将自签名证书与 .NET 的 HttpWebRequest/Response 一起使用

Question

我正在尝试连接到使用自签名 SSL 证书的 API。我这样做是使用 .NET 的 HttpWebRequest 和 HttpWebResponse 对象。我遇到了一个例外：

底层连接已关闭：无法为 SSL/TLS 安全通道建立信任关系。

我明白这意味着什么。我理解为什么 .NET 认为它应该警告我并关闭连接。但在这种情况下，我还是想连接到 API，中间人攻击该死。

那么，我该如何为这个自签名证书添加一个例外呢？或者是告诉 HttpWebRequest/Response 根本不验证证书的方法？我该怎么做？

Answer 1

事实证明，如果您只想完全禁用证书验证，您可以更改 ServicePointManager 上的 ServerCertificateValidationCallback，如下所示：

ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };

这将验证所有证书（包括无效、过期或自签名证书）。

Answer 2

@Domster：可行，但您可能希望通过检查证书哈希是否符合您的预期来加强一点安全性。所以扩展版本看起来有点像这样（基于我们正在使用的一些实时代码）：

static readonly byte[] apiCertHash = { 0xZZ, 0xYY, ....};

/// <summary>
/// Somewhere in your application's startup/init sequence...
/// </summary>
void InitPhase()
{
    // Override automatic validation of SSL server certificates.
    ServicePointManager.ServerCertificateValidationCallback =
           ValidateServerCertficate;
}

/// <summary>
/// Validates the SSL server certificate.
/// </summary>
/// <param name="sender">An object that contains state information for this
/// validation.</param>
/// <param name="cert">The certificate used to authenticate the remote party.</param>
/// <param name="chain">The chain of certificate authorities associated with the
/// remote certificate.</param>
/// <param name="sslPolicyErrors">One or more errors associated with the remote
/// certificate.</param>
/// <returns>Returns a boolean value that determines whether the specified
/// certificate is accepted for authentication; true to accept or false to
/// reject.</returns>
private static bool ValidateServerCertficate(
        object sender,
        X509Certificate cert,
        X509Chain chain,
        SslPolicyErrors sslPolicyErrors)
{
    if (sslPolicyErrors == SslPolicyErrors.None)
    {
        // Good certificate.
        return true;
    }

    log.DebugFormat("SSL certificate error: {0}", sslPolicyErrors);

    bool certMatch = false; // Assume failure
    byte[] certHash = cert.GetCertHash();
    if (certHash.Length == apiCertHash.Length)
    {
        certMatch = true; // Now assume success.
        for (int idx = 0; idx < certHash.Length; idx++)
        {
            if (certHash[idx] != apiCertHash[idx])
            {
                certMatch = false; // No match
                break;
            }
        }
    }

    // Return true => allow unauthenticated server,
    //        false => disallow unauthenticated server.
    return certMatch;
}

Answer 3

将自签名证书添加到本地计算机受信任的根证书颁发机构

您可以通过以管理员身份运行 MMC 来导入证书。

How to: View Certificates with the MMC Snap-in

Answer 4

Domster's answer 中使用的验证回调的范围可以限制为使用ServerCertificateValidationCallback 委托上的 sender 参数的特定请求。下面的简单作用域类使用这种技术来临时连接一个仅针对给定请求对象执行的验证回调。

public class ServerCertificateValidationScope : IDisposable
{
    private readonly RemoteCertificateValidationCallback _callback;

    public ServerCertificateValidationScope(object request,
        RemoteCertificateValidationCallback callback)
    {
        var previous = ServicePointManager.ServerCertificateValidationCallback;
        _callback = (sender, certificate, chain, errors) =>
            {
                if (sender == request)
                {
                    return callback(sender, certificate, chain, errors);
                }
                if (previous != null)
                {
                    return previous(sender, certificate, chain, errors);
                }
                return errors == SslPolicyErrors.None;
            };
        ServicePointManager.ServerCertificateValidationCallback += _callback;
    }

    public void Dispose()
    {
        ServicePointManager.ServerCertificateValidationCallback -= _callback;
    }
}

上述类可用于忽略特定请求的所有证书错误，如下所示：

var request = WebRequest.Create(uri);
using (new ServerCertificateValidationScope(request, delegate { return true; }))
{
    request.GetResponse();
}

Answer 5

基于devstuff 的回答，包括主题和发行人...欢迎 cmets...

