Using Rijndael encryption for large files

Question

Hi there!

I'm in a situation where I need to encrypt / decrypt a file of n length securely, ideally using Rijndael, but definitely at 256bit encryption.

I've played around with encryption before and have encrypted/decrypted strings and byte arrays quite happily. However, because I don't know the size of the file (and it's very feasible that the files in question could be quite large (~2.5gb) I can't just load them up into a byte array and enc/decrypt them in a single bound as I have before.

So, after a bit of reading around on Google, I knew the answer was to encrypt and decrypt the file in chunks, and so I produced the following code:

private static void Enc(string decryptedFileName, string encryptedFileName)
{            
   FileStream fsOutput = File.OpenWrite(encryptedFileName);
   FileStream fsInput = File.OpenRead(decryptedFileName);

   byte[] IVBytes = Encoding.ASCII.GetBytes("1234567890123456");

   fsOutput.Write(BitConverter.GetBytes(fsInput.Length), 0, 8);
   fsOutput.Write(IVBytes, 0, 16);

   RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC};
   ICryptoTransform encryptor = symmetricKey.CreateEncryptor(passwordDB.GetBytes(256 / 8), IVBytes);
   CryptoStream cryptoStream = new CryptoStream(fsOutput, encryptor, CryptoStreamMode.Write);

   for (long i = 0; i < fsInput.Length; i += chunkSize)
   {
      byte[] chunkData = new byte[chunkSize];
      fsInput.Read(chunkData, 0, chunkSize);
      cryptoStream.Write(chunkData, 0, chunkData.Length);
   }
   cryptoStream.Close();
   fsInput.Close();
   fsInput.Dispose();
   cryptoStream.Dispose();
}

private static void Dec(string encryptedFileName, string decryptedFileName)
{
    FileStream fsInput = File.OpenRead(encryptedFileName);
    FileStream fsOutput = File.OpenWrite(decryptedFileName);

    byte[] buffer = new byte[8];
    fsInput.Read(buffer, 0, 8);

    long fileLength = BitConverter.ToInt64(buffer, 0);

    byte[] IVBytes = new byte[16];
    fsInput.Read(IVBytes, 0, 16);

    RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC };
    ICryptoTransform decryptor = symmetricKey.CreateDecryptor(passwordDB.GetBytes(256 / 8), IVBytes);
    CryptoStream cryptoStream = new CryptoStream(fsOutput,decryptor,CryptoStreamMode.Write);

    for (long i = 0; i < fsInput.Length; i += chunkSize)
    {
        byte[] chunkData = new byte[chunkSize];
        fsInput.Read(chunkData, 0, chunkSize);
        cryptoStream.Write(chunkData, 0, chunkData.Length);
    }
    cryptoStream.Close();
    cryptoStream.Dispose();
    fsInput.Close();
    fsInput.Dispose();                      
} 

It all "looks" good to me, but sadly looks appear to be deceiving!

Encryption works without error, but during decryption, the "cryptoStream.Close()" method throws the following exception:

System.Security.Cryptography.CryptographicException was unhandled Message="Padding is invalid and cannot be removed."
Source="mscorlib" StackTrace: at System.Security.Cryptography.RijndaelManagedTransform.DecryptData(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[]& outputBuffer, Int32 outputOffset, PaddingMode paddingMode, Boolean fLast) at System.Security.Cryptography.RijndaelManagedTransform.TransformFinalBlock(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount) at System.Security.Cryptography.CryptoStream.FlushFinalBlock() at System.Security.Cryptography.CryptoStream.Dispose(Boolean disposing) at System.IO.Stream.Close()

It also appears that the unencrypted file size isn't matching the file size expected (ranging from around 8 bytes, to around 60)

I "fixed" the exception by altering the RijndaelManaged object creation lines to include a padding type, as below:

RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC,Padding=PaddingMode.None };

But the file sizes still don't match up and, predictably, the freshly unencrypted file is baloney!

I will admit that I'm now outside of my comfort zone with encryption/decryption, and it's probably a rookie mistake - but I can't spot it!

