JAVA RSA encrypt string with public key using bouncy castle Crypto APIs

The following sample code encrypts a String data using RSA public key. You can pass the public key file name and the String data to encrypt as input parameters and the program generates hex encoded encrypted 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.PublicKeyFactory;
 
import sun.misc.BASE64Decoder;
 
public class RSAEncryption {
 
    public static void main(String[] args)
    {
 
        String publicKeyFilename = null;
        String inputData = null;
        
        RSAEncryption rsaEncryption = new RSAEncryption();
 
        if (args.length < 2)
        {
            System.err.println("Usage: java "+ rsaEncryption.getClass().getName()+
            " Public_Key_Filename Input_String_data");
            System.exit(1);
        }
 
        publicKeyFilename = args[0].trim();
        inputData = args[1].trim();
        rsaEncryption.encrypt(publicKeyFilename, inputData);
 
    }
 
    private void encrypt (String publicKeyFilename, String inputFilename, String encryptedFilename){
 
        try {
 
            Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
 
            String key = readFileAsString(publicKeyFilename);
            BASE64Decoder b64 = new BASE64Decoder();
            AsymmetricKeyParameter publicKey =
                (AsymmetricKeyParameter) PublicKeyFactory.createKey(b64.decodeBuffer(key));
            AsymmetricBlockCipher e = new RSAEngine();
            e = new org.bouncycastle.crypto.encodings.PKCS1Encoding(e);
            e.init(true, publicKey);
 
            String inputdata = readFileAsString(inputFilename);
            byte[] messageBytes = inputdata.getBytes();
            byte[] hexEncodedCipher = e.processBlock(messageBytes, 0, messageBytes.length);
 
            System.out.println(getHexString(hexEncodedCipher));
            BufferedWriter out = new BufferedWriter(new FileWriter(encryptedFilename));
            out.write(getHexString(hexEncodedCipher));
            out.close();
 
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
    
    private String encrypt (String publicKeyFilename, String inputData){
 
        String encryptedData = null;
        try {
 
            Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
 
            BASE64Decoder b64 = new BASE64Decoder();
            String key = readFileAsString(publicKeyFilename);
            AsymmetricKeyParameter publicKey =
                (AsymmetricKeyParameter) PublicKeyFactory.createKey(b64.decodeBuffer(key));
            AsymmetricBlockCipher e = new RSAEngine();
            e = new org.bouncycastle.crypto.encodings.PKCS1Encoding(e);
            e.init(true, publicKey);
 
            byte[] messageBytes = inputData.getBytes();
            byte[] hexEncodedCipher = e.processBlock(messageBytes, 0, messageBytes.length);
 
            System.out.println(getHexString(hexEncodedCipher));
            encryptedData = getHexString(hexEncodedCipher);
    
        }
        catch (Exception e) {
            System.out.println(e);
        }
        
        return encryptedData;
    }
 
    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;
    }
 
    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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