Question

Write a java program to implement the RSA public-key cryptosystem.

Answer 1

Answer:

package com.sanfoundry.setandstring;

import java.io.DataInputStream;

import java.io.IOException;

import java.math.BigInteger;

import java.util.Random;

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 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;

}

@SuppressWarnings("deprecation")

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());

//decrypt

byte[ ] decrypted = rsa.decrypt(encrypted);

  System.out.println(" Decrypting Bytes: " + bytesToString(decrypted));

  System.out.println(" Decrypting String: " + new String(decrypted));

}

private static String bytesToString(byte[ ] encrypted)

{

String test = " ";

for (byte b : encrypted)

{

test += Byte.toString(b);

}

return test;

}

//Encrypt message

public byte[ ] encrypt(byte[ ] message)

{

return (new BigInteger(message)).modPow(e, N).toByteArray();

}

//Decrypt message

public byte[ ] decrypt(byte[ ] message)

{

return (new BigInteger(message)).modPow(d, N).toByteArray();

}

}

OUTPUT:

$ javac RSA.java

$ java RSA

Enter the plain text:

Sanfoundry

Encrypting String: Sanfoundry

String in Bytes: 8397110102111117110100114121

Decrypting Bytes: 8397110102111117110100114121

Decrypted String: Sanfoundry