diff --git a/Svrnty.CQRS.Altcha.Grpc/AltchaCallCredentials.cs b/Svrnty.CQRS.Altcha.Grpc/AltchaCallCredentials.cs
new file mode 100644
index 0000000..e673ab8
--- /dev/null
+++ b/Svrnty.CQRS.Altcha.Grpc/AltchaCallCredentials.cs
@@ -0,0 +1,27 @@
+using Grpc.Core;
+
+namespace Svrnty.CQRS.Altcha.Grpc;
+
+/// <summary>
+/// Helper that builds gRPC call metadata (an <c>Authorization</c> header)
+/// from <see cref="AltchaGrpcOptions.TokenProvider"/>. Kept as a separate
+/// shared helper so the verifier and challenge provider apply identical
+/// rules.
+/// </summary>
+internal static class AltchaCallCredentials
+{
+    public static async Task<Metadata?> BuildMetadataAsync(AltchaGrpcOptions options, CancellationToken cancellationToken)
+    {
+        if (options.TokenProvider is null)
+            return null;
+
+        var token = await options.TokenProvider(cancellationToken);
+        if (string.IsNullOrWhiteSpace(token))
+            return null;
+
+        return new Metadata
+        {
+            { "Authorization", $"Bearer {token}" }
+        };
+    }
+}
diff --git a/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcChallengeProvider.cs b/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcChallengeProvider.cs
new file mode 100644
index 0000000..797a2c2
--- /dev/null
+++ b/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcChallengeProvider.cs
@@ -0,0 +1,58 @@
+using Grpc.Core;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using Svrnty.CQRS.Altcha.Abstractions;
+
+namespace Svrnty.CQRS.Altcha.Grpc;
+
+/// <summary>
+/// Default <see cref="IAltchaChallengeProvider"/> backed by gRPC. Calls
+/// <c>AltchaService.CreateChallenge</c> on the configured endpoint and
+/// projects the response onto <see cref="AltchaChallenge"/>.
+/// </summary>
+public sealed class AltchaGrpcChallengeProvider : IAltchaChallengeProvider
+{
+    private readonly AltchaService.AltchaServiceClient _client;
+    private readonly IOptions<AltchaGrpcOptions> _options;
+    private readonly ILogger<AltchaGrpcChallengeProvider> _logger;
+
+    public AltchaGrpcChallengeProvider(
+        AltchaService.AltchaServiceClient client,
+        IOptions<AltchaGrpcOptions> options,
+        ILogger<AltchaGrpcChallengeProvider> logger)
+    {
+        _client = client;
+        _options = options;
+        _logger = logger;
+    }
+
+    public async Task<AltchaChallenge> CreateAsync(CancellationToken cancellationToken = default)
+    {
+        var opts = _options.Value;
+        var metadata = await AltchaCallCredentials.BuildMetadataAsync(opts, cancellationToken);
+        var deadline = DateTime.UtcNow.Add(opts.CallTimeout);
+
+        try
+        {
+            var response = await _client.CreateChallengeAsync(
+                new CreateChallengeRequest(),
+                headers: metadata,
+                deadline: deadline,
+                cancellationToken: cancellationToken);
+
+            return new AltchaChallenge
+            {
+                Algorithm = response.Algorithm,
+                Challenge = response.ChallengeHash,
+                Salt = response.Salt,
+                Signature = response.Signature,
+                MaxNumber = response.Maxnumber
+            };
+        }
+        catch (RpcException ex)
+        {
+            _logger.LogError(ex, "Altcha create-challenge failed against {Endpoint}: {Status}", opts.Endpoint, ex.StatusCode);
+            throw;
+        }
+    }
+}
diff --git a/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcOptions.cs b/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcOptions.cs
new file mode 100644
index 0000000..1b9d342
--- /dev/null
+++ b/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcOptions.cs
@@ -0,0 +1,31 @@
+namespace Svrnty.CQRS.Altcha.Grpc;
+
+/// <summary>
+/// Configuration for <see cref="AltchaGrpcVerifier"/> and
+/// <see cref="AltchaGrpcChallengeProvider"/>. Bind from configuration
+/// (e.g. <c>"Altcha"</c> section) or pass via the registration delegate.
+/// </summary>
+public sealed class AltchaGrpcOptions
+{
+    /// <summary>
+    /// gRPC endpoint of the altcha service. Typically the internal
+    /// docker / k8s address — e.g. <c>http://altcha:9090</c> or
+    /// <c>https://altcha.planb.svc.cluster.local:9090</c>.
