Add /service-parameters route to enable discovery via client CLI

Related: ietf-wg-scitt/draft-ietf-scitt-architecture#96
Signed-off-by: John Andersen <johnandersenpdx@gmail.com>

scitt_emulator/client.py

@@ -151,6 +151,16 @@ def retrieve_receipt(url: str, entry_id: Path, receipt_path: Path, client: HttpC
     print(f"Receipt written to {receipt_path}")
 
 
+def retrieve_service_parameters(url: str, service_parameters_path: Path, client: HttpClient):
+    response = client.get(f"{url}/service-parameters")
+    service_parameters = response.content
+
+    with open(service_parameters_path, "wb") as f:
+        f.write(service_parameters)
+
+    print(f"Service parameters written to {service_parameters_path}")
+
+
 def verify_receipt(cose_path: Path, receipt_path: Path, service_parameters_path: Path):
     with open(service_parameters_path) as f:
         service_parameters = json.load(f)
@@ -227,6 +237,20 @@ def cli(fn):
         )
     )
 
+    p = sub.add_parser("retrieve-service-parameters", description="Retrieve SCITT service parameters")
+    p.add_argument(
+        "--out", required=True, type=Path, help="Path to write the service parameters to"
+    )
+    p.add_argument("--url", required=False, default=DEFAULT_URL)
+    p.add_argument("--token", help="Bearer token to authenticate with")
+    p.add_argument("--cacert", type=Path, help="CA certificate to verify host against")
+    p.set_defaults(
+        func=lambda args: retrieve_service_parameters(
+            args.url, args.out,
+            HttpClient(args.token, args.cacert)
+        )
+    )
+
     p = sub.add_parser("verify-receipt", description="Verify a SCITT receipt")
     p.add_argument("--claim", required=True, type=Path)
     p.add_argument("--receipt", required=True, type=Path)

scitt_emulator/scitt.py

@@ -71,6 +71,9 @@ def create_receipt_contents(self, countersign_tbi: bytes, entry_id: str):
     def verify_receipt_contents(receipt_contents: list, countersign_tbi: bytes):
         raise NotImplementedError
 
+    def get_service_parameters(self) -> dict:
+        return self.service_parameters
+
     def get_operation(self, operation_id: str) -> dict:
         operation_path = self.operations_path / f"{operation_id}.json"
         try:

scitt_emulator/server.py

@@ -6,7 +6,7 @@
 from io import BytesIO
 import random
 
-from flask import Flask, request, send_file, make_response
+from flask import Flask, request, send_file, make_response, jsonify
 
 from scitt_emulator.tree_algs import TREE_ALGS
 from scitt_emulator.scitt import EntryNotFoundError, ClaimInvalidError, OperationNotFoundError
@@ -52,6 +52,12 @@ def create_flask_app(config):
     def is_unavailable():
         return random.random() <= error_rate
 
+    @app.route("/service-parameters", methods=["GET"])
+    def get_service_parameters():
+        if is_unavailable():
+            return make_unavailable_error()
+        return jsonify(app.scitt_service.get_service_parameters())
+
     @app.route("/entries/<string:entry_id>/receipt", methods=["GET"])
     def get_receipt(entry_id: str):
         if is_unavailable():

tests/test_cli.py

@@ -1,6 +1,7 @@
 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT License.
 import os
+import json
 import threading
 import pytest
 from werkzeug.serving import make_server
@@ -86,6 +87,21 @@ def test_client_cli(use_lro: bool, tmp_path):
         assert os.path.exists(receipt_path)
         assert os.path.exists(entry_id_path)
 
+        # retrieve service parameters
+        original_service_parameters = json.loads(service.service_parameters_path.read_text())
+        service.service_parameters_path.unlink()
+        assert not service.service_parameters_path.exists()
+        command = [
+            "client",
+            "retrieve-service-parameters",
+            "--out",
+            service.service_parameters_path,
+            "--url",
+            service.url
+        ]
+        execute_cli(command)
+        assert original_service_parameters == json.loads(service.service_parameters_path.read_text())
+
         # verify receipt
         command = [
             "client",