Implementation of Affine Cipher


The Affine cipher is a type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The formula used means that each letter encrypts to one other letter, and back again, meaning the cipher is essentially a standard substitution cipher with a rule governing which letter goes to which.
The whole process relies on working modulo m (the length of the alphabet used). In the affine cipher, the letters of an alphabet of size m are first mapped to the integers in the range 0 … m-1.
The ‘key’ for the Affine cipher consists of 2 numbers, we’ll call them a and b. The following discussion assumes the use of a 26 character alphabet (m = 26). a should be chosen to be relatively prime to m (i.e. a should have no factors in common with m).
affine cipher values

Encryption

It uses modular arithmetic to transform the integer that each plaintext letter corresponds to into another integer that correspond to a ciphertext letter. The encryption function for a single letter is



 E ( x ) = ( a x + b ) mod m 
modulus m: size of the alphabet
a and b: key of the cipher.
a must be chosen such that a and m are coprime.

Decryption

In deciphering the ciphertext, we must perform the opposite (or inverse) functions on the ciphertext to retrieve the plaintext. Once again, the first step is to convert each of the ciphertext letters into their integer values. The decryption function is

D ( x ) = a^-1 ( x - b ) mod m
a^-1 : modular multiplicative inverse of a modulo m. i.e., it satisfies the equation
1 = a a^-1 mod m .

To find a multiplicative inverse
We need to find a number x such that:
If we find the number x such that the equation is true, then x is the inverse of a, and we call it a^-1. The easiest way to solve this equation is to search each of the numbers 1 to 25, and see which one satisfies the equation.

[g,x,d] = gcd(a,m);    % we can ignore g and d, we dont need them
x = mod(x,m);      

If you now multiply x and a and reduce the result (mod 26), you will get the answer 1. Remember, this is just the definition of an inverse i.e. if a*x = 1 (mod 26), then x is an inverse of a (and a is an inverse of x)

Example:
affin cipher

C++

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//CPP program to illustate Affine Cipher
  
#include<bits/stdc++.h>
using namespace std;
  
//Key values of a and b
const int a = 17;
const int b = 20;
  
string encryptMessage(string msg)
{
    ///Cipher Text initially empty
    string cipher = ""
    for (int i = 0; i < msg.length(); i++)
    {
        // Avoid space to be encrypted 
        if(msg[i]!=' '
            /* applying encryption formula ( a x + b ) mod m
            {here x is msg[i] and m is 26} and added 'A' to 
            bring it in range of ascii alphabet[ 65-90 | A-Z ] */
            cipher = cipher + 
                        (char) ((((a * (msg[i]-'A') ) + b) % 26) + 'A');
        else
            //else simply append space character
            cipher += msg[i];     
    }
    return cipher;
}
  
string decryptCipher(string cipher)
{
    string msg = "";
    int a_inv = 0;
    int flag = 0;
      
    //Find a^-1 (the multiplicative inverse of a 
        //in the group of integers modulo m.) 
    for (int i = 0; i < 26; i++)
    {
        flag = (a * i) % 26;
          
        //Check if (a*i)%26 == 1,
                //then i will be the multiplicative inverse of a
        if (flag == 1)
        
            a_inv = i;
        }
    }
    for (int i = 0; i < cipher.length(); i++)
    {
        if(cipher[i]!=' ')
            /*Applying decryption formula a^-1 ( x - b ) mod m 
            {here x is cipher[i] and m is 26} and added 'A' 
            to bring it in range of ASCII alphabet[ 65-90 | A-Z ] */
            msg = msg + 
                       (char) (((a_inv * ((cipher[i]+'A' - b)) % 26)) + 'A');
        else
            //else simply append space characte
            msg += cipher[i]; 
    }
  
    return msg;
}
  
//Driver Program
int main(void)
{
    string msg = "AFFINE CIPHER";
      
    //Calling encryption function
    string cipherText = encryptMessage(msg);
    cout << "Encrypted Message is : " << cipherText<<endl;
      
    //Calling Decryption function
    cout << "Decrypted Message is: " << decryptCipher(cipherText);
  
    return 0;
}

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Java

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// Java program to illustate Affine Cipher
  
class GFG 
{
  
    // Key values of a and b
    static int a = 17;
    static int b = 20;
  
    static String encryptMessage(char[] msg) 
    {
        /// Cipher Text initially empty
        String cipher = "";
        for (int i = 0; i < msg.length; i++)
        {
            // Avoid space to be encrypted 
            /* applying encryption formula ( a x + b ) mod m
            {here x is msg[i] and m is 26} and added 'A' to 
            bring it in range of ascii alphabet[ 65-90 | A-Z ] */ 
            if (msg[i] != ' '
            {
                cipher = cipher
                        + (char) ((((a * (msg[i] - 'A')) + b) % 26) + 'A');
            } else // else simply append space character
            {
                cipher += msg[i];
            }
        }
        return cipher;
    }
  
    static String decryptCipher(String cipher) 
    {
        String msg = "";
        int a_inv = 0;
        int flag = 0;
  
        //Find a^-1 (the multiplicative inverse of a 
        //in the group of integers modulo m.) 
        for (int i = 0; i < 26; i++) 
        {
            flag = (a * i) % 26;
  
