Skip to content
Related Articles

Related Articles

Feistel Cipher
  • Difficulty Level : Easy
  • Last Updated : 14 Aug, 2020

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:
    f1= xor(R1, K1)

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

    R2= xor(f1, L1)
  • Second Round of Encryption
    a. Generate function f2 using R2 and K2 as follows:
    f2= xor(R2, K2)

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

    R3= xor(f2, L2)
  • Concatenation of R3 to L3 is the Cipher Text
  • Same algorithm is used for decryption to retrieve the Plain Text from the Cipher Text.


Plain Text is: Hello
Cipher Text:  E1!w(
Retrieved Plain Text is:  b'Hello'

Plain Text is: Geeks
Cipher Text: O;Q
Retrieved Plain Text is:  b'Geeks'

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


Plain Text is: Hello
Cipher Text:  E1!w(
Retrieved Plain Text is:  b'Hello'

Attention geek! Strengthen your foundations with the Python Programming Foundation Course and learn the basics.

To begin with, your interview preparations Enhance your Data Structures concepts with the Python DS Course.

My Personal Notes arrow_drop_up
Recommended Articles
Page :