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Cdm: Only store license request's context data 

This reduces the amount of data needing to be stored, but also simplifies the key derivation.
This commit is contained in:
rlaphoenix 2022-07-20 22:25:57 +01:00
parent 53f7c1dd62
commit 68db728bf0
1 changed files with 22 additions and 20 deletions
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42
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.license_request: Optional[SignedMessage] = None
self.context: Optional[tuple[bytes, bytes]] = None
def set_service_certificate(self, certificate: Union[bytes, str]) -> SignedMessage:
"""
@ -158,15 +158,14 @@ class Cdm:
new(self.device.private_key). \
sign(SHA1.new(license_message.msg))
# store it for later, we need it for deriving keys when parsing a license
self.license_request = license_message
self.context = 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 make self.license_request always match the license_message
if not self.license_request:
# 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:
@ -191,7 +190,7 @@ class Cdm:
new(self.device.private_key). \
decrypt(license_message.session_key)
enc_key, mac_key_server, mac_key_client = self.derive_keys(self.license_request.msg, session_key)
enc_key, mac_key_server, mac_key_client = self.derive_keys(*self.context, session_key)
license_signature = HMAC. \
new(mac_key_server, digestmod=SHA256). \
@ -306,7 +305,23 @@ class Cdm:
return enc_client_id
@staticmethod
def derive_keys(msg: bytes, key: bytes) -> tuple[bytes, bytes, bytes]:
def derive_context(message: bytes) -> tuple[bytes, bytes]:
"""Returns 2 Context Data used for computing the AES Encryption and HMAC Keys."""
def _get_enc_context(msg: bytes) -> bytes:
label = b"ENCRYPTION"
key_size = 16 * 8 # 128-bit
return label + b"\x00" + msg + key_size.to_bytes(4, "big")
def _get_mac_context(msg: bytes) -> bytes:
label = b"AUTHENTICATION"
key_size = 32 * 8 * 2 # 512-bit
return label + b"\x00" + msg + key_size.to_bytes(4, "big")
return _get_enc_context(message), _get_mac_context(message)
@staticmethod
def derive_keys(enc_context: bytes, mac_context: bytes, key: bytes) -> tuple[bytes, bytes, bytes]:
"""
Returns 3 keys derived from the input message.
Key can either be a pre-provision device aes key, provision key, or a session key.
@ -327,24 +342,11 @@ class Cdm:
keys and verify licenses.
"""
def get_enc_context(message: bytes) -> bytes:
label = b"ENCRYPTION"
key_size = 16 * 8 # 128-bit
return label + b"\x00" + message + key_size.to_bytes(4, "big")
def get_mac_context(message: bytes) -> bytes:
label = b"AUTHENTICATION"
key_size = 32 * 8 * 2 # 512-bit
return label + b"\x00" + message + key_size.to_bytes(4, "big")
def _derive(session_key: bytes, context: bytes, counter: int) -> bytes:
return CMAC.new(session_key, ciphermod=AES). \
update(counter.to_bytes(1, "big") + context). \
digest()
enc_context = get_enc_context(msg)
mac_context = get_mac_context(msg)
enc_key = _derive(key, enc_context, 1)
mac_key_server = _derive(key, mac_context, 1)
mac_key_server += _derive(key, mac_context, 2)