How to add and get specific number of bytes from a file using file input and output streams?

Question

I am working on a class to encrypt/decrypt large files so I am trying to use the streams instead of byte arrays to avoid OutOfMemory exceptions. In the encryption method I add random salt and iv to the beginning of the encrypted file and it works fine, and here is the code:

public File encryptFile(File inputFile, File outPutFile, String password, ProgressBar progressBar, Label progressPercentage){
        //Create IV
        byte[] ivBytes = new byte[16];
        SecureRandom random1 = new SecureRandom();
        random1.nextBytes(ivBytes);
        IvParameterSpec iv = new IvParameterSpec(ivBytes);


        //Create the key with the salt
        SecureRandom random = new SecureRandom();
        byte[] salt = new byte[SALT_SIZE];
        random.nextBytes(salt);

        SecretKeySpec keySpec = generateAesKey(password, salt);

        //Create and Init the cipher
        Cipher c = Cipher.getInstance("AES/CBC/"+padding);
        c.init(Cipher.ENCRYPT_MODE, keySpec, iv);

        byte[] buf = new byte[8192];
        FileInputStream in = new FileInputStream(inputFile);
        FileOutputStream out = new FileOutputStream(outPutFile);
        int nread;
        int progress = 0;

        byte[] ivAndSalt = new byte[ivBytes.length + salt.length];
        System.arraycopy(ivBytes,0, ivAndSalt,0, ivBytes.length );
        System.arraycopy(salt, 0, ivAndSalt, ivBytes.length, salt.length);
        out.write(ivAndSalt);

        while((nread = in.read(buf)) != -1) {
            byte[] enc = c.update(buf, 0, nread);
            out.write(enc);
            progress++;
        }

Then I try to get the iv and salt in decryption method and then decrypt the rest of the file into an output one with FileInputStream.getChannel.position():

 public File decryptFile(File inputFile, File outPutFile, String password, ProgressBar progressBar, Label progressPercentage) {
        //Create and Init Cipher
        Cipher c = Cipher.getInstance("AES/CBC/" + padding);

        FileInputStream in = new FileInputStream(inputFile);
        FileOutputStream out = new FileOutputStream(outPutFile);

        //Getting the iv and salt
        byte[] ivBytes = new byte[16];
        byte[] salt = new byte[SALT_SIZE];
        byte[] ivAndSalt = new byte[ivBytes.length+SALT_SIZE];
        in.read(ivAndSalt, 0, ivBytes.length+SALT_SIZE);
        System.arraycopy(ivAndSalt, 0, ivBytes, 0, ivBytes.length);
        System.arraycopy(ivAndSalt, ivBytes.length, salt, 0, SALT_SIZE);
        IvParameterSpec iv =new IvParameterSpec(ivBytes);
        SecretKeySpec keySpec = generateAesKey(password, salt);
        c.init(Cipher.DECRYPT_MODE, keySpec, iv);


        in.getChannel().position(ivAndSalt.length);

        int nread;
        int progress = 0;
        byte[] buf = new byte[8192];

        while((nread = in.read(buf)) != -1) {
                byte[] enc = c.update(buf, 0, nread);
                out.write(enc);
                progress++;

            /*if (enc.length / 8192 != 0)
                    System.out.println((nread*progress) + "%");*/
        }
        System.out.println("Size of out before doFinal(): " + out.getChannel().size());
        byte[] enc = c.doFinal();
        out.write(enc);
        System.out.println("Size of out after doFinal(): " + out.getChannel().size());
        return  outPutFile;
    }

I didn't get an error on calling decryptFile() but the produced file is corrupted and this means that there is an issue somewhere in the decrypting.

There is nothing obviously wrong with the algorithm, but the code is not structured well for debugging -- effectively it is doing too much for each of those single functions (IO, key derivation, looping through chunks, encoding or decoding). Break methods down to a few simple ones and write tests to narrow down the problem. E.g. do you know that writing and then reading the ivBytes + salt produces the values that were initially there? Write a test. Do you know if encrypting and then decrypting a single chunk of 8192 bytes produces result identical to input? Extract a function and write test. — Oleg Sklyar
Also stream.read(buf) != -1 does not guarantee you read 8192 bytes. — Oleg Sklyar
I'd just skip in.getChannel().position(ivAndSalt.length); though otherwise nothing obvious.. If the decryption woudn't be ok, you'd get an exception.. — gusto2
I tried to comprise the iv and salt and it was fine, I am sure it is something with skipping the salt and iv bytes in the encrypted file. Is there any advice to avoid that? — user10222681

Unknown Unknown · Accepted Answer · 2019-01-12T15:05:09

The problem was totally something silly, I forgot to close the input and output streams after doFinal() and writing the enc bytes array to output:

in.close();
out.close();

How to add and get specific number of bytes from a file using file input and output streams?

1 Answers