现在很多网页登陆的时候都使用了随机图片的方式，是一种简单、有效的防止黑客恶意攻击的手段。今天看了一些网上的资料，明白其生成原理：从样本中，获取随机字符串，随机字符串保存进session，并以位图的方式形成随机码图片。
实现：
添加命名空间
using
System.Drawing;
using
System.Drawing.Imaging;
using
System.Drawing.Drawing2D;
生成页代码
using
System;
using
System.Data;
using
System.Configuration;
using
System.Collections;
using
System.Web;
using
System.Web.Security;
using
System.Web.UI;
using
System.Web.UI.WebControls;
using
System.Web.UI.WebControls.WebParts;
using
System.Web.UI.HtmlControls;
using
System.Drawing;
using
System.Drawing.Imaging;
using
System.Drawing.Drawing2D;
public
partial
class
getRandImg : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{ //生成随机码图片 SetValidateCode(); //生成页面不保存到cache Response.Cache.SetNoStore();
} //设置验证码 private void SetValidateCode() { //新建位图 Bitmap newBitmap = new Bitmap( 71, 23, PixelFormat.Format32bppArgb ); //从位图获得绘图画面 Graphics g = Graphics.FromImage(newBitmap); //随机数生成器 Random r = new Random(); //绘图画面清空 g.Clear(Color.White); //绘图画面划线干扰 for (int i = 0; i < 50; i++) { int x1 = r.Next(newBitmap.Width); int x2 = r.Next(newBitmap.Width); int y1 = r.Next(newBitmap.Height); int y2 = r.Next(newBitmap.Height); g.DrawLine(new Pen( Color.FromArgb(r.Next())), x1, y1, x2, y2 ); } //绘图画面点数干扰 for (int i = 0; i < 100; i++) { int x = r.Next(newBitmap.Width); int y = r.Next(newBitmap.Height); newBitmap.SetPixel( x, y, Color.FromArgb(r.Next()) ); } //获得随机字符串（5位长度） string value = GenerateRandom(5); //随机字符串赋值给Session Session["RandCode"] = value; //定义图片显示字体样式 Font font = new Font( "Arial", 14, FontStyle.Bold ); Random rr = new Random(); int yy = rr.Next(1, 4); //定义随机字符串显示图片刷子 LinearGradientBrush brush = new LinearGradientBrush( new Rectangle(0, 0, 71, 23), Color.Red, Color.Blue, 1.2f, true ); g.DrawString(value, font, brush, 2, yy); g.DrawRectangle(new Pen( Color.Silver), 0, 0, 70, 22 ); System.IO.MemoryStream ms = new System.IO.MemoryStream(); newBitmap.Save(ms, ImageFormat.Gif); //输出图片 Response.ClearContent(); Response.ContentType = "image/gif"; Response.BinaryWrite(ms.ToArray()); } //常量集 private static char[] constant ={ '0','1','2','3','4','5','6','7','8','9', 'a','b','c','d','e','f','g','h','i','j', 'k','l','m','n','o','p','q','r','s','t', 'u','v','w','x','y','z' }; //生成随机字符串 public static string GenerateRandom(int Length) { System.Text.StringBuilder newRandom = new System.Text.StringBuilder(36); Random rd = new Random(); for (int i = 0; i < Length; i++) { newRandom.Append(constant[rd.Next(36)]); } return newRandom.ToString(); }
}
使用随机图片的页面，IMAGE控件的写法如下：
<
asp:Image
ID
="Image1"
ImageUrl
="~/getRandImg.aspx"
runat
="server"
/>
