I’m trying to write an encrypter/decrypter class that encrypts and decrypts files. My server receives the file as an InputStream, and I want to store it at a particular path, so those are the inputs to my encrypt() method. For testing purposes, encrypt() generates a new key for each file and stores it in a map with the file’s path. My decrypt() method just takes the file name and attempts to print the file contents as a string. encrypt() seems to work fine. The problem I’m running into is that when I call decrypt() with the file name, I get a ShortBufferException. I’ve tried changing the buffer size in my BufferedReader, but to no avail. I think I’m missing something fundamental about how CipherInputStream or InputStreams in general work. Any thoughts would be much appreciated! Here’s my FileEncrypterDecrypter class:

public class FileEncrypterDecrypter {
    private static final int ALGORITHM_NONCE_SIZE = 12;
    private static final String ALGORITHM_NAME = "AES/GCM/NoPadding";
    private final static int ALGORITHM_TAG_SIZE = 128;
    private final static HashMap<String, SecretKey> fileToKey = new HashMap<>();

    FileEncrypterDecrypter() {}

    public static void encrypt(InputStream plaintext, String filePath) throws IOException, InvalidKeyException, InvalidAlgorithmParameterException, NoSuchAlgorithmException, NoSuchPaddingException {
        // Generate a new SecretKey for this file
        SecretKey secretKey = KeyGenerator.getInstance("AES").generateKey();
        fileToKey.put(filePath, secretKey);

        // Generate a 96-bit nonce using a CSPRNG.
        SecureRandom rand = new SecureRandom();
        byte[] nonce = new byte[ALGORITHM_NONCE_SIZE];
        rand.nextBytes(nonce);
        System.out.println(Arrays.toString(nonce));

        // Create the cipher instance and initialize.
        Cipher cipher = Cipher.getInstance(ALGORITHM_NAME);
        cipher.init(Cipher.ENCRYPT_MODE, secretKey, new GCMParameterSpec(ALGORITHM_TAG_SIZE, nonce));

        // Write to the file using a CipherOutputStream
        FileOutputStream fileOut = new FileOutputStream(filePath);
        CipherOutputStream cipherOut = new CipherOutputStream(fileOut, cipher);
        fileOut.write(nonce);
        plaintext.transferTo(cipherOut);
    }

    public static String decrypt(String filePath) throws InvalidKeyException, IOException, InvalidAlgorithmParameterException, IllegalBlockSizeException, BadPaddingException, NoSuchAlgorithmException, NoSuchPaddingException {
        // Get the SecretKey for this file
        SecretKey secretKey = fileToKey.get(filePath);

        // Read nonce from the file
        FileInputStream fileIn = new FileInputStream(filePath);
        byte[] nonce = new byte[ALGORITHM_NONCE_SIZE];
        fileIn.read(nonce);
        System.out.println(Arrays.toString(nonce));
        fileIn = new FileInputStream(filePath);
        // Create the cipher instance and initialize
        Cipher cipher = Cipher.getInstance(ALGORITHM_NAME);
        cipher.init(Cipher.DECRYPT_MODE, secretKey, new GCMParameterSpec(128, nonce));

        CipherInputStream cipherIn = new CipherInputStream(fileIn, cipher);
        InputStreamReader inputReader = new InputStreamReader(cipherIn);
        BufferedReader reader = new BufferedReader(inputReader);

        // Decrypt and return result.
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            sb.append(line);
        }
        return sb.toString();
    }
}

And my error output:

Exception in thread "main" java.io.IOException: javax.crypto.ShortBufferException: Output buffer invalid
    at java.base/javax.crypto.CipherInputStream.getMoreData(CipherInputStream.java:148)
    at java.base/javax.crypto.CipherInputStream.read(CipherInputStream.java:261)
    at java.base/sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:270)
    at java.base/sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:313)
    at java.base/sun.nio.cs.StreamDecoder.read(StreamDecoder.java:188)
    at java.base/java.io.InputStreamReader.read(InputStreamReader.java:177)
    at java.base/java.io.BufferedReader.fill(BufferedReader.java:162)
    at java.base/java.io.BufferedReader.readLine(BufferedReader.java:329)
    at java.base/java.io.BufferedReader.readLine(BufferedReader.java:396)
    at FileEncrypterDecrypter.decrypt(FileEncrypterDecrypter.java:64)
    at Main.main(Main.java:22)
Caused by: javax.crypto.ShortBufferException: Output buffer invalid
    at java.base/com.sun.crypto.provider.GaloisCounterMode$GCMDecrypt.doFinal(GaloisCounterMode.java:1366)
    at java.base/com.sun.crypto.provider.GaloisCounterMode.engineDoFinal(GaloisCounterMode.java:432)
    at java.base/javax.crypto.Cipher.doFinal(Cipher.java:2152)
    at java.base/javax.crypto.CipherInputStream.getMoreData(CipherInputStream.java:145)
    ... 10 more

Here’s my main method:

public static void main (String[] args) throws BadPaddingException, IllegalBlockSizeException, IOException, InvalidKeyException, InvalidAlgorithmParameterException, NoSuchAlgorithmException, NoSuchPaddingException {
        InputStream is = new ByteArrayInputStream(Charset.forName("UTF-8").encode("HELLO WORLD").array());
        FileEncrypterDecrypter encrypterDecrypter = new FileEncrypterDecrypter();
        encrypterDecrypter.encrypt(is, "./message");
        encrypterDecrypter.decrypt("./message");
    }

I’ve found similar problems on Stack Overflow, but none which applies exactly to my situation.