feat(secrets): G-2b resolve OpcUaClient secret: Password/UserCertificatePassword (Task 8)

Resolve secret:-prefixed Password + UserCertificatePassword through the shared
ISecretResolver, fail-closed on absent, retiring the cleartext-in-DB path. The
driver-registry factory is synchronous (Func<string,string,IDriver>), so resolution
is done lazily in the async session-open (InitializeAsync, before BuildUserIdentity)
rather than at deserialize — mirroring Task 7's Galaxy pattern and matching its
re-resolve-on-reconnect behavior. Both consumers (username Password and the
certificate-password LoadPkcs12 path via BuildCertificateIdentity) see the resolved
connect-scoped options; _options stays raw (secret: refs intact), no long-lived
plaintext field.

Scope corrections vs the plan (verified against v3): the probe is unauthenticated
GetEndpoints-only and never reads either credential, so it is NOT a resolution site
(comment added, no dead code); OpcUaClientDriverOptions was a sealed class, converted
to sealed record for the with-expression (no positional params → identical JSON; no
reference-equality/dict-key/ToString-log usages → no behavior/leak change).

ISecretResolver threaded via factory Register/CreateInstance + DriverFactoryBootstrap
(real resolver from DI); NullSecretResolver null-object backs test/parse paths only,
fail-closed on secret: refs. TDD: 4 helper tests RED→GREEN; 142 OpcUaClient tests pass.

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

@@ -14,7 +14,9 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
 ///     exposes PLC data" flow. Tier A (pure managed, OPC Foundation reference SDK); universal
 ///     protections cover it.
 /// </remarks>
-public sealed class OpcUaClientDriverOptions
+// A record (not a plain class) so G-2b's OpcUaClientSecretResolution can produce a
+// credential-resolved copy with a `with` expression — every property keeps its init setter.
+public sealed record OpcUaClientDriverOptions
 {
     /// <summary>
     ///     Remote OPC UA endpoint URL, e.g. <c>opc.tcp://plc.internal:4840</c>. Convenience

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/NullSecretResolver.cs

@@ -0,0 +1,26 @@
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
+
+/// <summary>
+///     A null-object <see cref="ISecretResolver"/> that resolves every name to <c>null</c>
+///     (absent). Backs the driver's default (test/parse-only) construction path so callers that
+///     never author a <c>secret:</c> credential ref need not thread a real resolver. A production
+///     <c>OpcUaClientDriver</c> always receives the DI-registered resolver via the factory; a
+///     <c>secret:</c> ref resolved against this null object throws fail-closed (the secret is
+///     reported absent), which is the correct behaviour for a mis-wired deployment — the
+///     <c>secret:</c> literal is never sent verbatim as a password.
+/// </summary>
+internal sealed class NullSecretResolver : ISecretResolver
+{
+    /// <summary>The shared singleton instance.</summary>
+    public static readonly NullSecretResolver Instance = new();
+
+    private NullSecretResolver()
+    {
+    }
+
+    /// <inheritdoc />
+    public Task<string?> GetAsync(SecretName name, CancellationToken ct) =>
+        Task.FromResult<string?>(null);
+}

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

@@ -4,6 +4,7 @@ using Opc.Ua;
 using Opc.Ua.Client;
 using Opc.Ua.Configuration;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.Secrets.Abstractions;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
 
@@ -36,15 +37,25 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
     /// <param name="options">Driver configuration.</param>
     /// <param name="driverInstanceId">Stable logical ID from the config DB.</param>
     /// <param name="logger">Optional logger; defaults to NullLogger when not supplied.</param>
+    /// <param name="secretResolver">
+    ///     Optional shared secret resolver used to resolve <c>secret:</c>-prefixed
+    ///     <see cref="OpcUaClientDriverOptions.Password"/> /
+    ///     <see cref="OpcUaClientDriverOptions.UserCertificatePassword"/> at session-open.
+    ///     Defaults to the <see cref="NullSecretResolver"/> null-object (which reports every
+    ///     secret absent) for the test/parse-only path; the production factory threads the
+    ///     DI-registered resolver. A <c>secret:</c> ref against the null-object fails closed.
+    /// </param>
     public OpcUaClientDriver(OpcUaClientDriverOptions options, string driverInstanceId,
-        ILogger<OpcUaClientDriver>? logger = null)
+        ILogger<OpcUaClientDriver>? logger = null, ISecretResolver? secretResolver = null)
     {
         _options = options;
         _driverInstanceId = driverInstanceId;
         _logger = logger ?? NullLogger<OpcUaClientDriver>.Instance;
+        _secretResolver = secretResolver ?? NullSecretResolver.Instance;
     }
 
