How to implement Custom Audio Capcha in ASP.Net

Question

We have created a Custom Capcha generator using C#.Net. Now we have requirement to upgrade our Capcha to include audio feature(Listen Capcha). Our major constraint is that w

Answer 1

I made something like this in the past. There was a need for back noise (instrumental music) merged with the concatenated letters/characters. In addition recordings in use we generated from different voices. All these increased the complexity for "smart code" able to extract the captcha text from voice. I had also to transform the final WAVE to MP3 (using lame_enc.dll) for obvious reasons. It's not so easy to explain you how to do it, so I include the very first draft version needed for the job, leaving out the MP3 code as it uses a third party DLL.

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Drawing.Text;
using System.IO;
using System.Reflection;

namespace suryakiran
{
    internal static partial class Extensions
    {
        internal static byte[] AudioCaptcha(this string text)
        {
            String en = "abcdefghijkoprstvx0123456789", Location = string.Concat(System.Web.HttpContext.Current.Request.ServerVariables["APPL_PHYSICAL_PATH"].ToString(), @"\bin\wav\");
            Int32 dataLength = 0, length = 0, sampleRate = 0, plus = 37500, p = 0;
            Int16 bitsPerSample = 0, channels = 0;
            Byte[] music, wav;
            Random r = new Random();
            p = r.Next(1, 4000000);
            p += (p % 150) + 44;
            Byte[] rb = new Byte[9 * plus];
        // read music
        using (FileStream fs = new FileStream(String.Format(Location + @"z{0}.wav", (r.Next() % 12) + 1), FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
        {
            wav = new Byte[44];
            fs.Read(wav, 0, 44);
            fs.Position = (long)p;
            fs.Read(rb, 0, rb.Length);
        }
        // make music
        using (MemoryStream ms = new MemoryStream())
        {
            channels = BitConverter.ToInt16(wav, 22); sampleRate = BitConverter.ToInt32(wav, 24); bitsPerSample = BitConverter.ToInt16(wav, 34);
            length = rb.Length; dataLength = rb.Length;
            ms.Write(new Byte[44], 0, 44); ms.Write(rb, 0, rb.Length);
            ms.Position = 0;
            using (BinaryWriter bw = new BinaryWriter(ms))
            {
                bw.Write(new char[4] { 'R', 'I', 'F', 'F' }); bw.Write(length);
                bw.Write(new char[8] { 'W', 'A', 'V', 'E', 'f', 'm', 't', ' ' });
                bw.Write((Int32)16); bw.Write((Int16)1);
                bw.Write(channels); bw.Write(sampleRate);
                bw.Write((Int32)(sampleRate * ((bitsPerSample * channels) / 8)));
                bw.Write((Int16)((bitsPerSample * channels) / 8));
                bw.Write(bitsPerSample); bw.Write(new char[4] { 'd', 'a', 't', 'a' }); bw.Write(dataLength);
                music = ms.ToArray();
                p = 0;
            }
        }

        using (MemoryStream final = new MemoryStream())
        {
            final.Write(music, 0, 44);
            // make voice
            using (MemoryStream msvoice = new MemoryStream())
            {
                msvoice.Write(new Byte[plus / 2], 0, plus / 2);
                length += plus; dataLength += plus / 2; p += plus / 2;
                for (var i = 0; i < text.Length; i++)
                {
                    String fn = String.Format(Location + @"{0}\{1}.wav", (r.Next() % 3), en.Substring(en.IndexOf(text.Substring(i, 1)), 1)).Replace("?", "qm");
                    wav = File.ReadAllBytes(fn);
                    Int32 size = BitConverter.ToInt32(wav, 4);
                    {
                        msvoice.Write(new Byte[plus / 2], 0, plus / 2);
                        length += plus; dataLength += plus / 2; p += plus / 2;
                    }
                    msvoice.Write(wav, 44, wav.Length - 44);
                    length += size; dataLength += size - 36;
                }
                msvoice.Position = 0;
                MemoryStream msmusic = new MemoryStream();
                msmusic.Write(music, 0, music.Length);
                msmusic.Position = 44;
                //merge;
                while (final.Length < msmusic.Length)
                    final.WriteByte((byte)(msvoice.ReadByte() - msmusic.ReadByte()));
                return final.ToArray();
            }
        }
    }

    internal static Byte[] VisualCaptcha(this String source)
    {
        try
        {
            Random r = new Random();
            Int32 w = 250, h = 75;
            String family = "Arial Rounded MT Bold";
            using (var bmp = new Bitmap(w, h, PixelFormat.Format32bppArgb))
            {
                Int32 m = 0, nm = 0;
                Color tc;
                using (var g = Graphics.FromImage(bmp))
                {
                    g.TextRenderingHint = TextRenderingHint.AntiAlias;
                    g.SmoothingMode = SmoothingMode.HighQuality;
                    g.Clear(Color.White);
                    SizeF size;
                    m = r.Next() % 9 + 1;
                    nm = r.Next() % 3;
                    tc = Color.FromArgb(255, 255, 255);
                    size = g.MeasureString(source, new Font(family, h * 1.2f, FontStyle.Bold), new SizeF(w * 1F, h * 1F));
                    using (var brush = new LinearGradientBrush(new Rectangle(0, 0, w, h), Color.Black, Color.Black, 45, false))
                    {
                        ColorBlend blend = new ColorBlend(6);
                        for (var i = 0; i < 6; i++) { blend.Positions[i] = i * (1 / 5F); blend.Colors[i] = r.RandomColor(255, 64, 128); }
                        brush.InterpolationColors = blend;

