WGU C839 INTRODUCTION TO
CRYPTOGRAPHY QUESTIONS
AND ANSWERS WITH
RATIONALES/GRADED A+/2026
UPDATE/100% CORRECT
/INSTANT DOWNLOAD
Topic 1: Fundamental Principles & History
Q1. According to Kerckhoffs’s principle, a cryptosystem should remain secure even if
everything is public except which component?
A) The encryption algorithm
B) The ciphertext
C) The key
D) The plaintext
Correct Answer: C
Rationale: Kerckhoffs's principle states that the security of a system should depend
solely on the secrecy of the key, not the obscurity of the algorithm itself.
Q2. Which classical cipher is a mono-alphabetic substitution cipher where each letter
is shifted a fixed number of places down the alphabet?
A) Atbash
B) Vigenère
C) Caesar Cipher
D) Scytale
Correct Answer: C
Rationale: The Caesar cipher (or shift cipher) works by shifting each letter by a fixed
,number of positions. Atbash reverses the alphabet (A->Z), and Vigenère uses a
keyword for polyalphabetic shifts.
Q3. A cryptanalyst uses the Kasiski examination to find repeating strings in the
ciphertext. Which cipher is this technique specifically designed to break?
A) Rail Fence Cipher
B) Vigenère Cipher
C) One-Time Pad
D) Enigma
Correct Answer: B
Rationale: The Kasiski examination looks for repeated patterns in the ciphertext to
guess the length of the keyword used in the Vigenère cipher.
Q4. Why is the One-Time Pad (OTP) considered theoretically unbreakable?
A) It uses complex mathematical trapdoor functions
B) The key is as long as the message, truly random, and never reused
C) It uses a 256-bit key size
D) It relies on the factorization of prime numbers
Correct Answer: B
Rationale: Shannon proved the One-Time Pad provides perfect secrecy because the
key is random, matches the message length, and is used only once.
Q5. Which of the following best describes the purpose of diffusion in a secure
cipher?
A) To obscure the relationship between the key and the ciphertext
B) To spread the influence of a single plaintext bit across many ciphertext bits
C) To add random data to the plaintext
D) To compress the message before encryption
Correct Answer: B
Rationale: Diffusion dissipates the statistical structure of the plaintext. The avalanche
effect (changing one bit changes ~half the output) is a measure of good diffusion.
Topic 2: Symmetric Cryptography (Block Ciphers)
Q6. Which of the following key sizes is NOT supported by the Advanced Encryption
Standard (AES)?
A) 128 bits
B) 192 bits
, C) 256 bits
D) 512 bits
Correct Answer: D
Rationale: AES is a block cipher with a fixed block size of 128 bits and supports key
lengths of 128, 192, or 256 bits.
Q7. The Data Encryption Standard (DES) uses a 64-bit block size and a key size of
effectively how many bits?
A) 128 bits
B) 64 bits
C) 56 bits
D) 256 bits
Correct Answer: C
Rationale: While the key is 64 bits long, 8 bits are used for parity, leaving 56 bits for
the actual encryption key.
Q8. Which mode of operation for a block cipher is the most basic, where identical
plaintext blocks produce identical ciphertext blocks, making it unsuitable for
encrypting images or patterns?
A) Cipher Block Chaining (CBC)
B) Counter Mode (CTR)
C) Electronic Codebook (ECB)
D) Output Feedback (OFB)
Correct Answer: C
Rationale: ECB encrypts each block independently. Because it reveals patterns in the
plaintext (like a "penguin" image remaining visible), it is generally considered
insecure for most applications.
Q9. During Cipher Block Chaining (CBC) mode, what is the role of the Initialization
Vector (IV)?
A) It acts as the secret key for decryption
B) It ensures that encrypting the same plaintext twice results in different ciphertexts
C) It compresses the output
D) It performs the S-Box substitution
Correct Answer: B
Rationale: The IV is XORed with the first plaintext block. This randomization ensures
that even if the same message is sent twice, the ciphertext will be different.
Q10. Which algorithm is a lightweight symmetric block cipher specifically designated
as a Type 2 product by the NSA and uses an 80-bit key?