     private readonly OpcUaClientDriverOptions _options;
+    private readonly ISecretResolver _secretResolver;
     private readonly string _driverInstanceId;
     // ---- IAlarmSource state ----
 
@@ -159,7 +170,18 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
             var appConfig = await BuildApplicationConfigurationAsync(cancellationToken).ConfigureAwait(false);
             var candidates = ResolveEndpointCandidates(_options);
 
-            var identity = BuildUserIdentity(_options);
+            // G-2b: resolve any secret:-prefixed Password / UserCertificatePassword through the
+            // shared secret store ONCE, here in the async connect flow, just before the credentials
+            // are consumed. _options stays the raw config (with secret: refs) — only this
+            // connect-scoped local carries plaintext, and only for the duration of the connect.
+            // Resolving on every InitializeAsync (a full reconnect calls it) picks up rotations.
+            // Both credential consumers — the Username password below and the Certificate password
+            // in BuildCertificateIdentity — flow through BuildUserIdentity(resolvedOptions), so a
+            // single resolve covers both paths.
+            var resolvedOptions = await OpcUaClientSecretResolution
+                .ResolveSecretRefsAsync(_options, _secretResolver, cancellationToken).ConfigureAwait(false);
+
+            var identity = BuildUserIdentity(resolvedOptions);
 
             // Failover sweep: try each endpoint in order, return the session from the first
             // one that successfully connects. Per-endpoint failures are captured so the final

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriverFactoryExtensions.cs

@@ -2,6 +2,7 @@ using System.Text.Json;
 using System.Text.Json.Serialization;
 using Microsoft.Extensions.Logging;
 using ZB.MOM.WW.OtOpcUa.Core.Hosting;
+using ZB.MOM.WW.Secrets.Abstractions;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
 
@@ -31,22 +32,37 @@ public static class OpcUaClientDriverFactoryExtensions
         Converters = { new JsonStringEnumConverter() },
     };
 
-    /// <summary>Register the OpcUaClient factory with the driver registry.</summary>
+    /// <summary>Register the OpcUaClient factory with the driver registry, threading the shared secret resolver.</summary>
     /// <param name="registry">The driver factory registry to register with.</param>
     /// <param name="loggerFactory">Optional logger factory used to create per-instance loggers.</param>
-    public static void Register(DriverFactoryRegistry registry, ILoggerFactory? loggerFactory = null)
+    /// <param name="secretResolver">
+    ///     The shared secret resolver (from DI) used to resolve the <c>secret:</c> arm of each
+    ///     instance's <c>Password</c> / <c>UserCertificatePassword</c> at session-open. Defaults to
+    ///     the <see cref="NullSecretResolver"/> null-object for the test/standalone path — callers
+    ///     that need a working <c>secret:</c> credential ref must thread the DI resolver.
+    /// </param>
+    public static void Register(
+        DriverFactoryRegistry registry,
+        ILoggerFactory? loggerFactory = null,
+        ISecretResolver? secretResolver = null)
     {
         ArgumentNullException.ThrowIfNull(registry);
-        registry.Register(DriverTypeName, (id, json) => CreateInstance(id, json, loggerFactory));
+        var resolver = secretResolver ?? NullSecretResolver.Instance;
+        registry.Register(DriverTypeName, (id, json) => CreateInstance(id, json, loggerFactory, resolver));
     }
 
     /// <summary>Public for the Server-side bootstrapper + test consumers.</summary>
     /// <param name="driverInstanceId">The unique identifier for the driver instance.</param>
     /// <param name="driverConfigJson">The JSON configuration string for the driver.</param>
     /// <param name="loggerFactory">Optional logger factory for the per-instance logger.</param>
+    /// <param name="secretResolver">
+    ///     Optional shared secret resolver for the driver's <c>secret:</c> credential arm; defaults
+    ///     to the <see cref="NullSecretResolver"/> null-object (fail-closed on a <c>secret:</c> ref).
+    /// </param>
     /// <returns>A configured <see cref="OpcUaClientDriver"/>.</returns>
     public static OpcUaClientDriver CreateInstance(
-        string driverInstanceId, string driverConfigJson, ILoggerFactory? loggerFactory = null)
+        string driverInstanceId, string driverConfigJson, ILoggerFactory? loggerFactory = null,
+        ISecretResolver? secretResolver = null)
     {
         ArgumentException.ThrowIfNullOrWhiteSpace(driverInstanceId);
         ArgumentException.ThrowIfNullOrWhiteSpace(driverConfigJson);
@@ -55,6 +71,8 @@ public static class OpcUaClientDriverFactoryExtensions
             ?? throw new InvalidOperationException(
                 $"OpcUaClient driver config for '{driverInstanceId}' deserialised to null");
 
-        return new OpcUaClientDriver(options, driverInstanceId, loggerFactory?.CreateLogger<OpcUaClientDriver>());
+        return new OpcUaClientDriver(
+            options, driverInstanceId, loggerFactory?.CreateLogger<OpcUaClientDriver>(),
+            secretResolver ?? NullSecretResolver.Instance);
     }
 }

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriverProbe.cs

@@ -34,6 +34,12 @@ public sealed class OpcUaClientDriverProbe : IDriverProbe
     /// <inheritdoc />
     public async Task<DriverProbeResult> ProbeAsync(string configJson, TimeSpan timeout, CancellationToken ct)
     {
+        // G-2b note: the probe does an UNAUTHENTICATED GetEndpoints discovery preflight
+        // (BuildMinimalAppConfig sets no user identity) and never reads opts.Password /
+        // opts.UserCertificatePassword. Those credentials — including any secret: refs — are
+        // therefore intentionally NOT resolved here; resolving them would be dead code. Secret
+        // resolution happens lazily in OpcUaClientDriver's async session-open path, where the
+        // credentials are actually consumed.
         OpcUaClientDriverOptions? opts;
         try { opts = JsonSerializer.Deserialize<OpcUaClientDriverOptions>(configJson, _opts); }
         catch (Exception ex) { return new(false, $"Config JSON is invalid: {ex.Message}", null); }

