RSA.java

import java.io.DataInputStream; 
import java.io.IOException; 
import java.math.BigInteger; 
import java.util.Random; 
public class RSA 
{ 
    private BigInteger d, e, p, q, N, phi; 
    private int bitlength = 1024; 
    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())); 
         
        byte[] encrypted = rsa.encrypt(teststring.getBytes()); 
      
        byte[] decrypted = rsa.decrypt(encrypted); 
        System.out.println("Decrypting 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(); 
    } 
}