Cdm: Support providing Service Cert as any 3 schemas

Some service's might provide the Service Certificate as a SignedDrmCertificate instead of a SignedMessage so I added support for supplying such format certificates. I also added support for supplying a DrmCertificate directly, though it's unlikely for a service to provide it raw without a signature like that.

The Service Certificate is now also stored as just the DrmCertificate internally, as it will not be using the signature.

pywidevine/cdm.py

@@ -84,10 +84,10 @@ class Cdm:
             self.init_data = PSSH.get_as_box(pssh).init_data
 
         self.session_id = get_random_bytes(16)
-        self.service_certificate: Optional[SignedMessage] = None
+        self.service_certificate: Optional[DrmCertificate] = None
         self.context: dict[bytes, tuple[bytes, bytes]] = {}
 
-    def set_service_certificate(self, certificate: Union[bytes, str]) -> SignedMessage:
+    def set_service_certificate(self, certificate: Union[bytes, str]) -> DrmCertificate:
         """
         Set a Service Privacy Certificate for Privacy Mode. (optional but recommended)
 
@@ -96,7 +96,7 @@ class Cdm:
                 Some services have their own, but most use the common privacy cert,
                 (common_privacy_cert).
 
-        Returns the parsed Signed Message if successful, otherwise raises a DecodeError.
+        Returns the parsed Drm Certificate if successful, otherwise raises a DecodeError.
 
         The Service Certificate is used to encrypt Client IDs in Licenses. This is also
         known as Privacy Mode and may be required for some services or for some devices.
@@ -106,13 +106,48 @@ class Cdm:
             certificate = base64.b64decode(certificate)  # assuming base64
 
         signed_message = SignedMessage()
-        try:
-            signed_message.ParseFromString(certificate)
-        except DecodeError as e:
-            raise DecodeError(f"Could not parse certificate as a Signed Message: {e}")
+        signed_drm_certificate = SignedDrmCertificate()
+        drm_certificate = DrmCertificate()
 
-        self.service_certificate = signed_message
-        return signed_message
+        # Note: A secure CDM would likely reject any Service Certificate that is
+        # not either a SignedMessage or a SignedDrmCertificate. This is because
+        # the DrmCertificate itself is not signed. This CDM does not verify the
+        # signatures as I'm not sure what HMAC key is used. At this stage of the
+        # CDM flow, we wouldn't have any mac_keys, and those might not work for
+        # verifying service certificate signatures (likely not).
+
+        # All these 3 schemas can sort of parse each other in a minimal buggy way,
+        # so we have to parse down the full chain instead of relaying each step
+
+        try:  # SignedMessage input
+            signed_message.ParseFromString(certificate)
+            signed_drm_certificate.ParseFromString(signed_message.msg)
+            if not signed_drm_certificate.drm_certificate:
+                raise DecodeError()
+            drm_certificate.ParseFromString(signed_drm_certificate.drm_certificate)
+            self.service_certificate = drm_certificate
+            return self.service_certificate
+        except DecodeError:
+            pass
+
+        try:  # SignedDrmCertificate input
+            signed_drm_certificate.ParseFromString(certificate)
+            if not signed_drm_certificate.drm_certificate:
+                raise DecodeError()
+            drm_certificate.ParseFromString(signed_drm_certificate.drm_certificate)
+            self.service_certificate = drm_certificate
+            return self.service_certificate
+        except DecodeError:
+            pass
+
+        try:  # DrmCertificate input
+            drm_certificate.ParseFromString(certificate)
+            self.service_certificate = drm_certificate
+            return self.service_certificate
+        except DecodeError:
+            pass
+
+        raise DecodeError("Could not parse certificate as a Service Certificate")
 
     def get_license_challenge(self, type_: LicenseType = LicenseType.STREAMING, privacy_mode: bool = True) -> bytes:
         """
@@ -253,7 +288,7 @@ class Cdm:
     @staticmethod
     def encrypt_client_id(
         client_id: ClientIdentification,
-        service_certificate: Union[SignedMessage, SignedDrmCertificate, DrmCertificate],
+        service_certificate: DrmCertificate,
         key: bytes = None,
         iv: bytes = None
     ) -> EncryptedClientIdentification:
@@ -261,18 +296,8 @@ class Cdm:
         privacy_key = key or get_random_bytes(16)
         privacy_iv = iv or get_random_bytes(16)
 
-        if isinstance(service_certificate, SignedMessage):
-            signed_service_certificate = SignedDrmCertificate()
-            signed_service_certificate.ParseFromString(service_certificate.msg)
-            service_certificate = signed_service_certificate
-
-        if isinstance(service_certificate, SignedDrmCertificate):
-            service_service_drm_certificate = DrmCertificate()
-            service_service_drm_certificate.ParseFromString(service_certificate.drm_certificate)
-            service_certificate = service_service_drm_certificate
-
         if not isinstance(service_certificate, DrmCertificate):
-            raise ValueError(f"Service Certificate is in an unexpected type {service_certificate!r}")
+            raise ValueError(f"Expecting Service Certificate to be a DrmCertificate, not {service_certificate!r}")
 
         enc_client_id = EncryptedClientIdentification()
         enc_client_id.provider_id = service_certificate.provider_id