Cryptography & Network Security Exam
2026/2027 | 80 Practice Questions with
Explanations | Faculty of Computer Science
Description:
Master your final exam with 80 updated Cryptography practice questions covering AES,
RSA, Blockchain, & Light-Weight Cryptography. Includes detailed answers &
explanations. Faculty of Computer Science.
Download your complete 2026/2027 study guide now!
,Cryptography Exam 2026/2027: 80 Practice Questions & Answers
Section 1: Fundamentals of Cryptography
Question 1
When performing a binary XOR operation on the following two binary strings, what is the
correct result?
0101110101010111
1001100000111010
A. 1100010101101101
B. 1101010101011101
C. 0110101010101000
D. 1011001010100110
Answer: A
Explanation: The XOR (exclusive OR) operation returns 1 when the two bits being
compared are different and 0 when they are identical. Applying this rule bit by bit: 0⊕1=1,
1⊕0=1, 0⊕0=0, 1⊕1=0, and so on throughout the entire 16-bit string. Performing this
operation systematically across all bit positions yields 1100010101101101.
Question 2
Which statement accurately distinguishes between combinations and permutations in
cryptographic contexts?
A. Combinations consider sequence importance; permutations do not consider order
B. Combinations are not concerned with order; permutations consider all options including
sequence
C. Both combinations and permutations ignore sequence in different ways
D. Combinations include all possible sequences; permutations exclude ordered arrangements
Answer: B
Explanation: In cryptographic and mathematical contexts, combinations refer to selections
where the order of elements does not matter, while permutations consider every possible
arrangement where sequence is significant. This distinction becomes important in
understanding cryptographic key spaces and the number of possible encryption states.
,Question 3
A researcher observes that a particular three-character sequence appears frequently at the
beginning of encrypted messages. What cryptanalysis technique is being employed?
A. Rainbow table analysis
B. Dictionary attack
C. Frequency analysis
D. Birthday attack
Answer: C
Explanation: Frequency analysis examines patterns in ciphertext by identifying recurring
sequences and comparing their frequency of occurrence to known language patterns. For
example, frequent appearance of a three-character sequence at message beginnings might
suggest it represents the word "THE" in English, which is the most common three-letter
word.
Question 4
What is the mathematical result of 15 mod 7?
A. 2
B. 1
C. 8
D. 0
Answer: B
Explanation: The modulo operation finds the remainder after division. When 15 is divided
by 7, the quotient is 2 (since 7×2=14) and the remainder is 1. Thus, 15 mod 7 = 1. Modular
arithmetic forms the foundation for many cryptographic algorithms including RSA and
Diffie-Hellman.
Question 5
Which measure describes the degree of unpredictability in an encryption process?
A. Work factor
B. Collision resistance
, C. Entropy
D. Block size
Answer: C
Explanation: Entropy quantifies the level of uncertainty or randomness in a system. In
cryptography, higher entropy indicates greater unpredictability of encryption keys or outputs,
making cryptanalysis more difficult. Entropy is measured in bits and directly relates to the
strength of cryptographic systems against brute-force attacks.
Section 2: Historical and Classical Ciphers
Question 6
Which cipher employs a 5×5 matrix containing the alphabet excluding the letter J, with
encryption and decryption governed by rules involving column and row combinations?
A. Bifid cipher
B. Four-square cipher
C. Playfair cipher
D. Pigpen cipher
Answer: C
Explanation: The Playfair cipher uses a 5×5 grid containing 25 letters (typically combining I
and J). Encryption processes pairs of letters according to their positions in the grid, applying
rules based on whether letters appear in the same row, same column, or form a rectangle. This
was the first practical digraph substitution cipher.
Question 7
What was the primary cryptographic weakness that enabled Allied cryptanalysts to break the
Enigma machine's encryption?
