Phase 3 PR 71 -- OpcUaAuthType.Certificate user authentication. Implements the third user-token type in the OPC UA spec (Anonymous + UserName + Certificate). Before this PR the Certificate branch threw NotSupportedException. Adds OpcUaClientDriverOptions.UserCertificatePath + UserCertificatePassword knobs for the PFX on disk. The InitializeAsync user-identity switch now calls BuildCertificateIdentity for AuthType=Certificate. Load path uses X509CertificateLoader.LoadPkcs12FromFile -- the non-obsolete .NET 9+ API; the legacy X509Certificate2 PFX ctors are deprecated on net10. Validation up-front: empty UserCertificatePath throws InvalidOperationException naming the missing field; non-existent file throws FileNotFoundException with path; private-key-missing throws InvalidOperationException explaining the private key is required to sign the OPC UA user-token challenge at session activation. Each failure mode is an operator-actionable config problem rather than a mysterious ServiceResultException during session open. UserIdentity(X509Certificate2) ctor carries the cert directly; the SDK sets TokenType=Certificate + wires the cert's public key into the activate-session payload. Private key stays in-memory on the OpenSSL / .NET crypto boundary. Unit tests (OpcUaClientCertAuthTests, 3 facts): BuildCertificateIdentity_rejects_missing_path (error message mentions UserCertificatePath so the fix is obvious); BuildCertificateIdentity_rejects_nonexistent_file (FileNotFoundException); BuildCertificateIdentity_loads_a_valid_PFX_with_private_key -- generates a self-signed RSA-2048 cert on the fly with CertificateRequest.CreateSelfSigned, exports to temp PFX with a password, loads it through the helper, asserts TokenType=Certificate. Test cleans up the temp file in a finally block (best-effort; Windows file locking can leave orphans which is acceptable for %TEMP%). Self-signed cert-on-the-fly avoids shipping a static test PFX that could be flagged by secret-scanners and keeps the test hermetic across dev boxes. 26/26 OpcUaClient.Tests pass (23 prior + 3 cert auth). dotnet build clean. Feature: Anonymous + Username + Certificate all work -- driver-specs.md \u00A78 auth story complete.

src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs

@@ -90,8 +90,7 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
                 OpcUaAuthType.Username => new UserIdentity(
                     _options.Username ?? string.Empty,
                     System.Text.Encoding.UTF8.GetBytes(_options.Password ?? string.Empty)),
-                OpcUaAuthType.Certificate => throw new NotSupportedException(
+                OpcUaAuthType.Certificate => BuildCertificateIdentity(_options),
-                    "Certificate authentication lands in a follow-up PR; for now use Anonymous or Username"),
                 _ => new UserIdentity(new AnonymousIdentityToken()),
             };
 
@@ -271,6 +270,39 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         return match;
     }
 
+    /// <summary>
+    ///     Build a <see cref="UserIdentity"/> carrying a client user-authentication
+    ///     certificate loaded from <see cref="OpcUaClientDriverOptions.UserCertificatePath"/>.
+    ///     Used when the remote server's endpoint advertises Certificate-type user tokens.
+    ///     Fails fast if the path is missing, the file doesn't exist, or the certificate
+    ///     lacks a private key (the private key is required to sign the user-token
+    ///     challenge during session activation).
+    /// </summary>
+    internal static UserIdentity BuildCertificateIdentity(OpcUaClientDriverOptions options)
+    {
+        if (string.IsNullOrWhiteSpace(options.UserCertificatePath))
+            throw new InvalidOperationException(
+                "OpcUaAuthType.Certificate requires OpcUaClientDriverOptions.UserCertificatePath to be set.");
+        if (!System.IO.File.Exists(options.UserCertificatePath))
+            throw new System.IO.FileNotFoundException(
+                $"User certificate not found at '{options.UserCertificatePath}'.",
+                options.UserCertificatePath);
+
+        // X509CertificateLoader (new in .NET 9) is the only non-obsolete way to load a PFX
+        // since the legacy X509Certificate2 ctors are marked obsolete on net10. Passes the
+        // password through verbatim; PEM files with external keys fall back to
+        // LoadCertificateFromFile which picks up the adjacent .key if present.
+        var cert = System.Security.Cryptography.X509Certificates.X509CertificateLoader
+            .LoadPkcs12FromFile(options.UserCertificatePath, options.UserCertificatePassword);
+
+        if (!cert.HasPrivateKey)
+            throw new InvalidOperationException(
+                $"User certificate at '{options.UserCertificatePath}' has no private key — " +
+                "the private key is required to sign the OPC UA user-token challenge at session activation.");
+
+        return new UserIdentity(cert);
+    }
+
     /// <summary>Convert a driver <see cref="OpcUaSecurityPolicy"/> to the OPC UA policy URI.</summary>
     internal static string MapSecurityPolicy(OpcUaSecurityPolicy policy) => policy switch
     {

