Decrypt a media file and play it without saving to the HDD

I ne开发者_StackOverflowed to develop WinForms app, which will be able to decrypt a media file (a movie) and then play it without saving decrypted file to the HDD (the decrypted file finally will be stored in the memory stream) The problem is, how then play that movie from the memory stream ? Is it possible ?

It is possible, but I expect you will need to write your own DirectShow filter to do so, which once created will act as a file reader (implementing the IFileSourceFilter interface), and, as the video plays, will read successive frames from the file, decrypt them, and pass them up to the next filter.

This will only work however if the file is encrypted in a sequential form (i.e. each individual frame is encrypted as a seperate entity). Otherwise, you will have to decrypt the entire file at once, which could be intensive, slow, and probably have to hit the hard drive to store the end file.

But anyway, this link should get you started: http://msdn.microsoft.com/en-us/library/dd375454%28VS.85%29.aspx

I'm afraid that in order to create the DirectShow filter, you will need to use C++, and it isn't the easiest API to get your head around.

An alternate way to do it may be to use the Windows Media Format SDK, which allows you to pass custom video packets to a renderer in real time. There is also a good interop library for C# (WindowsMediaLib)

First of all, it's a good idea to encrypt source video piece by piece. So the encrypted video file is a set of encrypted parts. Just split original file into parts of the same size and encrypt them.

Here the scheme (OutputStream is a stream of encrypted video file, InputStream is original file stream, ChunkSize is a size of each part in the original file, also we write some metadata: sizes of original and encrypted pieces):

using (BinaryWriter Writer = new BinaryWriter(OutputStream))
{ 
    byte[] Buf = new byte[ChunkSize];

    List<int> SourceChunkSizeList = new List<int>();
    List<int> EncryptedChunkSizeList = new List<int>();

    int ReadBytes;
    while ((ReadBytes = InputStream.Read(Buf, 0, Buf.Length)) > 0)
    {
        byte[] EncryptedData = Encrypt(Buf, ReadBytes);
        OutputStream.Write(EncryptedData, 0, EncryptedData.Length);

        SourceChunkSizeList.Add(ReadBytes);
        EncryptedChunkSizeList.Add(EncryptedData.Length);
    }

    foreach (int SourceChunkSize in SourceChunkSizeList)
        Writer.Write(SourceChunkSize);

    foreach (int EncryptedChunkSize in EncryptedChunkSizeList)
        Writer.Write(EncryptedChunkSize);
}

Such metadata will help to find encrypted part rapidly.

Secondly, don't decrypt encrypted data in each read request. Cache it: video playing in the most case is just a sequential reading.

The tricky part is how to play encrypted video file. You may write either a DirectShow filter (video specific solution), or check a 3rd party product (multipurpose solution): BoxedApp, a virtualization SDK. What's cool is that they have an article that shows how to solve exact your task, look: http://boxedapp.com/encrypted_video_streaming.html