~~NOTOC~~
====== Szenario 9.1: mTLS Client-Authentifizierung ======

<WRAP round info>
**Kategorie:** [[.:start|Authentifizierung]] \\
**Komplexität:** ⭐⭐⭐⭐ (Hoch) \\
**Voraussetzungen:** Client- und Server-Zertifikate \\
**Geschätzte Zeit:** 20-30 Minuten
</WRAP>

----

===== Beschreibung =====

Dieses Szenario beschreibt die **Mutual TLS (mTLS) Authentifizierung**, bei der sowohl Server als auch Client sich gegenseitig mit Zertifikaten authentifizieren.

**Vorteile gegenüber Passwort-Authentifizierung:**
  * **Stärker** - Kryptographisch basiert
  * **Automatisierbar** - Keine Benutzerinteraktion
  * **Revozierbar** - Zentrale Widerrufsmöglichkeit
  * **Zero-Trust** - Jeder Request authentifiziert

----

===== Workflow =====

<mermaid>
sequenceDiagram
    participant Client
    participant Server

    Client->>Server: ClientHello
    Server->>Client: ServerHello + ServerCert
    Server->>Client: CertificateRequest
    Client->>Server: ClientCert + CertificateVerify
    Server->>Server: Client-Zertifikat validieren
    Server->>Client: Finished
    Client->>Server: Finished
    Note over Client,Server: mTLS-Verbindung etabliert
</mermaid>

----

===== Code-Beispiel: mTLS Server (ASP.NET Core) =====

<code csharp>
using Microsoft.AspNetCore.Authentication.Certificate;
using Microsoft.AspNetCore.Server.Kestrel.Https;

var builder = WebApplication.CreateBuilder(args);

// Kestrel für mTLS konfigurieren
builder.WebHost.ConfigureKestrel(options =>
{
    options.ConfigureHttpsDefaults(https =>
    {
        // Server-Zertifikat laden
        https.ServerCertificate = new X509Certificate2(
            "server.pfx", "ServerPassword!");

        // Client-Zertifikat anfordern
        https.ClientCertificateMode = ClientCertificateMode.RequireCertificate;

        // Client-Zertifikat validieren
        https.ClientCertificateValidation = (cert, chain, errors) =>
        {
            // Custom Validation
            if (errors != SslPolicyErrors.None)
            {
                Console.WriteLine($"SSL Policy Errors: {errors}");
                return false;
            }

            // Eigene CA prüfen
            var trustedRoots = new X509Certificate2Collection();
            trustedRoots.Add(new X509Certificate2("root-ca.crt"));

            var customChain = new X509Chain();
            customChain.ChainPolicy.CustomTrustStore.AddRange(trustedRoots);
            customChain.ChainPolicy.TrustMode = X509ChainTrustMode.CustomRootTrust;
            customChain.ChainPolicy.RevocationMode = X509RevocationMode.Online;

            return customChain.Build(cert);
        };
    });
});

// Certificate Authentication hinzufügen
builder.Services.AddAuthentication(CertificateAuthenticationDefaults.AuthenticationScheme)
    .AddCertificate(options =>
    {
        options.AllowedCertificateTypes = CertificateTypes.Chained;
        options.RevocationMode = X509RevocationMode.Online;

        options.Events = new CertificateAuthenticationEvents
        {
            OnCertificateValidated = context =>
            {
                // Claims aus Zertifikat extrahieren
                var claims = new[]
                {
                    new Claim(ClaimTypes.NameIdentifier, context.ClientCertificate.Subject),
                    new Claim(ClaimTypes.Name, context.ClientCertificate.GetNameInfo(X509NameType.SimpleName, false)),
                    new Claim("certificate_thumbprint", context.ClientCertificate.Thumbprint)
                };

                context.Principal = new ClaimsPrincipal(
                    new ClaimsIdentity(claims, context.Scheme.Name));

                context.Success();
                return Task.CompletedTask;
            },

            OnAuthenticationFailed = context =>
            {
                Console.WriteLine($"Auth failed: {context.Exception.Message}");
                return Task.CompletedTask;
            }
        };
    });

builder.Services.AddAuthorization();

var app = builder.Build();

app.UseAuthentication();
app.UseAuthorization();

app.MapGet("/api/protected", [Authorize] (HttpContext ctx) =>
{
    var subject = ctx.User.FindFirst(ClaimTypes.NameIdentifier)?.Value;
    return Results.Ok(new { message = "Authenticated", subject });
});

app.Run();
</code>

----

===== Code-Beispiel: mTLS Client (HttpClient) =====

<code csharp>
using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;

using var ctx = PqCryptoContext.Initialize();

// Client-Zertifikat und Schlüssel laden
var clientCert = ctx.LoadCertificateWithPrivateKey(
    "client.crt.pem",
    "client.key.pem",
    "ClientPassword!"
);

