Java Programming

I need to implement use the RSA public key cryptosystem with BouncyCastle, but not too sure on how to do it. I found the example below but not sure on how to change it from random gen keys to custom strings as private and public keys...any help?

 /*Public Key cryptography using the RSA algorithm.
*/

import java.security.*;
import javax.crypto.*;

public class PublicKeyCrypto {

    public static void main (String[] args) throws Exception {
        //
        // Check args and get plaintext
        if (args.length !=1) {
            System.err.println("Usage: java PublicExample text");
            System.exit(1);
        }
        byte[] plainText = args[0].getBytes("UTF8");
        //
        // Generate an RSA key
        System.out.println( "\nStart generating RSA key" );
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
        keyGen.initialize(1024);
        KeyPair key = keyGen.generateKeyPair();
        System.out.println( "Finish generating RSA key" );
        //
        // Creates an RSA Cipher object (specifying the algorithm, mode, and padding).
        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        //
        // Print the provider information
        System.out.println( "\n" + cipher.getProvider().getInfo() );
        System.out.println( "\nStart encryption" );
        //
        // Initializes the Cipher object.
        cipher.init(Cipher.ENCRYPT_MODE, key.getPublic());      
        //
        // Encrypt the plaintext using the public key
        byte[] cipherText = cipher.doFinal(plainText);
        System.out.println( "Finish encryption: " );
        System.out.println( new String(cipherText, "UTF8") );
        System.out.println( "\nStart decryption" );
        //
        // Initializes the Cipher object.
        cipher.init(Cipher.DECRYPT_MODE, key.getPrivate());
        //
        // Decrypt the ciphertext using the private key
        byte[] newPlainText = cipher.doFinal(cipherText);
        System.out.println( "Finish decryption: " );
        System.out.println( new String(newPlainText, "UTF8") );
    }
}

Message was edited by:
luqman