accessory: Integrate .plist reading into library

findmy/accessory.py

@@ -6,9 +6,10 @@ Accessories could be anything ranging from AirTags to iPhones.
 from __future__ import annotations
 
 import logging
+import plistlib
 from abc import ABC, abstractmethod
-from datetime import datetime, timedelta
-from typing import Generator, overload
+from datetime import datetime, timedelta, timezone
+from typing import IO, Generator, overload
 
 from typing_extensions import override
 
@@ -143,6 +144,30 @@ class FindMyAccessory(RollingKeyPairSource):
 
         return possible_keys
 
+    @classmethod
+    def from_plist(cls, plist: IO[bytes]) -> FindMyAccessory:
+        """Create a FindMyAccessory from a .plist file dumped from the FindMy app."""
+        device_data = plistlib.load(plist)
+
+        # PRIVATE master key. 28 (?) bytes.
+        master_key = device_data["privateKey"]["key"]["data"][-28:]
+
+        # "Primary" shared secret. 32 bytes.
+        skn = device_data["sharedSecret"]["key"]["data"]
+
+        # "Secondary" shared secret. 32 bytes.
+        if "secondarySharedSecret" in device_data:
+            # AirTag
+            sks = device_data["secondarySharedSecret"]["key"]["data"]
+        else:
+            # iDevice
+            sks = device_data["secureLocationsSharedSecret"]["key"]["data"]
+
+        # "Paired at" timestamp (UTC)
+        paired_at = device_data["pairingDate"].replace(tzinfo=timezone.utc)
+
+        return cls(master_key, skn, sks, paired_at)
+
 
 class AccessoryKeyGenerator(KeyGenerator[KeyPair]):
     """KeyPair generator. Uses the same algorithm internally as FindMy accessories do."""