// HttpClientHandler mit Client-Zertifikat
var handler = new HttpClientHandler
{
    ClientCertificateOptions = ClientCertificateOption.Manual,
    ServerCertificateCustomValidationCallback = (message, cert, chain, errors) =>
    {
        // Server-Zertifikat validieren
        if (errors != SslPolicyErrors.None)
        {
            Console.WriteLine($"Server certificate error: {errors}");
            // In Produktion: return false;
        }
        return true;
    }
};

// Client-Zertifikat hinzufügen
handler.ClientCertificates.Add(clientCert);

using var httpClient = new HttpClient(handler);

// Request senden
var response = await httpClient.GetAsync("https://api.example.com/api/protected");

if (response.IsSuccessStatusCode)
{
    var content = await response.Content.ReadAsStringAsync();
    Console.WriteLine($"Response: {content}");
}
else
{
    Console.WriteLine($"Error: {response.StatusCode}");
}
</code>

----

===== Zertifikats-basierte Autorisierung =====

<code csharp>
public class CertificateAuthorizationHandler : AuthorizationHandler<CertificateRequirement>
{
    protected override Task HandleRequirementAsync(
        AuthorizationHandlerContext context,
        CertificateRequirement requirement)
    {
        // Zertifikat aus Principal holen
        var thumbprint = context.User.FindFirst("certificate_thumbprint")?.Value;
        var subject = context.User.FindFirst(ClaimTypes.NameIdentifier)?.Value;

        if (string.IsNullOrEmpty(thumbprint))
        {
            return Task.CompletedTask;
        }

        // Prüfen ob Zertifikat autorisiert ist
        if (requirement.AllowedThumbprints.Contains(thumbprint) ||
            requirement.AllowedSubjectPatterns.Any(p => Regex.IsMatch(subject, p)))
        {
            context.Succeed(requirement);
        }

        return Task.CompletedTask;
    }
}

public class CertificateRequirement : IAuthorizationRequirement
{
    public HashSet<string> AllowedThumbprints { get; set; } = new();
    public List<string> AllowedSubjectPatterns { get; set; } = new();
}

// Registrierung
builder.Services.AddAuthorization(options =>
{
    options.AddPolicy("ServiceAccounts", policy =>
        policy.Requirements.Add(new CertificateRequirement
        {
            AllowedSubjectPatterns = new List<string>
            {
                @"^CN=.*\.svc\.cluster\.local$",  // Kubernetes services
                @"^CN=service-[a-z]+$"            // Named services
            }
        }));
});

builder.Services.AddSingleton<IAuthorizationHandler, CertificateAuthorizationHandler>();
</code>

----

===== Nginx als mTLS Reverse Proxy =====

<code nginx>
server {
    listen 443 ssl;
    server_name api.example.com;

    # Server-Zertifikat
    ssl_certificate     /etc/nginx/ssl/server-chain.pem;
    ssl_certificate_key /etc/nginx/ssl/server.key.pem;

    # Client-Zertifikats-Authentifizierung
    ssl_client_certificate /etc/nginx/ssl/ca-chain.pem;
    ssl_verify_client on;
    ssl_verify_depth 2;

    # TLS 1.3
    ssl_protocols TLSv1.3;
    ssl_prefer_server_ciphers on;

    # OCSP Stapling
    ssl_stapling on;
    ssl_stapling_verify on;

    location / {
        # Client-Zertifikat-Info an Backend weitergeben
        proxy_set_header X-Client-Cert $ssl_client_cert;
        proxy_set_header X-Client-Verify $ssl_client_verify;
        proxy_set_header X-Client-S-DN $ssl_client_s_dn;

        proxy_pass http://backend:8080;
    }
}
</code>

----

===== Branchenspezifische mTLS-Anforderungen =====

^  Branche  ^  Zertifikatslaufzeit  ^  Revocation  ^  Besonderheit  ^
| **Kubernetes** | 1 Jahr | CRL | SPIFFE/SPIRE |
| **Finanzsektor** | 90 Tage | OCSP | PCI-DSS compliant |
| **Healthcare** | 1 Jahr | CRL + OCSP | DiGAV-konform |
| **IoT/SCADA** | 5 Jahre | CRL | Offline-Fähigkeit |

----

===== Verwandte Szenarien =====

^  Beziehung  ^  Szenario  ^  Beschreibung  ^
| **Voraussetzung** | [[de:int:pqcrypt:szenarien:zertifikate:client_cert|3.2 Client-Zertifikat]] | Client-Cert erstellen |
| **Verwandt** | [[.:smartcard_login|9.2 Smartcard Login]] | Hardware-Token |
| **Verwandt** | [[de:int:pqcrypt:szenarien:tls:mtls_deployment|10.3 mTLS Deployment]] | TLS-Konfiguration |

----

<< [[.:start|← Authentifizierung-Übersicht]] | [[de:int:pqcrypt:szenarien:start|↑ Szenarien]] | [[.:smartcard_login|9.2 Smartcard Login →]] >>

{{tag>szenario authentifizierung mtls client-zertifikat zero-trust}}

----
//Wolfgang van der Stille @ EMSR DATA d.o.o. - Post-Quantum Cryptography Professional//