# Implementation of RSA Algorithm(Encryption and Decryption) in Java

Levels of difficulty: / perform operation:

Java program to encrypt and decrypt a given message using RSA algorithm. Open Command Prompt and compile & Run. RSA algorithm is used to changing message that no one can understand the communication between sender and receiver. Sender and Receiver have public and private key and they can only understand message.

### JAVA Program

```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 ) {
}
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:");
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;
}
//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();
}
}
```

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