Describe two methods for constructing block ciphers. What is a round in block ciphers? How a slide attack is realized?
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1. Two methods for constructing block ciphers:
a) Substitution-Permutation Networks (SPN): This method involves dividing the input data into multiple blocks and applying several substitution and permutation operations on each block. The process repeats for multiple rounds, with each round using a different set of substitution and permutation functions. This helps provide confusion and diffusion throughout the encryption process.
b) Feistel Network: In this method, the input is divided into two halves. The right half is processed through a function that takes the right half and a subkey as inputs and produces an output, which is then XORed with the left half. The left half and the new XORed output become the inputs for the next round. This process repeats for multiple rounds, with each round using a different subkey.
2. A round in block ciphers: A round refers to a single iteration of the encryption process in a block cipher. In each round, the input block undergoes a series of operations, including substitution, permutation, key mixing, or other mathematical transformations. The purpose of each round is to introduce non-linearity, confusion, and diffusion so that the original data becomes indistinguishable and less susceptible to attacks.
3. Slide attack realization: A slide attack is a type of cryptanalysis technique used to break block ciphers. In this attack, the attacker exploits the fact that a block cipher with multiple identical rounds can be represented as a set of equivalent keys and plaintexts. The attacker prepares a large amount of chosen plaintext-ciphertext pairs using known keys. Then, by identifying a pair that has the same intermediate state at the end of a round, the attacker can slide both the plaintext and key forward or backward to form new pairs with equivalent intermediate states. This enables the attacker to compute the subkey used in that round, compromising the overall security of the cipher.
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