feat: implement TLS cert-to-user mapping via X500 DN matching

src/NATS.Server/Auth/AuthService.cs

@@ -34,6 +34,13 @@ public sealed class AuthService
         var nonceRequired = false;
         Dictionary<string, User>? usersMap = null;
 
+        // TLS certificate mapping (highest priority when enabled)
+        if (options.TlsMap && options.TlsVerify && options.Users is { Count: > 0 })
+        {
+            authenticators.Add(new TlsMapAuthenticator(options.Users));
+            authRequired = true;
+        }
+
         // Priority order (matching Go): NKeys > Users > Token > SimpleUserPassword
 
         if (options.NKeys is { Count: > 0 })

src/NATS.Server/Auth/IAuthenticator.cs

@@ -1,3 +1,4 @@
+using System.Security.Cryptography.X509Certificates;
 using NATS.Server.Protocol;
 
 namespace NATS.Server.Auth;
@@ -11,4 +12,5 @@ public sealed class ClientAuthContext
 {
     public required ClientOptions Opts { get; init; }
     public required byte[] Nonce { get; init; }
+    public X509Certificate2? ClientCertificate { get; init; }
 }

src/NATS.Server/Auth/TlsMapAuthenticator.cs

@@ -0,0 +1,67 @@
+using System.Security.Cryptography.X509Certificates;
+
+namespace NATS.Server.Auth;
+
+/// <summary>
+/// Authenticates clients by mapping TLS certificate subject DN to configured users.
+/// Corresponds to Go server/auth.go checkClientTLSCertSubject.
+/// </summary>
+public sealed class TlsMapAuthenticator : IAuthenticator
+{
+    private readonly Dictionary<string, User> _usersByDn;
+    private readonly Dictionary<string, User> _usersByCn;
+
+    public TlsMapAuthenticator(IReadOnlyList<User> users)
+    {
+        _usersByDn = new Dictionary<string, User>(StringComparer.OrdinalIgnoreCase);
+        _usersByCn = new Dictionary<string, User>(StringComparer.OrdinalIgnoreCase);
+        foreach (var user in users)
+        {
+            _usersByDn[user.Username] = user;
+            _usersByCn[user.Username] = user;
+        }
+    }
+
+    public AuthResult? Authenticate(ClientAuthContext context)
+    {
+        var cert = context.ClientCertificate;
+        if (cert == null)
+            return null;
+
+        var dn = cert.SubjectName;
+        var dnString = dn.Name; // RFC 2253 format
+
+        // Try exact DN match first
+        if (_usersByDn.TryGetValue(dnString, out var user))
+            return BuildResult(user);
+
+        // Try CN extraction
+        var cn = ExtractCn(dn);
+        if (cn != null && _usersByCn.TryGetValue(cn, out user))
+            return BuildResult(user);
+
+        return null;
+    }
+
+    private static string? ExtractCn(X500DistinguishedName dn)
+    {
+        var dnString = dn.Name;
+        foreach (var rdn in dnString.Split(',', StringSplitOptions.TrimEntries))
+        {
+            if (rdn.StartsWith("CN=", StringComparison.OrdinalIgnoreCase))
+                return rdn[3..];
+        }
+        return null;
+    }
+
+    private static AuthResult BuildResult(User user)
+    {
+        return new AuthResult
+        {
+            Identity = user.Username,
+            AccountName = user.Account,
+            Permissions = user.Permissions,
+            Expiry = user.ConnectionDeadline,
+        };
+    }
+}

src/NATS.Server/NatsClient.cs

@@ -367,6 +367,7 @@ public sealed class NatsClient : IDisposable
             {
                 Opts = ClientOpts,
                 Nonce = _nonce ?? [],
+                ClientCertificate = TlsState?.PeerCert,
             };
 
             var result = _authService.Authenticate(context);

