fix(grpc): control-plane PSK credentials work on https endpoints (ScadaBridge#64)

WithSiteCredentials always composed the per-site PSK over
ChannelCredentials.Insecure. Grpc.Net.Client refuses that on an https
address ("insecure channel credentials and can't use a HttpClient with a
'https' scheme"), so every https control-plane endpoint threw at channel
construction - in SitePairChannelProvider the throw was caught and the
site address logged as malformed, i.e. a TLS site was silently
undialable. The old comment claiming TLS needed no change here was wrong.

WithSiteCredentials now takes the endpoint: https composes over
ChannelCredentials.SecureSsl (PSK retained inside TLS, no insecure
opt-in); anything else, including callers that omit it, keeps the
previous composition unchanged. SitePairChannelProvider,
SiteStreamGrpcClient and CentralChannelProvider pass their endpoint. The
three AuditLog pull clients are deliberately left for after #42.

Tests: real Kestrel HTTP/2-over-TLS (ALPN) with the real
ControlPlaneAuthInterceptor - correct PSK accepted, wrong PSK still
PermissionDenied, untrusted server cert fails with an
AuthenticationException cause, the pre-fix composition throws on https,
http composition unchanged; plus each production builder accepts https
(mutation check: disabling the TLS branch turns those red).

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/CentralChannelProvider.cs

@@ -117,7 +117,7 @@ public sealed class CentralChannelProvider : IDisposable
                     KeepAlivePingPolicy = HttpKeepAlivePingPolicy.Always,
                     EnableMultipleHttp2Connections = true,
                 },
-            }.WithMessageLimits().WithSiteCredentials(pskProvider, siteId);
+            }.WithMessageLimits().WithSiteCredentials(pskProvider, siteId, endpoints[i]);
 
             _channels[i] = GrpcChannel.ForAddress(endpoints[i], channelOptions);
             _clients[i] = new CentralControlService.CentralControlServiceClient(_channels[i]);

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/ISitePskProvider.cs

@@ -65,8 +65,16 @@ public interface ISitePskProvider
 /// credentials to a plaintext channel by default, precisely because a bearer token on h2c is
 /// readable and replayable by anyone on the path. That is a real and accepted limitation here:
 /// these listeners are h2c today and the boundary assumes a trusted network. The PSK raises the
-/// bar from "anyone who can reach the port" to "anyone who can read the traffic"; TLS on these
+/// bar from "anyone who can reach the port" to "anyone who can read the traffic".
-/// listeners is the follow-on hardening and requires no change to this code.
+/// </para>
+/// <para>
+/// <b>TLS (ScadaBridge#64).</b> An earlier version of this comment claimed TLS would need no change
+/// here. It did: Grpc.Net.Client refuses an <c>https://</c> address on a channel whose credentials
+/// are composed over <see cref="ChannelCredentials.Insecure"/> ("insecure channel credentials and
+/// can't use a HttpClient with a 'https' scheme"), so every https endpoint threw at channel
+/// construction. <see cref="WithSiteCredentials"/> now takes the endpoint and composes over
+/// <see cref="ChannelCredentials.SecureSsl"/> for <c>https</c> — the PSK still rides every call,
+/// now inside TLS — and keeps the insecure composition, byte for byte, for <c>http</c>.
 /// </para>
 /// </remarks>
 public static class ControlPlaneCredentials
@@ -119,9 +127,15 @@ public static class ControlPlaneCredentials
     /// <param name="options">Channel options being built.</param>
     /// <param name="provider">Key provider, or null to leave the channel unauthenticated.</param>
     /// <param name="siteId">The site this channel talks to.</param>
+    /// <param name="endpoint">
+    /// The address the channel will dial. An <c>https</c> address composes the call credentials over
+    /// TLS channel credentials (ScadaBridge#64); anything else — including null, for callers that
+    /// predate it — keeps the h2c composition. Must be the same address passed to
+    /// <c>GrpcChannel.ForAddress</c>, or an https channel throws at construction.
+    /// </param>
     /// <returns>The same options instance, for chaining.</returns>
     public static GrpcChannelOptions WithSiteCredentials(
-        this GrpcChannelOptions options, ISitePskProvider? provider, string? siteId)
+        this GrpcChannelOptions options, ISitePskProvider? provider, string? siteId, string? endpoint = null)
     {
         ArgumentNullException.ThrowIfNull(options);
 
