Mutual authentication with TLS

Mutual or mTLS means that just like how a client will only connect to servers with valid certificates, the server will also verify the client certificate and only allow connections if the client key pair is accepted by the server. This is useful for example in microservices where only other known services are allowed to interact with a service, and public access should be denied.

For mTLS to work the server must be set up with a keystore containing the CA (certificate authority) public key used to sign the individual certs for clients that are allowed to access the server, just like how in a regular TLS/HTTPS scenario the client must be able to verify the server certificate.

Since the CA is what controls what clients can access a service, it is likely an organisation or service specific CA rather than a normal public one like what you use for a public web server.

Setting the server up

A JKS store can be prepared with the right contents, or we can create it on the fly from certificate files in some location the server as read access to. In this example we use certificate and key files in the PEM format available from the file system and use the Akka HTTP convenience factories to load them.

The server is set up with its own private key and certificate as well as with a trust store containing the certificate authority (CA) to trust client certificates from:

Scala

copysourcepackage example.myapp.helloworld

import akka.actor.ActorSystem
import akka.http.scaladsl.ConnectionContext
import akka.http.scaladsl.Http
import akka.http.scaladsl.HttpsConnectionContext
import akka.http.scaladsl.common.SSLContextFactory
import akka.http.scaladsl.model.HttpRequest
import akka.http.scaladsl.model.HttpResponse
import example.myapp.helloworld.grpc._
import org.slf4j.LoggerFactory

import java.nio.file.Paths
import scala.concurrent.ExecutionContext
import scala.concurrent.Future
import scala.io.Source

object MtlsGreeterServer {
  def main(args: Array[String]): Unit = {
    val system = ActorSystem("MtlsHelloWorldServer")
    new MtlsGreeterServer(system).run()
    // ActorSystem threads will keep the app alive until `system.terminate()` is called
  }

}

class MtlsGreeterServer(system: ActorSystem) {

  private val log = LoggerFactory.getLogger(classOf[MtlsGreeterServer])
  def run(): Future[Http.ServerBinding] = {
    // Akka boot up code
    implicit val sys: ActorSystem = system
    implicit val ec: ExecutionContext = sys.dispatcher

    // Create service handlers
    val service: HttpRequest => Future[HttpResponse] =
      GreeterServiceHandler(new GreeterServiceImpl())

    // Bind service handler servers to localhost:8443
    val binding =
      Http().newServerAt("127.0.0.1", 8443).enableHttps(serverHttpContext).bind(service)

    // report successful binding
    binding.foreach { binding => log.info(s"gRPC server bound to: {}", binding.localAddress) }

    binding
  }

  private def serverHttpContext: HttpsConnectionContext =
    ConnectionContext.httpsServer { () =>
      val context = SSLContextFactory.createSSLContextFromPem(
        // Note: these are filesystem paths, not classpath
        certificatePath = Paths.get("src/main/resources/certs/localhost-server.crt"),
        privateKeyPath = Paths.get("src/main/resources/certs/localhost-server.key"),
        // client certs are issued by this CA
        trustedCaCertificatePaths = Seq(Paths.get("src/main/resources/certs/rootCA.crt")))

      val engine = context.createSSLEngine()
      engine.setUseClientMode(false)

      // require client certs
      engine.setNeedClientAuth(true)

      engine
    }

  private def classPathFileAsString(path: String): String =
    Source.fromResource(path).mkString
}

Java

copysourcepackage example.myapp.helloworld;

import akka.actor.ActorSystem;
import akka.http.javadsl.ConnectionContext;
import akka.http.javadsl.Http;
import akka.http.javadsl.HttpsConnectionContext;
import akka.http.javadsl.ServerBinding;
import akka.http.javadsl.common.SSLContextFactory;
import akka.http.javadsl.model.AttributeKeys;
import akka.http.javadsl.model.HttpRequest;
import akka.http.javadsl.model.HttpResponse;
import akka.http.javadsl.model.SslSessionInfo;
import akka.japi.function.Function;
import akka.pki.pem.DERPrivateKeyLoader;
import akka.pki.pem.PEMDecoder;
import akka.stream.Materializer;
import akka.stream.SystemMaterializer;
import example.myapp.helloworld.grpc.GreeterService;
import example.myapp.helloworld.grpc.GreeterServiceHandlerFactory;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.net.ssl.*;
import java.io.*;
import java.nio.charset.StandardCharsets;
import java.nio.file.Paths;
import java.security.KeyStore;
import java.security.PrivateKey;
import java.security.SecureRandom;
import java.security.cert.Certificate;
import java.security.cert.CertificateFactory;
import java.time.Duration;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.CompletionStage;
import java.util.stream.Collectors;

class MtlsGreeterServer {

  private static final Logger log = LoggerFactory.getLogger(MtlsGreeterServer.class);

  public static void main(String[] args) throws Exception {
    ActorSystem sys = ActorSystem.create("MtlsHelloWorldServer");

    run(sys).thenAccept(binding -> {
      log.info("gRPC server bound to {}", binding.localAddress());
    });

    // ActorSystem threads will keep the app alive until `system.terminate()` is called
  }

  public static CompletionStage<ServerBinding> run(ActorSystem sys) throws Exception {
    Materializer mat = SystemMaterializer.get(sys).materializer();

    // Instantiate implementation
    GreeterService impl = new GreeterServiceImpl(mat);

    Function<HttpRequest, CompletionStage<HttpResponse>> service =
      GreeterServiceHandlerFactory.create(impl, sys);

    return Http
      .get(sys)
      .newServerAt("127.0.0.1", 8443)
      .enableHttps(serverHttpContext())
      .bind(service);
  }

  private static HttpsConnectionContext serverHttpContext() {
    return ConnectionContext.httpsServer(() -> {
      SSLContext context = SSLContextFactory.createSSLContextFromPem(
        // Note: these are filesystem paths, not classpath
        Paths.get("src/main/resources/certs/localhost-server.crt"),
        Paths.get("src/main/resources/certs/localhost-server.key"),
        // client certs to trust are issued by this CA
        List.of(Paths.get("src/main/resources/certs/rootCA.crt")));

      SSLEngine engine = context.createSSLEngine();
      engine.setUseClientMode(false);

      // require client certs
      engine.setNeedClientAuth(true);
      return engine;
    });
  }
}

When run the server will only accept client connections that use a keypair that it considers valid, other connections will be denied and fail with a TLS protocol error.

It is possible to rotate the certificates without restarting the service, see the Akka HTTP documentation.

Setting the client up

In the client, the trust store must be set up to trust the server certificate, in our example it is signed with the same CA as the server:

Scala

copysourcepackage example.myapp.helloworld

import akka.actor.ActorSystem
import akka.grpc.GrpcClientSettings
import example.myapp.helloworld.grpc.GreeterServiceClient
import example.myapp.helloworld.grpc.HelloRequest

import java.nio.file.Paths
import javax.net.ssl.SSLContext
import akka.http.scaladsl.common.SSLContextFactory
import scala.concurrent.ExecutionContext
import scala.io.Source
import scala.util.Success
import scala.util.Failure

object MtlsGreeterClient {

  def main(args: Array[String]): Unit = {
    implicit val sys: ActorSystem = ActorSystem.create("MtlsHelloWorldClient")
    implicit val ec: ExecutionContext = sys.dispatcher

    val clientSettings = GrpcClientSettings.connectToServiceAt("localhost", 8443).withSslContext(sslContext())

    val client = GreeterServiceClient(clientSettings)

    val reply = client.sayHello(HelloRequest("Jonas"))

    reply.onComplete { tryResponse =>
      tryResponse match {
        case Success(reply) =>
          println(s"Successful reply: $reply")
        case Failure(exception) =>
          println("Request failed")
          exception.printStackTrace()
      }
      sys.terminate()
    }
  }

  def sslContext(): SSLContext = {
    SSLContextFactory.createSSLContextFromPem(
      // Note: these are filesystem paths, not classpath
      certificatePath = Paths.get("src/main/resources/certs/client1.crt"),
      privateKeyPath = Paths.get("src/main/resources/certs/client1.key"),
      // server cert is issued by this CA
      trustedCaCertificatePaths = Seq(Paths.get("src/main/resources/certs/rootCA.crt")))
  }

  private def classPathFileAsString(path: String): String =
    Source.fromResource(path).mkString

}

Java

copysourcepackage example.myapp.helloworld;

import akka.actor.ActorSystem;
import akka.grpc.GrpcClientSettings;
import akka.http.javadsl.common.SSLContextFactory;
import example.myapp.helloworld.grpc.GreeterServiceClient;
import example.myapp.helloworld.grpc.HelloReply;
import example.myapp.helloworld.grpc.HelloRequest;

import javax.net.ssl.*;
import java.nio.file.Paths;
import java.util.List;
import java.util.concurrent.CompletionStage;

public class MtlsGreeterClient {

  public static void main(String[] args) {
    ActorSystem system = ActorSystem.create("MtlsHelloWorldClient");

    GrpcClientSettings clientSettings =
      GrpcClientSettings.connectToServiceAt("localhost", 8443, system)
        .withSslContext(sslContext());

    GreeterServiceClient client = GreeterServiceClient.create(clientSettings, system);

    CompletionStage<HelloReply> reply = client.sayHello(HelloRequest.newBuilder().setName("Jonas").build());

    reply.whenComplete((response, error) -> {
      if (error == null) {
        System.out.println("Successful reply: " + reply);
      } else {
        System.out.println("Request failed");
        error.printStackTrace();
      }
      system.terminate();
    });
  }

  private static SSLContext sslContext() {
    try {
      return SSLContextFactory.createSSLContextFromPem(
        // Note: these are filesystem paths, not classpath
        Paths.get("src/main/resources/certs/client1.crt"),
        Paths.get("src/main/resources/certs/client1.key"),
        // server cert is issued by this CA
        List.of(Paths.get("src/main/resources/certs/rootCA.crt"))
      );
    } catch (Exception ex) {
      throw new RuntimeException("Failed to set up SSL context for the client", ex);
    }
  }
}

A client presenting a keypair will be able to connect to both servers requiring regular HTTPS gRPC services and mTLS servers that accept the client certificate.

It is possible to rotate the certificates without restarting the service, see the Akka HTTP documentation.

Further limiting of access using client certificate identities

In addition to requiring a trusted certificate it is possible to further limit access based on the identity present in the trusted client certificate ip or dns SAN (Subject Alternative Names) or CN (Common Name).

This is done by wrapping the service handler in the requireClientCertificateIdentity:

Scala

copysource// Create service handlers
val handler: HttpRequest => Future[HttpResponse] =
  GreeterServiceHandler(new GreeterServiceImpl())

// As a Route
val handlerRoute: Route = handle(handler)

val route = requireClientCertificateIdentity("""mycorp\..*\.client\d+}""".r) {
  handlerRoute
}

// Bind service handler servers to localhost:8082
val binding = Http().newServerAt("127.0.0.1", 8082).bind(route)

Java

copysource// Instantiate implementation
GreeterService impl = new GreeterServiceImpl(mat);
Function<HttpRequest, CompletionStage<HttpResponse>> handler = GreeterServiceHandlerFactory.create(impl, sys);

// As a Route
Route handlerRoute = handle(handler);

// Protect the handler route
Route protectedHandler =
  requireClientCertificateIdentity("mycorp\\..*\\.client\\d+}", () -> {
    return handlerRoute;

  });

return Http.get(sys)
  .newServerAt("127.0.0.1", 8090)
  .bind(protectedHandler);

FIXME provide sample snippets