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Implementation of RC4 algorithm
  • Difficulty Level : Medium
  • Last Updated : 25 Jan, 2021

RC4 is a symmetric stream cipher and variable key length algorithm. This symmetric key algorithm is used identically for encryption and decryption such that the data stream is simply XORed with the generated key sequence. The algorithm is serial as it requires successive exchanges of state entries based on the key sequence. The algorithm works in two phases:

Key Scheduling Algorithm(KSA):

  • It is used to generate a State array by applying a permutation using a variable-length key consisting of 0 to 256 bytes.
  • The state vector is identified as S[0], S[1]…. S[255] is initialized with {0, 1, 2, …, 255}. The key K[0], K[1], …., K[255] can be of any length from 0 to 256 bytes and is used to initialize permutation S. Each K[I] and S[I] is a byte.
  • K is a temporary array if the length of the key is 256 bytes copy it to K else after copying the remaining positions of K are filled with repeated Key Values until full.
S[] is permutation of 0, 1, ..., 255
key[] contains N bytes of key

for i = 0 to 255
    S[i] = i
    
    // Selects a keystream byte from
    // the table
    K[i] = key[i (mod N)]
    i++
j = 0

for i = 0 to 255
    j = (j + S[i] + K[i]) mod 256
    
    // Swaps elements in the current
    // lookup table
    swap(S[i], S[j])
i = j = 0

Pseudo-Random Generation Algorithm(PRGA): It used to generate keystream byte from State vector array after one more round of permutation.

Keystream Generation(i := 0, j := 0 )

while Generating Output:
    i = (i + 1) mod 256
    j = (j + S[i]) mod 256
    swap(S[i], S[j])
    t = (S[i] + S[j]) mod 256
    keystreamByte = S[t]

At each iteration, swap elements in table
and select keystream byte

Then, perform XOR between the keystream generated and the plain text for encryption.

Follow the same procedure as above for decryption, taking cipher text in place of plain text everywhere.



Examples:

Input: plain text = 001010010010, key = 101001000001, n= 3
Output: 
cipher text = 110011100011
decrypted text = 001010010010

Input: plain text = 1111000000001111, key = 0101010111001010, n= 4
Output:
cipher text = 0011011110100010
decrypted text = 1111000000001111

Below is the implementation of the above approach with a detailed output of all the important steps involved:

Python3




# Python3 program for the above approach
# of RC4 algorithm
  
# Function for encryption
def encryption():
  
    global key, plain_text, n
  
    # Given text and key
    plain_text = "001010010010"
    key = "101001000001"
  
    # n is the no: of bits to
    # be considered at a time
    n = 3
  
    print("Plain text : ", plain_text)
    print("Key : ", key)
    print("n : ", n)
  
    print(" ")
  
    # The initial state vector array
    S = [i for i in range(0, 2**n)]
    print("S : ", S)
  
    key_list = [key[i:i + n] for i in range(0, len(key), n)]
  
    # Convert to key_stream to decimal
    for i in range(len(key_list)):
        key_list[i] = int(key_list[i], 2)
  
    # Convert to plain_text to decimal
    global pt
  
    pt = [plain_text[i:i + n] for i in range(0, len(plain_text), n)]
  
    for i in range(len(pt)):
        pt[i] = int(pt[i], 2)
  
    print("Plain text ( in array form ): ", pt)
  
    # Making key_stream eqaul
    # to length of state vector
    diff = int(len(S)-len(key_list))
  
    if diff != 0:
        for i in range(0, diff):
            key_list.append(key_list[i])
  
    print("Key list : ", key_list)
    print(" ")
  
    # Perform the KSA algorithm
    def KSA():
        j = 0
        N = len(S)
          
        # Iterate over the range [0, N]
        for i in range(0, N):
            
            # Find the key
            j = (j + S[i]+key_list[i]) % N
              
            # Update S[i] and S[j]
            S[i], S[j] = S[j], S[i]
            print(i, " ", end ="")
              
            # Print S
            print(S)
  
        initial_permutation_array = S
          
        print(" ")
        print("The initial permutation array is : ",
              initial_permutation_array)
  
    print("KSA iterations : ")
    print(" ")
    KSA()
    print(" ")
  
    # Perform PGRA algorithm
    def PGRA():
  
        N = len(S)
        i = j = 0
        global key_stream
        key_stream = []
  
        # Iterate over [0, length of pt]
        for k in range(0, len(pt)):
            i = (i + 1) % N
            j = (j + S[i]) % N
              
            # Update S[i] and S[j]
            S[i], S[j] = S[j], S[i]
            print(k, " ", end ="")
            print(S)
            t = (S[i]+S[j]) % N
            key_stream.append(S[t])
  
        # Print the key stream
        print("Key stream : ", key_stream)
        print(" ")
  
    print("PGRA iterations : ")
    print(" ")
    PGRA()
  
    # Performing XOR between generated
    # key stream and plain text
    def XOR():
        global cipher_text
        cipher_text = []
        for i in range(len(pt)):
            c = key_stream[i] ^ pt[i]
            cipher_text.append(c)
  
    XOR()
  
