Cryptography and Network Security Exam

The primary purpose of S-boxes in DES is to provide confusion and substitution, which increases the security of the encrypted data.

The avalanche effect is a phenomenon where a small change in the input plaintext results in a significant change in the output ciphertext.

Kerberos is an authentication protocol that provides secure authentication and communication over an open network.

The Euclid algorithm works by repeatedly applying the division algorithm to find the remainder, until the remainder is zero, at which point the GCD is the last non-zero remainder.

A cryptographic checksum or MAC is used to ensure the integrity and authenticity of a message, by detecting any tampering or alterations to the message.

The RSA algorithm is a public-key encryption algorithm that uses the principles of prime numbers and modular exponentiation to encrypt and decrypt data.

Cryptography is the practice and study of techniques for secure communication in the presence of third-party adversaries. It is significant in modern computing as it enables secure data transmission and protects against unauthorized access, ensuring confidentiality, integrity, and authenticity of data.

Passive attacks involve unauthorized access or interception of data, whereas active attacks involve alteration or modification of data. An example of a passive attack is eavesdropping, while an example of an active attack is a man-in-the-middle attack.

A group in cryptography is a set of elements with a binary operation (such as addition or multiplication) that satisfies certain properties, including closure, associativity, and the existence of an identity element and inverse elements. A ring adds the property of distributivity, while a field is a commutative ring with a multiplicative inverse.

A substitution cipher replaces plaintext elements with corresponding ciphertext elements, whereas a transposition cipher rearranges plaintext elements. An example of a substitution cipher is the Caesar Cipher, and an example of a transposition cipher is the Rail Fence Cipher.

ECB (Electronic Codebook) mode encrypts each block independently, while CBC (Cipher Block Chaining) mode uses the previous block's ciphertext to encrypt the current block. ECB is vulnerable to block repetition attacks, while CBC is more secure but requires an initialization vector.

The Hill Cipher is a polygraphic substitution cipher that uses linear algebra to encrypt and decrypt data. To encrypt a message, the plaintext is converted into a numerical representation, and then multiplied by a key matrix. The ciphertext is obtained by taking the modulus of the result. For example, encrypting the message 'BALLOON' with key [3 2] would result in a ciphertext.