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) L2=R1
Second Round of Encryption
- a. Generate function f2 using R2 and K2 as follows:
f2= xor(R2, K2)
- b. Now the new left half(L3) and right half(R3) after round 2 are as follows:
R3= xor(f2, L2) L3=R2
- 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:
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'
Python3
# Python program to demonstrate# Feistel Cipher Algorithmimport binascii# Random bits key generationdef 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 exordef exor(a,b): temp = "" for i in range(n): if (a[i] == b[i]): temp += "0" else: temp += "1" return temp# Defining BinarytoDecimal() functiondef BinaryToDecimal(binary): # Using int function to convert to # string string = int(binary, 2) return string# Feistel CipherPT = "Hello"print("Plain Text is:", PT)# Converting the plain text to# ASCIIPT_Ascii = [ord(x) for x in PT]# Converting the ASCII to# 8-bit binary formatPT_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 roundK1= rand_key(m) # Generate Key K2 for the# second roundK2= rand_key(m) # first round of Feistelf1 = exor(R1,K1)R2 = exor(f1,L1)L2 = R1 # Second round of Feistelf2 = exor(R2,K2)R3 = exor(f2,L2)L3 = R2 # Cipher textbin_data = L3 + R3str_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)# DecryptionL4 = L3R4 = R3 f3 = exor(L4,K2)L5 = exor(R4,f3)R5 = L4 f4 = exor(L5,K1)L6 = exor(R5,f4)R6 = L5PT1 = L6+R6 PT1 = int(PT1, 2)RPT = binascii.unhexlify( '%x'% PT1)print("Retrieved Plain Text is: ", RPT) |
Output:
Plain Text is: Hello Cipher Text: E1!w( Retrieved Plain Text is: b'Hello'
Time Complexity: O(n)
Auxiliary Space: O(n)
