Preparing to enable SSL/TLS for gRPC server

.gitignore

@@ -4,6 +4,7 @@ data/
 doc/source/api_documentation
 doc/source/_build
 flwr_logs
+.cache
 
 # Datasets
 cifar-10-python.tar.gz

dev/certificates/certificate.conf

@@ -0,0 +1,20 @@
+[req]
+default_bits = 4096
+prompt = no
+default_md = sha256
+req_extensions = req_ext
+distinguished_name = dn
+
+[dn]
+C = DE
+ST = HH
+O = Flower
+CN = localhost
+
+[req_ext]
+subjectAltName = @alt_names
+
+[alt_names]
+DNS.1 = localhost
+IP.1 = ::1
+IP.2 = 127.0.0.1

dev/certificates/generate.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+# This script will generate all certificates if ca.crt does not exist
+
+set -e
+cd "$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"/../../
+
+CA_PASSWORD=notsafe
+
+CERT_DIR=.cache/certificates
+
+# Generate directories if not exists
+mkdir -p .cache/certificates
+
+# if [ -f ".cache/certificates/ca.crt" ]; then
+#     echo "Skipping certificate generation as they already exist."
+#     exit 0
+# fi
+
+rm -f $CERT_DIR/*
+
+# Generate the root certificate authority key and certificate based on key
+openssl genrsa -out $CERT_DIR/ca.key 4096
+openssl req \
+    -new \
+    -x509 \
+    -key $CERT_DIR/ca.key \
+    -sha256 \
+    -subj "/C=DE/ST=HH/O=CA, Inc." \
+    -days 365 -out $CERT_DIR/ca.crt
+
+# Generate a new private key for the server
+openssl genrsa -out $CERT_DIR/server.key 4096
+
+# Create a signing CSR
+openssl req \
+    -new \
+    -key $CERT_DIR/server.key \
+    -out $CERT_DIR/server.csr \
+    -config ./dev/certificates/certificate.conf
+
+# Generate a certificate for the server
+openssl x509 \
+    -req \
+    -in $CERT_DIR/server.csr \
+    -CA $CERT_DIR/ca.crt \
+    -CAkey $CERT_DIR/ca.key \
+    -CAcreateserial \
+    -out $CERT_DIR/server.pem \
+    -days 365 \
+    -sha256 \
+    -extfile ./dev/certificates/certificate.conf \
+    -extensions req_ext

src/py/flwr/client/grpc_client/connection_test.py

@@ -24,7 +24,7 @@
 
 from flwr.proto.transport_pb2 import ClientMessage, ServerMessage
 from flwr.server.client_manager import SimpleClientManager
-from flwr.server.grpc_server.grpc_server import start_insecure_grpc_server
+from flwr.server.grpc_server.grpc_server import start_grpc_server
 
 from .connection import insecure_grpc_connection
 
@@ -82,7 +82,7 @@ def test_integration_connection() -> None:
     # Prepare
     port = unused_tcp_port()
 
-    server = start_insecure_grpc_server(
+    server = start_grpc_server(
         client_manager=SimpleClientManager(), server_address=f"[::]:{port}"
     )
 

src/py/flwr/server/app.py

@@ -21,7 +21,7 @@
 from flwr.common import GRPC_MAX_MESSAGE_LENGTH
 from flwr.common.logger import log
 from flwr.server.client_manager import SimpleClientManager
-from flwr.server.grpc_server.grpc_server import start_insecure_grpc_server
+from flwr.server.grpc_server.grpc_server import start_grpc_server
 from flwr.server.history import History
 from flwr.server.server import Server
 from flwr.server.strategy import FedAvg, Strategy
@@ -70,7 +70,7 @@ def start_server(  # pylint: disable=too-many-arguments
     initialized_server, initialized_config = _init_defaults(server, config, strategy)
 
