[ASP.NET]驗證碼-登入常用機制

2011-09-21

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ASP.NET
2011-09-23

在登入時或從網頁寄Mail時常要用到驗證碼，利用驗證碼防止機器人攻擊. 所謂驗證碼，就是將一串隨機產生的數字、符號、文字等，變成一個圖案。


 Dim chkCode As String = String.Empty
        '顏色列表，用於驗證碼、噪線、噪點 
        Dim color As Color() = {Drawing.Color.Navy, Drawing.Color.Black, Drawing.Color.Red, Drawing.Color.Blue, Drawing.Color.Green, Drawing.Color.Orange, Drawing.Color.Brown, Drawing.Color.Brown, Drawing.Color.DarkBlue}
        '字體列表，用於驗證碼 
        Dim font As String() = {"Times New Roman", "MS Mincho", "Book Antiqua", "Gungsuh", "PMingLiU", "Impact"}

        '驗證碼的字元集，去掉了一些容易混淆的字元 
        Dim character As Char() = {"2"c, "3"c, "4"c, "5"c, "6"c, "8"c, _
        "9"c, "A"c, "B"c, "C"c, "D"c, "E"c, _
        "F"c, "G"c, "H"c, "J"c, "K"c, "L"c, _
        "M"c, "N"c, "P"c, "R"c, "S"c, "T"c, _
        "W"c, "X"c, "Y"c}
        Dim rnd As New Random()

        For i As Integer = 0 To 3
            '生成驗證碼字串 

            chkCode += character(rnd.[Next](character.Length))
        Next

        Dim bmp As New Bitmap(100, 40)
        Dim g As Graphics = Graphics.FromImage(bmp)
        g.Clear(Drawing.Color.White)

        For i As Integer = 0 To 9
            '畫噪線 
            Dim x1 As Integer = rnd.[Next](100)
            Dim y1 As Integer = rnd.[Next](40)
            Dim x2 As Integer = rnd.[Next](100)
            Dim y2 As Integer = rnd.[Next](40)
            Dim clr As Color = color(rnd.[Next](color.Length))
            g.DrawLine(New Pen(clr), x1, y1, x2, y2)
        Next
        For i As Integer = 0 To chkCode.Length - 1
            '畫驗證碼字串 
            Dim fnt As String = font(rnd.[Next](font.Length))
            Dim ft As New Font(fnt, 18)
            Dim clr As Color = color(rnd.[Next](color.Length))
            g.DrawString(chkCode(i).ToString(), ft, New SolidBrush(clr), CSng(i) * 20 + 8, CSng(8))
        Next
        For i As Integer = 0 To 99
            '畫噪點 
            Dim x As Integer = rnd.[Next](bmp.Width)
            Dim y As Integer = rnd.[Next](bmp.Height)
            Dim clr As Color = color(rnd.[Next](color.Length))
            bmp.SetPixel(x, y, clr)
        Next
        '清除該頁輸出緩存，設置該頁無緩存 
        Response.Buffer = True
        Response.ExpiresAbsolute = System.DateTime.Now.AddMilliseconds(0)
        Response.Expires = 0
        Response.CacheControl = "no-cache"
        Response.AppendHeader("Pragma", "No-Cache")
        '將驗證碼圖片寫入記憶體流，並將其以 "image/Png" 格式輸出 
        Dim ms As New MemoryStream()
        Try
            bmp.Save(ms, ImageFormat.Png)
            Response.ClearContent()
            Response.ContentType = "image/Png"
            Response.BinaryWrite(ms.ToArray()) '顯示圖形顯證碼
        Finally
            '顯式釋放資源 
            bmp.Dispose()
            g.Dispose()
        End Try

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