    # Convert the encrypted text to
    # bits form
    encrypted_to_bits = ""
    for i in cipher_text:
        encrypted_to_bits += '0'*(n-len(bin(i)[2:]))+bin(i)[2:]
  
    print(" ")
    print("Cipher text : ", encrypted_to_bits)
  
  
encryption()
  
print("---------------------------------------------------------")
  
# Function for decryption of data
def decryption():
  
    # The initial state vector array
    S = [i for i in range(0, 2**n)]
  
    key_list = [key[i:i + n] for i in range(0, len(key), n)]
  
    # Convert to key_stream to decimal
    for i in range(len(key_list)):
        key_list[i] = int(key_list[i], 2)
  
    # Convert to plain_text to decimal
    global pt
  
    pt = [plain_text[i:i + n] for i in range(0, len(plain_text), n)]
  
    for i in range(len(pt)):
        pt[i] = int(pt[i], 2)
  
    # making key_stream eqaul
    # to length of state vector
    diff = int(len(S)-len(key_list))
  
    if diff != 0:
        for i in range(0, diff):
            key_list.append(key_list[i])
  
    print(" ")
  
    # KSA algorithm
    def KSA():
        j = 0
        N = len(S)
          
        # Iterate over the range [0, N]
        for i in range(0, N):
            j = (j + S[i]+key_list[i]) % N
              
            # Update S[i] and S[j]
            S[i], S[j] = S[j], S[i]
            print(i, " ", end ="")
            print(S)
  
        initial_permutation_array = S
        print(" ")
        print("The initial permutation array is : ",
              initial_permutation_array)
  
    print("KSA iterations : ")
    print(" ")
    KSA()
    print(" ")
  
    # Perform PRGA algorithm
    def do_PGRA():
  
        N = len(S)
        i = j = 0
        global key_stream
        key_stream = []
  
        # Iterate over the range
        for k in range(0, len(pt)):
            i = (i + 1) % N
            j = (j + S[i]) % N
              
            # Update S[i] and S[j]
            S[i], S[j] = S[j], S[i]
            print(k, " ", end ="")
            print(S)
            t = (S[i]+S[j]) % N
            key_stream.append(S[t])
  
    print("Key stream : ", key_stream)
    print(" ")
  
    print("PGRA iterations : ")
    print(" ")
    do_PGRA()
  
    # Perform XOR between generated
    # key stream  and cipher text
    def do_XOR():
        global original_text
        original_text = []
        for i in range(len(cipher_text)):
            p = key_stream[i] ^ cipher_text[i]
            original_text.append(p)
  
    do_XOR()
  
    # convert the decrypted text to
    # the bits form
    decrypted_to_bits = ""
    for i in original_text:
        decrypted_to_bits += '0'*(n-len(bin(i)[2:]))+bin(i)[2:]
  
    print(" ")
    print("Decrypted text : ",
          decrypted_to_bits)
  
# Driver Code
decryption()
Output:
Plain text :  001010010010
Key :  101001000001
n :  3
 
S :  [0, 1, 2, 3, 4, 5, 6, 7]
Plain text ( in array form ):  [1, 2, 2, 2]
Key list :  [5, 1, 0, 1, 5, 1, 0, 1]
 
KSA iterations : 
 
0  [5, 1, 2, 3, 4, 0, 6, 7]
1  [5, 7, 2, 3, 4, 0, 6, 1]
2  [5, 2, 7, 3, 4, 0, 6, 1]
3  [5, 2, 7, 0, 4, 3, 6, 1]
4  [5, 2, 7, 0, 6, 3, 4, 1]
5  [5, 2, 3, 0, 6, 7, 4, 1]
6  [5, 2, 3, 0, 6, 7, 4, 1]
7  [1, 2, 3, 0, 6, 7, 4, 5]
 
The initial permutation array is :  [1, 2, 3, 0, 6, 7, 4, 5]
 
PGRA iterations : 
 
0  [1, 3, 2, 0, 6, 7, 4, 5]
1  [1, 3, 6, 0, 2, 7, 4, 5]
2  [1, 3, 6, 2, 0, 7, 4, 5]
3  [1, 3, 6, 2, 0, 7, 4, 5]
Key stream :  [7, 1, 6, 1]
 
 
Cipher text :  110011100011
---------------------------------------------------------
 
KSA iterations : 
 
0  [5, 1, 2, 3, 4, 0, 6, 7]
1  [5, 7, 2, 3, 4, 0, 6, 1]
2  [5, 2, 7, 3, 4, 0, 6, 1]
3  [5, 2, 7, 0, 4, 3, 6, 1]
4  [5, 2, 7, 0, 6, 3, 4, 1]
5  [5, 2, 3, 0, 6, 7, 4, 1]
6  [5, 2, 3, 0, 6, 7, 4, 1]
7  [1, 2, 3, 0, 6, 7, 4, 5]
 
The initial permutation array is :  [1, 2, 3, 0, 6, 7, 4, 5]
 
Key stream :  [7, 1, 6, 1]
 
PGRA iterations : 
 
0  [1, 3, 2, 0, 6, 7, 4, 5]
1  [1, 3, 6, 0, 2, 7, 4, 5]
2  [1, 3, 6, 2, 0, 7, 4, 5]
3  [1, 3, 6, 2, 0, 7, 4, 5]
 
Decrypted text :  001010010010

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