Decryption-Tools/pywidevine
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Cdm: Fix bug where context data may not correspond to the right license

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We are using a trick with the request_id to be able to add an identifier between get_license_challenge() and parse_license() without any middleman data needing to be passed by the user.

Otherwise the user would need to either create the context data themselves after get_license_challenge() and pass it to get_license(), or something that is similar at its core to that.
This commit is contained in:
rlaphoenix 2022-07-21 01:12:28 +01:00
parent 68db728bf0
commit 23165f92de
1 changed files with 10 additions and 9 deletions
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19
pywidevine/cdm.py
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@ -89,7 +89,7 @@ class Cdm:
self.session_id = self.create_session_id(self.device)
self.service_certificate: Optional[SignedMessage] = None
self.context: Optional[tuple[bytes, bytes]] = None
self.context: dict[bytes, tuple[bytes, bytes]] = {}
def set_service_certificate(self, certificate: Union[bytes, str]) -> SignedMessage:
"""
@ -131,6 +131,8 @@ class Cdm:
Returns a SignedMessage containing a LicenseRequest message. It's signed with
the Private Key of the device provision.
"""
request_id = get_random_bytes(16)
license_request = LicenseRequest()
license_request.type = LicenseRequest.RequestType.Value("NEW")
license_request.request_time = int(time.time())
@ -139,7 +141,7 @@ class Cdm:
license_request.content_id.widevine_pssh_data.pssh_data.append(self.init_data)
license_request.content_id.widevine_pssh_data.license_type = type_
license_request.content_id.widevine_pssh_data.request_id = self.session_id
license_request.content_id.widevine_pssh_data.request_id = request_id
if self.service_certificate and privacy_mode:
# encrypt the client id for privacy mode
@ -158,16 +160,11 @@ class Cdm:
new(self.device.private_key). \
sign(SHA1.new(license_message.msg))
self.context = self.derive_context(license_message.msg)
self.context[request_id] = self.derive_context(license_message.msg)
return license_message.SerializeToString()
def parse_license(self, license_message: Union[bytes, str]) -> list[Key]:
# TODO: What if the CDM generates a 2nd challenge, overwriting the previous making it mismatch
# for deriving keys? We need to get these key context's corresponding to their license_message
if not self.context:
raise ValueError("Cannot parse a license message without first making a license request")
if not license_message:
raise ValueError("Cannot parse an empty license_message as a SignedMessage")
@ -186,11 +183,15 @@ class Cdm:
licence = License()
licence.ParseFromString(license_message.msg)
context = self.context[licence.id.request_id]
if not context:
raise ValueError("Cannot parse a license message without first making a license request")
session_key = PKCS1_OAEP. \
new(self.device.private_key). \
decrypt(license_message.session_key)
enc_key, mac_key_server, mac_key_client = self.derive_keys(*self.context, session_key)
enc_key, mac_key_server, mac_key_client = self.derive_keys(*context, session_key)
license_signature = HMAC. \
new(mac_key_server, digestmod=SHA256). \