Question

I am writting a program where I take a string, encrypt it and then write it in a file. Then later, I read from the file the string, decrypt it and then modify it. Here’s my code for DES encryption/decryption:

/* class for crypting and decrypting a file */
class DESEncrypter
{
private Cipher encryptionCipher;
private Cipher decryptionCipher;

public DESEncrypter (SecretKey key) throws Exception
{
encryptionCipher = Cipher.getInstance("DES/ECB/PKCS5Padding");
encryptionCipher.init(Cipher.ENCRYPT_MODE, key);
decryptionCipher = Cipher.getInstance("DES/ECB/PKCS5Padding");
decryptionCipher.init(Cipher.DECRYPT_MODE, key);
}

/* write to 'out' the encryption of the information read from 'in' */
public String encrypt(String unencryptedString)
{
    String encryptedString = "";

    try {
        byte[] unencryptedByteArray = unencryptedString.getBytes("UTF8");

        byte[] encryptedBytes = this.encryptionCipher.doFinal(unencryptedByteArray);

        encryptedString = new sun.misc.BASE64Encoder().encode(encryptedBytes);

    } catch (Exception ex) {
        Logger.getLogger(DESEncrypter.class.getName()).log(Level.SEVERE, null, ex);
    }

    return encryptedString;
}

private static String bytes2String(byte[] bytes)
{

    StringBuffer stringBuffer = new StringBuffer();
    for (int i = 0; i < bytes.length; i++) 
    {
        stringBuffer.append((char) bytes[i]);
    }

    return stringBuffer.toString();
}

/* write to 'out' the information obtained by decrypting the information read from 'in' */
public String decrypt (String encryptedString) throws UnsupportedEncodingException
{
    byte[] unencryptedByteArray = new byte[4096];

    try {
        // Encode bytes to base64 to get a string
        byte[] decodedBytes = new sun.misc.BASE64Decoder().decodeBuffer(encryptedString);

       // Decrypt
       unencryptedByteArray = this.decryptionCipher.doFinal(decodedBytes);     
    } catch (Exception ex) {
        Logger.getLogger(DESEncrypter.class.getName()).log(Level.SEVERE, null, ex);
    }

    return bytes2String(unencryptedByteArray);
}
} 

And this is the function where I write a encrypted String in a file:

public void writeToFileEncrypted(String filename, String owner, String departament)
{
try 
    {
        BufferedReader br = new BufferedReader(new FileReader(new File("files_encrypted")));
        String crypt = "";
        String aux;
        while ((aux = br.readLine()) != null)
        {
            crypt += aux;
        }
        br.close();

        String info = this.server.crypt.decrypt(crypt);
        info += filename + " " + owner + " " + departament + "\n";

        /* delete the old encryption */
        File temp = new File("files_encrypted");
        temp.delete();

        String infoCrypt = this.server.crypt.encrypt(info); 

        File newFiles = new File("files_encrypted");
        if (newFiles.createNewFile() == false) 
        {
    log.severe("Failed to re-create the 'files_encrypted' file when trying to add a new file");
    return; 
        }

        BufferedWriter bw = new BufferedWriter(new FileWriter(newFiles));
        bw.write(infoCrypt);
        bw.close();
    }
    catch (Exception e)
    {
        log.warning("An exception was caught while trying to remove '" + clientName + "' from the banned list");
        e.printStackTrace();
        return;
}
}

While the server runs, I can make modification to that String from file(run that function many time). The problem is when I close the server and then I open it again because I get the error:
javax.crypto.BadPaddingException: Given final block not properly padded

This is how I read from file when the server opens:

BufferedReader br = new BufferedReader(new FileReader(new File("files_encrypted")));
String crypto = new String();
String aux;
while ((aux = br.readLine()) != null)
{
    crypto += aux;
    readBytes++;
}
br.close();
System.out.println(readBytes);
info = this.crypt.decrypt(crypto); 

Why do I get that error? What I’m doing wrong? I must write the encrypted String in file some other way?

