Playfair Cipher
Introduction#
The best-known multiple-letter encryption cipher is the Playfair, which treats digrams in the plaintext as single units and translates these units into ciphertext digrams The Playfair algorithm is based on the use of a 5 x 5 matrix of letters constructed using a keyword.
Example of Playfair Cipher Encryption along with Encryption and Decryption Rule
A keyword is “MONARCHY” then the matrix will look like
The matrix is constructed by filling in the letters of the keyword (minus duplicates) from left to right and from top to bottom, and then filling in the remainder of the matrix with the remaining letters in alphabetic order. Plaintext is encrypted two letters at a time, according to the following rules:
- Take the characters in the text(plain/cipher) and make a group of two characters. If the numbers of characters in the text is odd then add a filler letter (usually we use ‘x’).
Text=“HELLO” then it would be group as
HE | LL | OX
- Two plaintext letters that fall in the same row of the matrix are each replaced by the letter to
the right, with the first element of the row circularly following the last. For example, ar is encrypted as RM. 3. Two plaintext letters that fall in the same column are each replaced by the letter beneath, with the top element of the column circularly following the last. For example, mu is encrypted as CM. 4. Otherwise, each plaintext letter in a pair is replaced by the letter that lies in its own row and the column occupied by the other plaintext letter. Thus, hs becomes BP and ea becomes IM (or JM, as the encipherer wishes).
A Simple Java Program that implements the Playfair Cipher is given below:
import java.util.Scanner;
public class Playfair {
public static void main(String[] args) {
Scanner in=new Scanner(System.in);
System.out.print("Enter keyword: ");
String key=in.nextLine();
System.out.print("Enter message to encrypt: ");
String msg=in.nextLine();
PFEncryption pfEncryption=new PFEncryption();
pfEncryption.makeArray(key);
msg=pfEncryption.manageMessage(msg);
pfEncryption.doPlayFair(msg, "Encrypt");
String en=pfEncryption.getEncrypted();
System.out.println("Encrypting....\n\nThe encrypted text is: " + en);
System.out.println("=======================================");
pfEncryption.doPlayFair(en, "Decrypt");
System.out.print("\nDecrypting....\n\nThe encrypted text is: " + pfEncryption.getDecrypted());
}
}
class PFEncryption{
private char [][] alphabets= new char[5][5];
private char[] uniqueChar= new char[26];
private String ch="ABCDEFGHIKLMNOPQRSTUVWXYZ";
private String encrypted="";
private String decrypted="";
void makeArray(String keyword){
keyword=keyword.toUpperCase().replace("J","I");
boolean present, terminate=false;
int val=0;
int uniqueLen;
for (int i=0; i<keyword.length(); i++){
present=false;
uniqueLen=0;
if (keyword.charAt(i)!= ' '){
for (int k=0; k<uniqueChar.length; k++){
if (Character.toString(uniqueChar[k])==null){
break;
}
uniqueLen++;
}
for (int j=0; j<uniqueChar.length; j++){
if (keyword.charAt(i)==uniqueChar[j]){
present=true;
}
}
if (!present){
uniqueChar[val]=keyword.charAt(i);
val++;
}
}
ch=ch.replaceAll(Character.toString(keyword.charAt(i)), "");
}
for (int i=0; i<ch.length(); i++){
uniqueChar[val]=ch.charAt(i);
val++;
}
val=0;
for (int i=0; i<5; i++){
for (int j=0; j<5; j++){
alphabets[i][j]=uniqueChar[val];
val++;
System.out.print(alphabets[i][j] + "\t");
}
System.out.println();
}
}
String manageMessage(String msg){
int val=0;
int len=msg.length()-2;
String newTxt="";
String intermediate="";
while (len>=0){
intermediate=msg.substring(val, val+2);
if (intermediate.charAt(0)==intermediate.charAt(1)){
newTxt=intermediate.charAt(0) + "x" + intermediate.charAt(1);
msg=msg.replaceFirst(intermediate, newTxt);
len++;
}
len-=2;
val+=2;
}
if (msg.length()%2!=0){
msg=msg+'x';
}
return msg.toUpperCase().replaceAll("J","I").replaceAll(" ","");
}
void doPlayFair(String msg, String tag){
int val=0;
while (val<msg.length()){
searchAndEncryptOrDecrypt(msg.substring(val, val + 2), tag);
val+=2;
}
}
void searchAndEncryptOrDecrypt(String doubblyCh, String tag){
char ch1=doubblyCh.charAt(0);
char ch2=doubblyCh.charAt(1);
int row1=0, col1=0, row2=0, col2=0;
for (int i=0; i<5; i++){
for (int j=0; j<5; j++){
if (alphabets[i][j]==ch1){
row1=i;
col1=j;
}else if (alphabets[i][j]==ch2){
row2=i;
col2=j;
}
}
}
if (tag=="Encrypt")
encrypt(row1, col1, row2, col2);
else if(tag=="Decrypt")
decrypt(row1, col1, row2, col2);
}
void encrypt(int row1, int col1, int row2, int col2){
if (row1==row2){
col1=col1+1;
col2=col2+1;
if (col1>4)
col1=0;
if (col2>4)
col2=0;
encrypted+=(Character.toString(alphabets[row1][col1])+Character.toString(alphabets[row1][col2]));
}else if(col1==col2){
row1=row1+1;
row2=row2+1;
if (row1>4)
row1=0;
if (row2>4)
row2=0;
encrypted+=(Character.toString(alphabets[row1][col1])+Character.toString(alphabets[row2][col1]));
}else{
encrypted+=(Character.toString(alphabets[row1][col2])+Character.toString(alphabets[row2][col1]));
}
}
void decrypt(int row1, int col1, int row2, int col2){
if (row1==row2){
col1=col1-1;
col2=col2-1;
if (col1<0)
col1=4;
if (col2<0)
col2=4;
decrypted+=(Character.toString(alphabets[row1][col1])+Character.toString(alphabets[row1][col2]));
}else if(col1==col2){
row1=row1-1;
row2=row2-1;
if (row1<0)
row1=4;
if (row2<0)
row2=4;
decrypted+=(Character.toString(alphabets[row1][col1])+Character.toString(alphabets[row2][col1]));
}else{
decrypted+=(Character.toString(alphabets[row1][col2])+Character.toString(alphabets[row2][col1]));
}
}
String getEncrypted(){
return encrypted;
}
String getDecrypted(){
return decrypted;
}
}