PlayFair.java

import java.util.*;
import java.lang.*;
import java.math.*;

public class AutokeyEncrypt {

    private static String alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    public static void main(String[] args){
                    String text = "FOLLOWDIRECTION";
                    String key = "P";  //15 - P

                    if(key.matches("[-+]?\\d*\\.?\\d+"))
                        key = ""+alpha.charAt(Integer.parseInt(key));

                    String enc = AutoEncryption(text,key);
                    System.out.println("Plaintext : "+text);
                    System.out.println("Encrypted : "+enc);
                    System.out.println("Decrypted : "+AutoDecryption(enc,key));

                }

                public static String AutoEncryption(String text,String key){
                    int len = text.length();

                    String subkey = key + text;
                    subkey = subkey.substring(0,subkey.length()-key.length());

                    String sb = "";
                    for(int x=0;x<len;x++){
                        int get1 = alpha.indexOf(text.charAt(x));
                        int get2 = alpha.indexOf(subkey.charAt(x));

                        int total = (get1 + get2)%26;

                        sb += alpha.charAt(total);
                    }

                    return sb;
                }

                public static String AutoDecryption(String text,String key){
                    int len = text.length();

                    String current = key;
                    String sb ="";

                    for(int x=0;x<len;x++){
                        int get1 = alpha.indexOf(text.charAt(x));
                        int get2 = alpha.indexOf(current.charAt(x));

                        int total = (get1 - get2)%26;
                        total = (total<0)? total + 26 : total;
                        sb += alpha.charAt(total);

                        current += alpha.charAt(total);
                    }
                    return sb;
                }

        }