Any help on resolving this would be greatly appreciated!

Answer 1

The Stream.Read method, returns the number of bytes actually being read from the stream.

You should use this return value as the last parameter in the Write method on the next line.

My code would look like this:

byte[] chunkData = new byte[chunkSize];   
var bytesRead = 0;
while ((bytesRead = fsInput.Read(chunkData, 0, chunkSize)) > 0)
{
    cryptoStream.Write(chunkData, 0, bytesRead);
}

Answer 2

The problem is that I was using:

passwordDB.GetBytes(256 / 8)

within the constructor for the RijndaelManaged object in both the Encryption and Decryption methods, and I wasn't re-initialising the passwordDB object before attempting to decrypt.

The resolution was to simply including the construction of the passwordDB object within the first lines of both the Enc and Dec methods, as follows:

        private static void Enc(string decryptedFileName, string encryptedFileName)
        {
            PasswordDeriveBytes passwordDB = new PasswordDeriveBytes("ThisIsMyPassword", Encoding.ASCII.GetBytes("thisIsMysalt!"), "MD5", 2);
            byte[] passwordBytes = passwordDB.GetBytes(128 / 8);

            using (FileStream fsOutput = File.OpenWrite(encryptedFileName))
            {
                using(FileStream fsInput = File.OpenRead(decryptedFileName))
                {
                    byte[] IVBytes = Encoding.ASCII.GetBytes("1234567890123456");

                    fsOutput.Write(BitConverter.GetBytes(fsInput.Length), 0, 8);
                    fsOutput.Write(IVBytes, 0, 16);

                    RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC,Padding=PaddingMode.ANSIX923};
                    ICryptoTransform encryptor = symmetricKey.CreateEncryptor(passwordBytes, IVBytes);                   

                    using (CryptoStream cryptoStream = new CryptoStream(fsOutput, encryptor, CryptoStreamMode.Write))
                    {
                        for (long i = 0; i < fsInput.Length; i += chunkSize)
                        {
                            byte[] chunkData = new byte[chunkSize];
                            int bytesRead = 0;
                            while ((bytesRead = fsInput.Read(chunkData, 0, chunkSize)) > 0)
                            {
                                if (bytesRead != 16)
                                {
                                    for (int x = bytesRead - 1; x < chunkSize; x++)
                                    {
                                        chunkData[x] = 0;
                                    }
                                }
                                cryptoStream.Write(chunkData, 0, chunkSize);
                            }
                        }
                        cryptoStream.FlushFinalBlock();
                    }
                }
            }            
        }

        private static void Dec(string encryptedFileName, string decryptedFileName)
        {
            PasswordDeriveBytes passwordDB = new PasswordDeriveBytes("ThisIsMyPassword", Encoding.ASCII.GetBytes("thisIsMysalt!"), "MD5", 2);
            byte[] passwordBytes = passwordDB.GetBytes(128 / 8);

            using (FileStream fsInput = File.OpenRead(encryptedFileName))
            {
                using (FileStream fsOutput = File.OpenWrite(decryptedFileName))
                {
                    byte[] buffer = new byte[8];
                    fsInput.Read(buffer, 0, 8);

                    long fileLength = BitConverter.ToInt64(buffer, 0);

                    byte[] IVBytes = new byte[16];
                    fsInput.Read(IVBytes, 0, 16);


                    RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC,Padding=PaddingMode.ANSIX923};
                    ICryptoTransform decryptor = symmetricKey.CreateDecryptor(passwordBytes, IVBytes);

                    using (CryptoStream cryptoStream = new CryptoStream(fsOutput, decryptor, CryptoStreamMode.Write))
                    {
                        for (long i = 0; i < fsInput.Length; i += chunkSize)
                        {
                            byte[] chunkData = new byte[chunkSize];
                            int bytesRead = 0;
                            while ((bytesRead = fsInput.Read(chunkData, 0, chunkSize)) > 0)
                            {
                                cryptoStream.Write(chunkData, 0, bytesRead);
                            }
                        }
                    }
                }
            }
        }

Knew it had to be a schoolboy error :P