JAVA RSA decrypt file with private key using bouncy castle Crypto APIs

The following sample code decrypts a file using RSA private key. You can pass the private key file name, hex encoded data file name and file name to get the decrypted data. This program is very handy when it comes to decrypting xml file or text file. Before you can run the program you need to install the following jars in your classpath. Learn RSA Encryption and Decryption Basics

TIP: Get jars based on your java installation

• Apache log4j  
• http://logging.apache.org/log4j/1.2/download.html
• log4j-1.2.16
• Bouncy castle Crypto APIs  
• http://www.bouncycastle.org/latest_releases.html
• bcprov-jdk15-146

package com.as400samplecode;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.security.Security;

import org.bouncycastle.crypto.AsymmetricBlockCipher;
import org.bouncycastle.crypto.engines.RSAEngine;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.crypto.util.PrivateKeyFactory;

import sun.misc.BASE64Decoder;

public class RSADecryptFile {

    public static void main(String[] args)
    {

        String privateKeyFilename = null;
        String encryptedFilename = null;
        String outputFilename = null;

        RSADecryptFile rsaDecryptFile = new RSADecryptFile();

        if (args.length < 3)
        {
            System.err.println("Usage: java "+ rsaDecryptFile.getClass().getName()+
            " Private_Key_Filename Encrypted_Filename Output_Filename");
            System.exit(1);
        }

        privateKeyFilename = args[0].trim();
        encryptedFilename = args[1].trim();
        outputFilename = args[2].trim();
        rsaDecryptFile.decrypt(privateKeyFilename, encryptedFilename, outputFilename);

    }

    private void decrypt (String privateKeyFilename, String encryptedFilename, String outputFilename){

        try {

            Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());

            String value = "";
            String key = readFileAsString(privateKeyFilename);
            BASE64Decoder b64 = new BASE64Decoder();
            AsymmetricKeyParameter privateKey = 
                (AsymmetricKeyParameter) PrivateKeyFactory.createKey(b64.decodeBuffer(key));
            AsymmetricBlockCipher e = new RSAEngine();
            e = new org.bouncycastle.crypto.encodings.PKCS1Encoding(e);
            e.init(false, privateKey);

            String inputdata = readFileAsString(encryptedFilename);
            byte[] messageBytes = hexStringToByteArray(inputdata);

            int i = 0;
            int len = e.getInputBlockSize();
            while (i < messageBytes.length)
            {
                if (i + len > messageBytes.length)
                    len = messageBytes.length - i;

                byte[] hexEncodedCipher = e.processBlock(messageBytes, i, len);
                value = value + new String(hexEncodedCipher);
                i += e.getInputBlockSize();
            }


            System.out.println(value);

            BufferedWriter out = new BufferedWriter(new FileWriter(outputFilename));
            out.write(value);
            out.close();


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

    public static String getHexString(byte[] b) throws Exception {
        String result = "";
        for (int i=0; i < b.length; i++) {
            result +=
                Integer.toString( ( b[i] & 0xff ) + 0x100, 16).substring( 1 );
        }
        return result;
    }

    public static byte[] hexStringToByteArray(String s) {
        int len = s.length();
        byte[] data = new byte[len / 2];
        for (int i = 0; i < len; i += 2) {
            data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                    + Character.digit(s.charAt(i+1), 16));
        }
        return data;
    }

    private static String readFileAsString(String filePath)
    throws java.io.IOException{
        StringBuffer fileData = new StringBuffer(1000);
        BufferedReader reader = new BufferedReader(
                new FileReader(filePath));
        char[] buf = new char[1024];
        int numRead=0;
        while((numRead=reader.read(buf)) != -1){
            String readData = String.valueOf(buf, 0, numRead);
            fileData.append(readData);
            buf = new char[1024];
        }
        reader.close();
        System.out.println(fileData.toString());
        return fileData.toString();
    }

}