~~NOTOC~~
====== Scenario 9.1: mTLS Client Authentication ======

<WRAP round info>
**Category:** [[.:start|Authentication]] \\
**Complexity:** **** (High) \\
**Prerequisites:** Client and server certificates \\
**Estimated Time:** 20-30 minutes
</WRAP>

----

===== Description =====

This scenario describes **Mutual TLS (mTLS) authentication**, where both server and client authenticate each other using certificates.

**Advantages over password authentication:**
  * **Stronger** - Cryptographically based
  * **Automatable** - No user interaction required
  * **Revocable** - Central revocation capability
  * **Zero-Trust** - Every request is authenticated

----

===== Workflow =====

<mermaid>
sequenceDiagram
    participant Client
    participant Server

    Client->>Server: ClientHello
    Server->>Client: ServerHello + ServerCert
    Server->>Client: CertificateRequest
    Client->>Server: ClientCert + CertificateVerify
    Server->>Server: Validate client certificate
    Server->>Client: Finished
    Client->>Server: Finished
    Note over Client,Server: mTLS connection established
</mermaid>

----

===== Code Example: mTLS Server (ASP.NET Core) =====

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

var builder = WebApplication.CreateBuilder(args);

// Configure Kestrel for mTLS
builder.WebHost.ConfigureKestrel(options =>
{
    options.ConfigureHttpsDefaults(https =>
    {
        // Load server certificate
        https.ServerCertificate = new X509Certificate2(
            "server.pfx", "ServerPassword!");

        // Require client certificate
        https.ClientCertificateMode = ClientCertificateMode.RequireCertificate;

        // Validate client certificate
        https.ClientCertificateValidation = (cert, chain, errors) =>
        {
            // Custom validation
            if (errors != SslPolicyErrors.None)
            {
                Console.WriteLine($"SSL Policy Errors: {errors}");
                return false;
            }

            // Check against own CA
            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);
        };
    });
});

// Add certificate authentication
builder.Services.AddAuthentication(CertificateAuthenticationDefaults.AuthenticationScheme)
    .AddCertificate(options =>
    {
        options.AllowedCertificateTypes = CertificateTypes.Chained;
        options.RevocationMode = X509RevocationMode.Online;

        options.Events = new CertificateAuthenticationEvents
        {
            OnCertificateValidated = context =>
            {
                // Extract claims from certificate
                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 Example: mTLS Client (HttpClient) =====

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

using var ctx = PqCryptoContext.Initialize();

// Load client certificate and key
var clientCert = ctx.LoadCertificateWithPrivateKey(
    "client.crt.pem",
    "client.key.pem",
    "ClientPassword!"
);

// HttpClientHandler with client certificate
var handler = new HttpClientHandler
{
    ClientCertificateOptions = ClientCertificateOption.Manual,
    ServerCertificateCustomValidationCallback = (message, cert, chain, errors) =>
    {
        // Validate server certificate
        if (errors != SslPolicyErrors.None)
        {
            Console.WriteLine($"Server certificate error: {errors}");
            // In production: return false;
        }
        return true;
    }
};

// Add client certificate
handler.ClientCertificates.Add(clientCert);

using var httpClient = new HttpClient(handler);

// Send request
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>

----

===== Certificate-based Authorization =====

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

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

        // Check if certificate is authorized
        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();
}

// Registration
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 as mTLS Reverse Proxy =====

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

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

    # Client certificate authentication
    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 / {
        # Pass client certificate info to backend
        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>

----

===== Industry-Specific mTLS Requirements =====

^  Industry  ^  Certificate Lifetime  ^  Revocation  ^  Specifics  ^
| **Kubernetes** | 1 year | CRL | SPIFFE/SPIRE |
| **Financial Sector** | 90 days | OCSP | PCI-DSS compliant |
| **Healthcare** | 1 year | CRL + OCSP | DiGAV-compliant |
| **IoT/SCADA** | 5 years | CRL | Offline capability |

----

===== Related Scenarios =====

^  Relationship  ^  Scenario  ^  Description  ^
| **Prerequisite** | [[en:int:pqcrypt:szenarien:zertifikate:client_cert|3.2 Client Certificate]] | Create client cert |
| **Related** | [[.:smartcard_login|9.2 Smartcard Login]] | Hardware token |
| **Related** | [[en:int:pqcrypt:szenarien:tls:mtls_deployment|10.3 mTLS Deployment]] | TLS configuration |

----

<< [[.:start|<- Authentication Overview]] | [[en:int:pqcrypt:szenarien:start|^ Scenarios]] | [[.:smartcard_login|9.2 Smartcard Login ->]] >>

{{tag>scenario authentication mtls client-certificate zero-trust}}

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