A. The cipher repeated its polyalphabetic pattern every 24 hours
B. No plaintext letter could be encrypted as itself
C. The machine used a mono-alphabetic substitution system
D. The rotor mechanism was physically accessible during operation
Answer: B
2026/2027 | 80 Practice Questions with
Explanations | Faculty of Computer Science
Description:
Master your final exam with 80 updated Cryptography practice questions covering AES,
RSA, Blockchain, & Light-Weight Cryptography. Includes detailed answers &
explanations. Faculty of Computer Science.
Download your complete 2026/2027 study guide now!
,Cryptography Exam 2026/2027: 80 Practice Questions & Answers
Section 1: Fundamentals of Cryptography
Question 1
When performing a binary XOR operation on the following two binary strings, what is the
correct result?
0101110101010111
1001100000111010
A. 1100010101101101
B. 1101010101011101
C. 0110101010101000
D. 1011001010100110
Answer: A
Explanation: The XOR (exclusive OR) operation returns 1 when the two bits being
compared are different and 0 when they are identical. Applying this rule bit by bit: 0⊕1=1,
1⊕0=1, 0⊕0=0, 1⊕1=0, and so on throughout the entire 16-bit string. Performing this
operation systematically across all bit positions yields 1100010101101101.
Question 2
Which statement accurately distinguishes between combinations and permutations in
cryptographic contexts?
A. Combinations consider sequence importance; permutations do not consider order
B. Combinations are not concerned with order; permutations consider all options including
sequence
C. Both combinations and permutations ignore sequence in different ways
D. Combinations include all possible sequences; permutations exclude ordered arrangements
Answer: B
Explanation: In cryptographic and mathematical contexts, combinations refer to selections
where the order of elements does not matter, while permutations consider every possible
arrangement where sequence is significant. This distinction becomes important in
understanding cryptographic key spaces and the number of possible encryption states.
,Question 3
A researcher observes that a particular three-character sequence appears frequently at the
beginning of encrypted messages. What cryptanalysis technique is being employed?
A. Rainbow table analysis
B. Dictionary attack
C. Frequency analysis
D. Birthday attack
Answer: C
Explanation: Frequency analysis examines patterns in ciphertext by identifying recurring
sequences and comparing their frequency of occurrence to known language patterns. For
example, frequent appearance of a three-character sequence at message beginnings might
suggest it represents the word "THE" in English, which is the most common three-letter
word.
Question 4
What is the mathematical result of 15 mod 7?
A. 2
B. 1
C. 8
D. 0
Answer: B
Explanation: The modulo operation finds the remainder after division. When 15 is divided
by 7, the quotient is 2 (since 7×2=14) and the remainder is 1. Thus, 15 mod 7 = 1. Modular
arithmetic forms the foundation for many cryptographic algorithms including RSA and
Diffie-Hellman.
Question 5
Which measure describes the degree of unpredictability in an encryption process?
A. Work factor
B. Collision resistance
, C. Entropy
D. Block size
Answer: C
Explanation: Entropy quantifies the level of uncertainty or randomness in a system. In
cryptography, higher entropy indicates greater unpredictability of encryption keys or outputs,
making cryptanalysis more difficult. Entropy is measured in bits and directly relates to the
strength of cryptographic systems against brute-force attacks.
Section 2: Historical and Classical Ciphers
Question 6
Which cipher employs a 5×5 matrix containing the alphabet excluding the letter J, with
encryption and decryption governed by rules involving column and row combinations?
A. Bifid cipher
B. Four-square cipher
C. Playfair cipher
D. Pigpen cipher
Answer: C
Explanation: The Playfair cipher uses a 5×5 grid containing 25 letters (typically combining I
and J). Encryption processes pairs of letters according to their positions in the grid, applying
rules based on whether letters appear in the same row, same column, or form a rectangle. This
was the first practical digraph substitution cipher.
Question 7
What was the primary cryptographic weakness that enabled Allied cryptanalysts to break the
Enigma machine's encryption?
A. The cipher repeated its polyalphabetic pattern every 24 hours
B. No plaintext letter could be encrypted as itself
C. The machine used a mono-alphabetic substitution system
D. The rotor mechanism was physically accessible during operation
Answer: B