tests/NATS.Server.Tests/TlsMapAuthenticatorTests.cs

@@ -0,0 +1,134 @@
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using NATS.Server.Auth;
+
+namespace NATS.Server.Tests;
+
+public class TlsMapAuthenticatorTests
+{
+    private static X509Certificate2 CreateSelfSignedCert(string cn)
+    {
+        using var rsa = RSA.Create(2048);
+        var req = new CertificateRequest($"CN={cn}", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+        return req.CreateSelfSigned(DateTimeOffset.UtcNow, DateTimeOffset.UtcNow.AddYears(1));
+    }
+
+    private static X509Certificate2 CreateCertWithDn(string dn)
+    {
+        using var rsa = RSA.Create(2048);
+        var req = new CertificateRequest(dn, rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+        return req.CreateSelfSigned(DateTimeOffset.UtcNow, DateTimeOffset.UtcNow.AddYears(1));
+    }
+
+    [Fact]
+    public void Matches_user_by_cn()
+    {
+        var users = new List<User>
+        {
+            new() { Username = "alice", Password = "" },
+        };
+        var auth = new TlsMapAuthenticator(users);
+        var cert = CreateSelfSignedCert("alice");
+
+        var ctx = new ClientAuthContext
+        {
+            Opts = new Protocol.ClientOptions(),
+            Nonce = [],
+            ClientCertificate = cert,
+        };
+
+        var result = auth.Authenticate(ctx);
+        result.ShouldNotBeNull();
+        result.Identity.ShouldBe("alice");
+    }
+
+    [Fact]
+    public void Returns_null_when_no_cert()
+    {
+        var users = new List<User>
+        {
+            new() { Username = "alice", Password = "" },
+        };
+        var auth = new TlsMapAuthenticator(users);
+
+        var ctx = new ClientAuthContext
+        {
+            Opts = new Protocol.ClientOptions(),
+            Nonce = [],
+            ClientCertificate = null,
+        };
+
+        var result = auth.Authenticate(ctx);
+        result.ShouldBeNull();
+    }
+
+    [Fact]
+    public void Returns_null_when_cn_doesnt_match()
+    {
+        var users = new List<User>
+        {
+            new() { Username = "alice", Password = "" },
+        };
+        var auth = new TlsMapAuthenticator(users);
+        var cert = CreateSelfSignedCert("bob");
+
+        var ctx = new ClientAuthContext
+        {
+            Opts = new Protocol.ClientOptions(),
+            Nonce = [],
+            ClientCertificate = cert,
+        };
+
+        var result = auth.Authenticate(ctx);
+        result.ShouldBeNull();
+    }
+
+    [Fact]
+    public void Matches_by_full_dn_string()
+    {
+        var users = new List<User>
+        {
+            new() { Username = "CN=alice, O=TestOrg", Password = "" },
+        };
+        var auth = new TlsMapAuthenticator(users);
+        var cert = CreateCertWithDn("CN=alice, O=TestOrg");
+
+        var ctx = new ClientAuthContext
+        {
+            Opts = new Protocol.ClientOptions(),
+            Nonce = [],
+            ClientCertificate = cert,
+        };
+
+        var result = auth.Authenticate(ctx);
+        result.ShouldNotBeNull();
+        result.Identity.ShouldBe("CN=alice, O=TestOrg");
+    }
+
+    [Fact]
+    public void Returns_permissions_from_matched_user()
+    {
+        var perms = new Permissions
+        {
+            Publish = new SubjectPermission { Allow = ["foo.>"] },
+        };
+        var users = new List<User>
+        {
+            new() { Username = "alice", Password = "", Permissions = perms },
+        };
+        var auth = new TlsMapAuthenticator(users);
+        var cert = CreateSelfSignedCert("alice");
+
+        var ctx = new ClientAuthContext
+        {
+            Opts = new Protocol.ClientOptions(),
+            Nonce = [],
+            ClientCertificate = cert,
+        };
+
+        var result = auth.Authenticate(ctx);
+        result.ShouldNotBeNull();
+        result.Permissions.ShouldNotBeNull();
+        result.Permissions.Publish!.Allow!.ShouldContain("foo.>");
+    }
+}