AES padding and writing the ciphertext to a disk file

Question

I have a string which I encrypt with the following mehtod in C++ using Crypto++:

std::ifstream t(filename); //File to be encrypt
std::stringstream buffer;
buffer << t.rdbuf();

ofstream combined_file2(filename2); //Encrypted file
combined_file2 << encrypt(buffer.str());

string encrypt(string data)
{
  // Key and IV setup
  std::string key = "0123456789abcdef";
  std::string iv = "aaaaaaaaaaaaaaaa";

  //Alternative
  //byte key[CryptoPP::AES::DEFAULT_KEYLENGTH], iv[CryptoPP::AES::BLOCKSIZE];
  //memset(key, 0x00, CryptoPP::AES::DEFAULT_KEYLENGTH);
  //memset(iv, 0x00, CryptoPP::AES::BLOCKSIZE);

  std::string plaintext = data;
  std::string ciphertext;

  // Create Cipher Text
  CryptoPP::AES::Encryption aesEncryption((byte *)key.c_str(), CryptoPP::AES::DEFAULT_KEYLENGTH);
  CryptoPP::CBC_Mode_ExternalCipher::Encryption cbcEncryption(aesEncryption, (byte *)iv.c_str());

  //Alternative
  //CryptoPP::AES::Encryption aesEncryption(key, CryptoPP::AES::DEFAULT_KEYLENGTH);
  //CryptoPP::CBC_Mode_ExternalCipher::Encryption cbcEncryption(aesEncryption, iv);

  CryptoPP::StreamTransformationFilter stfEncryptor(cbcEncryption, new CryptoPP::StringSink(ciphertext));
  stfEncryptor.Put(reinterpret_cast<const unsigned char*>(plaintext.c_str()), plaintext.length() + 1);
  stfEncryptor.MessageEnd();

  return ciphertext;
}

When I try to decrypt the file in my C# Application, I get the message, that the length of the data is invalid. I think that the lenght of th byte array isn't a multiple of 16, so I get the error. I try to use a padding, but it is not working right.

Here is how I decrypt the file:

string plaintext = Decrypt(File.ReadAllBytes(path));

private static string Decrypt(byte[] cipherText)
{
   if (cipherText == null || cipherText.Length <= 0)
       throw new ArgumentNullException("cipherText");
   byte[] Key = GetBytes(ConfigurationManager.AppSettings["aes_key"]);
   byte[] IV = GetBytes(ConfigurationManager.AppSettings["aes_iv"]);

   // Declare the string used to hold the decrypted text.
   string plaintext = null;

   // Create an RijndaelManaged object with the specified key and IV.
   using (RijndaelManaged rijAlg = new RijndaelManaged())
   {
     rijAlg.Key = Key;
     //rijAlg.IV = IV;
     //for testing                
     rijAlg.IV = new byte[] { 97, 97, 97, 97, 97, 97, 97, 97, 97, 97, 97, 97, 97, 97, 97, 97 };

    // Create a decrytor to perform the stream transform.
    ICryptoTransform decryptor = rijAlg.CreateDecryptor(rijAlg.Key, rijAlg.IV);

    // Create the streams used for decryption.
    using (MemoryStream msDecrypt = new MemoryStream(cipherText))
    {
      using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
      {
        using (StreamReader srDecrypt = new StreamReader(csDecrypt))
        {
          // Read the decrypted bytes from the decrypting stream
          // and place them in a string.
          plaintext = srDecrypt.ReadToEnd();
        }
      }
    }
  }
  return plaintext;
}

How can I solve the problem? Is there any function to do the padding, or maybe padding is the wrong way?

Answer 1

Based on the comment:

In C++ I save a file combined_file2 << encrypt(buffer.str()); which I read in my C# program string plaintext = Decrypt(File.ReadAllBytes(path));

I don't think embedded NULLs is causing problems from the encrypt function since its returning a string, and that includes an explicit length.

However, an embedded NULL and the way the file is being written to disk in C++ (or read from disk in C#) will be a problem since this will stop at the first embedded NULL: combined_file2 << encrypt(buffer.str()).

Perhaps something like the following would be helpful:

StringSource ss(ciphertext, true /*pumpAll*/);
FileSink fs("my-encrypted-file.bin", true /*binary*/);
ss.TransferTo(fs);

If you are using a C++ stream, then use the write method on the stream object:

ofstream combined_file2;
...
combined_file2.write(ciphertext.data(), ciphertext.size());