Add Affine Cipher #hacktoberfest (#2587)

Ciphers/affineCipher.java

@@ -0,0 +1,83 @@
+package Ciphers;
+class affineCipher
+{
+ 
+    // 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 character
+            {
+                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));
+ 
+    }
+}
+