How to update part of the encrypted data with newly encrypted data?

Question

I need to encrypt an audio file while it is being generated. I am encrypting header with dummy data(because I don't know the actual size of audio data) at the starting and encrypting the audio data on the fly. My plan is to update the header at the end with actual data size of audio file.

But, When I tried to overwrite the encrypted header data with newly encrypted header data of same size by using same key and IV and try to decrypt later, I am getting junk data generated.

Why is this happening even though I am using same key and IV? In the below code I tried to simulate what I am doing. Encrypted file of size 64 bytes generated and decrypted file of size 50 bytes generated.

Without updation: abcdabcdab0123456789012345678901234567890123456789

With header updation: ABCDABCDAB÷‹þ@óMCKLZƒÖ^Ô234567890123456789

Expected output: ABCDABCDAB0123456789012345678901234567890123456789

Is this the right approach to achieve partial update of already encrypted data?

protected void Encrypt()
{

    byte[] numBytes = {'0','1','2','3','4','5','6','7','8','9','0','1','2','3','4','5','6','7','8','9', '0','1','2','3','4','5','6','7','8','9', '0','1','2','3','4','5','6','7','8','9'};
    byte[] smallCase = {'a','b','c','d','a','b','c','d','a','b','c','d','a','b','c','d'};
    byte[] capitalCase = {'A','B','C','D','A','B','C','D','A','B','C','D','A','B','C','D'};

    try {
        SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2withHmacSHA1And8BIT");
        KeySpec spec = new PBEKeySpec("junglebook".toCharArray(), "Salt".getBytes(), 65536, 256);
        SecretKey tmp = null;
        tmp = factory.generateSecret(spec);
        SecretKey secret = new SecretKeySpec(tmp.getEncoded(), "AES");

        /* Encryption cipher initialization. */
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.ENCRYPT_MODE, secret);

        AlgorithmParameters params = cipher.getParameters();
        byte[] iv = params.getParameterSpec(IvParameterSpec.class).getIV();

        Log.d("Encryption" + "iv data :", iv.toString());

        /*Open two Cipher ouput streams to the same encrypted file*/
        FileOutputStream os = new FileOutputStream(sdCard.getAbsolutePath() + "/Notes/sample.encrypted");
        CipherOutputStream cos = new CipherOutputStream(os,cipher);

        FileOutputStream os1 = new FileOutputStream(sdCard.getAbsolutePath() + "/Notes/sample.encrypted");
        CipherOutputStream cos1 = new CipherOutputStream(os1,cipher);

        int offset = 0;
        Log.d("Encryption", "Writing cipher text to output file");
        //Write 16 bytes header data with smallCase array
        cos.write(smallCase, offset, 16);
        // write 40 bytes actual data
        cos.write(numBytes, offset, 40);

        FileOutputStream ivStream = new FileOutputStream(sdCard.getAbsolutePath() + "/Notes/iv.dat");
        if (ivStream != null) {
            Log.d("Encryption", "Writing iv data to output file");
            ivStream.write(iv);
        }
        cos.close();

        // Overwrite header data with capitalCase array data
        cos1.write(capitalCase, offset, 16);
        cos1.close();

        ivStream.close();

    }catch (Exception e) {
        e.printStackTrace();
    }
}

protected void Decrypt()
{
    byte[] dBytes = new byte[200];

    try {

        Log.d("Decryption", "Reading iv data ");
        File f1 = new File(sdCard.getAbsolutePath()+"/Notes/iv.dat");
        byte[] newivtext = new byte[(int)f1.length()];
        FileInputStream readivStream = new FileInputStream(sdCard.getAbsolutePath()+"/Notes/iv.dat");
        if(readivStream != null) {
            readivStream.read(newivtext);
        }

        // Generate the secret key from same password and salt used in encryption
        SecretKeyFactory dfactory = SecretKeyFactory.getInstance("PBKDF2withHmacSHA1And8BIT");
        KeySpec dspec = new PBEKeySpec("junglebook".toCharArray(), "Salt".getBytes(), 65536, 256);
        SecretKey dtmp = dfactory.generateSecret(dspec);
        SecretKey dsecret = new SecretKeySpec(dtmp.getEncoded(), "AES");

        // Initialize dcipher
        Cipher dcipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        dcipher.init(Cipher.DECRYPT_MODE, dsecret, new IvParameterSpec(newivtext));

        FileInputStream inputStream = new FileInputStream(sdCard.getAbsolutePath()+"/Notes/sample.encrypted");
        CipherInputStream cis = new CipherInputStream(inputStream,dcipher);
        FileOutputStream os = new FileOutputStream(sdCard.getAbsolutePath() + "/Notes/sample.decrypted");

        int b = cis.read(dBytes);
        while(b != -1) {
            Log.d("Decryption","Bytes decrypted" + b);
            os.write(dBytes, 0, b);
            b = cis.read(dBytes);
        }
        cis.close();
        os.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Answer 1

I suggest you update several things:

1. you are opening multiple outputstreams to the SAME file, which is very strange, the runtime should not allow you to do that. So - write only with a single output if you want any predictable results.

2. You may read about the mode of operations see the CRT mode uses no padding and allows you to update only a portion of the ciphertext (assuming you use no authenticated encryption). So AES/CTR/NoPadding could solve your problem. (and there should be no extra bytes if you do it correctly)

3. you can update a portion of the file using the RandomAccessFile and overwrite portion of the ciphertext what is needed.