            // Check if (a*i)%26 == 1,
            // then i will be the multiplicative inverse of a
            if (flag == 1
            {
                a_inv = i;
            }
        }
        for (int i = 0; i < cipher.length(); i++) 
        {
            /*Applying decryption formula a^-1 ( x - b ) mod m 
            {here x is cipher[i] and m is 26} and added 'A' 
            to bring it in range of ASCII alphabet[ 65-90 | A-Z ] */ 
            if (cipher.charAt(i) != ' '
            {
                msg = msg + (char) (((a_inv * 
                        ((cipher.charAt(i) + 'A' - b)) % 26)) + 'A');
            
            else //else simply append space characte
            {
                msg += cipher.charAt(i);
            }
        }
  
        return msg;
    }
  
    // Driver code
    public static void main(String[] args) 
    {
        String msg = "AFFINE CIPHER";
  
        // Calling encryption function
        String cipherText = encryptMessage(msg.toCharArray());
        System.out.println("Encrypted Message is : " + cipherText);
  
        // Calling Decryption function
        System.out.println("Decrypted Message is: " + decryptCipher(cipherText));
  
    }
}
  
// This code contributed by Rajput-Ji

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Python

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# Implementation of Affine Cipher in Python
  
# Extended Euclidean Algorithm for finding modular inverse
# eg: modinv(7, 26) = 15
def egcd(a, b):
    x,y, u,v = 0,1, 1,0
    while a != 0:
        q, r = b//a, b%a
        m, n = x-u*q, y-v*q
        b,a, x,y, u,v = a,r, u,v, m,n
    gcd = b
    return gcd, x, y
  
def modinv(a, m):
    gcd, x, y = egcd(a, m)
    if gcd != 1:
        return None  # modular inverse does not exist
    else:
        return x % m
  
  
# affine cipher encrytion function 
# returns the cipher text
def affine_encrypt(text, key):
    '''
    C = (a*P + b) % 26
    '''
    return ''.join([ chr((( key[0]*(ord(t) - ord('A')) + key[1] ) % 26
                  + ord('A')) for t in text.upper().replace(' ', '') ])
  
  
# affine cipher decryption function 
# returns original text
def affine_decrypt(cipher, key):
    '''
    P = (a^-1 * (C - b)) % 26
    '''
    return ''.join([ chr((( modinv(key[0], 26)*(ord(c) - ord('A') - key[1])) 
                    % 26) + ord('A')) for c in cipher ])
  
  
# Driver Code to test the above functions
def main():
    # declaring text and key
    text = 'AFFINE CIPHER'
    key = [17, 20]
  
    # calling encryption function
    affine_encrypted_text = affine_encrypt(text, key)
  
    print('Encrypted Text: {}'.format( affine_encrypted_text ))
  
    # calling decryption function
    print('Decrypted Text: {}'.format
    ( affine_decrypt(affine_encrypted_text, key) ))
  
  
if __name__ == '__main__':
    main()
# This code is contributed by
# Bhushan Borole

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C#

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// C# program to illustate Affine Cipher
using System;
      
class GFG 
{
  
    // Key values of a and b
    static int a = 17;
    static int b = 20;
  
    static String encryptMessage(char[] msg) 
    {
        /// Cipher Text initially empty
        String cipher = "";
        for (int i = 0; i < msg.Length; i++)
        {
            // Avoid space to be encrypted 
            /* applying encryption formula ( a x + b ) mod m
            {here x is msg[i] and m is 26} and added 'A' to 
            bring it in range of ascii alphabet[ 65-90 | A-Z ] */
            if (msg[i] != ' '
            {
                cipher = cipher
                        + (char) ((((a * (msg[i] - 'A')) + b) % 26) + 'A');
            } else // else simply append space character
            {
                cipher += msg[i];
            }
        }
        return cipher;
    }
  
    static String decryptCipher(String cipher) 
    {
        String msg = "";
        int a_inv = 0;
        int flag = 0;
  
        //Find a^-1 (the multiplicative inverse of a 
        //in the group of integers modulo m.) 
        for (int i = 0; i < 26; i++) 
        {
            flag = (a * i) % 26;
  
            // Check if (a*i)%26 == 1,
            // then i will be the multiplicative inverse of a
            if (flag == 1) 
            {
                a_inv = i;
            }
        }
        for (int i = 0; i < cipher.Length; i++) 
        {
            /*Applying decryption formula a^-1 ( x - b ) mod m 
            {here x is cipher[i] and m is 26} and added 'A' 
            to bring it in range of ASCII alphabet[ 65-90 | A-Z ] */
            if (cipher[i] != ' '
            {
                msg = msg + (char) (((a_inv * 
                        ((cipher[i] + 'A' - b)) % 26)) + 'A');
            
            else //else simply append space characte
            {
                msg += cipher[i];
            }
        }
  
        return msg;
    }
  
    // Driver code
    public static void Main(String[] args) 
    {
        String msg = "AFFINE CIPHER";
  
        // Calling encryption function
        String cipherText = encryptMessage(msg.ToCharArray());
        Console.WriteLine("Encrypted Message is : " + cipherText);
  
        // Calling Decryption function
        Console.WriteLine("Decrypted Message is: " + decryptCipher(cipherText));
  
    }
}
  
/* This code contributed by PrinciRaj1992 */

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Output:

Encrypted Message is : UBBAHK CAPJKX
Decrypted Message is : AFFINE CIPHER

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