feat(auth): mTLS endpoint for Regional Access Boundaries

google-auth-library-java/oauth2_http/java/com/google/auth/mtls/MtlsHttpTransportFactory.java

@@ -31,6 +31,7 @@
 
 package com.google.auth.mtls;
 
+import com.google.api.client.http.HttpTransport;
 import com.google.api.client.http.javanet.NetHttpTransport;
 import com.google.api.core.InternalApi;
 import com.google.auth.http.HttpTransportFactory;
@@ -62,7 +63,7 @@ public MtlsHttpTransportFactory(KeyStore mtlsKeyStore) {
   }
 
   @Override
-  public NetHttpTransport create() {
+  public HttpTransport create() {
     try {
       // Build the mTLS transport using the provided KeyStore.
       return new NetHttpTransport.Builder().trustCertificates(null, mtlsKeyStore, "").build();

google-auth-library-java/oauth2_http/java/com/google/auth/mtls/MtlsUtils.java

@@ -31,14 +31,19 @@
 package com.google.auth.mtls;
 
 import com.google.api.core.InternalApi;
+import com.google.auth.http.HttpTransportFactory;
 import com.google.auth.oauth2.EnvironmentProvider;
+import com.google.auth.oauth2.OAuth2Utils;
 import com.google.auth.oauth2.PropertyProvider;
 import com.google.common.base.Strings;
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.security.KeyStore;
 import java.util.Locale;
+import java.util.logging.Logger;
+import javax.annotation.Nullable;
 
 /**
  * Utility class for mTLS related operations.
@@ -47,10 +52,27 @@
  */
 @InternalApi
 public class MtlsUtils {
+  private static final Logger LOGGER = Logger.getLogger(MtlsUtils.class.getName());
+
   static final String CERTIFICATE_CONFIGURATION_ENV_VARIABLE = "GOOGLE_API_CERTIFICATE_CONFIG";
   static final String WELL_KNOWN_CERTIFICATE_CONFIG_FILE = "certificate_config.json";
   static final String CLOUDSDK_CONFIG_DIRECTORY = "gcloud";
 
+  /**
+   * The policy determining when to use mutual TLS (mTLS) endpoints.
+   *
+   * <p>See <a href="https://google.aip.dev/auth/4114">AIP-4114</a> for the specification on mTLS
+   * endpoint usage.
+   */
+  public enum MtlsEndpointUsagePolicy {
+    /** Always use the mTLS endpoint, and fail if client certificates are not configured. */
+    ALWAYS,
+    /** Never use the mTLS endpoint. */
+    NEVER,
+    /** Use the mTLS endpoint if client certificates are configured (auto-discovery). */
+    AUTO
+  }
+
   private MtlsUtils() {
     // Prevent instantiation for Utility class
   }
@@ -76,38 +98,27 @@ public static String getCertificatePath(
   }
 
   /**
-   * Resolves and loads the workload certificate configuration.
+   * Resolves and parses the workload certificate configuration.
    *
-   * <p>The configuration file is resolved in the following order of precedence: 1. The provided
-   * certConfigPathOverride (if not null). 2. The path specified by the
-   * GOOGLE_API_CERTIFICATE_CONFIG environment variable. 3. The well-known certificate configuration
-   * file in the gcloud config directory.
+   * <p>This locates the certificate configuration file via {@link #resolveCertificateConfigFile}
+   * and parses its contents into a {@link WorkloadCertificateConfiguration}.
    *
    * @param envProvider the environment provider to use for resolving environment variables
    * @param propProvider the property provider to use for resolving system properties
    * @param certConfigPathOverride optional override path for the configuration file
    * @return the loaded WorkloadCertificateConfiguration
-   * @throws IOException if the configuration file cannot be found, read, or parsed
+   * @throws IOException if the configuration file cannot be resolved, read, or parsed
    */
   static WorkloadCertificateConfiguration getWorkloadCertificateConfiguration(
       EnvironmentProvider envProvider, PropertyProvider propProvider, String certConfigPathOverride)
       throws IOException {
-    File certConfig;
-    if (certConfigPathOverride != null) {
-      certConfig = new File(certConfigPathOverride);
-    } else {
-      String envCredentialsPath = envProvider.getEnv(CERTIFICATE_CONFIGURATION_ENV_VARIABLE);
-      if (!Strings.isNullOrEmpty(envCredentialsPath)) {
-        certConfig = new File(envCredentialsPath);
-      } else {
-        certConfig = getWellKnownCertificateConfigFile(envProvider, propProvider);
-      }
-    }
-
-    if (!certConfig.isFile()) {
+    File certConfig =
+        resolveCertificateConfigFile(envProvider, propProvider, certConfigPathOverride);
+    if (certConfig == null) {
+      File wellKnownConfig = getWellKnownCertificateConfigFile(envProvider, propProvider);
       throw new CertificateSourceUnavailableException(
           "Certificate configuration file does not exist or is not a file: "
-              + certConfig.getAbsolutePath());
+              + wellKnownConfig.getAbsolutePath());
     }
     try (InputStream certConfigStream = new FileInputStream(certConfig)) {
       return WorkloadCertificateConfiguration.fromCertificateConfigurationStream(certConfigStream);
@@ -137,4 +148,177 @@ private static File getWellKnownCertificateConfigFile(
     }
     return new File(cloudConfigPath, WELL_KNOWN_CERTIFICATE_CONFIG_FILE);
   }
+
+  /**
+   * Centralized helper method to determine if mutual TLS (mTLS) can be enabled.
+   *
+   * @param envProvider the environment provider to use for resolving environment variables
+   * @param propProvider the property provider to use for resolving system properties
+   * @param certConfigPathOverride optional override path for the configuration file
+   * @return true if mTLS should be enabled, false otherwise
+   * @throws IOException if the configuration file is present but contains missing or malformed
+   *     files
+   */
+  public static boolean canBeEnabled(
+      EnvironmentProvider envProvider, PropertyProvider propProvider, String certConfigPathOverride)
+      throws IOException {
+
+    // Check if client certificate usage is allowed
+    String useClientCertificate = envProvider.getEnv("GOOGLE_API_USE_CLIENT_CERTIFICATE");
+    MtlsEndpointUsagePolicy policy = getMtlsEndpointUsagePolicy(envProvider);
+    if ("false".equalsIgnoreCase(useClientCertificate)) {
+      if (policy == MtlsEndpointUsagePolicy.ALWAYS) {
+        throw new CertificateSourceUnavailableException(
+            "mTLS is configured to ALWAYS, but client certificate usage was explicitly disabled via GOOGLE_API_USE_CLIENT_CERTIFICATE=false.");
+      }
+      return false;
+    }
+
+    if (policy == MtlsEndpointUsagePolicy.NEVER) {
+      return false;
+    }
+
+    if (policy == MtlsEndpointUsagePolicy.ALWAYS) {
+      return true;
+    }
+
+    File certConfigFile =
+        resolveCertificateConfigFile(envProvider, propProvider, certConfigPathOverride);
+    return certConfigFile != null;
+  }
+
+  /**
+   * Resolves the mutual TLS (mTLS) certificate configuration file.
+   *
+   * <p>The configuration file is resolved in the following order of precedence:
+   *
+   * <ol>
+   *   <li>The developer-provided {@code certConfigPathOverride} (if not null).
+   *   <li>The path specified by the {@code GOOGLE_API_CERTIFICATE_CONFIG} environment variable.
+   *   <li>The well-known automatic gcloud workload identity provisioning location (via {@link
+   *       #getWellKnownCertificateConfigFile}).
+   * </ol>
+   *
+   * <p>If an explicit configuration file is specified (via override or environment variable) and it
+   * is missing or invalid, an exception is thrown. If no explicit file is specified and the default
+   * well-known file is missing, {@code null} is returned.
+   *
+   * @param envProvider the environment provider to use for resolving environment variables
+   * @param propProvider the property provider to use for resolving system properties
+   * @param certConfigPathOverride optional override path for the configuration file
+   * @return the resolved File object, or null if no configuration was found
+   * @throws IOException if an explicit configuration file is missing or malformed
+   */
+  @Nullable
+  static File resolveCertificateConfigFile(
+      EnvironmentProvider envProvider, PropertyProvider propProvider, String certConfigPathOverride)
+      throws IOException {
+    // 1. Check explicit developer override
+    if (certConfigPathOverride != null) {
+      File certConfigFile = new File(certConfigPathOverride);
+      if (!certConfigFile.isFile()) {
+        throw new CertificateSourceUnavailableException(
+            "Certificate configuration file does not exist or is not a file: "
+                + certConfigFile.getAbsolutePath());
+      }
+      return certConfigFile;
+    }
+
+    // 2. Check explicit environment variable
+    String envPath = envProvider.getEnv(CERTIFICATE_CONFIGURATION_ENV_VARIABLE);
+    if (!Strings.isNullOrEmpty(envPath)) {
+      File certConfigFile = new File(envPath);
+      if (!certConfigFile.isFile()) {
+        throw new CertificateSourceUnavailableException(
+            "Certificate configuration file does not exist or is not a file: "
+                + certConfigFile.getAbsolutePath());
+      }
+      return certConfigFile;
+    }
+
+    // 3. Check optional well-known automatic provisioning location
+    try {
+      File wellKnownConfig = getWellKnownCertificateConfigFile(envProvider, propProvider);
+      if (wellKnownConfig.isFile()) {
+        return wellKnownConfig;
+      }
+    } catch (IOException e) {
+      LOGGER.info(
+          "Could not get the mutual TLS (mTLS) client certificate configuration. The library will fall back to making standard non-mTLS requests.");
+    }
+
+    return null;
+  }
+
+  /**
+   * Returns the current mutual TLS endpoint usage policy.
+   *
+   * @param envProvider the environment provider to use for resolving environment variables
+   * @return the MtlsEndpointUsagePolicy enum value
+   */
+  public static MtlsEndpointUsagePolicy getMtlsEndpointUsagePolicy(
+      EnvironmentProvider envProvider) {
+    String mtlsEndpointUsagePolicy = envProvider.getEnv("GOOGLE_API_USE_MTLS_ENDPOINT");
+    if ("never".equals(mtlsEndpointUsagePolicy)) {
+      return MtlsEndpointUsagePolicy.NEVER;
+    } else if ("always".equals(mtlsEndpointUsagePolicy)) {
+      return MtlsEndpointUsagePolicy.ALWAYS;
+    }
+    return MtlsEndpointUsagePolicy.AUTO;
+  }
+
+  /**
+   * Prepares and upgrades the HTTP transport factory for mutual TLS (mTLS) if applicable.
+   *
+   * @param baseTransportFactory the base HTTP transport factory to upgrade
+   * @param envProvider the environment provider to use for resolving environment variables
+   * @param propProvider the property provider to use for resolving system properties
+   * @param certConfigPathOverride optional override path for the configuration file
+   * @return the mTLS-configured HTTP transport factory, or the base factory if mTLS is not enabled
+   * @throws IOException if mTLS is required/enabled but certificate initialization fails or an
+   *     incompatible transport factory was provided
+   */
+  public static HttpTransportFactory prepareTransportFactoryIfMtlsEnabled(
+      HttpTransportFactory baseTransportFactory,
+      EnvironmentProvider envProvider,
+      PropertyProvider propProvider,
+      String certConfigPathOverride)
+      throws IOException {
+
+    MtlsEndpointUsagePolicy mtlsPolicy = getMtlsEndpointUsagePolicy(envProvider);
+    try {
+      if (baseTransportFactory instanceof MtlsHttpTransportFactory) {
+        // A custom MtlsHttpTransportFactory was already pre-configured by the user.
+        // Keep using it as-is without re-initializing.
+        return baseTransportFactory;
+      }
+
+      if (!canBeEnabled(envProvider, propProvider, certConfigPathOverride)) {
+        return baseTransportFactory;
+      }
+
+      if (baseTransportFactory == OAuth2Utils.HTTP_TRANSPORT_FACTORY) {
+        // This is the default HttpTransportFactory assigned by credentials.
+        // Automatically discover and load client certificates to construct an mTLS factory.
+        X509Provider x509Provider =
+            new X509Provider(envProvider, propProvider, certConfigPathOverride);
+        KeyStore mtlsKeyStore = x509Provider.getKeyStore();
+        return new MtlsHttpTransportFactory(mtlsKeyStore);
+      }
+
+      // A user configured non-mTLS HttpTransportFactory was explicitly injected.
+      // Reject it to avoid bypassing mTLS enforcement or overriding the user's factory.
+      throw new IOException(
+          "mTLS is enabled on the system, but a user configured non-mTLS HttpTransportFactory was provided: "
+              + baseTransportFactory.getClass().getName());
+
+    } catch (Exception e) {
+      if (mtlsPolicy == MtlsEndpointUsagePolicy.ALWAYS) {
+        throw new IOException(
+            "mTLS is configured to ALWAYS, but initialization failed: " + e.getMessage(), e);
+      }
+      // Graceful fallback to standard transport if mTLS initialization fails under AUTO policy
+      return baseTransportFactory;
+    }
+  }
 }

google-auth-library-java/oauth2_http/java/com/google/auth/mtls/X509Provider.java

@@ -102,7 +102,7 @@ public X509Provider() {
    *
    * <ul>
    *   <li>The certificate config override path, if set.
-   *   <li>The path pointed to by the "GOOGLE_API_CERTIFICATE_CONFIG" environment variable
+   *   <li>The path pointed to by the "GOOGLE_API_CERTIFICATE_CONFIG" environment variable.
    *   <li>The well known gcloud location for the certificate configuration file.
    * </ul>
    *
@@ -113,40 +113,24 @@ public X509Provider() {
    */
   @Override
   public KeyStore getKeyStore() throws CertificateSourceUnavailableException, IOException {
+    // Attempt to load from resolved Config File
     WorkloadCertificateConfiguration workloadCertConfig =
         MtlsUtils.getWorkloadCertificateConfiguration(
             envProvider, propProvider, certConfigPathOverride);
 
-    // Read the certificate and private key file paths into streams.
     try (InputStream certStream = new FileInputStream(new File(workloadCertConfig.getCertPath()));
         InputStream privateKeyStream =
             new FileInputStream(new File(workloadCertConfig.getPrivateKeyPath()));
         SequenceInputStream certAndPrivateKeyStream =
             new SequenceInputStream(certStream, privateKeyStream)) {
-
-      // Build a key store using the combined stream.
       return SecurityUtils.createMtlsKeyStore(certAndPrivateKeyStream);
-    } catch (CertificateSourceUnavailableException e) {
-      // Throw the CertificateSourceUnavailableException without wrapping.
-      throw e;
     } catch (Exception e) {
-      // Wrap all other exception types to an IOException.
-      throw new IOException("X509Provider: Unexpected IOException:", e);
+      throw new IOException("X509Provider: Unexpected error loading from config file:", e);
     }
   }
 
-  /**
-   * Returns true if the X509 mTLS provider is available.
-   *
-   * @throws IOException if a general I/O error occurs while determining availability.
-   */
   @Override
   public boolean isAvailable() throws IOException {
-    try {
-      this.getKeyStore();
-    } catch (CertificateSourceUnavailableException e) {
-      return false;
-    }
-    return true;
+    return MtlsUtils.canBeEnabled(envProvider, propProvider, certConfigPathOverride);
   }
 }

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/GoogleCredentials.java

@@ -39,6 +39,7 @@
 import com.google.auth.RequestMetadataCallback;
 import com.google.auth.http.AuthHttpConstants;
 import com.google.auth.http.HttpTransportFactory;
+import com.google.auth.mtls.MtlsUtils;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.MoreObjects;
 import com.google.common.base.MoreObjects.ToStringHelper;
@@ -397,6 +398,12 @@ void refreshRegionalAccessBoundaryIfExpired(@Nullable URI uri, @Nullable AccessT
       return;
     }
 
+    // Automatically discover certificates or enforce mTLS policy if applicable
+    // TODO: https://github.com/googleapis/google-cloud-java/issues/13461
+    transportFactory =
+        MtlsUtils.prepareTransportFactoryIfMtlsEnabled(
+            transportFactory, getEnvironmentProvider(), getPropertyProvider(), null);
+
     regionalAccessBoundaryManager.triggerAsyncRefresh(
         transportFactory, (RegionalAccessBoundaryProvider) this, token);
   }
@@ -843,6 +850,14 @@ HttpTransportFactory getTransportFactory() {
     return null;
   }
 
+  EnvironmentProvider getEnvironmentProvider() {
+    return SystemEnvironmentProvider.getInstance();
+  }
+
+  PropertyProvider getPropertyProvider() {
+    return SystemPropertyProvider.getInstance();
+  }
+
   public static class Builder extends OAuth2Credentials.Builder {
     @Nullable protected String quotaProjectId;
     @Nullable protected String universeDomain;