                        for (int wave = 0; wave < 2; wave++)
                        {
                            Int32 min = (15 + wave * 20);
                            PointF[] pt = new PointF[] { new PointF(16f, (float)r.Next(min, min + 10)), new PointF(240f, (float)r.Next(min + 10, min + 20)) };
                            List PointList = new List();
                            float curDist = 0, distance = 0;
                            for (int i = 0; i < pt.Length - 1; i++)
                            {
                                PointF ptA = pt[i], ptB = pt[i + 1];
                                float deltaX = ptB.X - ptA.X, deltaY = ptB.Y - ptA.Y;
                                curDist = 0;
                                distance = (float)Math.Sqrt(Math.Pow(deltaX, 2) + Math.Pow(deltaY, 2));
                                while (curDist < distance)
                                {
                                    curDist++;
                                    float offsetX = (float)((double)curDist / (double)distance * (double)deltaX);
                                    float offsetY = (float)((double)curDist / (double)distance * (double)deltaY);
                                    PointList.Add(new PointF(ptA.X + offsetX, ptA.Y + offsetY));
                                }
                            }
                            for (int i = 0; i < PointList.Count - 24; i = i + 24)
                            {
                                float x1 = PointList[i].X, y1 = PointList[i].Y, x2 = PointList[i + 24].X, y2 = PointList[i + 24].Y;
                                float angle = (float)((Math.Atan2(y2 - y1, x2 - x1) * 180 / 3.14159265));
                                g.TranslateTransform(x1, y1);
                                g.RotateTransform(angle);
                                Int32 pm = r.Next() % 2 + 1;
                                Point[] p1 = new Point[] { new Point(0, 0), new Point(3, -3 * pm), new Point(6, -4 * pm), new Point(9, -3 * pm), new Point(12, 0), new Point(15, 3 * pm), new Point(18, 4 * pm), new Point(21, 3 * pm), new Point(24, 0) };
                                using (var path = new GraphicsPath()) g.DrawLines(new Pen(brush, 2f), p1);
                                g.RotateTransform(-angle);
                                g.TranslateTransform(-x1, -y1);
                            }
                        }
                        using (var path = new GraphicsPath())
                        {
                            PointF[] points = new PointF[] { };
                            if (m == 1 || m == 2 || m == 3) // star trek inverse
                            {
                                path.AddString(source, new FontFamily(family), 1, h * 0.75F, new PointF((w - size.Width) / 2F, (h * 0.9F - size.Height) / 2F), StringFormat.GenericTypographic);
                                points = new PointF[] { new PointF(0, 0), new PointF(w, 0), new PointF(w * 0.2F, h), new PointF(w * 0.8F, h) };
                            }
                            else if (m == 4 || m == 5) // star trek
                            {
                                path.AddString(source, new FontFamily(family), 1, h * 0.75F, new PointF((w - size.Width) / 2F, (h * 1.2F - size.Height) / 2F + 2F), StringFormat.GenericTypographic);
                                points = new PointF[] { new PointF(w * 0.2F, 0), new PointF(w * 0.8F, 0), new PointF(0, h), new PointF(w, h) };
                            }
                            else if (m == 6 || m == 7) // grow from left
                            {
                                path.AddString(source, new FontFamily(family), 1, h * 0.75F, new PointF((w * 1.15F - size.Width) / 2F, (h - size.Height) / 2F), StringFormat.GenericTypographic);
                                points = new PointF[] { new PointF(0, h * 0.25F), new PointF(w, 0), new PointF(0, h * 0.75F), new PointF(w, h) };
                            }
                            else if (m == 8 || m == 9) // grow from right
                            {
                                path.AddString(source, new FontFamily(family), 1, h * 0.75F, new PointF((w * 0.85F - size.Width) / 2F, (h - size.Height) / 2F), StringFormat.GenericTypographic);
                                points = new PointF[] { new PointF(w * 0.1F, 0), new PointF(w * 0.9F, h * 0.25F), new PointF(w * 0.1F, h), new PointF(w * 0.9F, h * 0.75F) };
                            }
                            path.Warp(points, new RectangleF(0, 0, w, h));
                            g.FillPath(Brushes.White, path);
                            g.DrawPath(new Pen(brush, 2F), path);
                        }
                    }
                }
                using (var thumb = new Bitmap(128, 40, PixelFormat.Format32bppArgb))
                {
                    using (var g = Graphics.FromImage(thumb))
                    {
                        g.CompositingQuality = CompositingQuality.HighQuality;
                        g.SmoothingMode = SmoothingMode.HighQuality;
                        g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                        Rectangle tr = new Rectangle(0, 0, thumb.Width, thumb.Height);
                        g.DrawImage(bmp, tr);
                        g.DrawRectangle(new Pen(Brushes.White), new Rectangle(0, 0, 127, 39));
                    }
                    using (var ms = new MemoryStream())
                    {
                        ((Image)thumb).Save(ms, ImageFormat.Png);
                        return ms.ToArray();
                    }
                }
            }
        }
        catch { return null; }
    }

    private static Color RandomColor(this Random rnd, Int32 alpha, Int32 min, Int32 max)
    {
        return Color.FromArgb(alpha, rnd.Next(min, max), rnd.Next(min, max), rnd.Next(min, max));
    }
}

}

For both visual and audio generation the code is independent from the captcha generation (source text). Each letter audio file in WAVE format was about 30KB but each music file was at least 8MB, giving me a wide range for random starting positions (avoiding - frequently - repeated patterns). You can compile the code as a dll or place it (modified) in a code behind file.