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientSecretResolution.cs

@@ -0,0 +1,90 @@
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
+
+/// <summary>
+///     Layer-B (G-2b) secret resolution for the OPC UA Client driver's credential fields. Resolves
+///     <c>secret:</c>-prefixed <see cref="OpcUaClientDriverOptions.Password"/> and
+///     <see cref="OpcUaClientDriverOptions.UserCertificatePassword"/> through the shared
+///     <see cref="ISecretResolver"/> (the encrypted-at-rest store) just before the async
+///     session-open path consumes them — retiring the cleartext password-in-DB model for
+///     production.
+/// </summary>
+/// <remarks>
+///     Resolution is lazy (called from the connect flow, not at config deserialization) so a
+///     full reconnect via <c>InitializeAsync</c> re-resolves and picks up secret rotations,
+///     mirroring the Galaxy driver's re-resolve-on-reconnect behaviour. Only <c>secret:</c>-prefixed
+///     values are resolved; a null/empty or non-<c>secret:</c> value passes through verbatim so the
+///     literal-password back-compat path is preserved. The <c>secret:</c> arm is <b>fail-closed</b>:
+///     an absent/tombstoned secret throws rather than leaving the <c>secret:</c> literal in place,
+///     which would otherwise be sent verbatim to the remote server as the password.
+/// </remarks>
+internal static class OpcUaClientSecretResolution
+{
+    private const string SecretPrefix = "secret:";
+
+    /// <summary>
+    ///     Return a copy of <paramref name="options"/> with any <c>secret:</c>-prefixed
+    ///     <see cref="OpcUaClientDriverOptions.Password"/> /
+    ///     <see cref="OpcUaClientDriverOptions.UserCertificatePassword"/> resolved to their
+    ///     plaintext via <paramref name="resolver"/>. Non-secret / null / empty fields are
+    ///     returned unchanged.
+    /// </summary>
+    /// <param name="options">The raw driver options (credential fields may carry <c>secret:</c> refs).</param>
+    /// <param name="resolver">The shared secret resolver used by the <c>secret:</c> arm.</param>
+    /// <param name="ct">Cancellation token for the async secret resolution.</param>
+    /// <returns>An options copy whose credential fields carry resolved plaintext.</returns>
+    /// <exception cref="InvalidOperationException">
+    ///     A <c>secret:</c> ref names a secret that is absent/tombstoned in the store (fail-closed).
+    /// </exception>
+    internal static async Task<OpcUaClientDriverOptions> ResolveSecretRefsAsync(
+        OpcUaClientDriverOptions options, ISecretResolver resolver, CancellationToken ct)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        ArgumentNullException.ThrowIfNull(resolver);
+
+        var password = await ResolveFieldAsync(options.Password, nameof(options.Password), resolver, ct)
+            .ConfigureAwait(false);
+        var certPassword = await ResolveFieldAsync(
+            options.UserCertificatePassword, nameof(options.UserCertificatePassword), resolver, ct)
+            .ConfigureAwait(false);
+
+        // Only re-materialize when something actually changed — a `with` on the reference-equal
+        // strings is harmless, but skipping it keeps the common (no-secret) case allocation-free.
+        if (ReferenceEquals(password, options.Password)
+            && ReferenceEquals(certPassword, options.UserCertificatePassword))
+        {
+            return options;
+        }
+
+        return options with { Password = password, UserCertificatePassword = certPassword };
+    }
+
+    /// <summary>
+    ///     Resolve a single credential field. Null/empty or non-<c>secret:</c> values pass through
+    ///     unchanged (the reference-equal original is returned). A <c>secret:NAME</c> value is
+    ///     resolved through <paramref name="resolver"/> and is fail-closed when the secret is absent.
+    /// </summary>
+    /// <param name="value">The raw field value (may be a <c>secret:</c> ref).</param>
+    /// <param name="fieldName">The field name, used in the fail-closed exception message.</param>
+    /// <param name="resolver">The shared secret resolver.</param>
+    /// <param name="ct">Cancellation token for the async resolution.</param>
+    /// <returns>The resolved plaintext, or the original value when it is not a <c>secret:</c> ref.</returns>
+    private static async Task<string?> ResolveFieldAsync(
+        string? value, string fieldName, ISecretResolver resolver, CancellationToken ct)
+    {
+        if (string.IsNullOrEmpty(value)
+            || !value.StartsWith(SecretPrefix, StringComparison.OrdinalIgnoreCase))
+        {
+            return value;
+        }
+
+        var name = value[SecretPrefix.Length..];
+        var resolved = await resolver.GetAsync(new SecretName(name), ct).ConfigureAwait(false);
+        return !string.IsNullOrEmpty(resolved)
+            ? resolved
+            : throw new InvalidOperationException(
+                $"OpcUaClientDriverOptions.{fieldName}='{value}' resolves secret '{name}', but it is " +
+                "absent from the store (fail-closed).");
+    }
+}

src/Server/ZB.MOM.WW.OtOpcUa.Host/Drivers/DriverFactoryBootstrap.cs

@@ -144,7 +144,7 @@ public static class DriverFactoryBootstrap
         Driver.FOCAS.FocasDriverFactoryExtensions.Register(registry);
         Driver.Galaxy.GalaxyDriverFactoryExtensions.Register(registry, secretResolver, loggerFactory);
         Driver.Modbus.ModbusDriverFactoryExtensions.Register(registry, loggerFactory);
-        Driver.OpcUaClient.OpcUaClientDriverFactoryExtensions.Register(registry, loggerFactory);
+        Driver.OpcUaClient.OpcUaClientDriverFactoryExtensions.Register(registry, loggerFactory, secretResolver);
         Driver.S7.S7DriverFactoryExtensions.Register(registry);
         Driver.TwinCAT.TwinCATDriverFactoryExtensions.Register(registry);
     }

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientSecretResolutionTests.cs

