JNTU B.Tech III CRYPTOGRAPHY AND NETWORK SECURITY Previous Question Paper (March - 2024 - R16)

An active attack is a type of network security threat where the attacker attempts to alter system resources or affect their operation. Unlike passive attacks, active attacks involve the modification of the data stream or the creation of a false stream, making them detectable by the target system. Common examples include masquerade, replay, modification of messages, and denial of service.

A substitution cipher is a method of encryption where units of plaintext are replaced with other ciphertext symbols, characters, or groups of characters according to a fixed system. The original message remains in its original order, but the identity of the characters is changed.

Key characteristics include:

• Monoalphabetic substitution: Each plaintext character is always replaced by the same ciphertext character (e.g., the Caesar cipher, where each letter is shifted by a fixed number of positions).
• Polyalphabetic substitution: Multiple substitution alphabets are used to encrypt the message, which helps defend against frequency analysis (e.g., the Vigenère cipher).

While simple to implement, basic substitution ciphers are generally vulnerable to frequency analysis, as they preserve the underlying statistical distribution of the plaintext language.