public class SelfSignedCertificateValidator
{
    private class CertificateAttributes
    {
        public string Subject { get; private set; }
        public string Issuer { get; private set; }
        public string Thumbprint { get; private set; }

        public CertificateAttributes(string subject, string issuer, string thumbprint)
        {
            Subject = subject;
            Issuer = issuer;                
            Thumbprint = thumbprint.Trim(
                new char[] { '\u200e', '\u200f' } // strip any lrt and rlt markers from copy/paste
                ); 
        }

        public bool IsMatch(X509Certificate cert)
        {
            bool subjectMatches = Subject.Replace(" ", "").Equals(cert.Subject.Replace(" ", ""), StringComparison.InvariantCulture);
            bool issuerMatches = Issuer.Replace(" ", "").Equals(cert.Issuer.Replace(" ", ""), StringComparison.InvariantCulture);
            bool thumbprintMatches = Thumbprint == String.Join(" ", cert.GetCertHash().Select(h => h.ToString("x2")));
            return subjectMatches && issuerMatches && thumbprintMatches; 
        }
    }

    private readonly List<CertificateAttributes> __knownSelfSignedCertificates = new List<CertificateAttributes> {
        new CertificateAttributes(  // can paste values from "view cert" dialog
            "CN = subject.company.int", 
            "CN = issuer.company.int", 
            "f6 23 16 3d 5a d8 e5 1e 13 58 85 0a 34 9f d6 d3 c8 23 a8 f4") 
    };       

    private static bool __createdSingleton = false;

    public SelfSignedCertificateValidator()
    {
        lock (this)
        {
            if (__createdSingleton)
                throw new Exception("Only a single instance can be instanciated.");

            // Hook in validation of SSL server certificates.  
            ServicePointManager.ServerCertificateValidationCallback += ValidateServerCertficate;

            __createdSingleton = true;
        }
    }

    /// <summary>
    /// Validates the SSL server certificate.
    /// </summary>
    /// <param name="sender">An object that contains state information for this
    /// validation.</param>
    /// <param name="cert">The certificate used to authenticate the remote party.</param>
    /// <param name="chain">The chain of certificate authorities associated with the
    /// remote certificate.</param>
    /// <param name="sslPolicyErrors">One or more errors associated with the remote
    /// certificate.</param>
    /// <returns>Returns a boolean value that determines whether the specified
    /// certificate is accepted for authentication; true to accept or false to
    /// reject.</returns>
    private bool ValidateServerCertficate(
        object sender,
        X509Certificate cert,
        X509Chain chain,
        SslPolicyErrors sslPolicyErrors)
    {
        if (sslPolicyErrors == SslPolicyErrors.None)
            return true;   // Good certificate.

        Dbg.WriteLine("SSL certificate error: {0}", sslPolicyErrors);
        return __knownSelfSignedCertificates.Any(c => c.IsMatch(cert));            
    }
}

Answer 6

注意，在 .NET 4.5 中，您可以覆盖每个 HttpWebRequest 本身的 SSL 验证（而不是通过影响所有请求的全局委托）：

http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.servercertificatevalidationcallback.aspx

HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(uri);
request.ServerCertificateValidationCallback = delegate { return true; };

Answer 7

要记住的一点是，拥有 ServicePointManager.ServerCertificateValidationCallback 似乎并不意味着没有完成 CRL 检查和服务器名称验证，它只是提供了一种覆盖其结果的方法。因此，您的服务可能仍需要一段时间才能获得 CRL，只有事后您才会知道它未通过某些检查。

Answer 8

为其他人添加可能的帮助...如果您希望它提示用户安装自签名证书，您可以使用此代码（从上面修改）。

不需要管理员权限，安装到本地用户受信任的配置文件：

    private static bool ValidateServerCertficate(
        object sender,
        X509Certificate cert,
        X509Chain chain,
        SslPolicyErrors sslPolicyErrors)
    {
        if (sslPolicyErrors == SslPolicyErrors.None)
        {
            // Good certificate.
            return true;
        }

        Common.Helpers.Logger.Log.Error(string.Format("SSL certificate error: {0}", sslPolicyErrors));
        try
        {
            using (X509Store store = new X509Store(StoreName.My, StoreLocation.CurrentUser))
            {
                store.Open(OpenFlags.ReadWrite);
                store.Add(new X509Certificate2(cert));
                store.Close();
            }
            return true;
        }
        catch (Exception ex)
        {
            Common.Helpers.Logger.Log.Error(string.Format("SSL certificate add Error: {0}", ex.Message));
        }

        return false;
    }

这似乎对我们的应用程序很有效，如果用户按否，通信将无法正常工作。

更新：2015-12-11 - 将 StoreName.Root 更改为 StoreName.My - 我将安装到本地用户存储中，而不是 Root。即使您“以管理员身份运行”，某些系统上的 Root 也无法正常工作