@@ -0,0 +1,95 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     G-2b — pins <see cref="OpcUaClientSecretResolution.ResolveSecretRefsAsync"/>, the Layer-B
+///     arm that resolves <c>secret:</c>-prefixed <see cref="OpcUaClientDriverOptions.Password"/>
+///     and <see cref="OpcUaClientDriverOptions.UserCertificatePassword"/> through the shared
+///     <see cref="ISecretResolver"/> just before the async session-open consumes them. The
+///     <c>secret:</c> arm is fail-closed — an absent secret throws rather than leaving the
+///     <c>secret:</c> literal in place (which would be sent verbatim as the password).
+///     Non-secret literals and null/empty pass through unchanged.
+/// </summary>
+public sealed class OpcUaClientSecretResolutionTests
+{
+    private const string KnownPwName = "opcua/inst/password";
+    private const string KnownPwValue = "resolved-password-plaintext";
+    private const string KnownCertPwName = "opcua/inst/cert-pw";
+    private const string KnownCertPwValue = "resolved-cert-pw-plaintext";
+
+    /// <summary>A fake resolver: returns known values for two known names, null otherwise.</summary>
+    private static ISecretResolver FakeResolver() => new StubSecretResolver();
+
+    /// <summary>(a) A <c>secret:</c> Password resolves to the store's plaintext.</summary>
+    [Fact]
+    public async Task Secret_password_resolves_to_store_plaintext()
+    {
+        var options = new OpcUaClientDriverOptions { Password = $"secret:{KnownPwName}" };
+
+        var resolved = await OpcUaClientSecretResolution.ResolveSecretRefsAsync(
+            options, FakeResolver(), CancellationToken.None);
+
+        resolved.Password.ShouldBe(KnownPwValue);
+    }
+
+    /// <summary>(b) A <c>secret:</c> UserCertificatePassword resolves to the store's plaintext.</summary>
+    [Fact]
+    public async Task Secret_cert_password_resolves_to_store_plaintext()
+    {
+        var options = new OpcUaClientDriverOptions { UserCertificatePassword = $"secret:{KnownCertPwName}" };
+
+        var resolved = await OpcUaClientSecretResolution.ResolveSecretRefsAsync(
+            options, FakeResolver(), CancellationToken.None);
+
+        resolved.UserCertificatePassword.ShouldBe(KnownCertPwValue);
+    }
+
+    /// <summary>(c) A non-secret literal Password and a null field pass through unchanged.</summary>
+    [Fact]
+    public async Task Literal_and_null_pass_through_unchanged()
+    {
+        var options = new OpcUaClientDriverOptions
+        {
+            Password = "plainpw",
+            UserCertificatePassword = null,
+        };
+
+        var resolved = await OpcUaClientSecretResolution.ResolveSecretRefsAsync(
+            options, FakeResolver(), CancellationToken.None);
+
+        resolved.Password.ShouldBe("plainpw");
+        resolved.UserCertificatePassword.ShouldBeNull();
+    }
+
+    /// <summary>(d) An unknown <c>secret:</c> ref (resolver returns null) is fail-closed — it throws
+    ///     and never leaves the <c>secret:</c> literal in place.</summary>
+    [Fact]
+    public async Task Unknown_secret_ref_is_fail_closed()
+    {
+        var options = new OpcUaClientDriverOptions { Password = "secret:opcua/inst/absent" };
+
+        var ex = await Should.ThrowAsync<InvalidOperationException>(() =>
+            OpcUaClientSecretResolution.ResolveSecretRefsAsync(
+                options, FakeResolver(), CancellationToken.None));
+
+        ex.Message.ShouldContain("Password");
+        ex.Message.ShouldContain("opcua/inst/absent");
+    }
+
+    /// <summary>A fake resolver: returns known values for two known names, null otherwise.</summary>
+    private sealed class StubSecretResolver : ISecretResolver
+    {
+        /// <inheritdoc />
+        public Task<string?> GetAsync(SecretName name, CancellationToken ct) =>
+            Task.FromResult<string?>(name.Value switch
+            {
+                KnownPwName => KnownPwValue,
+                KnownCertPwName => KnownCertPwValue,
+                _ => null,
+            });
+    }
+}