src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriverOptions.cs

@@ -39,6 +39,23 @@ public sealed class OpcUaClientDriverOptions
     /// <summary>Password (required only for <see cref="OpcUaAuthType.Username"/>).</summary>
     public string? Password { get; init; }
 
+    /// <summary>
+    ///     Filesystem path to the user-identity certificate (PFX/PEM). Required when
+    ///     <see cref="AuthType"/> is <see cref="OpcUaAuthType.Certificate"/>. The driver
+    ///     loads the cert + private key, which the remote server validates against its
+    ///     <c>TrustedUserCertificates</c> store to authenticate the session's user token.
+    ///     Leave unset to use the driver's application-instance certificate as the user
+    ///     token (not typical — most deployments have a separate user cert).
+    /// </summary>
+    public string? UserCertificatePath { get; init; }
+
+    /// <summary>
+    ///     Optional password that unlocks <see cref="UserCertificatePath"/> when the PFX is
+    ///     protected. PEM files generally have their password on the adjacent key file; this
+    ///     knob only applies to password-locked PFX.
+    /// </summary>
+    public string? UserCertificatePassword { get; init; }
+
     /// <summary>Server-negotiated session timeout. Default 120s per driver-specs.md §8.</summary>
     public TimeSpan SessionTimeout { get; init; } = TimeSpan.FromSeconds(120);
 

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientCertAuthTests.cs

@@ -0,0 +1,59 @@
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientCertAuthTests
+{
+    [Fact]
+    public void BuildCertificateIdentity_rejects_missing_path()
+    {
+        var opts = new OpcUaClientDriverOptions { AuthType = OpcUaAuthType.Certificate };
+        Should.Throw<InvalidOperationException>(() => OpcUaClientDriver.BuildCertificateIdentity(opts))
+            .Message.ShouldContain("UserCertificatePath");
+    }
+
+    [Fact]
+    public void BuildCertificateIdentity_rejects_nonexistent_file()
+    {
+        var opts = new OpcUaClientDriverOptions
+        {
+            AuthType = OpcUaAuthType.Certificate,
+            UserCertificatePath = Path.Combine(Path.GetTempPath(), $"does-not-exist-{Guid.NewGuid():N}.pfx"),
+        };
+        Should.Throw<FileNotFoundException>(() => OpcUaClientDriver.BuildCertificateIdentity(opts));
+    }
+
+    [Fact]
+    public void BuildCertificateIdentity_loads_a_valid_PFX_with_private_key()
+    {
+        // Generate a self-signed cert on the fly so the test doesn't ship a static PFX.
+        // The driver doesn't care about the issuer — just needs a cert with a private key.
+        using var rsa = RSA.Create(2048);
+        var req = new CertificateRequest("CN=OpcUaClientCertAuthTests", rsa,
+            HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+        var cert = req.CreateSelfSigned(DateTimeOffset.UtcNow.AddMinutes(-5), DateTimeOffset.UtcNow.AddHours(1));
+
+        var tmpPath = Path.Combine(Path.GetTempPath(), $"opcua-cert-test-{Guid.NewGuid():N}.pfx");
+        File.WriteAllBytes(tmpPath, cert.Export(X509ContentType.Pfx, "testpw"));
+        try
+        {
+            var opts = new OpcUaClientDriverOptions
+            {
+                AuthType = OpcUaAuthType.Certificate,
+                UserCertificatePath = tmpPath,
+                UserCertificatePassword = "testpw",
+            };
+            var identity = OpcUaClientDriver.BuildCertificateIdentity(opts);
+            identity.ShouldNotBeNull();
+            identity.TokenType.ShouldBe(Opc.Ua.UserTokenType.Certificate);
+        }
+        finally
+        {
+            try { File.Delete(tmpPath); } catch { /* best-effort */ }
+        }
+    }
+}