Aes decryption in java - problems with padding

Question

I'd like to implement a simple encryption/decryption tool in java. Therefore I found a little tutorial on: http://www.codejava.net/coding/file-encryption-and-decryption-simple-example

I changed some lines of code, to guarantee the encryption of large files. Now I got the problem, that the decryption doen't work.

I got the following error message/exception:

Error encrypting/decrypting file
    at Algorithmus.Encryptor.doCrypto(Encryptor.java:71)
    at Algorithmus.Encryptor.decrypt(Encryptor.java:39)
    at GUI.MainWindow$encryptThread.run(MainWindow.java:838)
Caused by: javax.crypto.BadPaddingException: Given final block not properly padded
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:966)
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:824)
    at com.sun.crypto.provider.AESCipher.engineDoFinal(AESCipher.java:436)
    at javax.crypto.Cipher.doFinal(Cipher.java:2165)
    at Algorithmus.Encryptor.doCrypto(Encryptor.java:60)
    ... 2 more

I tried to change the Transoformation parameter to AES/CBC/PKCS5Padding but that has no effect. Does anyone know, how to optimize the given code?

        private static final String ALGORITHM = "AES";

        private static final String TRANSFORMATION = "AES";

        public static  void encrypt(String key, File inputFile, File outputFile)
                throws ExtendedException {
            doCrypto(Cipher.ENCRYPT_MODE, key, inputFile, outputFile);
        }

        public static  void decrypt(String key, File inputFile, File outputFile)
                throws ExtendedException {
            doCrypto(Cipher.DECRYPT_MODE, key, inputFile, outputFile);
        }

        private static void doCrypto(int cipherMode, String key, File inputFile,
                File outputFile) throws ExtendedException {
            try {
                Key secretKey = new SecretKeySpec(key.getBytes(), ALGORITHM);
                Cipher cipher = Cipher.getInstance(TRANSFORMATION);
                cipher.init(cipherMode, secretKey);

                FileInputStream inputStream = new FileInputStream(inputFile);


                CipherOutputStream out = new CipherOutputStream(new FileOutputStream(outputFile), cipher);
                byte[] buffer = new byte[8192];
                byte[] outputBytes = null;
                FileOutputStream outputStream = new FileOutputStream(outputFile);
                int count;
                while ((count = inputStream.read(buffer)) > 0)
                {
                    out.write(buffer, 0, count);
                    outputBytes = cipher.doFinal(buffer);

                }


                inputStream.close();
                outputStream.close();

            } catch (NoSuchPaddingException | NoSuchAlgorithmException
                    | InvalidKeyException | BadPaddingException
                    | IllegalBlockSizeException | IOException ex) {
                throw new ExtendedException("Error encrypting/decrypting file", ex);
            }
        }

René Link · Accepted Answer · 2018-03-10 08:08:36Z

3

Just use the CipherOutputStream. Do not invoke cipher.doFinal(buffer) and don't forget to close the output stream.

FileInputStream inputStream = new FileInputStream(inputFile);
FileOutputStream fileout = new FileOutputStream(outputFile);
CipherOutputStream out = new CipherOutputStream(fileout , cipher);

try {
    byte[] buffer = new byte[8192];
    int count;

    while ((count = inputStream.read(buffer)) > 0) {
        out.write(buffer, 0, count);
    }
} finally {
    out.close();
    inputStream.close();
}

The CipherOutputStream manages the cipher for you. It invokes doFinal when the stream will be closed and flushes the internal buffer.

edited Mar 10, 2018 at 8:08

answered May 8, 2016 at 11:08

René Link

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2 Comments

jollepe Over a year ago

Thanks a lot. It works perfectly! But if I type in the wrong passprase, no exception will be thrown. The decrypted file will be created, but it's not readable for humans

zaph Over a year ago

@jollepe AES/CBC encryption by itself does not provide authentication of the correct password, if you need that add a MAC of the encrypted data and use it for verification or use AES/GCM. Returning an error to the user for bad padding must be avoided or an attacker can exploit it in a padding oracle attack.

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