+ README update

README.md

@@ -22,6 +22,11 @@ pyplayready test DEVICE.prd
 > [!IMPORTANT]  
 > There currently isn't a proper method of extracting Group Certificates/Keys. They can be found inside older Samsung phones/Smart TVs, Windows DLLs and set-top-boxes in encrypted form.
 
+Export a provisioned device to its raw .dat files
+```shell
+pyplayready export-device DEVICE.prd
+```
+
 ## Usage
 An example code snippet:
 

pyplayready/cdm.py

@@ -252,6 +252,9 @@ class Cdm:
         if not session:
             raise InvalidSession(f"Session identifier {session_id!r} is invalid.")
 
+        if not session.encryption_key or not session.signing_key:
+            raise InvalidSession("Cannot parse a license message without first making a license request")
+
         try:
             root = ET.fromstring(licence)
             license_elements = root.findall(".//{http://schemas.microsoft.com/DRM/2007/03/protocols}License")