CRYPTOGRAPHY CHAPTER WISE QUESTONS COLLECTION

 

Unit 1: Introduction and Classical Ciphers

1.Define the CIA Triad and explain its components. List out the services of security. Differentiate between block ciphers and stream ciphers with examples.

Solution:

CIA Triad:

The CIA Triad is a fundamental model in information security that represents three core principles required to protect information systems.

a) Confidentiality

Ensures that information is accessible only to authorized users.

Prevents unauthorized disclosure of sensitive data.

Achieved using encryption, passwords, and access control.

b) Integrity

Ensures that data remains accurate and unaltered.

Prevents unauthorized modification of information.

Maintained using hashing, checksums, and digital signatures.

c) Availability

Ensures that systems and data are available when needed.

Protects against failures and service disruptions.

Achieved through backups, redundancy, and fault tolerance.

The main security services provided by information security systems are:

Confidentiality – Prevents unauthorized disclosure of data

Integrity – Prevents unauthorized modification of data

Authentication – Verifies the identity of users or systems

Authorization / Access Control – Restricts access to resources

Non-repudiation – Prevents denial of sending or receiving data

Availability – Ensures continuous access to resources

Accountability / Auditing – Tracks user actions through logs

2.Illustrate the concept of security policy and mechanism with an example. Differentiate between block cipher and stream cipher.

Solution:

Security Policy

A security policy is a set of rules and guidelines that defines what is allowed and what is not in a system.

It specifies who can access what resources and under what conditions.

It focuses on “what to protect”.

Example:

“Only authorized staff can access the student database.”

Security Mechanism

A security mechanism is the method or tool used to enforce the security policy.

It implements the rules defined by the security policy.

It focuses on “how to protect”.

Example:

Username–password login and role-based access control to restrict database access.

Illustration (Policy vs Mechanism)

Policy: Only teachers can modify student grades.

Mechanism: Authentication system + access control list that allows grade editing only to teacher accounts.

Summary

Security Policy defines the rules.

Security Mechanism enforces those rules using technical controls.

3.How substitution ciphers are different from transposition ciphers? Given a message M=”CSIT PROGRAM IS A HOT CAKE”, encrypt M using Rail Fence cipher with rail size 3.

Solution:

4.Which one is more secure, monoalphabetic cipher or poly alphabetic cipher? Justify. Using rail fence cipher encrypt the text “LEARNING AND TEACHING ARE DIFFERENT” using 3 as rails.

Solution:

Polyalphabetic cipher is more secure than monoalphabetic cipher.

Justification:

Monoalphabetic cipher uses a single substitution alphabet, so the same plaintext letter always maps to the same ciphertext letter, making it vulnerable to frequency analysis.

Polyalphabetic cipher uses multiple substitution alphabets, so the same plaintext letter can be encrypted as different ciphertext letters.

Because of this variation, polyalphabetic ciphers are harder to break than monoalphabetic ciphers.

Example:

Monoalphabetic: Caesar Cipher

Polyalphabetic: Vigenère Cipher

5.Among monoalphabetic and polyalphabetic cipher, which one is more vulnerable? Justify your statement.

Solution:

Monoalphabetic cipher is more vulnerable than polyalphabetic cipher.

Justification:

In a monoalphabetic cipher, each plaintext letter is always replaced by the same ciphertext letter, creating predictable patterns.

These patterns make it easy to break using frequency analysis.

In contrast, a polyalphabetic cipher uses multiple substitution alphabets, reducing visible patterns and increasing security.

Conclusion:

Because of fixed substitutions and easy pattern detection, monoalphabetic ciphers are more vulnerable.

6.Given the key “HELLOWORLD”, encrypt the plaintext “TURINGTEST” using Play fair cipher.

Solution:

7.Show the encryption of plain text “ALGORITHM” using the key “PSEUDOCODE” using playfair cipher.

Solution:

8.Encrypt the message “INFORMATION” using the Playfair cipher with the keyword “SECURITY”.

Solution:

9.The message “IMOGUN” was encrypted with a Playfair cipher using keyword “GALOIS”. Decrypt the message.

Solution:

10.Using Vignere cipher with key = “worlds”, encrypt the plain text “hello everyone”.

Solution:

11.Differentiate between Symmetric and Asymmetric cipher. Encrypt the message “HELL” using the key “FAIL” using Vernanm cipher.

Solution:

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Unit 2: Symmetric Ciphers

1. Describe the Fiestel Cipher structure.

2. Explain the single round operation in DES. Describe the F function in DES.

3. Write down the encryption and decryption process at 2-DES and 3-DES.

4. State the encryption process of double and triple DES. What is the task of S-Box in DES?

5. Explain the process of key expansion in AES.

6. Explain the procedure of mix column transformation in AES with an example.

7. How encryption is done using IDEA algorithm.

8. Explain the round operation in IDEA.

9. Let us consider the 4 bits key set … Now trace the first full round operation of IDEA algorithm.

10. Describe any two modes of block cipher encryption.

11. Explain the different modes of block cipher.

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Unit 3: Asymmetric Ciphers

1. Illustrate the man in middle attack in Diffie–Hellman key exchange protocol.

2. Describe the meet in middle attack in Diffie Hellman key exchange protocol.

3. Define discrete logarithm. Explain the procedure of sharing the secret key in Diffie Hellman.

4. Why do we need discrete logarithm? Illustrate with an example.

5. Define discrete logarithms. How key generation, encryption and decryption is done in RSA.

6. Given p = 61, q = 53. Calculate the public key, private key, and encrypt the message “42”.

7. Explain the process of public key distribution and secret key distribution using public key cryptography.

8. Find whether 3 is primitive root of 7 or not.

9. Show that Z5 is a field. Encrypt the message using ElGamal cryptosystem.

10. Encrypt the message m=13 using ElGamal cryptosystem.

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Unit 4: Cryptographic Hash Functions and Digital Signatures

1. List the properties of hash function. Describe the algorithm for SHA-1.

2. Describe the properties of hash functions. Discuss how hash value is generated using SHA-1 algorithm.

3. Compare the SHA parameters between SHA-1 and SHA-2 families.

4. Explain the process of generating message digests using MD4.

5. List the operation of computing digest value in different passes of MD4.

6. Describe the working mechanism of digital signature algorithm.

7. Explain the concept of digital signatures and differentiate between direct and arbitrated digital signatures.

8. Explain the generic model of digital signature process.

9. How digital signature generation and verification is done using RSA.

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Unit 5: Authentication

1. Define authentication system. Discuss about challenge response system.

2. Define authentication system. Illustrate the need of mutual authentication over one way authentication.

3. Give the formal definition of authentication system. Describe about one way and mutual authentication system.

4. What is Message Authentication Code?

5. Write down any two limitations of MAC.

6. Describe about Needham-Schroeder protocol.

7. Discuss the working mechanism of kerberos protocol.

8. Why do we need Kerberos?

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Unit 6: Network Security and Public Key Infrastructure

1. What is digital certificate? Discuss the certificate life cycle.

2. List the stage of certificate life cycle.

3. Describe the PKI trust model.

4. Define PKI with its architecture model.

5. Explain IP Security (IPSec) and its components.

6. Describe about IPSec.

7. Describe the services provided by Pretty Good Privacy protocol to secure email.

8. Define SSL protocol. Mention the services provided by PGP.

9. List and explain the types of firewall.

10. Define firewall and types of firewalls.

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Unit 7: Malicious Logic

1. Describe any three types of malicious logic.

2. What is malicious logic? How zombies are different from trojan horses?

3. Differentiate between Trojan horse and virus.

4. What is intrusion? Explain any two types of intrusion detection system.

5. Explain the concept of a denial of service attack and provide examples.

6. How does the nature of worms differ from viruses?

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Miscellaneous / Cross-Chapter Questions

1. Show that the set of integers is Ring under addition and multiplication.

2. Find the multiplicative inverse of polynomial using extended Euclidean Algorithm.

3. Divide polynomials over GF(7) / GF(9).

4. Define Euler totient function with an example.

5. Find GCD using Euclidean algorithm.