@@ -130,9 +144,20 @@ public static class ControlPlaneCredentials
             return options;
         }
 
+        if (IsTlsEndpoint(endpoint))
+        {
+            // TLS: the server certificate is validated by the HTTP handler (OS trust store by
+            // default, or the handler's own SslOptions), and the call credentials need no opt-in.
+            options.Credentials = ChannelCredentials.Create(
+                ChannelCredentials.SecureSsl, ForSite(provider, siteId));
+            options.UnsafeUseInsecureChannelCallCredentials = false;
+        }
+        else
+        {
             options.Credentials = ChannelCredentials.Create(
                 ChannelCredentials.Insecure, ForSite(provider, siteId));
             options.UnsafeUseInsecureChannelCallCredentials = true;
+        }
         if (options.HttpHandler is not PskRejectionRefreshHandler)
         {
             // Null means "let GrpcChannel build its default", which is this same handler shape.
@@ -144,6 +169,13 @@ public static class ControlPlaneCredentials
 
         return options;
     }
+
+    /// <summary>True when <paramref name="endpoint"/> is an absolute <c>https</c> URI.</summary>
+    /// <param name="endpoint">The channel address, or null.</param>
+    /// <returns>Whether the channel will run over TLS.</returns>
+    internal static bool IsTlsEndpoint(string? endpoint) =>
+        Uri.TryCreate(endpoint, UriKind.Absolute, out var uri)
+        && string.Equals(uri.Scheme, Uri.UriSchemeHttps, StringComparison.OrdinalIgnoreCase);
 }
 
 /// <summary>

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SitePairChannelProvider.cs

@@ -584,7 +584,7 @@ public sealed class SitePairChannelProvider : IDisposable
                     KeepAlivePingPolicy = HttpKeepAlivePingPolicy.Always,
                     EnableMultipleHttp2Connections = true
                 }
-        }.WithMessageLimits().WithSiteCredentials(_pskProvider, siteId);
+        }.WithMessageLimits().WithSiteCredentials(_pskProvider, siteId, endpoint);
 
         return GrpcChannel.ForAddress(endpoint, channelOptions);
     }

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteStreamGrpcClient.cs

@@ -94,7 +94,7 @@ public class SiteStreamGrpcClient : IAsyncDisposable, IDisposable
                 KeepAlivePingTimeout = options.GrpcKeepAlivePingTimeout,
                 KeepAlivePingPolicy = HttpKeepAlivePingPolicy.Always
             }
-        }.WithMessageLimits().WithSiteCredentials(pskProvider, siteIdentifier));
+        }.WithMessageLimits().WithSiteCredentials(pskProvider, siteIdentifier, endpoint));
         _client = new SiteStreamService.SiteStreamServiceClient(_channel);
         _logger = logger;
     }

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/HttpsEndpointChannelTests.cs

@@ -0,0 +1,88 @@
+using Grpc.Net.Client;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.Tests.Grpc;
+
+/// <summary>
+/// Every production builder of a PSK-credentialed control-plane channel must accept an
+/// <c>https://</c> endpoint (ScadaBridge#64). Before the endpoint was threaded into
+/// <see cref="ControlPlaneCredentials.WithSiteCredentials"/>, each composed its credentials over
+/// insecure channel credentials and Grpc.Net.Client threw at construction for any https address —
+/// in <see cref="SitePairChannelProvider"/> that throw was caught and the site address logged as
+/// malformed, so a TLS site was silently undialable.
+/// </summary>
+public class HttpsEndpointChannelTests
+{
+    private const string Site = "site-a";
+
+    /// <summary>Verifies central's per-site pair provider builds a channel for an https site address.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task SitePairChannelProvider_builds_an_https_site_channel()
+    {
+        using var provider = new SitePairChannelProvider(
+            new FixedKeyProvider(),
+            Options.Create(new CommunicationOptions()),
+            NullLogger<SitePairChannelProvider>.Instance,
+            handlerFactory: null,
+            reachabilityProbe: (_, _) => Task.FromResult(false));
+
+        provider.UpdateSite(Site, "https://site-a-node-a:8083", nodeBEndpoint: null);
+
+        var target = await provider.ExecuteAsync(
+            Site, (channel, _) => Task.FromResult(channel.Target), CancellationToken.None);
+        Assert.Equal("site-a-node-a:8083", target);
+    }
+
+    /// <summary>Verifies a site's central channel provider builds https channels for both centrals.</summary>
+    [Fact]
+    public void CentralChannelProvider_builds_https_central_channels()
+    {
+        using var provider = new CentralChannelProvider(
+            new[] { "https://central-a:8083", "https://central-b:8083" },
+            new FixedKeyProvider(),
+            Site,
+            new CommunicationOptions(),
+            NullLogger.Instance);
+    }
+
+    /// <summary>Verifies the site stream client builds an https channel with credentials.</summary>
+    [Fact]
+    public void SiteStreamGrpcClient_builds_an_https_channel()
+    {
+        using var client = new SiteStreamGrpcClient(
+            "https://site-a-node-a:8083",
+            NullLogger<SiteStreamGrpcClient>.Instance,
+            new CommunicationOptions(),
+            new FixedKeyProvider(),
+            Site);
+
+        Assert.Equal("https://site-a-node-a:8083", client.Endpoint);
+    }
+
+    /// <summary>Verifies the scheme check behind the composition choice.</summary>
+    /// <param name="endpoint">The endpoint.</param>
+    /// <param name="expected">Whether it is TLS.</param>
+    [Theory]
+    [InlineData("https://host:8083", true)]
+    [InlineData("HTTPS://host:8083", true)]
+    [InlineData("http://host:8083", false)]
+    [InlineData(null, false)]
+    [InlineData("", false)]
+    [InlineData("not a url", false)]
+    public void IsTlsEndpoint_matches_only_absolute_https(string? endpoint, bool expected)
+    {
+        Assert.Equal(expected, ControlPlaneCredentials.IsTlsEndpoint(endpoint));
+    }
+
+    private sealed class FixedKeyProvider : ISitePskProvider
+    {
+        /// <inheritdoc />
+        public ValueTask<string> GetAsync(string siteId, CancellationToken ct) => new("k");
+
+        /// <inheritdoc />
+        public void Invalidate(string siteId) { }
+    }
+}

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ControlPlaneTlsEndToEndTests.cs

@@ -0,0 +1,229 @@
+using System.Net;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using Grpc.Core;
+using Grpc.Net.Client;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Hosting.Server;
+using Microsoft.AspNetCore.Hosting.Server.Features;
+using Microsoft.AspNetCore.Server.Kestrel.Core;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.ScadaBridge.Communication;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
+
+/// <summary>
+/// The control-plane PSK over a real TLS listener (ScadaBridge#64): a Kestrel HTTP/2 endpoint with
+/// a certificate (h2 negotiated by ALPN), the real <see cref="ControlPlaneAuthInterceptor"/>, and a
+/// client channel built by <see cref="ControlPlaneCredentials.WithSiteCredentials"/> exactly as the
+/// production channel builders do.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <see cref="Microsoft.AspNetCore.TestHost.TestServer"/> (what <c>ControlPlaneAuthEndToEndTests</c>
+/// uses) has no TLS, so this binds real Kestrel on a loopback ephemeral port. The client trusts the
+/// generated certificate by pinning its thumbprint in the handler's validation callback — the test
+/// stand-in for "the node trusts the issuing CA"; production uses the OS trust store.
+/// </para>
+/// <para>
+/// The first test is the reason the endpoint argument exists: before it, every channel composed its
+/// PSK over insecure channel credentials, and Grpc.Net.Client refuses that on an https address.
+/// </para>
+/// </remarks>
+public sealed class ControlPlaneTlsEndToEndTests : IAsyncLifetime
+{
+    private const string SiteKey = "the-site-a-preshared-key";
+
+    private X509Certificate2 _certificate = null!;
+    private IHost _host = null!;
+    private string _httpsAddress = null!;
+
+    /// <summary>Generates a localhost certificate and starts a TLS gRPC host with the real interceptor.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    public async Task InitializeAsync()
+    {
+        using (var rsa = RSA.Create(2048))
+        {
+            var request = new CertificateRequest("CN=localhost", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+            var san = new SubjectAlternativeNameBuilder();
+            san.AddDnsName("localhost");
+            request.CertificateExtensions.Add(san.Build());
+            request.CertificateExtensions.Add(new X509EnhancedKeyUsageExtension(
+                new OidCollection { new Oid("1.3.6.1.5.5.7.3.1") }, critical: false)); // serverAuth
+            using var ephemeral = request.CreateSelfSigned(DateTimeOffset.UtcNow.AddMinutes(-5), DateTimeOffset.UtcNow.AddDays(1));
+            // Round-trip through PFX so the private key is usable by SslStream on every platform.
+            _certificate = X509CertificateLoader.LoadPkcs12(ephemeral.Export(X509ContentType.Pfx), password: null);
+        }
+
+        _host = await new HostBuilder()
+            .ConfigureWebHost(web => web
+                .UseKestrel(kestrel => kestrel.Listen(IPAddress.Loopback, 0, listen =>
+                {
+                    listen.Protocols = HttpProtocols.Http2;
+                    listen.UseHttps(_certificate);
+                }))
+                .ConfigureServices(services =>
+                {
+                    // By TYPE, interceptor not in DI — exactly as Program.cs registers it.
+                    services.AddGrpc(o => o.Interceptors.Add<ControlPlaneAuthInterceptor>());
+                    services.AddSingleton(Options.Create(new CommunicationOptions { GrpcPsk = SiteKey }));
+                })
+                .Configure(app =>
+                {
+                    app.UseRouting();
+                    app.UseEndpoints(e => e.MapGrpcService<EchoSiteStreamService>());
+                }))
+            .StartAsync();
+
+        var bound = _host.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()!.Addresses.Single();
+        _httpsAddress = $"https://localhost:{new Uri(bound).Port}";
+    }
+
+    /// <summary>Stops the host and releases the certificate.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    public async Task DisposeAsync()
+    {
+        await _host.StopAsync();
+        _host.Dispose();
+        _certificate.Dispose();
+    }
+
+    private SocketsHttpHandler PinnedHandler() => new()
+    {
+        EnableMultipleHttp2Connections = true,
+        SslOptions =
+        {
+            RemoteCertificateValidationCallback = (_, cert, _, _) =>
+                cert is not null && string.Equals(
+                    cert.GetCertHashString(), _certificate.Thumbprint, StringComparison.OrdinalIgnoreCase),
+        },
+    };
+
+    private GrpcChannel Channel(string key, string? endpointForCredentials) =>
+        GrpcChannel.ForAddress(_httpsAddress, new GrpcChannelOptions { HttpHandler = PinnedHandler() }
+            .WithMessageLimits()
+            .WithSiteCredentials(new FixedPskProvider(key), "site-a", endpointForCredentials));
+
+    /// <summary>
+    /// Verifies the pre-#64 composition (no endpoint → insecure credentials) cannot even build an
+    /// https channel. This is the defect the endpoint argument fixes; if Grpc.Net.Client ever stops
+    /// refusing it, this test says so.
+    /// </summary>
+    [Fact]
+    public void Https_channel_with_insecure_composition_throws_at_construction()
+    {
+        var ex = Assert.Throws<InvalidOperationException>(() => Channel(SiteKey, endpointForCredentials: null));
+
+        Assert.Contains("insecure channel credentials", ex.Message);
+    }
+
+    /// <summary>Verifies a correctly keyed call succeeds over TLS and carries the site header.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task Correct_key_is_accepted_over_tls()
+    {
+        using var channel = Channel(SiteKey, _httpsAddress);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        var reply = await client.PullAuditEventsAsync(new PullAuditEventsRequest { BatchSize = 1 });
+
+        Assert.False(reply.MoreAvailable);
+    }
+
+    /// <summary>
+    /// Verifies TLS does not replace the PSK: a wrong key over a perfectly good TLS channel is still
+    /// refused by the interceptor.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task Wrong_key_is_still_rejected_over_tls()
+    {
+        using var channel = Channel("some-other-sites-key", _httpsAddress);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(
+            async () => await client.PullAuditEventsAsync(new PullAuditEventsRequest()));
+
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    /// <summary>
+    /// Verifies the client validates the server certificate: a handler that does not trust it fails
+    /// the call rather than sending the PSK to an unverified peer.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task Untrusted_server_certificate_fails_the_call()
+    {
+        var distrusting = new SocketsHttpHandler
+        {
+            SslOptions = { RemoteCertificateValidationCallback = (_, _, _, _) => false },
+        };
+        using var channel = GrpcChannel.ForAddress(_httpsAddress, new GrpcChannelOptions { HttpHandler = distrusting }
+            .WithSiteCredentials(new FixedPskProvider(SiteKey), "site-a", _httpsAddress));
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(
+            async () => await client.PullAuditEventsAsync(new PullAuditEventsRequest()));
+
+        // Grpc.Net.Client surfaces a failed handshake as a transport error; what matters is that the
+        // cause is the certificate check, i.e. the call never reached the server.
+        Assert.NotEqual(StatusCode.OK, ex.StatusCode);
+        Assert.Contains(Causes(ex.Status.DebugException), e => e is System.Security.Authentication.AuthenticationException);
+    }
+
+    private static IEnumerable<Exception> Causes(Exception? ex)
+    {
+        for (; ex is not null; ex = ex.InnerException)
+        {
+            yield return ex;
+        }
+    }
+
+    /// <summary>
+    /// Verifies the h2c composition is unchanged for an http endpoint: insecure channel credentials
+    /// with the explicit opt-in, as before #64.
+    /// </summary>
+    [Fact]
+    public void Http_endpoint_keeps_the_h2c_composition()
+    {
+        var options = new GrpcChannelOptions()
+            .WithSiteCredentials(new FixedPskProvider(SiteKey), "site-a", "http://site-a:8083");
+
+        Assert.True(options.UnsafeUseInsecureChannelCallCredentials);
+        // Building the channel on the http address must still succeed.
+        using var channel = GrpcChannel.ForAddress("http://site-a:8083", options);
+    }
+
+    /// <summary>Verifies the https endpoint drops the insecure opt-in.</summary>
+    [Fact]
+    public void Https_endpoint_drops_the_insecure_opt_in()
+    {
+        var options = new GrpcChannelOptions()
+            .WithSiteCredentials(new FixedPskProvider(SiteKey), "site-a", "https://site-a:8083");
+
+        Assert.False(options.UnsafeUseInsecureChannelCallCredentials);
+    }
+
+    private sealed class FixedPskProvider(string key) : ISitePskProvider
+    {
+        /// <inheritdoc />
+        public ValueTask<string> GetAsync(string siteId, CancellationToken ct) => new(key);
+
+        /// <inheritdoc />
+        public void Invalidate(string siteId) { }
+    }
+
+    /// <summary>Stub service: the interceptor is under test, not the service.</summary>
+    private sealed class EchoSiteStreamService : SiteStreamService.SiteStreamServiceBase
+    {
+        /// <inheritdoc />
+        public override Task<PullAuditEventsResponse> PullAuditEvents(
+            PullAuditEventsRequest request, ServerCallContext context) =>
+            Task.FromResult(new PullAuditEventsResponse { MoreAvailable = false });
+    }
+}