Steganography IV - Reading and Writing AVI files

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Introduction

The video stream in an AVI file is nothing more than a sequence of bitmaps. This article is about extracting these bitmaps and re-building the stream, in order to hide a message in the video.

Background

Before reading this article, you should have read at least part one, Steganography - Hiding messages in the Noise of a Picture. This one uses the application described in parts 1-3, but you don’t need the extended features to understand it.

Reading the Video Stream

The Windows AVI library is a set of functions in avifil32.dll. Before it is ready to use, it has to be initialized with AVIFileInit. AVIFileOpen opens the file, AVIFileGetStream finds the video stream. Each of these functions allocates memory which has to be released.

//Initialize the AVI library

[DllImport("avifil32.dll")]
public static extern void AVIFileInit();

//Open an AVI file

[DllImport("avifil32.dll", PreserveSig=true)]
public static extern int AVIFileOpen(
    ref int ppfile,
    String szFile,
    int uMode,
    int pclsidHandler);

//Get a stream from an open AVI file

[DllImport("avifil32.dll")]
public static extern int AVIFileGetStream(
    int pfile,
    out IntPtr ppavi,  
    int fccType,       
    int lParam);

//Release an open AVI stream

[DllImport("avifil32.dll")]
public static extern int AVIStreamRelease(IntPtr aviStream);

//Release an ope AVI file

[DllImport("avifil32.dll")]
public static extern int AVIFileRelease(int pfile);

//Close the AVI library

[DllImport("avifil32.dll")]
public static extern void AVIFileExit();

Now we are able to open an AVI file and get the video stream. AVI files can contain many streams of four different types (Video, Audio, Midi and Text). Usually there is only one stream of each type, and we are only interested in the video stream.

private int aviFile = 0;
private IntPtr aviStream;

public void Open(string fileName) {
    AVIFileInit(); //Intitialize AVI library

    
    //Open the file

    int result = AVIFileOpen(
        ref aviFile,
        fileName,
        OF_SHARE_DENY_WRITE, 0);
    
    //Get the video stream

    result = AVIFileGetStream(
        aviFile,
        out aviStream,
        streamtypeVIDEO, 0);
}

Before we start reading the frames, we must know what exactly we want to read:

• Where does the first frame begin?
• How many frames are available?
• What is the height/width of the images?

The AVI library contains a function for every question.

//Get the start position of a stream

[DllImport("avifil32.dll", PreserveSig=true)]
public static extern int AVIStreamStart(int pavi);

//Get the length of a stream in frames

[DllImport("avifil32.dll", PreserveSig=true)]
public static extern int AVIStreamLength(int pavi);

//Get header information about an open stream

[DllImport("avifil32.dll")]
public static extern int AVIStreamInfo(
    int pAVIStream,
    ref AVISTREAMINFO psi,
    int lSize);

With these functions we can fill a BITMAPINFOHEADER structure. To extract the images, we need three more functions.

//Get a pointer to a GETFRAME object (returns 0 on error)

[DllImport("avifil32.dll")]
public static extern int AVIStreamGetFrameOpen(
    IntPtr pAVIStream,
    ref BITMAPINFOHEADER bih);

//Get a pointer to a packed DIB (returns 0 on error)

[DllImport("avifil32.dll")]
public static extern int AVIStreamGetFrame(
    int pGetFrameObj,
    int lPos);

//Release the GETFRAME object

[DllImport("avifil32.dll")]
public static extern int AVIStreamGetFrameClose(int pGetFrameObj);

Finally we are ready to decompress the frames...

//get start position and count of frames

int firstFrame = AVIStreamStart(aviStream.ToInt32());
int countFrames = AVIStreamLength(aviStream.ToInt32());

//get header information            

AVISTREAMINFO streamInfo = new AVISTREAMINFO();
result = AVIStreamInfo(aviStream.ToInt32(), ref streamInfo,
    Marshal.SizeOf(streamInfo));

//construct the expected bitmap header

BITMAPINFOHEADER bih = new BITMAPINFOHEADER();
bih.biBitCount = 24;
bih.biCompression = 0; //BI_RGB;

bih.biHeight = (Int32)streamInfo.rcFrame.bottom;
bih.biWidth = (Int32)streamInfo.rcFrame.right;
bih.biPlanes = 1;
bih.biSize = (UInt32)Marshal.SizeOf(bih);

//prepare to decompress DIBs (device independend bitmaps)

int getFrameObject = AVIStreamGetFrameOpen(aviStream, ref bih);

...

//Export the frame at the specified position

public void ExportBitmap(int position, String dstFileName){
    //Decompress the frame and return a pointer to the DIB

    int pDib = Avi.AVIStreamGetFrame(getFrameObject, firstFrame + position);

    //Copy the bitmap header into a managed struct

    BITMAPINFOHEADER bih = new BITMAPINFOHEADER();
    bih = (BITMAPINFOHEADER)Marshal.PtrToStructure(new IntPtr(pDib),
        bih.GetType());
    
    //Copy the image

    byte[] bitmapData = new byte;
    int address = pDib + Marshal.SizeOf(bih);
    for(int offset=0; offset<bitmapData.Length; offset++){
        bitmapData[offset] = Marshal.ReadByte(new IntPtr(address));
        address++;
    }

    //Copy bitmap info

    byte[] bitmapInfo = new byte[Marshal.SizeOf(bih)];
    IntPtr ptr;
    ptr = Marshal.AllocHGlobal(bitmapInfo.Length);
    Marshal.StructureToPtr(bih, ptr, false);
    address = ptr.ToInt32();
    for(int offset=0; offset<bitmapInfo.Length; offset++){
        bitmapInfo[offset] = Marshal.ReadByte(new IntPtr(address));
        address++;
    }

...and store them in bitmap files.

    //Create file header

    Avi.BITMAPFILEHEADER bfh = new Avi.BITMAPFILEHEADER();
    bfh.bfType = Avi.BMP_MAGIC_COOKIE;
    //size of file as written to disk


    bfh.bfSize = (Int32)(55 + bih.biSizeImage);
    bfh.bfOffBits = Marshal.SizeOf(bih) + Marshal.SizeOf(bfh);

    //Create or overwrite the destination file

    FileStream fs = new FileStream(dstFileName, System.IO.FileMode.Create);
    BinaryWriter bw = new BinaryWriter(fs);

    //Write header

    bw.Write(bfh.bfType);
    bw.Write(bfh.bfSize);
    bw.Write(bfh.bfReserved1);
    bw.Write(bfh.bfReserved2);
    bw.Write(bfh.bfOffBits);
    //Write bitmap info

    bw.Write(bitmapInfo);
    //Write bitmap data

    bw.Write(bitmapData);
    bw.Close();
    fs.Close();
} //end of ExportBitmap

The application can use the extracted bitmaps just like any other image file. If one carrier file is an AVI video, it extracts the first frame to a temporary file, opens it and hides a part of the message. Then it writes the resulting bitmap to a new video stream, and continues with the next frame. After the last frame the application closes both video files, deletes the temporary bitmap file, and continues with the next carrier file.

Writing to a Video Stream

When the application opens an AVI carrier file, it creates another AVI file for the resulting bitmaps. The new video stream must have the same size and frame rate as the original stream, so we cannot create it in the Open() method. When the first bitmap arrives, we know the frame size and are able to create a video stream. The functions to create streams and write frames are AVIFileCreateStream, AVIStreamSetFormat and AVIStreamWrite:

//Create a new stream in an open AVI file

[DllImport("avifil32.dll")]
public static extern int AVIFileCreateStream(
    int pfile,
    out IntPtr ppavi, 
    ref AVISTREAMINFO ptr_streaminfo);

//Set the format for a new stream

[DllImport("avifil32.dll")]
public static extern int AVIStreamSetFormat(
    IntPtr aviStream, Int32 lPos, 
    ref BITMAPINFOHEADER lpFormat, Int32 cbFormat);

//Write a sample to a stream

[DllImport("avifil32.dll")]
public static extern int AVIStreamWrite(
    IntPtr aviStream, Int32 lStart, Int32 lSamples, 
    IntPtr lpBuffer, Int32 cbBuffer, Int32 dwFlags, 
    Int32 dummy1, Int32 dummy2);

Now we can create a stream...

//Create a new video stream

private void CreateStream() {
    //describe the stream to create

    AVISTREAMINFO strhdr = new AVISTREAMINFO();
    strhdr.fccType = this.fccType; //mmioStringToFOURCC("vids", 0)

    strhdr.fccHandler = this.fccHandler; //"Microsoft Video 1"

    strhdr.dwScale = 1;
    strhdr.dwRate = frameRate;
    strhdr.dwSuggestedBufferSize = (UInt32)(height * stride);
    //use highest quality! Compression destroys the hidden message.

    strhdr.dwQuality = 10000;
    strhdr.rcFrame.bottom = (UInt32)height;
    strhdr.rcFrame.right = (UInt32)width;
    strhdr.szName = new UInt16[64];
    
    //create the stream

    int result = AVIFileCreateStream(aviFile, out aviStream, ref strhdr);

    //define the image format

    BITMAPINFOHEADER bi = new BITMAPINFOHEADER();
    bi.biSize      = (UInt32)Marshal.SizeOf(bi);
    bi.biWidth     = (Int32)width;
    bi.biHeight    = (Int32)height;
    bi.biPlanes    = 1;
    bi.biBitCount  = 24;
    bi.biSizeImage = (UInt32)(this.stride * this.height);

    //format the stream

    result = Avi.AVIStreamSetFormat(aviStream, 0, ref bi, Marshal.SizeOf(bi));
}

...and write video frames.

//Create an empty AVI file

public void Open(string fileName, UInt32 frameRate) {
    this.frameRate = frameRate;

    Avi.AVIFileInit();
    
    int hr = Avi.AVIFileOpen(
        ref aviFile, fileName, 
        OF_WRITE | OF_CREATE, 0);
}

//Add a sample to the stream - for first sample: create the stream 

public void AddFrame(Bitmap bmp) {
    BitmapData bmpDat = bmp.LockBits(
        new Rectangle(0, 0, bmp.Width, bmp.Height),
        ImageLockMode.ReadOnly,PixelFormat.Format24bppRgb);

    //this is the first frame - get size and create a new stream

    if (this.countFrames == 0) {
        this.stride = (UInt32)bmpDat.Stride;
        this.width = bmp.Width;
        this.height = bmp.Height;
        CreateStream(); //a method to create a new video stream

    }

    //add the bitmap to the stream

    int result = AVIStreamWrite(aviStream,
        countFrames, 1, 
        bmpDat.Scan0, //pointer to the beginning of the image data

        (Int32) (stride  * height), 
        0, 0, 0); 

    bmp.UnlockBits(bmpDat);
    this.countFrames ++;
}

That's all we need to read and write video streams. Non-video streams and compression are not interesting at the moment, because compression destroys the hidden message by changing colours, and sound would make the files even larger - uncompressed AVI files are big enough! ;-)

Changes in CryptUtility

The HideOrExtract() method used to load all carrier images at once. This was no good from the beginning, and now it became impossible. From now on HideOrExtract() loads only one bitmap, and disposes it before loading the next one. The currently used carrier image - simple bitmap or extracted AVI frame - is stored in a BitmapInfo structure, which is passed around by ref.

public struct BitmapInfo{
    //uncompressed image

    public Bitmap bitmap;
    
    //position of the frame in the AVI stream, -1 for non-avi bitmaps

    public int aviPosition;
    //count of frames in the AVI stream, or 0 for non-avi bitmaps

    public int aviCountFrames;
    
    //path and name of the bitmap file

    //this file will be deleted later, if aviPosition is 0 or greater

    public String sourceFileName;
    //how many bytes will be hidden in this image

    public long messageBytesToHide;

    public void LoadBitmap(String fileName){
        bitmap = new Bitmap(fileName);
        sourceFileName = fileName;
    }
}

Whenever MovePixelPosition moves the position into the next carrier bitmap, it checks the field aviPosition. If aviPosition is < 0, it saves and disposes the bitmap. If aviPosition is 0 or greater, it is a frame in an AVI stream. It is not saved to a file, but added to the open AVI stream. If the bitmap was a simple image or the last frame of an AVI stream, the method opens the next carrier file. If there are more frames in the AVI stream, is exports and loads the next bitmap.

Using the code

There are three new classes in the project:

• AviReader opens existing AVI files and copies frames to bitmap files.
• AviWriter creates new AVI files and combines bitmaps to a video stream.
• Avi contains the function declarations and structure definitions.

Thanks

Thanks to Shrinkwrap Visual Basic, for the "Visual Basic AVIFile Tutorial". The examples could not be easily converted to VB.NET or C#, but they gave me important hints on how to copy DIBs.

Thanks to Rene N., who posted an AviWriter class in microsoft.public.dotnet.languages.csharp.

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video streaming sdeniz 4:49 26 Sep '08   Hi I want to take the video frame from the web cam and later I will send the video frame by using windows socket programming in c++.I have encountered your name in the codeproject forum.I really thank you if you can help me about video streaming.I can take the image with opencv and then send the image as bmpfile format in real time from one computer to the other by using windows socket programming.How can I convert the bitmap frames to the video file format or is there any other way for video streaming. Sign In·View Thread·PermaLink Avi to BMp #Jet 3:25 1 Apr '08   thanks for the article on AVI.. my main problem is with building the image data.. can u pls explain what really happen within this block to me.. it seems to take a long time before finishing this loop . this slows down the process of writing the bitmap. i would be happy if you could write down a sample windows app code on how to do this extraction. i would be happy to see the code that starts the extraction.. this is what i have //extract button eventhandler. reader is an AVIFILEREADER object for (int i = first; i <= last; i++) { reader.GenerateBitmaps(i, path + String.Format("\\image{0}.bmp", i)); } //inside the GenerateBitmaps this loop eats up all the time.. looking for help on it //Copy the image byte[] bitmapData = new byte[bih.biSizeImage]; int address = pDib + Marshal.SizeOf(bih); for (int offset = 0; offset < bitmapData.Length; offset++) { bitmapData[offset] = Marshal.ReadByte(new IntPtr(address)); address++; } int pDib = Avifill32dll.AVIStreamGetFrame(getframeobj, firstframe + position); //Copy the bitmap header into a managed struct Avifill32dll.BITMAPINFOHEADER bih = new Avifill32dll.BITMAPINFOHEADER(); bih = (Avifill32dll.BITMAPINFOHEADER)Marshal.PtrToStructure(new IntPtr(pDib), bih.GetType()); //Copy the image byte[] bitmapData = new byte[bih.biSizeImage]; int address = pDib + Marshal.SizeOf(bih); for (int offset = 0; offset < bitmapData.Length; offset++) { bitmapData[offset] = Marshal.ReadByte(new IntPtr(address)); address++; } Sign In·View Thread·PermaLink 1.50/5 (2 votes) BMP files Derek Loftis 16:12 17 Nov '07   I was wondering if you could tell me what I am doing wrong... I am trying to use the export as BMP function you have, but then later, I need to delete the BMP. The program then tells me that I can't delete it because the BMP file is being used by another process. That process is my program, I was wondering if your section of the export code in AVIReader does not release the BMP file? If that is true how do I releace it? I modeified the export function some... Thank You for your help. Here is my code: public Image GetBMP(int position) { string dstFileName = "deleteThis.bmp"; if (position > countFrames) { throw new Exception("Invalid frame position"); } //Decompress the frame and return a pointer to the DIB int pDib = Avi.AVIStreamGetFrame(getFrameObject, firstFrame + position); //Copy the bitmap header into a managed struct Avi.BITMAPINFOHEADER bih = new Avi.BITMAPINFOHEADER(); bih = (Avi.BITMAPINFOHEADER)Marshal.PtrToStructure(new IntPtr(pDib), bih.GetType()); /*if(bih.biBitCount < 24){ throw new Exception("Not enough colors! DIB color depth is less than 24 bit."); }else */ if (bih.biSizeImage < 1) { throw new Exception("Exception in AVIStreamGetFrame: Not bitmap decompressed."); } //Copy the image byte[] bitmapData = new byte[bih.biSizeImage]; int address = pDib + Marshal.SizeOf(bih); for (int offset = 0; offset < bitmapData.Length; offset++) { bitmapData[offset] = Marshal.ReadByte(new