如何验证贝宝支付的真实性或不是C#

using System;
using System.Collections.Specialized;
using System.Net;
using System.Text;
using System.Web;

public class PayPalIPNHandler
{
    private const string PayPalUrl = "https://www.paypal.com/cgi-bin/webscr";
    private const string VerifyUrl = "cmd=_notify-validate";

    public bool ProcessIPN(string requestBody)
    {
        // 解析请求体中的参数
        NameValueCollection formData = HttpUtility.ParseQueryString(requestBody);

        // 向PayPal发送验证请求
        string response = SendVerifyRequest(formData);

        // 验证响应
        return response.Equals("VERIFIED", StringComparison.OrdinalIgnoreCase);
    }

    private string SendVerifyRequest(NameValueCollection formData)
    {
        using (WebClient client = new WebClient())
        {
            // 添加cmd参数
            formData.Add(VerifyUrl, string.Empty);

            // 构建POST数据
            byte[] postData = Encoding.ASCII.GetBytes(formData.ToString());

            // 发送POST请求到PayPal
            byte[] responseData = client.UploadValues(PayPalUrl, "POST", postData);

            // 返回响应字符串
            return Encoding.ASCII.GetString(responseData);
        }
    }
}

protected void Page_Load(object sender, EventArgs e)
{
    if (Request.HttpMethod == "POST")
    {
        string requestBody = Request.Form.ToString();
        PayPalIPNHandler handler = new PayPalIPNHandler();
        bool isVerified = handler.ProcessIPN(requestBody);

        if (isVerified)
        {
            // 处理有效的支付
            // 更新数据库，发送确认邮件等
        }
        else
        {
            // 处理无效的支付
            // 记录日志，通知管理员等
        }
    }
}