RSA Encryption/Decryption example

import java.security.*;
import java.security.spec.*;
import javax.crypto.*;
import java.lang.*;
import java.io.*;
import java.math.*;

/*
 * Greatest thanks for this example go to the address mentioned below:
 */

// We're following http://www.javamex.com/tutorials/cryptography/asymmetric.shtml
// to show how to do RSA enc/dec
public class RSA {

    private static final String algorithm = "RSA";

    // 64 bytes
    private static final String msgChecksum = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";

    // Unlike in the original example, we want a TEXT file to actually SEE what it stores
    private static void saveToFile(String fileName, BigInteger mod, BigInteger exp) throws IOException {
        BufferedWriter out = new BufferedWriter(new FileWriter(fileName));
        out.write(mod.toString());
        out.newLine();
        out.write(exp.toString());
        out.newLine();
        out.close();
    }

    private static BigInteger[] readTwoNumbersFromFile(String fileName) throws IOException {
        BigInteger[] ret = new BigInteger[2];
        BufferedReader in = new BufferedReader(new FileReader(fileName));
        ret[0] = new BigInteger(in.readLine());
        ret[1] = new BigInteger(in.readLine());
        in.close();
        return ret;
    }

    public static void main(String[] args) throws Exception {
        if(args.length == 0) {
            System.out.println("Usage: $0 1|2|3");
            System.out.println("1 generate keys");
            System.out.println("2 encrypt msg-checksum with endor key");
            System.out.println("3 decrypt msg-checksum with gbz key");
            System.out.println("4 AnDenNutzer proposal");
            return;
        }
        switch(args[0].charAt(0)) {
            case '1': {
                // Generate keys
                KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
                kpg.initialize(2048);
                KeyPair kp = kpg.genKeyPair();
                Key publicKey = kp.getPublic();
                Key privateKey = kp.getPrivate();
                // Store keys to file
                KeyFactory fact = KeyFactory.getInstance("RSA");
                RSAPublicKeySpec pub = fact.getKeySpec(kp.getPublic(), RSAPublicKeySpec.class);
                RSAPrivateKeySpec priv = fact.getKeySpec(kp.getPrivate(), RSAPrivateKeySpec.class);
                saveToFile("public.key", pub.getModulus(), pub.getPublicExponent());
                saveToFile("private.key", priv.getModulus(), priv.getPrivateExponent());
                // Actually, this is it!
                break;
            }
            case '2': {
                // Encrypt "msgChecksum" field with the "public.key" file (later this file shall NOT be distributed as it is THE important file... in the AnDenNutzer layout, endor actually plays the role of a client as he connects to gbz... or s04... that have a server (started with --client) running)
                // We'll later store our key files in setting-fields of our INI

                // Read data from public.key
                BigInteger[] numbers = readTwoNumbersFromFile("public.key");
                RSAPublicKeySpec keySpec = new RSAPublicKeySpec(numbers[0], numbers[1]);
                KeyFactory fact = KeyFactory.getInstance("RSA");
                PublicKey pubKey = fact.generatePublic(keySpec);
                // Actual encryption
                Cipher cipher = Cipher.getInstance("RSA");
                cipher.init(Cipher.ENCRYPT_MODE, pubKey);
                byte[] cipherData = cipher.doFinal(msgChecksum.getBytes());
                // Write to file
                BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream("msg_checksum_encrypted.bin"));
                out.write(cipherData);
                out.close();
                break;
            }
            case '3': {
                // Decrypt
                // modulus, exponent (reihenfolge!)
                BigInteger[] numbers = readTwoNumbersFromFile("private.key");
                RSAPrivateKeySpec keySpec = new RSAPrivateKeySpec(numbers[0], numbers[1]);
                KeyFactory fact = KeyFactory.getInstance("RSA");
                PrivateKey privateKey = fact.generatePrivate(keySpec);
                // Actual decryption
                Cipher cipher = Cipher.getInstance("RSA");
                cipher.init(Cipher.DECRYPT_MODE, privateKey);
                BufferedInputStream in = new BufferedInputStream(new FileInputStream("msg_checksum_encrypted.bin"));
                byte[] buffer = new byte[0x100];
                int len;
                while ((len = in.read(buffer)) > 0) {
                    cipher.update(buffer, 0, len);
                }
                System.out.println(new String(cipher.doFinal()));
                break;
            }
            case '4': {
                // AnDenNutzer Variante
                // TODO PROBLEM BEI DER GANZEN SACHE: pwd-exponent auf dem Server ist zu leicht zu erraten (weil Standardwert) -- wir müssen auf dem Server sichere Werte als auf dem Client haben, aber der Client ist derjenige, der entschlüsseln muss.
                System.out.println("Generation in progress... Please wait a while...");
                KeyPairGenerator kpg = KeyPairGenerator.getInstance(algorithm);
                kpg.initialize(0x800);
                KeyPair kp = kpg.genKeyPair();
                KeyFactory fact = KeyFactory.getInstance(algorithm);
                RSAPublicKeySpec pub = fact.getKeySpec(kp.getPublic(), RSAPublicKeySpec.class);
                RSAPrivateKeySpec priv = fact.getKeySpec(kp.getPrivate(), RSAPrivateKeySpec.class);
                System.out.println("done.");
                System.out.println();
                System.out.println("Configuration for your server (that you invoke via --client=CLIENT MSG)");
                System.out.println("pwd-modulus=");
                printIndentedBigInteger(pub.getModulus());
                System.out.println("pwd-exponent=");
                printIndentedBigInteger(pub.getPublicExponent());
                System.out.println();
                System.out.println("Configuration for your client (that you invoke via --client)");
                System.out.println("pwd-modulus=");
                printIndentedBigInteger(priv.getModulus());
                System.out.println("pwd-exponent=");
                printIndentedBigInteger(priv.getPrivateExponent());
                System.out.println();
                System.out.println("Remember that you will need a different ini on server and client!");
                System.out.println("The numbers are quite large... you may want to redirect them into a file.");
                System.out.println("You will also have to remove the newlines and spaces between the numbers.");
            }
            default: {
                System.out.println("Use no param for help.");
            }
        }
    }

    private static void printIndentedBigInteger(BigInteger integer) {
        char[] chrData = integer.toString().toCharArray();
        char[] buffer = new char[79];
        int pos = 0;
        for(int i = 0; i < chrData.length; i++) {
            if(pos == 0) {
                pos = 8;
                for(int j = 0; j < 8; j++) {
                    buffer[j] = ' ';
                }
            }
            buffer[pos++] = chrData[i];
            if(pos > 78) {
                System.out.println(new String(buffer));
                pos = 0;
            }
        }
        StringBuffer lastLine = new StringBuffer("");
        for(int i = 0; i <= pos; i++) {
            lastLine.append(buffer[i]);
        }
        System.out.println(lastLine);
    }

}