IntPtr(address)); address++; } //Copy bitmap info byte[] bitmapInfo = new byte[Marshal.SizeOf(bih)]; IntPtr ptr; ptr = Marshal.AllocHGlobal(bitmapInfo.Length); Marshal.StructureToPtr(bih, ptr, false); address = ptr.ToInt32(); for (int offset = 0; offset < bitmapInfo.Length; offset++) { bitmapInfo[offset] = Marshal.ReadByte(new IntPtr(address)); address++; } //Create file header Avi.BITMAPFILEHEADER bfh = new Avi.BITMAPFILEHEADER(); bfh.bfType = Avi.BMP_MAGIC_COOKIE; bfh.bfSize = (Int32)(55 + bih.biSizeImage); //size of file as written to disk bfh.bfReserved1 = 0; bfh.bfReserved2 = 0; bfh.bfOffBits = Marshal.SizeOf(bih) + Marshal.SizeOf(bfh); //Create or overwrite the destination file FileStream fs = new FileStream(dstFileName, System.IO.FileMode.Create); BinaryWriter bw = new BinaryWriter(fs); //Write header bw.Write(bfh.bfType); bw.Write(bfh.bfSize); bw.Write(bfh.bfReserved1); bw.Write(bfh.bfReserved2); bw.Write(bfh.bfOffBits); //Write bitmap info bw.Write(bitmapInfo); //Write bitmap data bw.Write(bitmapData); bw.Close(); fs.Close(); // Added By Derek Image i = Image.FromFile(dstFileName); return i; // Done adding } Derek Loftis Дерек Лофтис Sign In·View Thread·PermaLink Re: BMP files Corinna John 7:43 18 Nov '07   Hi Derek, you don't need to write the bitmap to a file at all. Have you tried using a MemoryStream instead of the FileStream? I suggest only you save bitmaps you want to keep... For a more reliable implementation, please copy the code from the newer article. http://www.codeproject.com/cs/media/avifilewrapper.asp ____________________________________ There is no proof for this sentence. Sign In·View Thread·PermaLink Re: BMP files [modified] Derek Loftis 15:20 18 Nov '07   Thank you for the idea. Derek Sign In·View Thread·PermaLink 2.00/5 (1 vote) please reply suvarna_bvb 23:01 13 May '07   hi! i tried with avi file but its giving AviStreamGetFrameOpen exception on clicking add to image files button. please help thank you Sign In·View Thread·PermaLink plzzz reply... suvarna_bvb 19:54 10 May '07   we hav taken steganography as academics projects so we want to know how this work... why this project is askin bitmap files as input. plz explain how to run the application... Sign In·View Thread·PermaLink 2.00/5 (1 vote) Re: plzzz reply... Corinna John 5:32 11 May '07   Alright, I tried to answer your questions. You do not seem to be aware of the fact that your postings are very hard to read. If you expect serious answers, please do not use strange pronounciation-based abbreviations like "ur" or "y". I have to read those sentences three times to get their meaning. ____________________________________ There is no proof for this sentence. Sign In·View Thread·PermaLink Need the help.. atans 17:32 8 May '07   I'm is the beginner in C++ programming and I was using the Microsoft Visual Studio.net 2003 as a software to do the C++ code.. I need to convert the sequence images in bitmap files to avi files.. can u tell me how I can do it by your code?... Is that the reading or writing avi file portion?.. besides that what the other things that I must add to the code?.. I hope u can help me about this matter.. Thanks.. Sign In·View Thread·PermaLink Re: Need the help.. Corinna John 3:05 9 May '07   Hi atans, the code from this article won't write compressed avi files. Try that one: http://www.codeproject.com/cs/media/avifilewrapper.asp[^] The AVIFile Wrapper creates avi files from bitmaps like that: AviManager aviManager = new AviManager("aviFile.avi", true); VideoStream aviStream = aviManager.GetVideoStream(); Bitmap bitmap; foreach(string bitmapFileName in myFiles){ bitmap = (Bitmap)Bitmap.FromFile(bitmapFileName); aviStream.AddFrame(bitmap); bitmap.Dispose(); } aviManager.Close(); ____________________________________ There is no proof for this sentence. Sign In·View Thread·PermaLink need urgent help... [modified] suvarna_bvb 19:50 26 Apr '07   hi, i downloaded ur source code. its asking for bitmap file in place of avi file. please tel y is it so. how to give avi file as input? thanx reply soon plzzzzz reply...... why u r not replyin to us plzzzzzzzz -- modified at 0:47 Friday 11th May, 2007 Sign In·View Thread·PermaLink Re: need urgent help... Corinna