微软的SSL诊断工具可能能够帮助识别问题。

更新链接已经修复。

下面的代码片段将修复您正在调用的服务器上的SSL证书有问题的情况。例如，证书可能是自签名的，或者证书和服务器之间的主机名可能不匹配。

如果您正在调用您直接控制之外的服务器，这是危险的，因为您不能再确定您正在与您认为连接到的服务器通信。但是，如果您处理的是内部服务器，而获得“正确的”证书是不现实的，请使用以下命令告诉web服务忽略证书问题并勇敢地继续工作。

前两个使用lambda表达式，第三个使用常规代码。第一个接受任何证书。后两者至少检查证书中的主机名是否是您所期望的主机名。 ．.． 希望对你有帮助

//Trust all certificates
System.Net.ServicePointManager.ServerCertificateValidationCallback =
    ((sender, certificate, chain, sslPolicyErrors) => true);

// trust sender
System.Net.ServicePointManager.ServerCertificateValidationCallback
                = ((sender, cert, chain, errors) => cert.Subject.Contains("YourServerName"));

// validate cert by calling a function
ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback(ValidateRemoteCertificate);

// callback used to validate the certificate in an SSL conversation
private static bool ValidateRemoteCertificate(object sender, X509Certificate cert, X509Chain chain, SslPolicyErrors policyErrors)
{
    bool result = cert.Subject.Contains("YourServerName");
    return result;
}

试试这个:

System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;

请注意，您必须至少使用4.5 .NET框架

我已经用了一段时间了如果有用的话。

调用方必须显式地请求需要不受信任的认证，并在完成时将回调放回默认状态。

    /// <summary>
    /// Helper method for returning the content of an external webpage
    /// </summary>
    /// <param name="url">URL to get</param>
    /// <param name="allowUntrustedCertificates">Flags whether to trust untrusted or self-signed certificates</param>
    /// <returns>HTML of the webpage</returns>
    public static string HttpGet(string url, bool allowUntrustedCertificates = false) {
        var oldCallback = ServicePointManager.ServerCertificateValidationCallback;
        string webPage = "";
        try {
            WebRequest req = WebRequest.Create(url);

            if (allowUntrustedCertificates) {
                // so we can query self-signed certificates
                ServicePointManager.ServerCertificateValidationCallback = 
                    ((sender, certification, chain, sslPolicyErrors) => true);
            }

            WebResponse resp = req.GetResponse();
        
            using (StreamReader sr = new StreamReader(resp.GetResponseStream())) {
                webPage = sr.ReadToEnd().Trim();
                sr.Close();
            }
            return webPage;
        }
        catch {
            // if the remote site fails to response (or we have no connection)
            return null;
        }
        finally {
            ServicePointManager.ServerCertificateValidationCallback = oldCallback;
        }
    }

如果你使用的是Windows 2003，你可以这样做:

Open Microsoft Management Console (Start --> Run --> mmc.exe); Choose File --> Add/Remove Snap-in; In the Standalone tab, choose Add; Choose the Certificates snap-in, and click Add; In the wizard, choose the Computer Account, and then choose Local Computer. Press Finish to end the wizard; Close the Add/Remove Snap-in dialog; Navigate to Certificates (Local Computer) and choose a store to import: If you have the Root CA certificate for the company that issued the certificate, choose Trusted Root Certification Authorities; If you have the certificate for the server itself, choose Other People Right-click the store and choose All Tasks --> Import Follow the wizard and provide the certificate file you have; After that, simply restart IIS and try calling the web service again.

参考:http://www.outsystems.com/NetworkForums/ViewTopic.aspx?Topic=Web-Services: -Could-not-establish-trust-relationship-for-the-SSL / TLS -…

我刚刚遇到了这个问题。我的解决方案是通过手动同步时间服务器来更新系统时间。要做到这一点，你可以:

右键单击任务栏中的时钟 选择“调整日期/时间” 选择“Internet Time”选项卡 单击“更改设置” 选择立即更新

在我的情况下，这是不正确的同步，所以我必须点击它几次才能正确更新。如果它继续不正确地更新，您甚至可以尝试从服务器下拉菜单中使用不同的时间服务器。