D334 ITAS 2141 Introduction to
Cryptography
Final Assessment Review
(Questions & Solutions)
2025
©2025
, 1. Case Study – RSA Key Generation and Encryption
A financial institution uses RSA encryption to secure sensitive customer
data. During key generation, two large primes, p and q , are selected.
The public modulus n is computed as n = p × q .
Question: Which of the following best describes the role of the private
key in the RSA system?
A. It is used to generate the modulus n .
B. It is used to encrypt messages before transmission.
C. It is used to decrypt ciphertext that has been encrypted with the
matching public key.
D. It is publicly distributed to verify digital signatures.
ANS: C. It is used to decrypt ciphertext that has been encrypted with
the matching public key.
Rationale: In RSA, the private key (typically consisting of the decryption
exponent d ) is kept confidential and used to decrypt data that has been
encrypted with the corresponding public key, ensuring that only the key
owner can recover the original message.
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2. Case Study – Diffie–Hellman Key Exchange Process
A multinational company deploys a secure channel between branch
offices using Diffie–Hellman key exchange to agree upon a shared secret
over an insecure network.
Question: Which aspect of Diffie–Hellman key exchange provides its
security?
A. The use of symmetric keys exclusively
B. The difficulty of solving the discrete logarithm problem
C. The quadratic residue property of primes
D. The fact that public keys remain secret
ANS: B. The difficulty of solving the discrete logarithm problem.
Rationale: Security in Diffie–Hellman relies on the computational
intractability of the discrete logarithm problem; even with knowledge of
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, public parameters, computing the shared secret is infeasible within
reasonable time.
---
3. Case Study – Symmetric vs. Asymmetric Encryption
A company implements both symmetric and asymmetric encryption to
secure internal communications and external document exchange.
Question: Which statement accurately contrasts symmetric and
asymmetric encryption?
A. Symmetric encryption uses a pair of keys, whereas asymmetric
encryption uses one single key.
B. Asymmetric encryption is generally faster than symmetric encryption
for bulk data encryption.
C. Symmetric encryption requires secure key distribution, while
asymmetric encryption allows public distribution of one key.
D. Asymmetric keys are typically shorter than symmetric keys.
ANS: C. Symmetric encryption requires secure key distribution, while
asymmetric encryption allows public distribution of one key.
Rationale: In symmetric encryption, the same key is used for both
encryption and decryption and must be shared securely. In contrast,
asymmetric encryption uses a public/private key pair, allowing the public
key to be shared openly.
---
4. Case Study – Block vs. Stream Ciphers
A cybersecurity team evaluates encryption algorithms for real-time
communication systems. They compare block ciphers (e.g., AES) with
stream ciphers such as RC4.
Question: What is a primary difference between block ciphers and
stream ciphers?
A. Block ciphers encrypt data as individual bits, while stream ciphers
encrypt data in fixed-size blocks.
B. Block ciphers operate on fixed-size blocks of data, whereas stream
©2025
Cryptography
Final Assessment Review
(Questions & Solutions)
2025
©2025
, 1. Case Study – RSA Key Generation and Encryption
A financial institution uses RSA encryption to secure sensitive customer
data. During key generation, two large primes, p and q , are selected.
The public modulus n is computed as n = p × q .
Question: Which of the following best describes the role of the private
key in the RSA system?
A. It is used to generate the modulus n .
B. It is used to encrypt messages before transmission.
C. It is used to decrypt ciphertext that has been encrypted with the
matching public key.
D. It is publicly distributed to verify digital signatures.
ANS: C. It is used to decrypt ciphertext that has been encrypted with
the matching public key.
Rationale: In RSA, the private key (typically consisting of the decryption
exponent d ) is kept confidential and used to decrypt data that has been
encrypted with the corresponding public key, ensuring that only the key
owner can recover the original message.
---
2. Case Study – Diffie–Hellman Key Exchange Process
A multinational company deploys a secure channel between branch
offices using Diffie–Hellman key exchange to agree upon a shared secret
over an insecure network.
Question: Which aspect of Diffie–Hellman key exchange provides its
security?
A. The use of symmetric keys exclusively
B. The difficulty of solving the discrete logarithm problem
C. The quadratic residue property of primes
D. The fact that public keys remain secret
ANS: B. The difficulty of solving the discrete logarithm problem.
Rationale: Security in Diffie–Hellman relies on the computational
intractability of the discrete logarithm problem; even with knowledge of
©2025
, public parameters, computing the shared secret is infeasible within
reasonable time.
---
3. Case Study – Symmetric vs. Asymmetric Encryption
A company implements both symmetric and asymmetric encryption to
secure internal communications and external document exchange.
Question: Which statement accurately contrasts symmetric and
asymmetric encryption?
A. Symmetric encryption uses a pair of keys, whereas asymmetric
encryption uses one single key.
B. Asymmetric encryption is generally faster than symmetric encryption
for bulk data encryption.
C. Symmetric encryption requires secure key distribution, while
asymmetric encryption allows public distribution of one key.
D. Asymmetric keys are typically shorter than symmetric keys.
ANS: C. Symmetric encryption requires secure key distribution, while
asymmetric encryption allows public distribution of one key.
Rationale: In symmetric encryption, the same key is used for both
encryption and decryption and must be shared securely. In contrast,
asymmetric encryption uses a public/private key pair, allowing the public
key to be shared openly.
---
4. Case Study – Block vs. Stream Ciphers
A cybersecurity team evaluates encryption algorithms for real-time
communication systems. They compare block ciphers (e.g., AES) with
stream ciphers such as RC4.
Question: What is a primary difference between block ciphers and
stream ciphers?
A. Block ciphers encrypt data as individual bits, while stream ciphers
encrypt data in fixed-size blocks.
B. Block ciphers operate on fixed-size blocks of data, whereas stream
©2025
