# Simple RSA public key encryption algorithm implementation. : RSA « Security « Java

Simple RSA public key encryption algorithm implementation.

```
//package atnf.atoms.mon.util;

import java.math.BigInteger;
import java.security.SecureRandom;

/**
* Simple RSA public key encryption algorithm implementation.
* <P>
* Taken from "Paj's" website:
* <TT>http://pajhome.org.uk/crypt/rsa/implementation.html</TT>
* <P>
*/
public class RSA {
private BigInteger n, d, e;

private int bitlen = 1024;

/** Create an instance that can encrypt using someone elses public key. */
public RSA(BigInteger newn, BigInteger newe) {
n = newn;
e = newe;
}

/** Create an instance that can both encrypt and decrypt. */
public RSA(int bits) {
bitlen = bits;
SecureRandom r = new SecureRandom();
BigInteger p = new BigInteger(bitlen / 2, 100, r);
BigInteger q = new BigInteger(bitlen / 2, 100, r);
n = p.multiply(q);
BigInteger m = (p.subtract(BigInteger.ONE)).multiply(q
.subtract(BigInteger.ONE));
e = new BigInteger("3");
while (m.gcd(e).intValue() > 1) {
}
d = e.modInverse(m);
}

/** Encrypt the given plaintext message. */
public synchronized String encrypt(String message) {
return (new BigInteger(message.getBytes())).modPow(e, n).toString();
}

/** Encrypt the given plaintext message. */
public synchronized BigInteger encrypt(BigInteger message) {
return message.modPow(e, n);
}

/** Decrypt the given ciphertext message. */
public synchronized String decrypt(String message) {
return new String((new BigInteger(message)).modPow(d, n).toByteArray());
}

/** Decrypt the given ciphertext message. */
public synchronized BigInteger decrypt(BigInteger message) {
return message.modPow(d, n);
}

/** Generate a new public and private key set. */
public synchronized void generateKeys() {
SecureRandom r = new SecureRandom();
BigInteger p = new BigInteger(bitlen / 2, 100, r);
BigInteger q = new BigInteger(bitlen / 2, 100, r);
n = p.multiply(q);
BigInteger m = (p.subtract(BigInteger.ONE)).multiply(q
.subtract(BigInteger.ONE));
e = new BigInteger("3");
while (m.gcd(e).intValue() > 1) {
}
d = e.modInverse(m);
}

/** Return the modulus. */
public synchronized BigInteger getN() {
return n;
}

/** Return the public key. */
public synchronized BigInteger getE() {
return e;
}

/** Trivial test program. */
public static void main(String[] args) {
RSA rsa = new RSA(1024);

String text1 = "Yellow and Black Border Collies";
System.out.println("Plaintext: " + text1);
BigInteger plaintext = new BigInteger(text1.getBytes());

BigInteger ciphertext = rsa.encrypt(plaintext);
System.out.println("Ciphertext: " + ciphertext);
plaintext = rsa.decrypt(ciphertext);

String text2 = new String(plaintext.toByteArray());
System.out.println("Plaintext: " + text2);
}
}

```