Cdm: Save Service Certificate in SignedMessage form

We may need the signature for external verification, and most APIs require it to be in a SignedMessage to be accepted, even though the SignedMessage is pretty much empty (not even actually signed lol).

pywidevine/cdm.py

@@ -150,6 +150,10 @@ class Cdm:
                 signed_drm_certificate.ParseFromString(certificate)
                 if signed_drm_certificate.SerializeToString() != certificate:
                     raise DecodeError()
+                # Craft a SignedMessage as it's stored as a SignedMessage
+                signed_message.Clear()
+                signed_message.msg = signed_drm_certificate.SerializeToString()
+                # we don't need to sign this message, this is normal
         except DecodeError:
             # could be a direct unsigned DrmCertificate, but reject those anyway
             raise DecodeError("Could not parse certificate as a SignedDrmCertificate")
@@ -164,9 +168,9 @@ class Cdm:
         except (ValueError, TypeError):
             raise SignatureMismatch("Signature Mismatch on SignedDrmCertificate, rejecting certificate")
         else:
+            session.service_certificate = signed_message
             drm_certificate = DrmCertificate()
             drm_certificate.ParseFromString(signed_drm_certificate.drm_certificate)
-            session.service_certificate = drm_certificate
             return drm_certificate.provider_id
 
     def get_license_challenge(

pywidevine/session.py

@@ -3,12 +3,12 @@ from typing import Optional
 from Crypto.Random import get_random_bytes
 
 from pywidevine.key import Key
-from pywidevine.license_protocol_pb2 import DrmCertificate
+from pywidevine.license_protocol_pb2 import SignedMessage
 
 
 class Session:
     def __init__(self):
         self.id = get_random_bytes(16)
-        self.service_certificate: Optional[DrmCertificate] = None
+        self.service_certificate: Optional[SignedMessage] = None
         self.context: dict[bytes, tuple[bytes, bytes]] = {}
         self.keys: list[Key] = []