在我的例子中，运行应用程序的服务帐户没有访问私钥的权限。一旦我给予了这个许可，错误就消失了

mmc 证书 扩展到个人 选择证书 右击 所有任务 管理私钥 添加业务帐户用户

我在尝试访问https://ct.mob0.com/Styles/Fun.png时遇到了这个问题，这是CloudFlare在其CDN上分发的一个图像，支持SPDY和奇怪的重定向SSL证书等疯狂的东西。

我没有像Simons的答案那样指定Ssl3，而是通过下面的Tls12来修复它:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
new WebClient().DownloadData("https://ct.mob0.com/Styles/Fun.png");

这为我固定，添加网络服务的权限。 右键单击证书>所有任务>管理私钥…>添加…>添加“网络服务”。

我最近也遇到了同样的问题。我的环境运行在。net 4.6.1和VB.NET下。这就是我解决问题的方法:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3
ServicePointManager.ServerCertificateValidationCallback = New RemoteCertificateValidationCallback(AddressOf util.ValidateServerCertificate)

还有util。ValidateServerCertificate函数为:

Public Function ValidateServerCertificate(ByVal sender As Object, ByVal certificate As X509Certificate, ByVal chain As X509Chain, ByVal sslPolicyErrors As SslPolicyErrors) As Boolean
    Return True
End Function

以上大多数答案都提到了会话算法或密钥交换算法。

在我的情况下，两者都是OK的，问题是在服务器的证书哈希算法，在客户端PC上没有启用。

我在应用程序的配置中添加了一个部分。

<system.diagnostics>
    <trace autoflush="true" />
    <sources>
        <source name="System.Net">
            <listeners>
                <add name="System.Net" />
            </listeners>
        </source>
        <source name="System.Net.Sockets">
            <listeners>
                <add name="System.Net" />
            </listeners>
        </source>
        <source name="System.Net.Cache">
            <listeners>
                <add name="System.Net" />
            </listeners>
        </source>
    </sources>
    <sharedListeners>
        <add
            name="System.Net"
            type="System.Diagnostics.TextWriterTraceListener"
            initializeData="System.Net.trace.log"
        />
    </sharedListeners>
    <switches>
        <add name="System.Net" value="Verbose" />
        <add name="System.Net.Sockets" value="Verbose" />
        <add name="System.Net.Cache" value="Verbose" />
    </switches>
</system.diagnostics>

然后log中的错误让我得到了这个解

After days of pulling what hair I have left out, we solved the problem. I tried everything suggested on this post and nothing worked for me. For us, we had a basic .Net Framework 4.8 console app running on a customers Windows VM. The on-premise server we were communicating with required that SSL Certificate Validation was turned off. One of our guys discovered that the server required we were using TLS 1.0 and on the registry settings of the VM, TLS 1.0 was disabled. Once we enabled that, it worked. I also needed to added the following two lines as mentioned many times above:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls;
ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };