验证码 captcha 是对抗密码强力破解、垃圾信息的有效方式，一般用于用户注册、登录，当检测到频繁发帖时也会启用验证码。下面介绍用golang 生成防机器识别的图片验证码。
验证码字符
如果考虑用户输入体验，一般只用数字验证码，考虑到安全，防识别、碰撞，要生成6个数字的验证码，并且使字符弯曲变形、变色、加干扰。如下图效果：
captcha
github 上的 github.com/dchest/captcha 库已经做得很好，下面是参照官方的例子做一下改动。
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package main
import (
"fmt"
"github.com/dchest/captcha"
"io"
"log"
"net/http"
"text/template"
)
var formTemplate = template.Must(template.New("example").Parse(formTemplateSrc))
func showFormHandler(w http.ResponseWriter, r *http.Request) {
if r.URL.Path != "/" {
http.NotFound(w, r)
return
}
d := struct {
CaptchaId string
}{
captcha.New(),
}
if err := formTemplate.Execute(w, &d); err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
}
func processFormHandler(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "text/html; charset=utf-8")
if !captcha.VerifyString(r.FormValue("captchaId"), r.FormValue("captchaSolution")) {
io.WriteString(w, "Wrong captcha solution! No robots allowed!\n")
} else {
io.WriteString(w, "Great job, human! You solved the captcha.\n")
}
io.WriteString(w, "<br><a href='/'>Try another one</a>")
}
func main() {
http.HandleFunc("/", showFormHandler)
http.HandleFunc("/process", processFormHandler)
http.Handle("/captcha/", captcha.Server(captcha.StdWidth, captcha.StdHeight))
fmt.Println("Server is at localhost:8666")
if err := http.ListenAndServe("localhost:8666", nil); err != nil {
log.Fatal(err)
}
}
const formTemplateSrc = `<!doctype html>
<head><title>Captcha Example</title></head>
<body>
<script>
function setSrcQuery(e, q) {
var src = e.src;
var p = src.indexOf('?');
if (p >= 0) {
src = src.substr(0, p);
}
e.src = src + "?" + q
}
function reload() {
setSrcQuery(document.getElementById('image'), "reload=" + (new Date()).getTime());
return false;
}
</script>
<form action="/process" method=post>
<p>Type the numbers you see in the picture below:</p>
<p><img id=image src="/captcha/{{.CaptchaId}}.png" alt="Captcha image"></p>
<a href="#" onclick="reload()">Reload</a>
<input type=hidden name=captchaId value="{{.CaptchaId}}"><br>
<input name=captchaSolution>
<input type=submit value=Submit>
</form>
`
注意事项
当验证以后 CaptchaId 会销毁，如果验证码验证不对，需要重新生成CaptchaId并把它传给前端显示。
captcha 项目地址 https://github.com/dchest/captcha