Add Affine Cipher (TheAlgorithms#2668)

Author: vatsal-dp

Ciphers/AffineCipher.java

@@ -0,0 +1,98 @@
+//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).
+
+package Ciphers;
+
+import java.util.Scanner;
+
+class AffineCipher
+{
+    static Scanner in = new Scanner(System.in);
+
+    static String encryptMessage(char[] msg)
+    {
+        System.out.println("Enter key value a for encryption : ");
+        int a = in.nextInt();
+        
+        System.out.println("Enter key value b for encryption : ");
+        int b = in.nextInt();
+        
+        /// Initially empty cipher String
+        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 // append space character
+            {
+                cipher += msg[i];
+            }
+        }
+        return cipher;
+    }
+
+    static String decryptCipher(String cipher)
+    {
+        System.out.println("Enter key value a for decryption : ");
+        int a = in.nextInt();
+        
+        System.out.println("Enter key value b for decryption : ");
+        int b = in.nextInt();
+        
+        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,
+            // if so, 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 // append space character
+            {
+                msg += cipher.charAt(i);
+            }
+        }
+
+        return msg;
+    }
+
+    // Main method
+    public static void main(String[] args)
+    {
+        String msg = "AFFINE CIPHER";
+
+        // Encrypting message
+        String cipherText = encryptMessage(msg.toCharArray());
+        System.out.println("Encrypted Message is : " + cipherText);
+
+        // Decrypting message
+        System.out.println("Decrypted Message is: " + decryptCipher(cipherText));
+
+    }
+}