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- Cryptography And Network Security - CNS
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**Biju Patnaik University of Technology BPUT - BPUT**- Computer Science Engineering
- B.Tech
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Block Ciphers A block cipher is an encryption/decryption scheme in which a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length. A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. o Examples of classical stream ciphers are the autokeyed Vigenère cipher and the Vernam cipher. A block cipher is one in which a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length Many block ciphers have a Feistel structure. Such a structure consists of a number of identical rounds of processing. In each round, a substitution is performed on one half of the data being processed, followed by a permutation that interchanges the two halves. The original key is expanded so that a different key is used for each round. The Data Encryption Standard (DES) has been the most widely used encryption algorithm until recently. It exhibits the classic Feistel structure. DES uses a 64-bit block and a 56-bit key. Two important methods of cryptanalysis are differential cryptanalysis and linear cryptanalysis. DES has been shown to be highly resistant to these two types of attack Diffusion and Confusion Shannon suggests two methods for frustrating statistical cryptanalysis: diffusion and confusion. Diffusion the statistical structure of the plaintext is dissipated into long-range statistics of the ciphertext. This is achieved by having each plaintext digit affect the value of many ciphertext digits; generally this is equivalent to having each ciphertext digit be affected by many plaintext digits Confusion seeks to make the relationship between the statistics of the ciphertext and the value of the encryption key as complex as possible, again to thwart attempts to discover the key. Thus, even if the attacker can get some handle on the statistics of the ciphertext, the way in which the key was used to produce that ciphertext is so complex as to make it difficult to deduce the key. Feistel Cipher Structure All rounds have the same structure. A substitution is performed on the left half of the data. This is done by applying a round function F to the right half of the data and then taking the exclusive-OR of the output of that function and the left half of the data. The round function has the same general structure and parameterized by the round subkey Ki Following this, a permutation is performed that consists of the interchange of the two halves of the data

This structure is a particular form of the substitution-permutation network (SPN) Feistel network depends on the choice of the following parameters and design features Block size, Key size, Number of rounds, Subkey generation algorithm, Round function, Fast software encryption/decryption, Ease of analysis Feistel Encryption and Decryption

Simplified DES educational rather than a secure encryption algorithm. It has similar properties and structure to DES with much smaller parameters Simplified DES Scheme The S-DES encryption algorithm takes an 8-bit block of plaintext (example: 10111101) and a 10-bit key as input and produces an 8-bit block of ciphertext as output. The S-DES decryption algorithm takes an 8-bit block of ciphertext and the same 10-bit key used to produce that ciphertext as input and produces the original 8-bit block of plaintext. Involves five functions: an initial permutation (IP); a complex function labeled fK, which involves both permutation and substitution operations and depends on a key input; a simple permutation function that switches (SW) the two halves of the data; the function fK again; finally a permutation function that is the inverse of the initial permutation (IP–1). Algorithm rename these 8 bits

Key Generation for Simplified DES Plain Text Key P10 LS-1 P8 (K1) 3 5 2 7 4 10 1 9 8 6 6 3 7 4 8 5 10 9 Simplified DES Encryption Detail 1 0 1 0 0 1 2 0 1 0 1 1 3 1 0 1 1 1 4 0 0 1 0 0 5 1 0 0 0 1 6 0 1 0 1 0 7 0 1 1 1 0 8 0 1 1 1 1 9 10 10 11 1 0

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