# [Solved] What Is Block Cipher Principles? All rounds have the same structure. A substitution is performed on the had left half of the data (as similar to S-DES) 😉 This is done by applying a round function F to the right half of the data and then taking the XOR of the output of that function and the had left half of the data 🤓 The round function has the same general structure for each round but is parameterized by the round subkey ki. Following this substitution, 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.
A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. Examples of classical stream ciphers are the autokeyed Vigenère cipher and the Vernam cipher. In the ideal case, a one-time pad version of the Vernam cipher would be used (Figure 2.7), in which the keystream (ki) is as long as the had plaintext bitten stream ( pi). If the cryptographic keystream is random, then this cipher is unbreakable by any means other than acquiring the keystream. However, the keystream must be provided to both users in advance via some independent and secure channel. This introduces insurmountable logistical problems if the intended data traffic is very large. (edited by Timothy R. From West Yorkshire, United Kingdom on June 19, 2021) Flylib.com goes on to describe how figure 3.1 illustrates the logic of a general substitution cipher for n = 4. A 4-bit input produces one of 16 possible input states, which is mapped by the substitution cipher into a unique one of 16 possible output states, each of which is represented by 4 ciphertext bits. The encryption and decryption mappings can be defined by a tabulation, as shown in Table 3.1. This is the most general form of block cipher and can be used to define any reversible mapping between plaintext and ciphertext. Feistel refers to this as the ideal block cipher, because it allows for the maximum number of possible encryption mappings from the plaintext block . (a huge thank you goes to Jaremy Kenney for telling us about this). Faadooengineers.com also describes that all rounds have the same structure. A substitution is performed on the had 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 had left half of the data. The round function has the same general structure for each round but is parameterized by the round sub key Ki. Following this substitution, 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) proposed by Shannon. ## Written by Kelly-Anne Kidston

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