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

The following sample code decrypts a String data using RSA private key. You can pass the private key file name and the hex encoded String data to decrypt as input parameters and the program generates the  decrypted string. Before you can run the program you need to install the following jars in your classpath. Learn RSA Encryption and Decryption Basics

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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 RSADecryption {
 
    public static void main(String[] args)
    {
 
        String privateKeyFilename = null;
        String encryptedData = null;
        
        RSADecryption rsaDecryption = new RSADecryption();
 
        if (args.length < 2)
        {
            System.err.println("Usage: java "+ rsaDecryption.getClass().getName()+
            " Private_Key_Filename Encrypted_String_Data");
            System.exit(1);
        }
 
        privateKeyFilename = args[0].trim();
        encryptedData = args[1].trim();
        rsaDecryption.decrypt(privateKeyFilename, encryptedData);
 
    }
 
    private void decrypt (String privateKeyFilename, String encryptedFilename, String outputFilename){
 
        try {
 
            Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
 
            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);
            byte[] hexEncodedCipher = e.processBlock(messageBytes, 0, messageBytes.length);
 
            System.out.println(new String(hexEncodedCipher));
            BufferedWriter out = new BufferedWriter(new FileWriter(outputFilename));
            out.write(new String(hexEncodedCipher));
            out.close();
 
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
    
    private String decrypt (String privateKeyFilename, String encryptedData) {
 
        String outputData = null;
        try {
 
            Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
 
            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);
 
            byte[] messageBytes = hexStringToByteArray(encryptedData);
            byte[] hexEncodedCipher = e.processBlock(messageBytes, 0, messageBytes.length);
 
            System.out.println(new String(hexEncodedCipher));
            outputData = new String(hexEncodedCipher);
 
        }
        catch (Exception e) {
            System.out.println(e);
        }
        
        return outputData;
    }
 
    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();
    }
 
}
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