+    /// </summary>
+    public string Endpoint { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Optional per-call HMAC service-token provider. When set, the
+    /// returned string is sent as <c>Authorization: Bearer &lt;token&gt;</c>
+    /// on every outbound gRPC call. Use this to integrate with whatever
+    /// service-auth scheme the rest of the deployment uses (e.g. plan-b's
+    /// <c>ServiceTokenIssuer.GetToken("altcha")</c>).
+    /// </summary>
+    public Func<CancellationToken, Task<string>>? TokenProvider { get; set; }
+
+    /// <summary>
+    /// Per-call timeout for both <c>CreateChallenge</c> and
+    /// <c>VerifyChallenge</c>. Defaults to 5s.
+    /// </summary>
+    public TimeSpan CallTimeout { get; set; } = TimeSpan.FromSeconds(5);
+}
diff --git a/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcVerifier.cs b/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcVerifier.cs
new file mode 100644
index 0000000..5fc62d1
--- /dev/null
+++ b/Svrnty.CQRS.Altcha.Grpc/AltchaGrpcVerifier.cs
@@ -0,0 +1,66 @@
+using Grpc.Core;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using Svrnty.CQRS.Altcha.Abstractions;
+
+namespace Svrnty.CQRS.Altcha.Grpc;
+
+/// <summary>
+/// Default <see cref="IAltchaVerifier"/> backed by gRPC. Calls
+/// <c>AltchaService.VerifyChallenge</c> on the configured endpoint and
+/// maps any failure (transport error, deadline, server-reported failure)
+/// to <see cref="AltchaVerifyResult.Fail"/>. Verification failures are
+/// safe defaults — callers see an <c>Unauthorized</c> outcome from the
+/// auth check.
+/// </summary>
+public sealed class AltchaGrpcVerifier : IAltchaVerifier
+{
+    private readonly AltchaService.AltchaServiceClient _client;
+    private readonly IOptions<AltchaGrpcOptions> _options;
+    private readonly ILogger<AltchaGrpcVerifier> _logger;
+
+    public AltchaGrpcVerifier(
+        AltchaService.AltchaServiceClient client,
+        IOptions<AltchaGrpcOptions> options,
+        ILogger<AltchaGrpcVerifier> logger)
+    {
+        _client = client;
+        _options = options;
+        _logger = logger;
+    }
+
+    public async Task<AltchaVerifyResult> VerifyAsync(string payload, CancellationToken cancellationToken = default)
+    {
+        var opts = _options.Value;
+        try
+        {
+            var metadata = await AltchaCallCredentials.BuildMetadataAsync(opts, cancellationToken);
+            var deadline = DateTime.UtcNow.Add(opts.CallTimeout);
+
+            var response = await _client.VerifyChallengeAsync(
+                new VerifyChallengeRequest { Payload = payload },
+                headers: metadata,
+                deadline: deadline,
+                cancellationToken: cancellationToken);
+
+            return response.Ok
+                ? AltchaVerifyResult.Success
+                : AltchaVerifyResult.Fail(string.IsNullOrEmpty(response.Reason) ? "invalid" : response.Reason);
+        }
+        catch (RpcException ex) when (ex.StatusCode == StatusCode.DeadlineExceeded)
+        {
+            _logger.LogWarning(ex, "Altcha verify timed out against {Endpoint}.", opts.Endpoint);
+            return AltchaVerifyResult.Fail("verify-timeout");
+        }
+        catch (RpcException ex) when (ex.StatusCode == StatusCode.Unavailable)
+        {
+            _logger.LogWarning(ex, "Altcha service unavailable at {Endpoint}.", opts.Endpoint);
+            return AltchaVerifyResult.Fail("service-unavailable");
+        }
+        catch (RpcException ex)
+        {
+            _logger.LogWarning(ex, "Altcha verify failed against {Endpoint}: {Status}", opts.Endpoint, ex.StatusCode);
+            return AltchaVerifyResult.Fail("rpc-error");
+        }
+    }
+}
diff --git a/Svrnty.CQRS.Altcha.Grpc/Protos/altcha.proto b/Svrnty.CQRS.Altcha.Grpc/Protos/altcha.proto
new file mode 100644
index 0000000..678c490
--- /dev/null
+++ b/Svrnty.CQRS.Altcha.Grpc/Protos/altcha.proto
@@ -0,0 +1,69 @@
+syntax = "proto3";
+
+package svrnty.cqrs.altcha.v1;
+
+option go_package = "svrnty.cqrs.altcha.v1;altchapb";
+option csharp_namespace = "Svrnty.CQRS.Altcha.Grpc";
+
+// AltchaService is the wire contract between any backend that gates
+// commands/queries with the [Altcha] attribute and a self-hosted Altcha
+// implementation. The default .NET client lives in this package; a
+// reference Go server lives in plan-b's projects/altcha (which vendors
+// this proto). Keep this file the source of truth for both sides.
+service AltchaService {
+  // Mint a fresh challenge for the widget to solve. Server-stateless:
+  // the response embeds an HMAC signature that VerifyChallenge re-checks.
+  rpc CreateChallenge(CreateChallengeRequest) returns (Challenge);
+
+  // Verify a widget-produced solution payload. The server checks
+  // signature + expiry + PoW correctness, then atomically claims the
+  // challenge in a replay-protection cache (e.g. Redis SETNX) so each
+  // solution is single-use across all server replicas.
+  rpc VerifyChallenge(VerifyChallengeRequest) returns (VerifyChallengeResponse);
+}
+
+message CreateChallengeRequest {
+  // Optional per-request complexity override (PoW search-space upper
+  // bound). Higher = slower client solve. Server clamps to its
+  // configured min/max. Omit to use the server default.
+  optional uint32 complexity = 1;
+}
+
+message Challenge {
+  // Hashing algorithm — e.g. "SHA-256" (v2 default). Future versions may
+  // upgrade to PBKDF2 / Argon2; the field lets the widget pick the
+  // right solver.
+  string algorithm = 1;
+
+  // Hex-encoded hash the client must find a preimage for. Field is
+  // named "challenge_hash" rather than "challenge" to avoid a C#
+  // property/class name collision (the generated message class is
+  // also named Challenge); JSON projection for the widget remaps it.
+  string challenge_hash = 2;
+
+  // Hex-encoded random salt. Typically embeds the expiry timestamp so
+  // verifiers can re-derive the TTL without server state.
+  string salt = 3;
+
+  // HMAC-SHA256(secret, algorithm|challenge|salt|maxnumber). Lets
+  // VerifyChallenge confirm the challenge was issued by this server.
+  string signature = 4;
+
+  // PoW search-space upper bound. Equals the complexity used.
+  uint32 maxnumber = 5;
+}
+
+message VerifyChallengeRequest {
+  // Base64-encoded JSON payload produced by the Altcha widget — the
+  // value carried over the wire on IHasAltchaSolution.AltchaSolution.
+  string payload = 1;
+}
+
+message VerifyChallengeResponse {
+  bool ok = 1;
+
+  // Diagnostic only; not surfaced to end users. Suggested values:
+  //   "signature-invalid", "expired", "pow-incorrect", "replayed",
+  //   "redis-unreachable", "malformed".
+  string reason = 2;
+}
diff --git a/Svrnty.CQRS.Altcha.Grpc/ServiceCollectionExtensions.cs b/Svrnty.CQRS.Altcha.Grpc/ServiceCollectionExtensions.cs
new file mode 100644
index 0000000..c50f6d4
--- /dev/null
+++ b/Svrnty.CQRS.Altcha.Grpc/ServiceCollectionExtensions.cs
@@ -0,0 +1,64 @@
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using Svrnty.CQRS.Altcha.Abstractions;
+
+namespace Svrnty.CQRS.Altcha.Grpc;
+
+public static class ServiceCollectionExtensions
+{
+    /// <summary>
+    /// Registers the gRPC-backed <see cref="IAltchaVerifier"/> and
+    /// <see cref="IAltchaChallengeProvider"/>. Configure the endpoint
+    /// and optional service-auth token provider via the
+    /// <paramref name="configure"/> delegate.
+    /// </summary>
+    /// <example>
+    /// <code>
+    /// services.AddSvrntyAltcha();
+    /// services.AddSvrntyAltchaGrpcVerifier(opts =>
+    /// {
+    ///     opts.Endpoint = "http://altcha:9090";
+    ///     opts.TokenProvider = async ct => await tokenIssuer.GetTokenAsync("altcha", ct);
+    /// });
+    /// </code>
+    /// </example>
+    public static IServiceCollection AddSvrntyAltchaGrpcVerifier(
+        this IServiceCollection services,
+        Action<AltchaGrpcOptions> configure)
+    {
+        services.Configure(configure);
+        RegisterCore(services);
+        return services;
+    }
+
+    /// <summary>
+    /// Binds <see cref="AltchaGrpcOptions"/> from a configuration section
+    /// (typically <c>"Altcha:Grpc"</c>) and registers the gRPC verifier
+    /// and challenge provider.
+    /// </summary>
+    public static IServiceCollection AddSvrntyAltchaGrpcVerifier(
+        this IServiceCollection services,
+        IConfiguration configuration)
+    {
+        services.Configure<AltchaGrpcOptions>(configuration);
+        RegisterCore(services);
+        return services;
+    }
+
+    private static void RegisterCore(IServiceCollection services)
+    {
+        services.AddGrpcClient<AltchaService.AltchaServiceClient>((sp, client) =>
+        {
+            var opts = sp.GetRequiredService<Microsoft.Extensions.Options.IOptions<AltchaGrpcOptions>>().Value;
+            if (string.IsNullOrWhiteSpace(opts.Endpoint))
+                throw new InvalidOperationException(
+                    "Altcha gRPC endpoint not configured. Set AltchaGrpcOptions.Endpoint " +
+                    "(e.g. http://altcha:9090).");
+            client.Address = new Uri(opts.Endpoint);
+        });
+
+        services.TryAddSingleton<IAltchaVerifier, AltchaGrpcVerifier>();
+        services.TryAddSingleton<IAltchaChallengeProvider, AltchaGrpcChallengeProvider>();
+    }
+}
diff --git a/Svrnty.CQRS.Altcha.Grpc/Svrnty.CQRS.Altcha.Grpc.csproj b/Svrnty.CQRS.Altcha.Grpc/Svrnty.CQRS.Altcha.Grpc.csproj
new file mode 100644
index 0000000..3a900b8
--- /dev/null
+++ b/Svrnty.CQRS.Altcha.Grpc/Svrnty.CQRS.Altcha.Grpc.csproj
@@ -0,0 +1,47 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <IsAotCompatible>false</IsAotCompatible>
+    <LangVersion>14</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+
+    <Company>Svrnty</Company>
+    <Authors>David Lebee, Mathias Beaulieu-Duncan</Authors>
+    <PackageIcon>icon.png</PackageIcon>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
+    <RepositoryUrl>https://git.openharbor.io/svrnty/dotnet-cqrs</RepositoryUrl>
+    <RepositoryType>git</RepositoryType>
+    <PublishRepositoryUrl>true</PublishRepositoryUrl>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
+
+    <DebugType>portable</DebugType>
+    <DebugSymbols>true</DebugSymbols>
+    <IncludeSymbols>true</IncludeSymbols>
+    <IncludeSource>true</IncludeSource>
+    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Include="..\icon.png" Pack="true" PackagePath="" CopyToOutputDirectory="Always" />
+    <None Include="..\README.md" Pack="true" PackagePath="" CopyToOutputDirectory="Always" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Google.Protobuf" Version="3.30.2" />
+    <PackageReference Include="Grpc.Net.ClientFactory" Version="2.71.0" />
+    <PackageReference Include="Grpc.Tools" Version="2.71.0">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" Version="10.0.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Svrnty.CQRS.Altcha.Abstractions\Svrnty.CQRS.Altcha.Abstractions.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Protobuf Include="Protos\altcha.proto" GrpcServices="Client" />
+  </ItemGroup>
+</Project>
diff --git a/Svrnty.CQRS.sln b/Svrnty.CQRS.sln
index 9212eca..4b753ab 100644
--- a/Svrnty.CQRS.sln
+++ b/Svrnty.CQRS.sln
@@ -47,6 +47,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Svrnty.CQRS.Altcha.Abstract
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Svrnty.CQRS.Altcha", "Svrnty.CQRS.Altcha\Svrnty.CQRS.Altcha.csproj", "{9986C034-D585-4045-9F6C-99896B8A385B}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Svrnty.CQRS.Altcha.Grpc", "Svrnty.CQRS.Altcha.Grpc\Svrnty.CQRS.Altcha.Grpc.csproj", "{628DE10C-FCDB-418B-8341-FA246BBCF70E}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -285,6 +287,18 @@ Global
 		{9986C034-D585-4045-9F6C-99896B8A385B}.Release|x64.Build.0 = Release|Any CPU
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 		{9986C034-D585-4045-9F6C-99896B8A385B}.Release|x86.Build.0 = Release|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Debug|x64.Build.0 = Debug|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Debug|x86.Build.0 = Debug|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Release|Any CPU.Build.0 = Release|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Release|x64.ActiveCfg = Release|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Release|x64.Build.0 = Release|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Release|x86.ActiveCfg = Release|Any CPU
+		{628DE10C-FCDB-418B-8341-FA246BBCF70E}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE