Question

I’m using CipherInputStream and CipherOutputStream to encrypt files using AES.

encrypt(...) seems to be working fine, but my decrypt(...) function only decrypt the first 16 bytes of my files.

Here is my class :

public class AESFiles {

    private byte[] getKeyBytes(final byte[] key) throws Exception {
        byte[] keyBytes = new byte[16];
        System.arraycopy(key, 0, keyBytes, 0, Math.min(key.length, keyBytes.length));
        return keyBytes;
    }

    public Cipher getCipherEncrypt(final byte[] key) throws Exception {
        byte[] keyBytes = getKeyBytes(key);
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        SecretKeySpec secretKeySpec = new SecretKeySpec(keyBytes, "AES");
        IvParameterSpec ivParameterSpec = new IvParameterSpec(keyBytes);
        cipher.init(Cipher.ENCRYPT_MODE, secretKeySpec, ivParameterSpec);
        return cipher;
    }

    public Cipher getCipherDecrypt(byte[] key) throws Exception {
        byte[] keyBytes = getKeyBytes(key);
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        SecretKeySpec secretKeySpec = new SecretKeySpec(keyBytes, "AES");
        IvParameterSpec ivParameterSpec = new IvParameterSpec(keyBytes);
        cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, ivParameterSpec);
        return cipher;
    }

    public void encrypt(File inputFile, File outputFile, byte[] key) throws Exception {
        Cipher cipher = getCipherEncrypt(key);
        FileOutputStream fos = null;
        CipherOutputStream cos = null;
        FileInputStream fis = null;
        try {
            fis = new FileInputStream(inputFile);
            fos = new FileOutputStream(outputFile);
            cos = new CipherOutputStream(fos, cipher);
            byte[] data = new byte[1024];
            int read = fis.read(data);
            while (read != -1) {
                cos.write(data, 0, read);
                read = fis.read(data);
                System.out.println(new String(data, "UTF-8").trim());
            }
            cos.flush();
        } finally {
            fos.close();
            cos.close();
            fis.close();
        }
    }

    public void decrypt(File inputFile, File outputFile, byte[] key) throws Exception {
        Cipher cipher = getCipherDecrypt(key);
        FileOutputStream fos = null;
        CipherInputStream cis = null;
        FileInputStream fis = null;
        try {
            fis = new FileInputStream(inputFile);
            cis = new CipherInputStream(fis, cipher);
            fos = new FileOutputStream(outputFile);
            byte[] data = new byte[1024];
            int read = cis.read(data);
            while (read != -1) {
                fos.write(data, 0, read);
                read = cis.read(data);
                System.out.println(new String(data, "UTF-8").trim());
            }
        } finally {
            fos.close();
            cis.close();
            fis.close();
        }
    }

    public static void main(String args[]) throws Exception {
        byte[] key = "mykey".getBytes("UTF-8");
        new AESFiles().encrypt(new File("C:\\Tests\\secure.txt"), new File("C:\\Tests\\secure.txt.aes"), key);
        new AESFiles().decrypt(new File("C:\\Tests\\secure.txt.aes"), new File("C:\\Tests\\secure.txt.1"), key);
    }
}

So my question is, why the decrypt function only read the first 16 bytes ?

Answer 1

This is very subtle. Your problem is that you are closing your fos before your cos. In your encrypt(...) method you are doing:

} finally {
    fos.close();
    cos.close();
    fis.close();
}

That closes the FileOutputStream out from under the CipherOutputStream so the final padded block of encrypted output never gets written to the output file. If you close the fos after the cos then your code should work fine.

Really, you should consider doing something like:

    FileOutputStream fos = null;
    CipherOutputStream cos = null;
    FileInputStream fis = null;
    try {
        fis = new FileInputStream(inputFile);
        fos = new FileOutputStream(outputFile);
        cos = new CipherOutputStream(fos, cipher);
        // once cos wraps the fos, you should set it to null
        fos = null;
        ...
    } finally {
        if (cos != null) {
           cos.close();
        }
        if (fos != null) {
           fos.close();
        }
        if (fis != null) {
           fis.close();
        }
    }

FYI: org.apache.commons.io.IOUtils has a great closeQuietly(...) method which handles null checks and catches the exceptions for you.