Improve verification of proto parsing across Cdm, RemoteCdm and Device

This ensures that a partially parsing input (because of optional flags in the proto) does not get past any verification checks.

This prevents issues like an invalid License Challenging from getting an exception later down the line, as well as possibility of it also passing that check by pure luck, resulting in hard to debug issues.

pywidevine/cdm.py

@@ -360,6 +360,8 @@ class Cdm:
             signed_message = SignedMessage()
             try:
                 signed_message.ParseFromString(license_message)
+                if signed_message.SerializeToString() != license_message:
+                    raise DecodeError(license_message)
             except DecodeError as e:
                 raise InvalidLicenseMessage(f"Could not parse license_message as a SignedMessage, {e}")
             license_message = signed_message

pywidevine/device.py

@@ -110,20 +110,37 @@ class Device:
         self.client_id = ClientIdentification()
         try:
             self.client_id.ParseFromString(client_id)
-        except DecodeError:
+            if self.client_id.SerializeToString() != client_id:
-            raise ValueError("Failed to parse client_id as a ClientIdentification")
+                raise DecodeError("partial parse")
+        except DecodeError as e:
+            raise ValueError(f"Failed to parse client_id as a ClientIdentification, {e}")
 
         self.vmp = FileHashes()
         if self.client_id.vmp_data:
             try:
                 self.vmp.ParseFromString(self.client_id.vmp_data)
-            except DecodeError:
+                if self.vmp.SerializeToString() != self.client_id.vmp_data:
-                raise ValueError("Failed to parse Client ID's VMP data as a FileHashes")
+                    raise DecodeError("partial parse")
+            except DecodeError as e:
+                raise ValueError(f"Failed to parse Client ID's VMP data as a FileHashes, {e}")
 
         signed_drm_certificate = SignedDrmCertificate()
-        signed_drm_certificate.ParseFromString(self.client_id.token)
         drm_certificate = DrmCertificate()
+
+        try:
+            signed_drm_certificate.ParseFromString(self.client_id.token)
+            if signed_drm_certificate.SerializeToString() != self.client_id.token:
+                raise DecodeError("partial parse")
+        except DecodeError as e:
+            raise DecodeError(f"Failed to parse the Signed DRM Certificate of the Client ID, {e}")
+
+        try:
             drm_certificate.ParseFromString(signed_drm_certificate.drm_certificate)
+            if drm_certificate.SerializeToString() != signed_drm_certificate.drm_certificate:
+                raise DecodeError("partial parse")
+        except DecodeError as e:
+            raise DecodeError(f"Failed to parse the DRM Certificate of the Client ID, {e}")
+
         self.system_id = drm_certificate.system_id
 
     def __repr__(self) -> str:
@@ -190,6 +207,8 @@ class Device:
             if data.vmp:
                 try:
                     vmp.ParseFromString(data.vmp)
+                    if vmp.SerializeToString() != data.vmp:
+                        raise DecodeError("partial parse")
                 except DecodeError as e:
                     raise DecodeError(f"Failed to parse VMP data as FileHashes, {e}")
                 data.vmp = vmp
@@ -197,6 +216,8 @@ class Device:
                 client_id = ClientIdentification()
                 try:
                     client_id.ParseFromString(data.client_id)
+                    if client_id.SerializeToString() != data.client_id:
+                        raise DecodeError("partial parse")
                 except DecodeError as e:
                     raise DecodeError(f"Failed to parse VMP data as FileHashes, {e}")
 

pywidevine/remotecdm.py

@@ -175,8 +175,11 @@ class RemoteCdm(Cdm):
         r = r["data"]
 
         try:
+            challenge = base64.b64decode(r["challenge_b64"])
             license_message = SignedMessage()
-            license_message.ParseFromString(base64.b64decode(r["challenge_b64"]))
+            license_message.ParseFromString(challenge)
+            if license_message.SerializeToString() != challenge:
+                raise DecodeError("partial parse")
         except DecodeError as e:
             raise InvalidLicenseMessage(f"Failed to parse license request, {e}")
 
@@ -196,6 +199,8 @@ class RemoteCdm(Cdm):
             signed_message = SignedMessage()
             try:
                 signed_message.ParseFromString(license_message)
+                if signed_message.SerializeToString() != license_message:
+                    raise DecodeError("partial parse")
             except DecodeError as e:
                 raise InvalidLicenseMessage(f"Could not parse license_message as a SignedMessage, {e}")
             license_message = signed_message