examples: support dsse in example client

Add `--use-dsse` flag to `download` subcommand of example client, which
can be used to indicate that all metadata is expected to come in a DSSE
envelope.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer

examples/client/client

@@ -14,12 +14,13 @@ from pathlib import Path
 from urllib import request
 
 from tuf.api.exceptions import DownloadError, RepositoryError
-from tuf.ngclient import Updater
+from tuf.ngclient import Updater, UpdaterConfig
 
 # constants
 DOWNLOAD_DIR = "./downloads"
 CLIENT_EXAMPLE_DIR = os.path.dirname(os.path.abspath(__file__))
 
+
 def build_metadata_dir(base_url: str) -> str:
     """build a unique and reproducible directory name for the repository url"""
     name = sha256(base_url.encode()).hexdigest()[:8]
@@ -46,7 +47,7 @@ def init_tofu(base_url: str) -> bool:
     return True
 
 
-def download(base_url: str, target: str) -> bool:
+def download(base_url: str, target: str, use_dsse: bool) -> bool:
     """
     Download the target file using ``ngclient`` Updater.
 
@@ -72,12 +73,16 @@ def download(base_url: str, target: str) -> bool:
     if not os.path.isdir(DOWNLOAD_DIR):
         os.mkdir(DOWNLOAD_DIR)
 
+    config = UpdaterConfig()
+    config.use_dsse = use_dsse
+
     try:
         updater = Updater(
             metadata_dir=metadata_dir,
             metadata_base_url=f"{base_url}/metadata/",
             target_base_url=f"{base_url}/targets/",
             target_dir=DOWNLOAD_DIR,
+            config=config,
         )
         updater.refresh()
 
@@ -146,6 +151,13 @@ def main() -> None:
         help="Target file",
     )
 
+    download_parser.add_argument(
+        "--use-dsse",
+        help="Parse TUF metadata as DSSE",
+        default=False,
+        action="store_true",
+    )
+
     command_args = client_args.parse_args()
 
     if command_args.verbose == 0:
@@ -164,7 +176,9 @@ def main() -> None:
         if not init_tofu(command_args.url):
             return "Failed to initialize local repository"
     elif command_args.sub_command == "download":
-        if not download(command_args.url, command_args.target):
+        if not download(
+            command_args.url, command_args.target, command_args.use_dsse
+        ):
             return f"Failed to download {command_args.target}"
     else:
         client_args.print_help()