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How to encrypt and decrypt data using RSA in java?

Description

RSA.java generates the assysmetric key pair (public key and private key) using RSA algorithm. Encryption and decryption method is written based on RSA algorithm. Message to encrypt can be given as input. Encrypted and decrypted text is displayed in message dialog.

Sample Code
• Filename: RSA.java

import java.math.BigInteger ;
import java.util.Random ;
import java.io.* ;
import java.io.*;
import java.util.*;
import java.sql.*;
import javax.swing.*;

public class RSA
{
/**
* Bit length of each prime number.
*/
int primeSize ;

/**
* Two distinct large prime numbers p and q.
*/
BigInteger p, q ;

/**
* Modulus N.
*/
BigInteger N ;

/**
* r = ( p – 1 ) * ( q – 1 )
*/
BigInteger r ;

/**
* Public exponent E and Private exponent D
*/
BigInteger E, D ;

String nt,dt,et;
/**
* Constructor.
*
* @param primeSize Bit length of each prime number.
*/

String publicKey;
String privateKey;
String randomNumber;

BigInteger[] ciphertext;
int m[] = new int[1000];
String st[] = new String[1000];
String str = "";
String sarray1[] = new String[100000];

StringBuffer sb1 = new StringBuffer();
String inputMessage,encryptedData,decryptedMessage;

public RSA( int primeSize )
{

this.primeSize = primeSize ;

// Generate two distinct large prime numbers p and q.

// Generate Public and Private Keys.
generatePublicPrivateKeys() ;

BigInteger publicKeyB = getE();
BigInteger privateKeyB = getD();
BigInteger randomNumberB = getN();
publicKey = publicKeyB.toString();
privateKey = privateKeyB.toString();
randomNumber = randomNumberB.toString();
System.out.println("Public Key (E,N): "+publicKey+","+randomNumber);
System.out.println("Private Key (D,N): "+privateKey+","+randomNumber);

//Encrypt data
inputMessage=JOptionPane.showInputDialog(null,"Enter message to encrypt");
encryptedData=RSAencrypt(inputMessage);
System.out.println("Encrypted message"+encryptedData);
JOptionPane.showMessageDialog(null,"Encrypted Data "+"\n"+encryptedData);

//Decrypt data
JOptionPane.showMessageDialog(null,"Decrypted Data "+"\n"+decryptedMessage);

}

/**
* Generate two distinct large prime numbers p and q.
*/
{
p = new BigInteger( primeSize, 10, new Random() ) ;

do
{
q = new BigInteger( primeSize, 10, new Random() ) ;
}
while( q.compareTo( p ) == 0 ) ;
}

/**
* Generate Public and Private Keys.
*/
public void generatePublicPrivateKeys()
{
// N = p * q
N = p.multiply( q ) ;

// r = ( p – 1 ) * ( q – 1 )
r = p.subtract( BigInteger.valueOf( 1 ) ) ;
r = r.multiply( q.subtract( BigInteger.valueOf( 1 ) ) ) ; //(p-1)(q-1)

// Choose E, coprime to and less than r
do
{
E = new BigInteger( 2 * primeSize, new Random() ) ;
}
while( ( E.compareTo( r ) != -1 ) || ( E.gcd( r ).compareTo( BigInteger.valueOf( 1 ) ) != 0 ) ) ;

// Compute D, the inverse of E mod r
D = E.modInverse( r ) ;

}

/**
* Get prime number p.
*
* @return Prime number p.
*/
public BigInteger getp()
{
return( p ) ;
}

/**
* Get prime number q.
*
* @return Prime number q.
*/
public BigInteger getq()
{
return( q ) ;
}

/**
* Get r.
*
* @return r.
*/
public BigInteger getr()
{
return( r ) ;
}

/**
* Get modulus N.
*
* @return Modulus N.
*/
public BigInteger getN()
{
return( N ) ;
}

/**
* Get Public exponent E.
*
* @return Public exponent E.
*/
public BigInteger getE()
{
return( E ) ;
}

/**
* Get Private exponent D.
*
* @return Private exponent D.
*/
public BigInteger getD()
{
return( D ) ;
}

/** Encryption */

public String RSAencrypt(String info) {

E = new BigInteger(publicKey);
N = new BigInteger(randomNumber);
try {
ciphertext = encrypt( info ) ;
for( int i = 0 ; i < ciphertext.length ; i++ )
{
m[i] = ciphertext[i].intValue();
st[i] = String.valueOf(m[i]);
sb1.append(st[i]);
sb1.append(" ");
str = sb1.toString();

}
}
catch (Exception e) {
System.out.println(e);
}

return str;
}
public BigInteger[] encrypt( String message )
{
int i ;
byte[] temp = new byte[1] ;
byte[] digits = new byte[8];
try {
digits = message.getBytes() ;
String ds = new String(digits);

System.out.println("ds="+ds);

}
catch (Exception e) {
System.out.println(e);
}
BigInteger[] bigdigits = new BigInteger[digits.length] ;
for( i = 0 ; i < bigdigits.length ; i++ )
{
temp[0] = digits[i] ;
bigdigits[i] = new BigInteger( temp ) ;
}
BigInteger[] encrypted = new BigInteger[bigdigits.length] ;
for( i = 0 ; i < bigdigits.length ; i++ )
encrypted[i] = bigdigits[i].modPow( E, N ) ;

return( encrypted ) ;
}

/** Decrption */

D = new BigInteger(privateKey);
N = new BigInteger(randomNumber);

System.out.println("D = " + D);
System.out.println("N = " + N);

int k1= 0;
StringTokenizer st = new StringTokenizer(encryptedData);
while (st.hasMoreTokens()) {
sarray1[k1] = st.nextToken(" ");
k1++;
}

BigInteger[] ciphertext1 = new BigInteger[100000];

for( int i = 0 ; i ciphertext1[i] = new BigInteger(sarray1[i]);
}
String recoveredPlaintext = decrypt( ciphertext1,D,N,k1) ;
System.out.println(recoveredPlaintext);
return recoveredPlaintext;
}
public String decrypt( BigInteger[] encrypted,BigInteger D,BigInteger N,int size )
{
int i ;
String rs="";
BigInteger[] decrypted = new BigInteger[size] ;
for( i = 0 ; i < decrypted.length ; i++ ) {
decrypted[i] = encrypted[i].modPow( D, N ) ;
}
char[] charArray = new char[decrypted.length] ;
byte[] byteArray = new byte[decrypted.length] ;
for( i = 0 ; i < charArray.length ; i++ ) {
charArray[i] = (char) ( decrypted[i].intValue() ) ;
Integer iv = new Integer(0);
iv=decrypted[i].intValue() ;
byteArray[i] = iv.byteValue();
}
try {
rs=new String( byteArray );
}
catch (Exception e) {
System.out.println(e);
}
return(rs) ;
}

/**
* KeyGeneration Main program for Unit Testing.
*/
public static void main( String[] args ) throws IOException
{
RSA akg = new RSA(8);

}

}