practical 3

Aim : Design and implement a symmetric encryption algorithm based on Feistel structure.

code

# Python program to demonstrate
# Feistel Cipher Algorithm

import binascii

# Random bits key generation
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)

# Function to implement bit exor
def exor(a,b):
    
    temp = ""
    
    for i in range(n):
        
        if (a[i] == b[i]):
            temp += "0"
            
        else:
            temp += "1"
            
    return temp

# Defining BinarytoDecimal() function
def BinaryToDecimal(binary):
    
    # Using int function to convert to
    # string
    string = int(binary, 2)
    
    return string

# Feistel Cipher
PT = "Hello"
print("Plain Text is:", PT)

# Converting the plain text to
# ASCII
PT_Ascii = [ord(x) for x in PT]

# Converting the ASCII to
# 8-bit binary format
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)

# Generate Key K1 for the
# first round
K1= rand_key(m)

# Generate Key K2 for the
# second round
K2= rand_key(m)

# first round of Feistel
f1 = exor(R1,K1)
R2 = exor(f1,L1)
L2 = R1

# Second round of Feistel
f2 = exor(R2,K2)
R3 = exor(f2,L2)
L3 = R2

# Cipher text
bin_data = L3 + R3
str_data =' '

for i in range(0, len(bin_data), 7):
    
    # slicing the bin_data from index range [0, 6]
    # and storing it in temp_data
    temp_data = bin_data[i:i + 7]
        
    # passing temp_data in BinarytoDecimal() function
    # to get decimal value of corresponding temp_data
    decimal_data = BinaryToDecimal(temp_data)
        
    # Decoding the decimal value returned by
    # BinarytoDecimal() function, using chr()
    # function which return the string corresponding
    # character for given ASCII value, and store it
    # in str_data
    str_data = str_data + chr(decimal_data)
    
print("Cipher Text:", str_data)

# Decryption
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)

To run

save this file with -> pr3.py (add your preference name)

python pr3.py