(SEM VII) THEORY EXAMINATION 2017-18 CRYPTOGRAPHY & NETWORK SECURITY

SECTION A

(Short questions – explained clearly)

1. What is Cryptanalysis?

Cryptanalysis is the study of methods used to break cryptographic systems and obtain secret information without knowing the key. The main aim of cryptanalysis is to analyze encryption algorithms to find weaknesses that allow attackers to recover plaintext or keys. It is used both by attackers to break security and by designers to improve the strength of cryptographic systems.

2. What is a Security Attack? Discuss its types.

A security attack is any attempt to compromise the confidentiality, integrity, or availability of information. Security attacks are mainly classified into passive and active attacks. Passive attacks involve monitoring or eavesdropping on communication without altering data, such as traffic analysis. Active attacks involve modifying or disrupting data, such as masquerade, replay, message modification, and denial-of-service attacks.

SECTION B

(Medium questions – conceptual explanation)

3. How is E-mail security achieved? Explain S/MIME.

E-mail security is achieved by ensuring confidentiality, authentication, integrity, and non-repudiation of messages. S/MIME (Secure/Multipurpose Internet Mail Extensions) provides these services by using encryption and digital signatures. In S/MIME, the sender encrypts the message using the recipient’s public key and signs it using their private key. The receiver decrypts the message using their private key and verifies the signature using the sender’s public key, ensuring secure communication.

4. Explain DES (Data Encryption Standard) in brief.

DES is a symmetric key block cipher that encrypts data in blocks of 64 bits using a 56-bit secret key. The encryption process consists of an initial permutation, followed by 16 rounds of substitution and permutation, and finally an inverse permutation. Each round uses a different sub-key derived from the main key. Although DES was widely used, it is now considered insecure due to its small key size.

SECTION C

(Long questions – detailed explanation)

5. Explain Digital Signature and its signing and verification process.

A digital signature is a cryptographic technique used to verify the authenticity and integrity of a message. In the signing process, the sender creates a hash of the message and encrypts it using their private key to form a digital signature. The message and signature are then sent to the receiver. During verification, the receiver decrypts the signature using the sender’s public key and compares the result with the hash of the received message. If both match, the signature is valid, ensuring authentication, integrity, and non-repudiation.

6. What is Diffie-Hellman Key Exchange? Explain its working.

Diffie-Hellman key exchange is a method that allows two parties to securely generate a shared secret key over an insecure channel. Both parties agree on a public prime number and a generator. Each party selects a private key and computes a public value. These public values are exchanged, and each party computes the shared secret using their private key and the other party’s public value. The resulting secret key is the same for both parties and is used for secure communication.