Write a short notes on i) Feistel Block Cipher structure ii)Block cipher Design Principles in CNS
Answers
Explanation:
eistel Cipher is not a specific scheme of block cipher. It is a design model from which many different block ciphers are derived. DES is just one example of a Feistel Cipher. A cryptographic system based on Feistel cipher structure uses the same algorithm for both encryption and decryption.
1 Block cipher principles
most symmetric block ciphers are based on a Feistel Cipher Structure
needed since must be able to decrypt ciphertext to recover messages efficiently
block ciphers look like an extremely large substitution
would need table of 264 entries for a 64-bit block
instead create from smaller building blocks
using idea of a product cipher in 1949 Claude Shannon introduced idea of substitution-permutation (S-P) networks called modern substitution-transposition product cipher these form the basis of modern block ciphers
S-P networks are based on the two primitive cryptographic operations we have seen before:
substitution (S-box)
permutation (P-box)
provide confusion and diffusion of message
diffusion – dissipates statistical structure of plaintext over bulk of ciphertext
confusion – makes relationship between ciphertext and key as complex as possible
2 Feistel cipher structure
The input to the encryption algorithm are a plaintext block of length 2w bits and a key K. the plaintext block is divided into two halves L0 and R0. The two halves of the data pass through „n‟ rounds of processing and then combine to produce the ciphertext block. Each round „i‟ has inputs Li-1 and Ri-1, derived from the previous round, as well as the subkey Ki, derived from the overall key K. in general, the subkeys Ki are different from K and from each other.
All rounds have the same structure. A substitution is performed on the 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 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.