John 5:29 11 May '07   The application should ask for one of these file types: - "Bitmaps (*.bmp) - Tagged Image File Format (*.tif) - PNG-24 (*.png) - Video (*.avi) If you choose a single image, the message will be hidden only in that image. If you choose an AVI file, the message will be distributed into multiple bitmaps inside the video stream. ____________________________________ There is no proof for this sentence. Sign In·View Thread·PermaLink AVI steganography Musharaf Zaheer Qureshi 20:53 30 Jan '07   hi Is there any case for AVI file that we can extract sound in the form of wav and then hide message in sound file and then add that sound in AVI file i implement it but size of AVI file will be double that is if size is 18 MB then new size will be 36 MB is there any case to reduce size up to original size is this AVI steganography or not Musharaf Sign In·View Thread·PermaLink Re: AVI steganography Corinna John 8:06 31 Jan '07   The AviFile Wrapper can extract and embed sound. You could extract the sound to a wave file, hide a message in it and then re-embed it in an avi. This article's code produces large files, because it removes the compression to preserve the pixel values. You don't need that, if you use the sound as a carrier. Musharaf Zaheer Qureshi wrote: is this AVI steganography or not AVI stands for "Audio Video Interlace". So, AVI steganography can be both: Hiding bits in the video and/or in the sound. BTW: I just looked at your profile. Have you registered here only for stego? Why do you need it so much, what do you have to hide? ____________________________________ There is no proof for this sentence. Sign In·View Thread·PermaLink Re: AVI steganography lonelywind1982 18:32 29 Aug '07   Hi, if I extract wave from avi file and embed data to wave file,then re-embed it in an avi and then if avi file is modified, may the embed data on wave file in avi be destroyed? Thanks. Sign In·View Thread·PermaLink Re: AVI steganography Corinna John 21:09 29 Aug '07   Hi, is depends on the codec. If your avi codec does not compress the wave stream, it should work. ____________________________________ There is no proof for this sentence. Sign In·View Thread·PermaLink AVI files caohieu 0:13 13 Dec '06   i don't know about avi structure, can you help me thanks, hi Sign In·View Thread·PermaLink Re: AVI files Corinna John 6:11 13 Dec '06   That article can guide you through the whole AVI hell: http://www.codeproject.com/cs/media/avifilewrapper.asp ____________________________________ There is no proof for this sentence. Sign In·View Thread·PermaLink Re: AVI files caohieu 18:13 9 Jan '07   thanks! hi Sign In·View Thread·PermaLink Don't work with avi file ductri 1:39 22 Oct '06   I try to hide information in bmp file and it work well but i try to hide information in a avi file, it raise a error, I work on VS2005, Who can help me? Thanks very much! Sign In·View Thread·PermaLink 1.00/5 (1 vote) Please help me ana_cool 15:50 28 Aug '06   i have some problem about embeding text to file video use the steganography teknik.Would you help me,maybe about what kind a visual program i must used..or about the language program?i'll use delphi 7 for the visual but my frend told me sometime delphi 7 have problem ??so...what must i do??you think it's work or not??can you help me?? thnks...alot regard ana i have to doing my final test...soo i need your source code for referensi Sign In·View Thread·PermaLink Performance improvement Timmay! 8:36 1 Jul '06   When you copy the image bytes in ExportBitmap (the for loop under the "//Copy the image" comment) you'll get far better performance if you do a block copy... Marshal.Copy(new IntPtr(address), bitmapData, 0, (int)bih.biSizeImage); Sign In·View Thread·PermaLink sound?? brian scott 9:59 29 Dec '05   Hi Corinne! Your article seems to deal only with the video stream. I'm afraid I don't know C#, so this question may be redundant. If I understand correctly, you create bitmaps from the video stream, process them with the message to be hidden and then create a new video stream from the processed bitmaps? So what if the avi has a sound track? How do you recombine the processed video with the original soundtrack? Brian