Question

I want to compress and then encrypt my data, and for improved speed (by not having to write to byte arrays and back) decided to chain the streams used for compression and encryption together.

It works perfectly when I write (compress and encrypt) the data, but when I try to read the data (decompress and decrypt), the Read operation breaks – simply calling Read once reads exactly 0 bytes, because the first Read always returns 0. Looping as in the below code almost works, except that at a certain point, Read stops returning anything > 0 even though there’s still data to be read.

Everything before those last few bytes are decompressed and decrypted perfectly.

The number of bytes left when that happens remains the same for the same plaintext; for example, it’s always 9 bytes for a certain string, but always 1 byte for another.

The following is the relevant encryption and decryption code; any ideas as to what could be going wrong?

Encryption:

// Create the streams used for encryption.
using (MemoryStream msEncrypt = new MemoryStream())
{
    using (ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV))
    using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
    using (DeflateStream zip = new DeflateStream(csEncrypt, CompressionMode.Compress, true))
    {
        zip.Write(stringBytes, 0, stringBytes.Length);
        csEncrypt.FlushFinalBlock();

Decryption:

// Create the streams used for decryption.
using (MemoryStream msDecrypt = new MemoryStream())
{
    // Writes the actual data (sans prepended headers) to the stream
    msDecrypt.Write(stringBytes, prependLength, stringBytes.Length - prependLength);
    // Reset position to prepare for read
    msDecrypt.Position = 0;
    // init buffer to read to
    byte[] buffer = new byte[originalSize];

    using (ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV))
    using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
    using (DeflateStream zip = new DeflateStream(csDecrypt, CompressionMode.Decompress))
    {
        // Hangs with "offset" at a small, deterministic number away from originalSize (I've gotten 9 less and 1 less for different strings)
        // Loop fixed as per advice
        int offset = 0;
        while (offset < originalSize)
        {
            int read = zip.Read(buffer, offset, originalSize - offset);
            if (read > 0)
                offset += read;
            else if (read < 0)
                Console.WriteLine(read); // Catch it if it happens.
        }
        // Hangs with "left" at a small, deterministic number (I've gotten 9 and 1 for different strings)
        /*
        for (int left = buffer.Length; left > 0; )
            left -= zip.Read(buffer, 0, left);
        */

Solution (Modification to Encryption):

// Create the streams used for encryption.
using (MemoryStream msEncrypt = new MemoryStream())
{
    using (ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV))
    using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
    {
        using (DeflateStream zip = new DeflateStream(csEncrypt, CompressionMode.Compress, true))
            zip.Write(stringBytes, 0, stringBytes.Length);
        //Flush after DeflateStream is disposed.
        csEncrypt.FlushFinalBlock();

Answer 1

The problem lies in the following line:

csEncrypt.FlushFinalBlock();

If you remove that, the code will work.

The reason is that when you write to DeflateStream, not all data is written to the underlying stream. That happens only when you call Close() or Dispose() explicitly or implicitly by leaving the using block.

So in your code, this happens:

1. You Write() all of the data to the DeflateStream, which in turn writes most of the data to the underlying CryptoStream.
2. You call csEncrypt.FlushFinalBlock(), which closes the CryptoStream.
3. You leave the using block of the DeflateStream, which tries to write the rest of the data to the already closed CryptoStream.
4. You leave the using block of the CryptoStream, which would call FlushFinalBlock(), if it wasn’t called already.

The correct sequence of events is:

1. You Write() all of the data to the DeflateStream, which in turn writes most of the data to to the underlying CryptoStream.
2. You leave the using block of the DeflateStream, which writes the rest of the data to the already closed CryptoStream.
3. You leave the using block of the CryptoStream, which calls FlushFinalBlock().

Although I would expect that writing to a closed stream would fail with an exception. I’m not sure why that doesn’t happen.