Answer 9

我遇到了与 OP 相同的问题，其中 Web 请求会引发确切的异常。我认为所有设置都正确，证书已安装，我可以很好地在机器存储中找到它并将其附加到 Web 请求中，并且我已禁用请求上下文中的证书验证。

原来我是在我的用户帐户下运行的，并且证书已安装到机器商店。这导致 Web 请求抛出此异常。为了解决这个问题，我必须以管理员身份运行或将证书安装到用户存储并从那里读取。

似乎 C# 能够在机器存储中找到证书，即使它不能用于 Web 请求，这会导致一旦发出 Web 请求就会引发 OP 的异常。

Answer 10

首先 - 我很抱歉，因为我使用了 @devstuff 描述的解决方案。不过，我已经找到了一些改进方法。

• 添加自签名证书处理
• 证书原始数据对比
• 实际的证书颁发机构验证
• 一些额外的 cmets 和改进

这是我的修改：

private static X509Certificate2 caCertificate2 = null;

/// <summary>
/// Validates the SSL server certificate.
/// </summary>
/// <param name="sender">An object that contains state information for this validation.</param>
/// <param name="cert">The certificate used to authenticate the remote party.</param>
/// <param name="chain">The chain of certificate authorities associated with the remote certificate.</param>
/// <param name="sslPolicyErrors">One or more errors associated with the remote certificate.</param>
/// <returns>Returns a boolean value that determines whether the specified certificate is accepted for authentication; true to accept or false to reject.</returns>
private static bool ValidateServerCertficate(
        object sender,
        X509Certificate cert,
        X509Chain chain,
        SslPolicyErrors sslPolicyErrors)
{
    if (sslPolicyErrors == SslPolicyErrors.None)
    {
        // Good certificate.
        return true;
    }

    // If the following line is not added, then for the self-signed cert an error will be (not tested with let's encrypt!):
    // "A certificate chain processed, but terminated in a root certificate which is not trusted by the trust provider. (UntrustedRoot)"
    chain.ChainPolicy.VerificationFlags = X509VerificationFlags.AllowUnknownCertificateAuthority;

    // convert old-style cert to new-style cert
    var returnedServerCert2 = new X509Certificate2(cert);

    // This part is very important. Adding known root here. It doesn't have to be in the computer store at all. Neither do certificates.
    chain.ChainPolicy.ExtraStore.Add(caCertificate2);

    // 1. Checks if ff the certs are OK (not expired/revoked/etc) 
    // 2. X509VerificationFlags.AllowUnknownCertificateAuthority will make sure that untrusted certs are OK
    // 3. IMPORTANT: here, if the chain contains the wrong CA - the validation will fail, as the chain is wrong!
    bool isChainValid = chain.Build(returnedServerCert2);
    if (!isChainValid)
    {
        string[] errors = chain.ChainStatus
            .Select(x => String.Format("{0} ({1})", x.StatusInformation.Trim(), x.Status))
            .ToArray();

        string certificateErrorsString = "Unknown errors.";

        if (errors != null && errors.Length > 0)
        {
            certificateErrorsString = String.Join(", ", errors);
        }

        Log.Error("Trust chain did not complete to the known authority anchor. Errors: " + certificateErrorsString);
        return false;
    }

    // This piece makes sure it actually matches your known root
    bool isValid = chain.ChainElements
        .Cast<X509ChainElement>()
        .Any(x => x.Certificate.RawData.SequenceEqual(caCertificate2.GetRawCertData()));

    if (!isValid)
    {
        Log.Error("Trust chain did not complete to the known authority anchor. Thumbprints did not match.");
    }

    return isValid;
}

设置证书：

caCertificate2 = new X509Certificate2("auth/ca.crt", "");
var clientCertificate2 = new X509Certificate2("auth/client.pfx", "");

传递委托方法

ServerCertificateValidationCallback(ValidateServerCertficate)

client.pfx 是使用 KEY 和 CERT 生成的：

openssl pkcs12 -export -in client.crt -inkey client.key -out client.pfx