     # Start gRPC server
-    grpc_server = start_insecure_grpc_server(
+    grpc_server = start_grpc_server(
         client_manager=initialized_server.client_manager(),
         server_address=server_address,
         max_message_length=grpc_max_message_length,

src/py/flwr/server/grpc_server/grpc_server.py

@@ -12,45 +12,102 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 # ==============================================================================
-"""Implements utility function to create a grpc server."""
+"""Implements utility function to create a gRPC server."""
+
+
 import concurrent.futures
+import sys
+from logging import ERROR
+from pathlib import Path
+from typing import ByteString, Optional, Tuple, Union
 
 import grpc
 
 from flwr.common import GRPC_MAX_MESSAGE_LENGTH
+from flwr.common.logger import log
 from flwr.proto import transport_pb2_grpc
 from flwr.server.client_manager import ClientManager
 from flwr.server.grpc_server import flower_service_servicer as fss
 
+FILELIKE = Union[str, bytes]
+SSLFILES = Tuple[FILELIKE, FILELIKE, FILELIKE]
+
+INVALID_SSL_FILES_ERR_MSG = """
+    When setting any of root_certificate, certificate, or private_key,
+    all of them need to be set.
+"""
+
+
+def read_to_byte_string(file_like: FILELIKE) -> ByteString:
+    """Read file_like and return as ByteString."""
+    with open(file_like, "rb") as file:
+        return file.read()
+
+
+def valid_ssl_files(ssl_files: SSLFILES) -> bool:
+    """Validate type SSLFILES."""
+    all_files = [Path(ssl_file).is_file() for ssl_file in ssl_files]
+    is_valid = all(all_files) and len(all_files) == 3
+
+    if not is_valid:
+        log(ERROR, INVALID_SSL_FILES_ERR_MSG)
+
+    return is_valid
 
-def start_insecure_grpc_server(
+
+def start_grpc_server(
     client_manager: ClientManager,
     server_address: str,
     max_concurrent_workers: int = 1000,
     max_message_length: int = GRPC_MAX_MESSAGE_LENGTH,
+    ssl_files: Optional[SSLFILES] = None,
 ) -> grpc.Server:
-    """Create grpc server and return registered FlowerServiceServicer instance.
+    """Create gRPC server and return instance of grpc.Server.
 
     If used in a main function server.wait_for_termination(timeout=None)
     should be called as otherwise the server will immediately stop.
 
+    **SSL/TLS**
+    To enable SSL/TLS you have to pass all of root_certificate, certificate,
+    and private_key. Setting only some will make the process exit with code 1.
+
     Parameters
     ----------
     client_manager : ClientManager
         Instance of ClientManager
     server_address : str
         Server address in the form of HOST:PORT e.g. "[::]:8080"
     max_concurrent_workers : int
-        Set the maximum number of clients you want the server to process
-        before returning RESOURCE_EXHAUSTED status (default: 1000)
+        Maximum number of clients the server can process before returning
+        RESOURCE_EXHAUSTED status (default: 1000)
     max_message_length : int
         Maximum message length that the server can send or receive.
         Int valued in bytes. -1 means unlimited. (default: GRPC_MAX_MESSAGE_LENGTH)
+    ssl_files : Tuple[Union[str, bytes]]
+        Tuple containing root certificate, server certificate, and private key to start
+        a secure SSL/TLS server. The tuple is expected to have three file-like
+        elements in the following order:
+
+            * CA certificate.
+            * server certificate.
+            * server private key.
+
+        (default: None)
 
     Returns
     -------
     server : grpc.Server
         An instance of a gRPC server which is already started
+
+    Examples
+    --------
+    Starting a SSL/TLS enabled server.
+
+    >>> start_grpc_server(
+    >>>     client_manager=ClientManager(),
+    >>>     server_address="localhost:8080",
+    >>>     ssl_files=("/crts/root.pem", "/crts/localhost.crt", "/crts/localhost.key")
+    >>> )
     """
     server = grpc.server(
         concurrent.futures.ThreadPoolExecutor(max_workers=max_concurrent_workers),
@@ -73,7 +130,28 @@ def start_insecure_grpc_server(
     servicer = fss.FlowerServiceServicer(client_manager)
     transport_pb2_grpc.add_FlowerServiceServicer_to_server(servicer, server)
 
-    server.add_insecure_port(server_address)
+    if ssl_files is not None:
+        if not valid_ssl_files(ssl_files):
+            sys.exit(1)
+
+        root_certificate_b, certificate_b, private_key_b = [
+            read_to_byte_string(file_path) for file_path in ssl_files
+        ]
+
+        server_credentials = grpc.ssl_server_credentials(
+            ((private_key_b, certificate_b),),
+            root_certificates=root_certificate_b,
+            # A boolean indicating whether or not to require clients to be
+            # authenticated. May only be True if root_certificates is not None.
+            # We are explicitly setting the current gRPC default to document
+            # the option. For further reference see:
+            # https://grpc.github.io/grpc/python/grpc.html#create-server-credentials
+            require_client_auth=False,
+        )
+        server.add_secure_port(server_address, server_credentials)
+    else:
+        server.add_insecure_port(server_address)
+
     server.start()
 
     return server

src/py/flwr/server/grpc_server/grpc_server_test.py

@@ -15,11 +15,32 @@
 """Tests for module server."""
 
 import socket
+import subprocess
 from contextlib import closing
-from typing import cast
+from os.path import abspath, dirname, join
+from typing import Tuple, cast
 
 from flwr.server.client_manager import SimpleClientManager
-from flwr.server.grpc_server.grpc_server import start_insecure_grpc_server
+from flwr.server.grpc_server.grpc_server import start_grpc_server, valid_ssl_files
+
+root_dir = dirname(abspath(join(__file__, "../../../../..")))
+
+
+def load_certificates() -> Tuple[str, str, str]:
+    """Generate and load SSL/TLS credentials/certificates.
+
+    Utility function for loading for SSL/TLS enabled gRPC servertests.
+    """
+    # Trigger script which generates the certificates
+    subprocess.run(["bash", "./dev/certificates/generate.sh"], check=True, cwd=root_dir)
+
+    ssl_files = (
+        join(root_dir, ".cache/certificates/ca.crt"),
+        join(root_dir, ".cache/certificates/server.pem"),
+        join(root_dir, ".cache/certificates/server.key"),
+    )
+
+    return ssl_files
 
 
 def unused_tcp_port() -> int:
@@ -30,16 +51,59 @@ def unused_tcp_port() -> int:
         return cast(int, sock.getsockname()[1])
 
 
-def test_integration_start_and_shutdown_server() -> None:
+def test_valid_ssl_files_correctly() -> None:
+    """Test is validation function works correctly when passed valid list."""
+    # Prepare
+    ssl_files = load_certificates()
+
+    # Execute
+    is_valid = valid_ssl_files(ssl_files)
+
+    # Assert
+    assert is_valid
+
+
+def test_valid_ssl_files_correctly() -> None:
+    """Test is validation function works correctly when passed invalid list."""
+    # Prepare
+    ssl_files = ["/dev/null", "/dev/null"]
+
+    # Execute
+    is_valid = valid_ssl_files(ssl_files)
+
+    # Assert
+    assert not is_valid
+
+
+def test_integration_start_and_shutdown_insecure_server() -> None:
     """Create server and check if FlowerServiceServicer is returned."""
     # Prepare
     port = unused_tcp_port()
     client_manager = SimpleClientManager()
 
     # Execute
-    server = start_insecure_grpc_server(
+    server = start_grpc_server(
         client_manager=client_manager, server_address=f"[::]:{port}"
     )
 
     # Teardown
     server.stop(1)
+
+
+def test_integration_start_and_shutdown_secure_server() -> None:
+    """Create server and check if FlowerServiceServicer is returned."""
+    # Prepare
+    port = unused_tcp_port()
+    client_manager = SimpleClientManager()
+
+    ssl_files = load_certificates()
+
+    # Execute
+    server = start_grpc_server(
+        client_manager=client_manager,
+        server_address=f"[::]:{port}",
+        ssl_files=ssl_files,
+    )
+
+    # Teardown
+    server.stop(1)