验证码生成原理的解剖

目前，许多网站的会员登录时都要求输入验证码，尽管验证码的形式五花八门，但是所使用的原理基本是一样的，都是生成随机字符串，然后描绘成图片的形式输出。

     验证码的生产主要分两部分：1是随机字符串的生成；2是生产验证码图片

1   随机字符串的生成：

     随机字符串生成有很多方法，这里介绍一种利用字符数组生产随机串的方法

     代码如图1所示，自定义自己的随机字符数组，然后使用随机函数随机抽取4个字符组成一个随机字符串

 

/// <summary>

   /// 生成随机字符串

   /// </summary>

   /// <returns></returns>

   private string GenerateCheckCode()

   {

       //  PageUtils.VerifyCode = checkCode.ToLower();

 

 

       //自定义自己验证码需要显示的字符

       //如果仅仅是数字和26个字母，您可以写一个循环遍历

       char[] myCodeChar = { '1', '2', '3', '4', '5', 'a', 'b', 'c', 'd', 'e', 'f' };

       int number;

       char code;

       string checkCode = string.Empty;

 

 

       //使用随机函数，产生一个随机数

       System.Random random = new Random();

       for (int i = 0; i < 4; i++)

       {

           number = random.Next(myCodeChar.Length);

 

           code = (char)(myCodeChar[number]);

 

           checkCode += code.ToString();

       }

 

       Session["checkCode"] = checkCode.ToLower();//把生成的验证码存入session

       return checkCode;

   }

                                                           图1

2 生产验证码图片，其实就是用c#的Graphics把生成德随机串描绘到图片上，具体请看图2代码示意：

 

 

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/// <summary>    /// 生成验证码图片，并输出    /// </summary>    /// <param name="checkCode"></param>    private void CreateCheckCodeImage(string checkCode)    {        if (checkCode == null || checkCode.Trim() == String.Empty) return;          Matrix m = new Matrix();//定义几何变换        Bitmap charbmp = new Bitmap(90, 30);//图片前景色，即生成背景透明的随机字符串图片          //定义字体        Font[] fonts = {                                        new Font(new FontFamily("Times New Roman"), 17, FontStyle.Regular),                                        new Font(new FontFamily("Georgia"), 17, FontStyle.Regular),                                        new Font(new FontFamily("Arial"), 17, FontStyle.Regular),                                        new Font(new FontFamily("Comic Sans MS"), 17, FontStyle.Regular)                                     };                   //定义图片背景色        System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length * 22.5)), 30);          //开始描绘        Graphics g = Graphics.FromImage(image);          g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighSpeed;          //定义背景色为白色        g.Clear(Color.White);        try        {            Random random = new Random();       //生成随机生成器            g.Clear(Color.White);               //清空图片背景色            for (int i = 0; i < 2; i++)              //画图片的背景噪音线，i表示画多少条噪音线            {                int x1 = random.Next(image.Width);                int x2 = random.Next(image.Width);                int y1 = random.Next(image.Height);                int y2 = random.Next(image.Height);                g.DrawLine(new Pen(Color.Black), x1, y1, x2, y2);            }                            //开始描绘前景图            Graphics charg = Graphics.FromImage(charbmp);              SolidBrush drawBrush = new SolidBrush(Color.FromArgb(random.Next(101), random.Next(101), random.Next(101)));            float charx = -18;              //把随机字符串，逐个写入前景图            for (int i = 0; i < checkCode.Length; i++)            {                m.Reset();                m.RotateAt(random.Next(31) - 25, new PointF(random.Next(4) + 7, random.Next(4) + 7));                  charg.Clear(Color.Transparent);//定义前景图为透明                charg.Transform = m;                //定义前景色为黑色                drawBrush.Color = Color.Black;                  charx = charx + 20 + random.Next(3);                PointF drawPoint = new PointF(charx, 0.1F);                charg.DrawString(checkCode[i].ToString(), fonts[random.Next(fonts.Length)], drawBrush, new PointF(0, 0));//通过特定的几何变换，旋转或变形随机字符，写入前景图                  charg.ResetTransform();                  g.DrawImage(charbmp, drawPoint);            }                //画图片的前景噪音点            for (int i = 0; i < 25; i++)            {                int x = random.Next(image.Width);                int y = random.Next(image.Height);                  image.SetPixel(x, y, Color.FromArgb(random.Next()));            }              //画图片的边框线            g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);              //输出            System.IO.MemoryStream ms = new System.IO.MemoryStream();            image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);            Response.ClearContent();            Response.ContentType = "image/Gif";            Response.BinaryWrite(ms.ToArray());        }        finally        {            g.Dispose();            image.Dispose();        }    }