LATER EDIT:

I’ve changed the function that read a String from a file, decrypt it, modify it , encrypt it and then write it in file.

public void writeToFileEncrypted(String filename, String owner, String departament)
{
try 
    {
        File f = new File("files_encrypted");
        int nrRead = 0;
        String info = null;
        FileInputStream fis = new FileInputStream(f);
        StringBuffer sb = new StringBuffer();
        int ch;
        while ((ch = fis.read()) != -1)
        {
            sb.append((char)ch);
            nrRead++;
        }
        fis.close();

        StringBuilder sba = null;
        if (nrRead != 0)
        {
            info = this.server.crypt.decrypt(new String(sb.toString().getBytes("UTF-8"), "UTF-8"));
            sba = new StringBuilder(info);
            sba.append(filename + " " + owner + " " + departament + " ");
        }
        else
        {
            sba = new StringBuilder(filename + " " + owner + " " + departament + " ");
        }

        /* delete the old encryption */
        File temp = new File("files_encrypted");
        temp.delete();
        //System.out.println("before: " + sba.toString());
        String infoCrypt = this.server.crypt.encrypt(sba.toString()); 
        //System.out.println("after: " + infoCrypt);
        File newFiles = new File("files_encrypted");
        if (newFiles.createNewFile() == false) 
        {
    log.severe("Failed to re-create the 'files_encrypted' file when trying to add a new file");
    return; 
        }

        FileOutputStream fos = new FileOutputStream(newFiles);
        fos.write(infoCrypt.getBytes("UTF-8"));
        fos.flush();
        fos.close();
    }
    catch (Exception e)
    {
        log.warning("An exception was caught while trying to remove '" + clientName + "' from the banned list");
        e.printStackTrace();
        return;
}
}

I’ve also modified where I read the info from file when server opens for the first time:

FileInputStream fis = new FileInputStream(f);
StringBuffer sb = new StringBuffer();
int ch;
while ((ch = fis.read()) != -1)
{
    sb.append((char)ch);
    readBytes++;
}

fis.close();
if (readBytes != 0)
{
    System.out.println("on: " + sb.toString());
    info = this.crypt.decrypt(new String(sb.toString().getBytes("UTF-8"), "UTF-8"));                
    System.out.println("load: " + info);
}
} 

At the System.out.println with “on: ” what I read from file is exactly what I’ve written encrypted, without any spaces or new lines. If I read with read(buffer), where buffer is byte[], it seems that adds a lot of spaces.

Although I’ve made all this modifications I still get the error javax.crypto.BadPaddingException: Given final block not properly padded

Has somebody any idea what’s going on here?

Answer 1

There are a few things here.

private static String bytes2String(byte[] bytes)

Is dodgy, you are casting a byte to a char in this method so there is no character encoding specified here. To convert bytes to characters you should just use the String constructor that takes an array of bytes and an encoding. e.g.

    byte[] tmp = new byte[10];      
    String a = new String(tmp, "UTF-8");

Be careful using BufferedReaders + .readLine() – this will strip out any newline characters from your file as you read it unless you add them back into your buffer. Although I don’t think this is your problem.

But I think the best way to simplify your code is to write the encoded bytes via an OutputStream directly to the file. Unless you need to send the contents of the file over a transport that doesn’t like binary data, there is no need to base64 encode. Just use Input/OutputStreams to write the encrypted bytes direct to disk.

RESPONSE TO LATER EDIT:

You are still mixing up your use of binary data (bytes) and character data (String/chars). You can’t do things like:

    int ch;
    while ((ch = fis.read()) != -1)
    {
        sb.append((char)ch);

The input stream is retuning bytes, a byte is not a character and just casting it to one is going to cause problems. When using encryption the output from the encryption operation is binary data, and the input to the decryption operation is also binary data. The fact that your are encrypting text is something you deal with before the encryption occurs, and after the decryption occurs. You basic operation should go along the following lines.

• Take the text you want to encrypt and convert it to bytes, specifying an encoding using the .getBytes(String charsetName) on your String.
• Pass these bytes into your encryption routine
• Write the resulting bytes directly to disk

To decrypt:

• Read the bytes from the file
• Pass the bytes to your decryption routine (as bytes! no Strings/ text involved)
• Take the out put bytes and re-construct you String using new String(byte[] bytes, String charsetName) specifying the same encoding as before.

You might find the following (untested, but should work) methods useful:

public byte[] readBinaryFile(File f) throws IOException
{       
    byte[] contents = new byte[(int)f.length()];
    BufferedInputStream bis = null;
    try
    {
        bis = new BufferedInputStream(new FileInputStream(f));
        DataInputStream dis = new DataInputStream(bis);
        dis.readFully(contents);
    }
    finally
    {
        if(bis != null)
        {
            bis.close();
        }
    }           
    return contents;            
}

public void writeBinaryFile(byte[] contents, File f) throws IOException
{
    BufferedOutputStream bos = null;
    try
    {
        bos = new BufferedOutputStream(new FileOutputStream(f));
        bos.write(contents);
    }
    finally
    {
        if(bos != null)
        {
            bos.close();
        }
    }           
}

So you will also need to change the interface, and internals of your encrypt and decrypt methods so they take and return byte arrays, and ditch the base64 encoding.