JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV INFORMATION SECURITY Previous Question Paper (July/August - 2022 - R18)

The matrix illustrates that encipherment and traffic padding are primarily effective against passive attacks (e.g., traffic analysis, release of message contents), whereas mechanisms like digital signatures, access control, and data integrity are essential for mitigating active attacks (e.g., masquerade, replay, modification of messages, and denial of service).

The strength of the Data Encryption Standard (DES) is determined by the following key factors:

• Key Size: DES uses a 56-bit key, providing 2 to the power of 56 possible keys, which is approximately 7.2 x 10 to the power of 16. While this was secure in the 1970s, it is now considered vulnerable to brute-force attacks due to advances in computing power.
• Algorithm Design: The algorithm relies on the Feistel structure, which involves 16 rounds of processing. Each round uses a combination of substitution (S-boxes) and permutation (P-boxes) to achieve confusion and diffusion, making the relationship between the plaintext and ciphertext highly complex.
• S-Box Design: The S-boxes are the core of DES security, providing non-linearity. They are specifically designed to resist differential cryptanalysis.
• Brute-Force Vulnerability: The primary weakness of DES is its short key length. Modern distributed computing and specialized hardware (like FPGAs) can exhaust the key space in a relatively short time, rendering single-DES insecure for sensitive data.
• Avalanche Effect: DES exhibits a strong avalanche effect, where a small change in the input (plaintext or key) results in a significant, unpredictable change in the output ciphertext, which is a critical requirement for a secure block cipher.