RSA code in Java

      To encrypt and decrypt a string using RSA algorithm

The following code helps the user to provide a string for encryption and decryption using RSA algorithm with the help of Big Integer in java.

Source code for RSA.java:

import java.math.BigInteger; 
import java.util.Random;
import java.io.*;
 
 
public class RSA { 
     
    private BigInteger p; 
    private BigInteger q; 
    private BigInteger N; 
    private BigInteger phi; 
    private BigInteger e; 
    private BigInteger d; 
    private int bitlength = 1024; 
    private int blocksize = 256; //blocksize in byte 
     
    private Random r; 
     public RSA() { 
        r = new Random(); 
        p = BigInteger.probablePrime(bitlength, r); 
        q = BigInteger.probablePrime(bitlength, r); 
          N = p.multiply(q); 
           
        phi = p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE)); 
          e = BigInteger.probablePrime(bitlength/2, r); 
         
        while (phi.gcd(e).compareTo(BigInteger.ONE) > 0 && e.compareTo(phi) < 0 ) { 
            e.add(BigInteger.ONE); 
        } 
 d = e.modInverse(phi);  
    } 
     
    public RSA(BigInteger e, BigInteger d, BigInteger N) { 
        this.e = e; 
        this.d = d; 
        this.N = N; 
    } 
     
    public static void main (String[] args) throws IOException
{ 
        RSA rsa = new RSA(); 
           DataInputStream in=new DataInputStream(System.in);  
        String teststring ;
         System.out.println("Enter the plain text:");
        teststring=in.readLine();
        System.out.println("Encrypting String: " + teststring); 
        System.out.println("String in Bytes: " + bytesToString(teststring.getBytes())); 
 
        // encrypt 
        byte[] encrypted = rsa.encrypt(teststring.getBytes());                   
        System.out.println("Encrypted String in Bytes: " + bytesToString(encrypted)); 
         
        // decrypt 
        byte[] decrypted = rsa.decrypt(encrypted);       
        System.out.println("Decrypted String in Bytes: " +  bytesToString(decrypted)); 
         
        System.out.println("Decrypted String: " + new String(decrypted)); 
         
    } 
 
   private static String bytesToString(byte[] encrypted) { 
        String test = ""; 
        for (byte b : encrypted) { 
            test += Byte.toString(b); 
        } 
        return test; 
    } 
     
     public byte[] encrypt(byte[] message) {      
        return (new BigInteger(message)).modPow(e, N).toByteArray(); 
    } 
       
    public byte[] decrypt(byte[] message) { 
        return (new BigInteger(message)).modPow(d, N).toByteArray(); 
    } 
     
}

OUTPUT: