JNTU B.Tech IV NETWORK SECURITY AND CRYPTOGRAPHY Previous Question Paper (December - 2020 - R16)

A passive threat involves monitoring or eavesdropping on network traffic without altering the data or the system resources. The primary goal is to obtain information, such as passwords or sensitive data, while remaining undetected. Examples include traffic analysis and release of message contents.

An active threat involves unauthorized modification of the data stream or the creation of a false stream. Unlike passive threats, these attacks disrupt normal system operation or compromise data integrity and availability. Examples include masquerade, replay, modification of messages, and denial of service.

The design of a block cipher, such as the Data Encryption Standard (DES) or Advanced Encryption Standard (AES), relies on the principle of Shannon's Confusion and Diffusion to ensure security against cryptanalysis.

1. Confusion: This principle seeks to make the relationship between the statistics of the ciphertext and the value of the encryption key as complex as possible. This is typically achieved through complex non-linear substitution functions (S-boxes), ensuring that each bit of the ciphertext depends on several parts of the key.
2. Diffusion: This principle aims to spread the influence of a single plaintext bit over as many ciphertext bits as possible. This is usually implemented through permutation layers and bit-shuffling, ensuring that if a single bit of the plaintext is changed, approximately half of the bits in the ciphertext change, thereby hiding the statistical structure of the plaintext.
3. Product Cipher: Modern block ciphers are designed as product ciphers, which consist of a sequence of identical rounds. Each round applies a combination of substitution and permutation to the data. By iterating these rounds multiple times, the cipher achieves a high level of security that would be impossible with a single round.
4. Feistel Network Structure: Many block ciphers utilize the Feistel structure, which splits the data block into two halves. In each round, one half is modified by a function of the other half and the round key, and then the halves are swapped. This structure ensures that the encryption process is reversible, allowing the same algorithm to be used for decryption by simply reversing the order of the round keys.