address from the last address in base 256.
The result = 0.0.3.255
Therefore, number of addresses = (3 X 256 + 255) + 1 = 1024.

11. The first address in a range of addresses is 14.11.45.96. If the number of addresses in the range is 32, what is the last address?
a) 14.11.44.64
b) 14.11.44.128
c) 14.12.44.128
d) 14.11.45.127

Answer: d
Clarification: Last Address = (14.11.45.96 + 0.0.0.31)_256 = 14.11.45.127.

Cryptography Assessment Questions on “The Data Encryption Standard (DES) and It’s Strength”.

1. During decryption, we use the Inverse Initial Permutation (IP-1) before the IP.
a) True
b) False

Answer: a
Clarification: IP-1 is the first step and the last step is IP during decryption.

2. A preferable cryptographic algorithm should have a good avalanche effect.
a) True
b) False

Answer: a
Clarification: 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.

3. The number of tests required to break the DES algorithm are
a) 2.8×10¹⁴
b) 4.2×10⁹
c) 1.84×10¹⁹
d) 7.2×10¹⁶

Answer: d
Clarification: There are 2⁵⁶ keys =7.2×10¹⁶.

4. The number of tests required to break the Double DES algorithm are
a) 2112
b) 2111
c) 2128
d) 2119

Answer: b
Clarification: For Double DES key is 2112 bits, should require 2111 tests to break.

5. How many keys does the Triple DES algorithm use?
a) 2
b) 3
c) 2 or 3
d) 3 or 4

Answer: c
Clarification: 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))).

6. 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) 2168,2112

Answer: d
Clarification: The key size is 2168 and meet in the middle attack takes 2112 tests to break.

7. Using Differential Crypt-analysis, the minimum computations required to decipher the DES algorithm is
a) 2⁵⁶
b) 2⁴³
c) 2⁵⁵
d) 2⁴⁷

Answer: d
Clarification: Differential Crypt-analysis requires only 2⁴⁷ computations to decipher the DES algorithm.

8.Using Linear Crypt-analysis, the minimum computations required to decipher the DES algorithm is
a) 2⁴⁸
b) 2⁴³
c) 2⁵⁶
d) 2⁶⁴

Answer: b
Clarification: Linear Crypt-analysis requires only 2⁴³ computations to decipher the DES algorithm.