Lecture "On safety and security of information systems: Symmetric ciphers" provide students with knowledge about: Block cipher principles; Data encryption standard; Feistel cipher; . Please refer to the detailed content of the lecture! | SYMMETRIC CIPHERS Contents 1 Block Cipher Principles 2 Feistel Cipher 3 Data Encryption Standard DEC CRYPTOLOGY CRYPTOGRAPHY CRYPTANALYSIS SYMMETRIC ASYMMETRIC CLASSICAL MODERN Substitution Block ciphers Transposition Stream ciphers 1. Block Cipher Principles Stream Ciphers and Block Ciphers A stream cipher is a type pf symmetric encryption in which input data is encrypted one bit byte at a time. Stream Ciphers and Block Ciphers Block Ciphers is one in which the plaintext is divided in blocks and one block is encrypted at a time producing a ciphertext of equal length Block size 2. Feistel Cipher The Feistel Cipher Horst Feistel devised the feistel cipher Most symmetric block ciphers are based on a feistel cipher structure. Feistel proposed the use of a cipher that alternates substitutions and permutations where these terms are defined as follows Substitution Each plaintext element or group of elements is uniquely replaced by a corresponding ciphertext element or group of elements Permutation A sequence of plaintext elements is replaced by a permutation of that sequence Feistel Cipher Structure The Plaintext block 2w bits The plaintext block is divided into two halves L0 and R0. L R pass through n rounds of processing and then combine to produce the ciphertext block. Each round i 1- gt 16 Feistel Cipher Principles The exact realization of a Feistel network depends on the choice of the following parameters and design features Block size Larger block sizes mean greater security but reduced encryption decryption speed for a given algorithm. Key size Larger key size means greater security but may decrease encryption decryption speed. Number of rounds increase number improves security but slows cipher Feistel Cipher Principles cont. Subkey generation algorithm Greater complexity in this algorithm should lead to greater difficulty of cryptanalysis. Round function F greater complexity generally means greater resistance to cryptanalysis. Feistel decryption algorithm .
