如何使用密钥对字符串进行异或运算并存储在文件中以供以后解密和使用？

Question

I have researched a lot on this question and finally understood this, that encrypting and decryption using just XOR is not a suggested way of encryption but as it is an academic question I have to do it.

In my scenario I have to save java objects(space delimited strings with xor encryption by a key of our choice) in a file(.txt file) line by line and retrieve them later on if needed. So saving a xor string in a file is not a problem but while decrpytion its getting truncated. Below is a sample similar code which I have implemented for my project.

public class TestingXor {
    private static String XorString(String input){
        String key = "computing";
        Charset charSet = Charset.forName("UTF-8");
        byte[] inputBytes = input.getBytes(charSet);
        byte[] keyBytes = key.getBytes(charSet);
        for(int i=0;i<inputBytes.length;i++){
            inputBytes[i] = (byte)(inputBytes[i] ^ keyBytes[i%keyBytes.length]);
        }
        return (new String(inputBytes,charSet));
    }


    public static void main(String[] args) {
        // TODO Auto-generated method stub
        try {
            File testFile = new     File("addressBook.txt");
            Scanner scanObj = new Scanner(testFile);
            String testString = "asd asdasd 07/08/2015 asdasdasd 198546125";
            BufferedWriter writer = new BufferedWriter(new FileWriter(testFile));
            writer.write(XorString(testString));
            writer.close();
            System.out.println(XorString(scanObj.nextLine()));          
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

The above code returns "asd as" where as I need the whole string .If I am not saving the file I am able to encrypt and decrypt properly, but If I am saving the string to a file and retrieving the string from the file and decrypting it does'nt work. I think it is related to saving of the string(\\0 getting saved in the middle of the line) but what is the work around this. I dont know the length of the string when I am reading from a file. If this question is already answered you can redirect me to that question.

Answer 1

The problem is that you XorString method can return String that contain newlines or other other line separator characters (basically \\r\\n\
\
\… and the end of the file. Bad luck for you, your test generated multiple '\\r' which makes the Scanner stop looking for the rest of the string.

One way to get away from this is to use some kind of escaping mechanisms. For example, Properties provide such a mechanism, but feel free to implement your own.

Small example:

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.charset.Charset;
import java.util.Properties;

public class TestingXor {
    private static String xorString(String input) {
        String key = "computing";
        Charset charSet = Charset.forName("UTF-8");
        byte[] inputBytes = input.getBytes(charSet);
        byte[] keyBytes = key.getBytes(charSet);
        for (int i = 0; i < inputBytes.length; i++) {
            inputBytes[i] = (byte) (inputBytes[i] ^ keyBytes[i % keyBytes.length]);
        }
        return new String(inputBytes, charSet);
    }

    public static void main(String[] args) throws IOException {
        File testFile = new File("addressBook.txt");
        store(testFile);
        load(testFile);
    }

    private static void load(File file) throws IOException, FileNotFoundException {
        Properties props = new Properties();
        try (InputStream in = new BufferedInputStream(new FileInputStream(file));) {
            props.load(in);
        }
        for (int i = 1; i < props.size() + 1; i++) {
            System.out.println(xorString(props.getProperty("entry-" + i)));
        }
    }

    private static void store(File file) throws IOException, FileNotFoundException {
        Properties props = new Properties();
        String testString = "asd asdasd 07/08/2015 asdasdasd 198546125";
        props.put("entry-1", xorString(testString));
        try (OutputStream out = new BufferedOutputStream(new FileOutputStream(file));) {
            props.store(out, "Some comment");
        }
    }

}