A) Skipjack
CRYPTOGRAPHY QUESTIONS
AND ANSWERS WITH
RATIONALES/GRADED A+/2026
UPDATE/100% CORRECT
/INSTANT DOWNLOAD
Topic 1: Fundamental Principles & History
Q1. According to Kerckhoffs’s principle, a cryptosystem should remain secure even if
everything is public except which component?
A) The encryption algorithm
B) The ciphertext
C) The key
D) The plaintext
Correct Answer: C
Rationale: Kerckhoffs's principle states that the security of a system should depend
solely on the secrecy of the key, not the obscurity of the algorithm itself.
Q2. Which classical cipher is a mono-alphabetic substitution cipher where each letter
is shifted a fixed number of places down the alphabet?
A) Atbash
B) Vigenère
C) Caesar Cipher
D) Scytale
Correct Answer: C
Rationale: The Caesar cipher (or shift cipher) works by shifting each letter by a fixed
,number of positions. Atbash reverses the alphabet (A->Z), and Vigenère uses a
keyword for polyalphabetic shifts.
Q3. A cryptanalyst uses the Kasiski examination to find repeating strings in the
ciphertext. Which cipher is this technique specifically designed to break?
A) Rail Fence Cipher
B) Vigenère Cipher
C) One-Time Pad
D) Enigma
Correct Answer: B
Rationale: The Kasiski examination looks for repeated patterns in the ciphertext to
guess the length of the keyword used in the Vigenère cipher.
Q4. Why is the One-Time Pad (OTP) considered theoretically unbreakable?
A) It uses complex mathematical trapdoor functions
B) The key is as long as the message, truly random, and never reused
C) It uses a 256-bit key size
D) It relies on the factorization of prime numbers
Correct Answer: B
Rationale: Shannon proved the One-Time Pad provides perfect secrecy because the
key is random, matches the message length, and is used only once.
Q5. Which of the following best describes the purpose of diffusion in a secure
cipher?
A) To obscure the relationship between the key and the ciphertext
B) To spread the influence of a single plaintext bit across many ciphertext bits
C) To add random data to the plaintext
D) To compress the message before encryption
Correct Answer: B
Rationale: Diffusion dissipates the statistical structure of the plaintext. The avalanche
effect (changing one bit changes ~half the output) is a measure of good diffusion.
Topic 2: Symmetric Cryptography (Block Ciphers)
Q6. Which of the following key sizes is NOT supported by the Advanced Encryption
Standard (AES)?
A) 128 bits
B) 192 bits
, C) 256 bits
D) 512 bits
Correct Answer: D
Rationale: AES is a block cipher with a fixed block size of 128 bits and supports key
lengths of 128, 192, or 256 bits.
Q7. The Data Encryption Standard (DES) uses a 64-bit block size and a key size of
effectively how many bits?
A) 128 bits
B) 64 bits
C) 56 bits
D) 256 bits
Correct Answer: C
Rationale: While the key is 64 bits long, 8 bits are used for parity, leaving 56 bits for
the actual encryption key.
Q8. Which mode of operation for a block cipher is the most basic, where identical
plaintext blocks produce identical ciphertext blocks, making it unsuitable for
encrypting images or patterns?
A) Cipher Block Chaining (CBC)
B) Counter Mode (CTR)
C) Electronic Codebook (ECB)
D) Output Feedback (OFB)
Correct Answer: C
Rationale: ECB encrypts each block independently. Because it reveals patterns in the
plaintext (like a "penguin" image remaining visible), it is generally considered
insecure for most applications.
Q9. During Cipher Block Chaining (CBC) mode, what is the role of the Initialization
Vector (IV)?
A) It acts as the secret key for decryption
B) It ensures that encrypting the same plaintext twice results in different ciphertexts
C) It compresses the output
D) It performs the S-Box substitution
Correct Answer: B
Rationale: The IV is XORed with the first plaintext block. This randomization ensures
that even if the same message is sent twice, the ciphertext will be different.
Q10. Which algorithm is a lightweight symmetric block cipher specifically designated
as a Type 2 product by the NSA and uses an 80-bit key?
A) Skipjack
