Skip method in CipherInputStream

I am facing an issue where the code that works with an InputStream supported by FileInputStream does not work when a CipherInputStream is used.

See example below:

  // skipCount is same as n in a FileInputStream
  FileInputStream fis;
  ... 
  skipCount = fis.skip(n)

      

        
        
        
      

    

Get different behavior if using CipherInputStream

  // skipCount is always 0
  CipherInputStream cis;
  ...
  skipCount = cis.skip(n)

      

        
        
        
      

    

After further debugging, it looks like this: skip will work (i.e. return values> 0) if used in conjunction with a read () call.

Is there a better way to get the pass to work with the CipherInputStream than wrapping my own "pass" method that relies on the read call?

Also, is there a way to tell the CipherInputStream to automatically “read” as part of the missed call? Otherwise, it looks like the skip API is being passed on to the CipherInputStream.

MCVE

public class TestSkip {
  public static final String ALGO = "AES/CBC/PKCS5Padding";
  public static final String CONTENT = "Strive not to be a success, but rather to be of value";
  private static int BlockSizeBytes = 16;
  private static SecureRandom random = null;

  static {
    try {
      random = SecureRandom.getInstance("SHA1PRNG");
    } catch (NoSuchAlgorithmException e) {
      throw new RuntimeException("Could not initialize AES encryption", e);
    }
  }

  static byte[] getKeyBytes() throws NoSuchAlgorithmException, UnsupportedEncodingException
  {
    byte[] key = "Not a secure string!".getBytes("UTF-8");
    MessageDigest sha = MessageDigest.getInstance("SHA-1");
    key = sha.digest(key);
    key = Arrays.copyOf(key, 16); // use only first 128 bit

    return key;
  }

  static KeySpec getKeySpec() throws GeneralSecurityException, UnsupportedEncodingException
  {
    return new SecretKeySpec(getKeyBytes(), "AES");
  }

  static byte[] getIv ()
  {
    byte[] iv = new byte[BlockSizeBytes];
    random.nextBytes(iv);

    return iv;
  }

  static Cipher initCipher (int mode, byte[] iv) throws GeneralSecurityException, UnsupportedEncodingException
  {
    KeySpec spec = getKeySpec();
    Cipher cipher = Cipher.getInstance(ALGO);
    cipher.init(mode, (SecretKey) spec, new IvParameterSpec(iv));
    return cipher;
  }

  static void encrypt(String fileName) throws
      GeneralSecurityException,
      IOException
  {
    FileOutputStream fos = new FileOutputStream(fileName);
    byte[] iv = getIv();
    fos.write(iv);

    Cipher cipher = initCipher(Cipher.ENCRYPT_MODE, iv);
    CipherOutputStream cos = new CipherOutputStream(fos, cipher);
    PrintWriter pw = new PrintWriter(cos);
    pw.println(CONTENT);
    pw.close();
  }

  static void skipAndCheck(String fileName) throws
      GeneralSecurityException,
      IOException
  {
    FileInputStream fis = new FileInputStream(fileName);
    byte[] iv = new byte[BlockSizeBytes];
    if (fis.read(iv) != BlockSizeBytes) {
      throw new GeneralSecurityException("Could not retrieve IV from AES encrypted stream");
    }

    Cipher cipher = initCipher(Cipher.DECRYPT_MODE, iv);
    CipherInputStream cis = new CipherInputStream(fis, cipher);

    // This does not skip
    long count = cis.skip(32);
    System.out.println("Bytes skipped: " + count);

    // Read a line
    InputStreamReader is = new InputStreamReader(cis);
    BufferedReader br = new BufferedReader(is);
    String read = br.readLine();
    System.out.println("Content after skipping 32 bytes is: " + read);

    br.close();
  }

  static InputStream getWrapper(CipherInputStream cis) {
    return new SkipInputStream(cis);
  }

  public static void main(String[] args) throws
      IOException,
      GeneralSecurityException
  {
    String fileName = "EncryptedSample.txt";

    encrypt(fileName);
    skipAndCheck(fileName); 
  }

}