Which among the following algorithms is designed based on the Feistel cipher?

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Question

Which Among The Following Algorithms Is Designed Based On The Feistel Cipher?

Answer

Answer:Feistel Cipher is not a specific scheme of block cipher. It is a design model from which many differentblock ciphers are derived. DES is just one example of a Feistel Cipher. A cryptographic system based on Feistel cipher structure uses the samealgorithm for both encryption and decryption.

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Feistel Cipher model is a structure or a design used to develop many block ciphers such as DES. Feistel cipher may have invertible, non-invertible and self invertible components in its design. Same encryption as well as decryption algorithm is used. A separate key is used for each round. However same round keys are used for encryption as well as decryption. 
 

Feistel cipher algorithm

• Create a list of all the Plain Text characters. 
   
• Convert the Plain Text to Ascii and then 8-bit binary format. 
   
• Divide the binary Plain Text string into two halves: left half (L1)and right half (R1) 
   
• Generate a random binary keys (K1 and K2) of length equal to the half the length of the Plain Text for the two rounds. 
   

First Round of Encryption 

• a. Generate function f1 using R1 and K1 as follows:

• b. Now the new left half(L2) and right half(R2) after round 1 are as follows:

Second Round of Encryption 

• a. Generate function f2 using R2 and K2 as follows: 
   

• b. Now the new left half(L3) and right half(R3) after round 2 are as follows:

• Concatenation of R3 to L3 is the Cipher Text
• Same algorithm is used for decryption to retrieve the Plain Text from the Cipher Text.

Examples: 

Python3

import binascii

def rand_key(p):

    import random

    key1 = ""

    p = int(p)

    for i in range(p):

        temp = random.randint(0,1)

        temp = str(temp)

        key1 = key1 + temp

    return(key1)

def exor(a,b):

    temp = ""

    for i in range(n):

        if (a[i] == b[i]):

            temp += "0"

        else:

            temp += "1"

    return temp

def BinaryToDecimal(binary):

    string = int(binary, 2)

    return string

PT = "Hello"

print("Plain Text is:", PT)

PT_Ascii = [ord(x) for x in PT]

PT_Bin = [format(y,'08b') for y in PT_Ascii]

PT_Bin = "".join(PT_Bin)

n = int(len(PT_Bin)//2)

L1 = PT_Bin[0:n]

R1 = PT_Bin[n::]

m = len(R1)

K1= rand_key(m)

K2= rand_key(m)

f1 = exor(R1,K1)

R2 = exor(f1,L1)

L2 = R1

f2 = exor(R2,K2)

R3 = exor(f2,L2)

L3 = R2

bin_data = L3 + R3

str_data =' '

for i in range(0, len(bin_data), 7):

    temp_data = bin_data[i:i + 7]

    decimal_data = BinaryToDecimal(temp_data)

    str_data = str_data + chr(decimal_data)

print("Cipher Text:", str_data)

L4 = L3

R4 = R3

f3 = exor(L4,K2)

L5 = exor(R4,f3)

R5 = L4

f4 = exor(L5,K1)

L6 = exor(R5,f4)

R6 = L5

PT1 = L6+R6

PT1 = int(PT1, 2)

RPT = binascii.unhexlify( '%x'% PT1)

print("Retrieved Plain Text is: ", RPT)

Output: