googleapis · arithmetic1728 · Mar 19, 2020 · Feb 20, 2020 · Feb 24, 2020 · Feb 25, 2020
diff --git a/google/auth/transport/_mtls_helper.py b/google/auth/transport/_mtls_helper.py
@@ -0,0 +1,116 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Helper functions for getting mTLS cert and key, for internal use only."""
+
+import json
+import logging
+from os import path
+import re
+import subprocess
+
+CONTEXT_AWARE_METADATA_PATH = "~/.secureConnect/context_aware_metadata.json"
+_CERT_PROVIDER_COMMAND = "cert_provider_command"
+_CERT_REGEX = re.compile(
+    b"-----BEGIN CERTIFICATE-----.+-----END CERTIFICATE-----\r?\n?", re.DOTALL
+)
+
+# support various format of key files, e.g.
+# "-----BEGIN PRIVATE KEY-----...",
+# "-----BEGIN EC PRIVATE KEY-----...",
+# "-----BEGIN RSA PRIVATE KEY-----..."
+_KEY_REGEX = re.compile(
+    b"-----BEGIN [A-Z ]*PRIVATE KEY-----.+-----END [A-Z ]*PRIVATE KEY-----\r?\n?",
+    re.DOTALL,
+)
+
+_LOGGER = logging.getLogger(__name__)
+
+
+def _check_dca_metadata_path(metadata_path):
+    """Checks for context aware metadata. If it exists, returns the absolute path;
+    otherwise returns None.
+
+    Args:
+        metadata_path (str): context aware metadata path.
+
+    Returns:
+        str: absolute path if exists and None otherwise.
+    """
+    metadata_path = path.expanduser(metadata_path)
+    if not path.exists(metadata_path):
+        _LOGGER.debug("%s is not found, skip client SSL authentication.", metadata_path)
+        return None
+    return metadata_path
+
+
+def _read_dca_metadata_file(metadata_path):
+    """Loads context aware metadata from the given path.
+
+    Args:
+        metadata_path (str): context aware metadata path.
+
+    Returns:
+        Dict[str, str]: The metadata.
+
+    Raises:
+        ValueError: If failed to parse metadata as JSON.
+    """
+    with open(metadata_path) as f:
+        metadata = json.load(f)
+
+    return metadata
+
+
+def get_client_ssl_credentials(metadata_json):
+    """Returns the client side mTLS cert and key.
+
+    Args:
+        metadata_json (Dict[str, str]): metadata JSON file which contains the cert
+            provider command.
+
+    Returns:
+        Tuple[bytes, bytes]: client certificate and key, both in PEM format.
+
+    Raises:
+        OSError: If the cert provider command failed to run.
+        RuntimeError: If the cert provider command has a runtime error.
+        ValueError: If the metadata json file doesn't contain the cert provider command or if the command doesn't produce both the client certificate and client key.
+    """
+    # TODO: implement an in-memory cache of cert and key so we don't have to
+    # run cert provider command every time.
+
+    # Check the cert provider command existence in the metadata json file.
+    if _CERT_PROVIDER_COMMAND not in metadata_json:
+        raise ValueError("Cert provider command is not found")
+
+    # Execute the command. It throws OsError in case of system failure.
+    command = metadata_json[_CERT_PROVIDER_COMMAND]
+    process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    stdout, stderr = process.communicate()
+
+    # Check cert provider command execution error.
+    if process.returncode != 0:
+        raise RuntimeError(
+            "Cert provider command returns non-zero status code %s" % process.returncode
+        )
+
+    # Extract certificate (chain) and key.
+    cert_match = re.findall(_CERT_REGEX, stdout)
+    if len(cert_match) != 1:
+        raise ValueError("Client SSL certificate is missing or invalid")
+    key_match = re.findall(_KEY_REGEX, stdout)
+    if len(key_match) != 1:
+        raise ValueError("Client SSL key is missing or invalid")
+    return cert_match[0], key_match[0]
diff --git a/google/auth/transport/grpc.py b/google/auth/transport/grpc.py
@@ -17,9 +17,12 @@
 from __future__ import absolute_import
 
 from concurrent import futures
+import logging
 
 import six
 
+from google.auth.transport import _mtls_helper
+
 try:
     import grpc
 except ImportError as caught_exc:  # pragma: NO COVER
@@ -31,6 +34,8 @@
         caught_exc,
     )
 
+_LOGGER = logging.getLogger(__name__)
+
 
 class AuthMetadataPlugin(grpc.AuthMetadataPlugin):
     """A `gRPC AuthMetadataPlugin`_ that inserts the credentials into each
@@ -92,7 +97,12 @@ def __del__(self):
 
 
 def secure_authorized_channel(
-    credentials, request, target, ssl_credentials=None, **kwargs
+    credentials,
+    request,
+    target,
+    ssl_credentials=None,
+    client_cert_callback=None,
+    **kwargs
 ):
     """Creates a secure authorized gRPC channel.
 
@@ -114,11 +124,86 @@ def secure_authorized_channel(
 
         # Create a channel.
         channel = google.auth.transport.grpc.secure_authorized_channel(
-            credentials, 'speech.googleapis.com:443', request)
+            credentials, regular_endpoint, request,
+            ssl_credentials=grpc.ssl_channel_credentials())
 
         # Use the channel to create a stub.
         cloud_speech.create_Speech_stub(channel)
 
+    Usage:
+
+    There are actually a couple of options to create a channel, depending on if
+    you want to create a regular or mutual TLS channel.
+
+    First let's list the endpoints (regular vs mutual TLS) to choose from::
+
+        regular_endpoint = 'speech.googleapis.com:443'
+        mtls_endpoint = 'speech.mtls.googleapis.com:443'
+
+    Option 1: create a regular (non-mutual) TLS channel by explicitly setting
+    the ssl_credentials::
+
+        regular_ssl_credentials = grpc.ssl_channel_credentials()
+
+        channel = google.auth.transport.grpc.secure_authorized_channel(
+            credentials, regular_endpoint, request,
+            ssl_credentials=regular_ssl_credentials)
+
+    Option 2: create a mutual TLS channel by calling a callback which returns
+    the client side certificate and the key::
+
+        def my_client_cert_callback():
+            code_to_load_client_cert_and_key()
+            if loaded:
+                return (pem_cert_bytes, pem_key_bytes)
+            raise MyClientCertFailureException()
+
+        try:
+            channel = google.auth.transport.grpc.secure_authorized_channel(
+                credentials, mtls_endpoint, request,
+                client_cert_callback=my_client_cert_callback)
+        except MyClientCertFailureException:
+            # handle the exception
+
+    Option 3: use application default SSL credentials. It searches and uses
+    the command in a context aware metadata file, which is available on devices
+    with endpoint verification support.
+    See https://cloud.google.com/endpoint-verification/docs/overview::
+
+        try:
+            default_ssl_credentials = SslCredentials()
+        except:
+            # Exception can be raised if the context aware metadata is malformed.
+            # See :class:`SslCredentials` for the possible exceptions.
+
+        # Choose the endpoint based on the SSL credentials type.
+        if default_ssl_credentials.is_mtls:
+            endpoint_to_use = mtls_endpoint
+        else:
+            endpoint_to_use = regular_endpoint
+        channel = google.auth.transport.grpc.secure_authorized_channel(
+            credentials, endpoint_to_use, request,
+            ssl_credentials=default_ssl_credentials)
+
+    Option 4: not setting ssl_credentials and client_cert_callback. For devices
+    without endpoint verification support, a regular TLS channel is created;
+    otherwise, a mutual TLS channel is created, however, the call should be
+    wrapped in a try/except block in case of malformed context aware metadata.
+
+    The following code uses regular_endpoint, it works the same no matter the
+    created channle is regular or mutual TLS. Regular endpoint ignores client
+    certificate and key::
+
+        channel = google.auth.transport.grpc.secure_authorized_channel(
+            credentials, regular_endpoint, request)
+
+    The following code uses mtls_endpoint, if the created channle is regular,
+    and API mtls_endpoint is confgured to require client SSL credentials, API
+    calls using this channel will be rejected::
+
+        channel = google.auth.transport.grpc.secure_authorized_channel(
+            credentials, mtls_endpoint, request)
+
     Args:
         credentials (google.auth.credentials.Credentials): The credentials to
             add to requests.
@@ -129,23 +214,118 @@ def secure_authorized_channel(
         target (str): The host and port of the service.
         ssl_credentials (grpc.ChannelCredentials): Optional SSL channel
             credentials. This can be used to specify different certificates.
+            This argument is mutually exclusive with client_cert_callback;
+            providing both will raise an exception.
+            If ssl_credentials and client_cert_callback are None, application
+            default SSL credentials will be used.
+        client_cert_callback (Callable[[], (bytes, bytes)]): Optional
+            callback function to obtain client certicate and key for mutual TLS
+            connection. This argument is mutually exclusive with
+            ssl_credentials; providing both will raise an exception.
+            If ssl_credentials and client_cert_callback are None, application
+            default SSL credentials will be used.
         kwargs: Additional arguments to pass to :func:`grpc.secure_channel`.
 
     Returns:
         grpc.Channel: The created gRPC channel.
+
+    Raises:
+        OSError: If the cert provider command launch fails during the application
+            default SSL credentials loading process on devices with endpoint
+            verification support.
+        RuntimeError: If the cert provider command has a runtime error during the
+            application default SSL credentials loading process on devices with
+            endpoint verification support.
+        ValueError:
+            If the context aware metadata file is malformed or if the cert provider
+            command doesn't produce both client certificate and key during the
+            application default SSL credentials loading process on devices with
+            endpoint verification support.
     """
     # Create the metadata plugin for inserting the authorization header.
     metadata_plugin = AuthMetadataPlugin(credentials, request)
 
     # Create a set of grpc.CallCredentials using the metadata plugin.
     google_auth_credentials = grpc.metadata_call_credentials(metadata_plugin)
 
-    if ssl_credentials is None:
-        ssl_credentials = grpc.ssl_channel_credentials()
+    if ssl_credentials and client_cert_callback:
+        raise ValueError(
+            "Received both ssl_credentials and client_cert_callback; "
+            "these are mutually exclusive."
+        )
+
+    # If SSL credentials are not explicitly set, try client_cert_callback and ADC.
+    if not ssl_credentials:
+        if client_cert_callback:
+            # Use the callback if provided.
+            cert, key = client_cert_callback()
+            ssl_credentials = grpc.ssl_channel_credentials(
+                certificate_chain=cert, private_key=key
+            )
+        else:
+            # Use application default SSL credentials.
+            adc_ssl_credentils = SslCredentials()
+            ssl_credentials = adc_ssl_credentils.ssl_credentials
 
     # Combine the ssl credentials and the authorization credentials.
     composite_credentials = grpc.composite_channel_credentials(
         ssl_credentials, google_auth_credentials
     )
 
     return grpc.secure_channel(target, composite_credentials, **kwargs)
+
+
+class SslCredentials:
+    """Class for application default SSL credentials.
+
+    For devices with endpoint verification support, a device certificate will be
+    automatically loaded and mutual TLS will be established.
+    See https://cloud.google.com/endpoint-verification/docs/overview.
+    """
+
+    def __init__(self):
+        # Load client SSL credentials.
+        self._context_aware_metadata_path = _mtls_helper._check_dca_metadata_path(
+            _mtls_helper.CONTEXT_AWARE_METADATA_PATH
+        )
+        if self._context_aware_metadata_path:
+            self._is_mtls = True
+        else:
+            self._is_mtls = False
+
+    @property
+    def ssl_credentials(self):
+        """Get the created SSL channel credentials.
+
+        For devices with endpoint verification support, if the device certificate
+        loading has any problems, corresponding exceptions will be raised. For
+        a device without endpoint verification support, no exceptions will be
+        raised.
+
+        Returns:
+            grpc.ChannelCredentials: The created grpc channel credentials.
+
+        Raises:
+            OSError: If the cert provider command launch fails.
+            RuntimeError: If the cert provider command has a runtime error.
+            ValueError:
+                If the context aware metadata file is malformed or if the cert provider
+                command doesn't produce both the client certificate and key.
+        """
+        if self._context_aware_metadata_path:
+            metadata = _mtls_helper._read_dca_metadata_file(
+                self._context_aware_metadata_path
+            )
+            cert, key = _mtls_helper.get_client_ssl_credentials(metadata)
+            self._ssl_credentials = grpc.ssl_channel_credentials(
+                certificate_chain=cert, private_key=key
+            )
+        else:
+            self._ssl_credentials = grpc.ssl_channel_credentials()
+
+        return self._ssl_credentials
+
+    @property
+    def is_mtls(self):
+        """Indicates if the created SSL channel credentials is mutual TLS."""
+        return self._is_mtls
diff --git a/tests/data/context_aware_metadata.json b/tests/data/context_aware_metadata.json
@@ -0,0 +1,6 @@
+{
+  "cert_provider_command":[
+    "/opt/google/endpoint-verification/bin/SecureConnectHelper",
+    "--print_certificate"],
+  "device_resource_ids":["11111111-1111-1111"]
+}