fix(exchange): preserve old archives during server key rotation

src/apps/exchange/services.py

@@ -412,6 +412,30 @@ class ExchangePackageImportService:
 
         return header, bin_bytes[header_end:]
 
+    @classmethod
+    def _decryption_token(cls, header: dict[str, Any]) -> str:
+        token = str(getattr(settings, "EXCHANGE_SHARED_TOKEN", "") or "").strip()
+        if not token:
+            raise ExchangeImportError("EXCHANGE_SHARED_TOKEN не настроен")
+
+        previous_token = str(
+            getattr(settings, "EXCHANGE_PREVIOUS_SHARED_TOKEN", "") or ""
+        ).strip()
+        previous_id = str(
+            getattr(settings, "EXCHANGE_PREVIOUS_KEY_ID", "") or ""
+        ).strip()
+        current_id = str(getattr(settings, "EXCHANGE_KEY_ID", "") or "").strip()
+        if previous_token or previous_id:
+            if not previous_token or not previous_id or previous_id == current_id:
+                raise ExchangeImportError(
+                    "Некорректно настроен предыдущий ключ расшифровки обмена"
+                )
+            if header.get("key_id") == previous_id:
+                return previous_token
+        # Preserve the existing current-key contract, including old containers
+        # without key_id. Never try the previous secret as a decryption fallback.
+        return token
+
     @classmethod
     def _decrypt_payload(
         cls,
@@ -419,10 +443,7 @@ class ExchangePackageImportService:
         header: dict[str, Any],
         encrypted_payload: bytes,
     ) -> dict[str, Any]:
-        token = str(getattr(settings, "EXCHANGE_SHARED_TOKEN", "") or "").strip()
-        if not token:
-            raise ExchangeImportError("EXCHANGE_SHARED_TOKEN не настроен")
-
+        token = cls._decryption_token(header)
         raw_key = hashlib.sha256(token.encode("utf-8")).digest()
         nonce = cls._decode_base64_field(header, "nonce")
         aad = cls._decode_base64_field(header, "aad")

src/settings/base.py

@@ -19,6 +19,9 @@ BACKUP_ENCRYPTION_KEY = os.getenv("BACKUP_ENCRYPTION_KEY", "")
 BACKUP_KEY_ID = os.getenv("BACKUP_KEY_ID", "default")
 EXCHANGE_SHARED_TOKEN = os.getenv("EXCHANGE_SHARED_TOKEN", "")
 EXCHANGE_KEY_ID = os.getenv("EXCHANGE_KEY_ID", "dev-shared-token")
+# Read old encrypted archives after rotation; never accepted as HTTP credentials.
+EXCHANGE_PREVIOUS_SHARED_TOKEN = os.getenv("EXCHANGE_PREVIOUS_SHARED_TOKEN", "")
+EXCHANGE_PREVIOUS_KEY_ID = os.getenv("EXCHANGE_PREVIOUS_KEY_ID", "")
 
 warnings.filterwarnings(
     "ignore",