javax.crypto.BadPaddingException: Given final block not properly padded while decryption

Question

I am trying to decrypt a file in java. The first 16 bytes of decrypted file are IV (initialization vector). Please help in resolving the above exception.

I am trying to prepend the IV in the output file in AESFileEncryption() and then reading it while decryption.

Thank You.

public class AESFileEncryption {
public static void encrypt(String path,String pwd) throws Exception {

    FileOutputStream outFile;

    try ( 
            FileInputStream inFile = new FileInputStream(path)) {

        String fileName=path;

        System.out.println(path);

        outFile = new FileOutputStream(fileName+".aes");
        // password to encrypt the file
        String password = pwd;
        byte[] salt = {
        (byte)0xc7, (byte)0x73, (byte)0x21, (byte)0x8c,
        (byte)0x7e, (byte)0xc8, (byte)0xee, (byte)0x99
       };

        SecretKeyFactory factory = SecretKeyFactory
                .getInstance("PBKDF2WithHmacSHA1");
        KeySpec keySpec = new PBEKeySpec(password.toCharArray(),salt,65536,128);// user-chosen password that can be used with password-based encryption (PBE).
        SecretKey secretKey = factory.generateSecret(keySpec);
        SecretKey secret = new SecretKeySpec(secretKey.getEncoded(), "AES");//Secret KeySpec is a class and implements inteface SecretKey

        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        SecureRandom random = new SecureRandom();
        byte bytes[] = new byte[16];
        random.nextBytes(bytes);
     IvParameterSpec ivspec = new IvParameterSpec(bytes);   
        cipher.init(Cipher.ENCRYPT_MODE, secret,ivspec);//opmode,key
        outFile.write(bytes);
        byte[] input = new byte[64];
        int bytesRead;
        while ((bytesRead = inFile.read(input)) != -1) {
            byte[] output = cipher.update(input, 0, bytesRead);
            if (output != null)
                Files.write(Paths.get(fileName+".aes"), output, StandardOpenOption.APPEND);

        }   byte[] output = cipher.doFinal();
        if (output != null)
            Files.write(Paths.get(fileName+".aes"), output, StandardOpenOption.APPEND);
    }
    outFile.flush();
    outFile.close();
    File f=new File(path);
    boolean x=f.delete();
    if(x){
        System.out.println("File deleted");
    }
    JOptionPane.showMessageDialog(null,"File Encrypted.");

}
}

Decryption code

public class AESFileDecryption {
public static void decrypt(String path,String pwd) throws Exception {

    String password = pwd;
    String fileName=path;
    File file=new File(path);
        //System.out.println(inFile.toString());
        String fileNameWithOutExt = path.replaceFirst("[.][^.]+$", "");
        System.out.println(fileName);
        System.out.println(fileNameWithOutExt);
        byte[] salt = {
        (byte)0xc7, (byte)0x73, (byte)0x21, (byte)0x8c,
        (byte)0x7e, (byte)0xc8, (byte)0xee, (byte)0x99
    };
    System.out.println("1");
    FileInputStream fis = new FileInputStream(path);
    SecretKeyFactory factory = SecretKeyFactory
            .getInstance("PBKDF2WithHmacSHA1");
    KeySpec keySpec = new PBEKeySpec(password.toCharArray(),salt,65536,128);
    SecretKey tmp = factory.generateSecret(keySpec);
    SecretKey secret = new SecretKeySpec(tmp.getEncoded(), "AES");
    System.out.println("2");
    // file decryption
    Cipher cipher=null;
    byte bytes[]=new byte[16];
    fis.read(bytes, 0, 16); 
    IvParameterSpec ivspec = new IvParameterSpec(bytes);

    cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
    cipher.init(Cipher.DECRYPT_MODE, secret, ivspec);
    System.out.println("3");
    FileOutputStream fos = new FileOutputStream(fileNameWithOutExt);
    System.out.println("4");
    byte[] in   = new byte[64];
    int read;
    while ((read = fis.read(in,16,(int)file.length()-16)) != -1) {
        byte[] output = cipher.update(in, 0, read);
        if (output != null)
            fos.write(output);
    }
    try{
    byte[] output = cipher.doFinal();
    if (output != null)
        fos.write(output);
    fis.close();
    fos.flush();
    fos.close();
    System.out.println("File Decrypted.");
}
catch(IOException | BadPaddingException | IllegalBlockSizeException e)
{
    System.out.println(e+"");
}
}
}

Answer 1

There are a few problems with the small example, but the one that is most immediately your problem is the line

while ((read = fis.read(in,16,(int)file.length()-16)) != -1) {

You seem to be confused about the meaning of the offset parameter to the read(). It is not the offset into the file, but rather the offset into the array (in) that is specified in the first parameter.

A non-exhaustive list of other problems that I see include:

• writing to the file using the two independent mechanisms (FileOutputStream.write() and Files.write()). This actually worked ok when I ran your program but it seems like it's asking for trouble. There's no reason to use Files.write() here.
• fis.read(bytes, 0, 16); does not check the return value.

It seems you are struggling with finding some IO idioms that you're comfortable with. Or perhaps just experimenting. At the risk of giving you even more options to juggle, you might consider investigating google's open source Guava library. Many people find it has just what they needed.