JNTU B.Tech IV Year NETWORK SECURITY AND CRYPTOGRAPHY Previous Question Paper (July/August - 2022 - R18)

Security services are provided by a protocol layer of communicating open systems and are designed to ensure adequate security of the systems or data transfers. The X.800 standard defines these as: Authentication (verifying identity), Access Control (limiting resource access), Data Confidentiality (protecting data from unauthorized disclosure), Data Integrity (ensuring data is not altered), and Non-repudiation (preventing denial of transmission or receipt).

Security mechanisms are the methods used to implement these services, including encipherment, digital signatures, access control mechanisms, data integrity mechanisms, authentication exchanges, traffic padding, and routing control. Security attacks are categorized into Passive Attacks, where the attacker monitors transmissions without altering the system (e.g., release of message contents, traffic analysis), and Active Attacks, which involve modification of data or creation of false streams (e.g., masquerade, replay, modification of messages, and denial of service).

Block ciphers process plaintext in fixed-size blocks (e.g., 64 or 128 bits) rather than one bit or byte at a time. Most modern block ciphers are based on the Feistel Cipher structure, which utilizes an iterative process of substitution and permutation to achieve confusion and diffusion.

Key principles include:

• Confusion: Obscures the relationship between the plaintext and the ciphertext, typically achieved through complex substitution functions (S-boxes).
• Diffusion: Spreads the influence of a single plaintext digit over many ciphertext digits, typically achieved through permutations (P-boxes) and arithmetic operations.
• Iterative Structure: The algorithm performs multiple rounds of processing. Each round uses a subkey derived from the original secret key.
• Reversibility: The decryption process is essentially the same as encryption, but with subkeys applied in reverse order, ensuring the algorithm is a permutation of the input space.