Generate AES KeyPair lazily (#687)

* Generate AES KeyPair lazily

* Fix coverage

* Update post-review

* Fix pragma

* Make json dumps consistent between python and orjson

* Add comment

* Add comments re json parameter in HttpClient

kasa/aestransport.py

@@ -8,7 +8,7 @@ import base64
 import hashlib
 import logging
 import time
-from typing import Dict, Optional, cast
+from typing import TYPE_CHECKING, AsyncGenerator, Dict, Optional, cast
 
 from cryptography.hazmat.primitives import padding, serialization
 from cryptography.hazmat.primitives.asymmetric import padding as asymmetric_padding
@@ -55,6 +55,8 @@ class AesTransport(BaseTransport):
         "requestByApp": "true",
         "Accept": "application/json",
     }
+    CONTENT_LENGTH = "Content-Length"
+    KEY_PAIR_CONTENT_LENGTH = 314
 
     def __init__(
         self,
@@ -86,6 +88,8 @@ class AesTransport(BaseTransport):
 
         self._login_token = None
 
+        self._key_pair: Optional[KeyPair] = None
+
         _LOGGER.debug("Created AES transport for %s", self._host)
 
     @property
@@ -204,34 +208,44 @@ class AesTransport(BaseTransport):
         self._handle_response_error_code(resp_dict, "Error logging in")
         self._login_token = resp_dict["result"]["token"]
 
+    async def _generate_key_pair_payload(self) -> AsyncGenerator:
+        """Generate the request body and return an ascyn_generator.
+
+        This prevents the key pair being generated unless a connection
+        can be made to the device.
+        """
+        _LOGGER.debug("Generating keypair")
+        self._key_pair = KeyPair.create_key_pair()
+        pub_key = (
+            "-----BEGIN PUBLIC KEY-----\n"
+            + self._key_pair.get_public_key()  # type: ignore[union-attr]
+            + "\n-----END PUBLIC KEY-----\n"
+        )
+        handshake_params = {"key": pub_key}
+        _LOGGER.debug(f"Handshake params: {handshake_params}")
+        request_body = {"method": "handshake", "params": handshake_params}
+        _LOGGER.debug(f"Request {request_body}")
+        yield json_dumps(request_body).encode()
+
     async def perform_handshake(self):
         """Perform the handshake."""
         _LOGGER.debug("Will perform handshaking...")
-        _LOGGER.debug("Generating keypair")
 
+        self._key_pair = None
         self._handshake_done = False
         self._session_expire_at = None
         self._session_cookie = None
 
         url = f"http://{self._host}/app"
-        key_pair = KeyPair.create_key_pair()
-
-        pub_key = (
-            "-----BEGIN PUBLIC KEY-----\n"
-            + key_pair.get_public_key()
-            + "\n-----END PUBLIC KEY-----\n"
-        )
-        handshake_params = {"key": pub_key}
-        _LOGGER.debug(f"Handshake params: {handshake_params}")
-
-        request_body = {"method": "handshake", "params": handshake_params}
-
-        _LOGGER.debug(f"Request {request_body}")
-
+        # Device needs the content length or it will response with 500
+        headers = {
+            **self.COMMON_HEADERS,
+            self.CONTENT_LENGTH: str(self.KEY_PAIR_CONTENT_LENGTH),
+        }
         status_code, resp_dict = await self._http_client.post(
             url,
-            json=request_body,
-            headers=self.COMMON_HEADERS,
+            json=self._generate_key_pair_payload(),
+            headers=headers,
             cookies_dict=self._session_cookie,
         )
 
@@ -259,8 +273,10 @@ class AesTransport(BaseTransport):
             self._session_cookie = {self.SESSION_COOKIE_NAME: cookie}
 
         self._session_expire_at = time.time() + 86400
+        if TYPE_CHECKING:
+            assert self._key_pair is not None  # pragma: no cover
         self._encryption_session = AesEncyptionSession.create_from_keypair(
-            handshake_key, key_pair
+            handshake_key, self._key_pair
         )
 
         self._handshake_done = True