Cdm: Verify Signatures of Security Certificates
This improves Cdm security and prevents a trivial exploit on Privacy Mode allowing an attacker to bypass Privacy Mode by controlling their own Public/Private Key Pair on Service Certificates. The attack is simple in which you create your own RSA-2048 key pair, replace the public key of a service certificate with your own, and now you have the corresponding private key to be able to decrypt Encrypted Client IDs. This trivial attack is often used on CDM re-implementations, proxies, and APIs to obtain sensitive Device Client ID information. With this commit this attack is prevented on this Cdm implementation, making it more secure from attacks. A signed DRM Certificate must be provided now as the ability to provide a direct DrmCertificate has been removed. The root certificate added alongside this commit has no private key and cannot be used to re-sign an altered DrmCertificate.
This commit is contained in:
rlaphoenix
parent
d9d8074f73
commit
a3102ded18
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@ -41,6 +41,21 @@ class Cdm:
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"+Gf2Ogsrf9s2LFvE7NVV2FvKqcWTw4PIV9Sdqrd+QLeFHd/SSZiAjjWyWOddeOrAyhb3BHMEwg2T7eTo/xxvF+YkP"
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"j89qPwXCYcOxF+6gjomPwzvofcJOxkJkoMmMzcFBDopvab5tDQsyN9UPLGhGC98X/8z8QSQ+spbJTYLdgFenFoGq4"
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"7gLwDS6NWYYQSqzE3Udf2W7pzk4ybyG4PHBYV3s4cyzdq8amvtE/sNSdOKReuHpfQ=")
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root_signed_cert = SignedDrmCertificate()
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root_signed_cert.ParseFromString(base64.b64decode(
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"CpwDCAASAQAY3ZSIiwUijgMwggGKAoIBgQC0/jnDZZAD2zwRlwnoaM3yw16b8udNI7EQ24dl39z7nzWgVwNTTPZtNX2meNuzNtI/nECplSZy"
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"f7i+Zt/FIZh4FRZoXS9GDkPLioQ5q/uwNYAivjQji6tTW3LsS7VIaVM+R1/9Cf2ndhOPD5LWTN+udqm62SIQqZ1xRdbX4RklhZxTmpfrhNfM"
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"qIiCIHAmIP1+QFAn4iWTb7w+cqD6wb0ptE2CXMG0y5xyfrDpihc+GWP8/YJIK7eyM7l97Eu6iR8nuJuISISqGJIOZfXIbBH/azbkdDTKjDOx"
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"+biOtOYS4AKYeVJeRTP/Edzrw1O6fGAaET0A+9K3qjD6T15Id1sX3HXvb9IZbdy+f7B4j9yCYEy/5CkGXmmMOROtFCXtGbLynwGCDVZEiMg1"
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"7B8RsyTgWQ035Ec86kt/lzEcgXyUikx9aBWE/6UI/Rjn5yvkRycSEbgj7FiTPKwS0ohtQT3F/hzcufjUUT4H5QNvpxLoEve1zqaWVT94tGSC"
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"UNIzX5ECAwEAARKAA1jx1k0ECXvf1+9dOwI5F/oUNnVKOGeFVxKnFO41FtU9v0KG9mkAds2T9Hyy355EzUzUrgkYU0Qy7OBhG+XaE9NVxd0a"
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"y5AeflvG6Q8in76FAv6QMcxrA4S9IsRV+vXyCM1lQVjofSnaBFiC9TdpvPNaV4QXezKHcLKwdpyywxXRESYqI3WZPrl3IjINvBoZwdVlkHZV"
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"dA8OaU1fTY8Zr9/WFjGUqJJfT7x6Mfiujq0zt+kw0IwKimyDNfiKgbL+HIisKmbF/73mF9BiC9yKRfewPlrIHkokL2yl4xyIFIPVxe9enz2F"
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"RXPia1BSV0z7kmxmdYrWDRuu8+yvUSIDXQouY5OcCwEgqKmELhfKrnPsIht5rvagcizfB0fbiIYwFHghESKIrNdUdPnzJsKlVshWTwApHQh7"
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"evuVicPumFSePGuUBRMS9nG5qxPDDJtGCHs9Mmpoyh6ckGLF7RC5HxclzpC5bc3ERvWjYhN0AqdipPpV2d7PouaAdFUGSdUCDA=="
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))
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root_cert = DrmCertificate()
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root_cert.ParseFromString(root_signed_cert.drm_certificate)
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NUM_OF_SESSIONS = 0
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MAX_NUM_OF_SESSIONS = 50 # most common limit
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@ -91,63 +106,67 @@ class Cdm:
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"""
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Set a Service Privacy Certificate for Privacy Mode. (optional but recommended)
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Parameters:
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certificate: Signed Message in Base64 or Bytes form obtained from the Service.
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Some services have their own, but most use the common privacy cert,
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(common_privacy_cert).
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Returns the parsed Drm Certificate if successful, otherwise raises a DecodeError.
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The Service Certificate is used to encrypt Client IDs in Licenses. This is also
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known as Privacy Mode and may be required for some services or for some devices.
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Chrome CDM requires it as of the enforcement of VMP (Verified Media Path).
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We reject direct DrmCertificates as they do not have signature verification and
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cannot be verified. You must provide a SignedDrmCertificate or a SignedMessage
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containing a SignedDrmCertificate.
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Parameters:
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certificate: SignedDrmCertificate (or SignedMessage containing one) in Base64
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or Bytes form obtained from the Service. Some services have their own,
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but most use the common privacy cert, (common_privacy_cert).
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Raises:
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DecodeError: If the certificate could not be parsed as a SignedDrmCertificate
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nor a SignedMessage containing a SignedDrmCertificate.
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ValueError: If the SignedDrmCertificate signature is invalid.
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Returns the parsed and verified DrmCertificate if successful.
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"""
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if isinstance(certificate, str):
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certificate = base64.b64decode(certificate) # assuming base64
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# All these 3 schemas can sort of parse each other in a minimal buggy way,
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# so we have to parse down the full chain instead of relaying each step.
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# We also parse fully down to the DrmCertificate before parsing signatures
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# for the same reason. The data may not parse at a lower level.
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signed_message = SignedMessage()
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signed_drm_certificate = SignedDrmCertificate()
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drm_certificate = DrmCertificate()
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# Note: A secure CDM would likely reject any Service Certificate that is
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# not either a SignedMessage or a SignedDrmCertificate. This is because
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# the DrmCertificate itself is not signed. This CDM does not verify the
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# signatures as I'm not sure what HMAC key is used. At this stage of the
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# CDM flow, we wouldn't have any mac_keys, and those might not work for
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# verifying service certificate signatures (likely not).
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# All these 3 schemas can sort of parse each other in a minimal buggy way,
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# so we have to parse down the full chain instead of relaying each step
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try: # SignedMessage input
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signed_message.ParseFromString(certificate)
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signed_drm_certificate.ParseFromString(signed_message.msg)
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if not signed_drm_certificate.drm_certificate:
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raise DecodeError()
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DrmCertificate().ParseFromString(signed_drm_certificate.drm_certificate)
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except DecodeError:
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try: # SignedDrmCertificate input
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signed_drm_certificate.ParseFromString(certificate)
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if not signed_drm_certificate.drm_certificate:
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raise DecodeError()
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DrmCertificate().ParseFromString(signed_drm_certificate.drm_certificate)
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except DecodeError:
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# could be a direct unsigned DrmCertificate, but reject those anyway
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raise DecodeError("Could not parse certificate as a SignedDrmCertificate")
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try:
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pss. \
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new(RSA.import_key(self.root_cert.public_key)). \
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verify(
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msg_hash=SHA1.new(signed_drm_certificate.drm_certificate),
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signature=signed_drm_certificate.signature
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)
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except (ValueError, TypeError):
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raise ValueError("Signature Mismatch on SignedDrmCertificate, rejecting certificate")
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else:
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drm_certificate = DrmCertificate()
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drm_certificate.ParseFromString(signed_drm_certificate.drm_certificate)
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self.service_certificate = drm_certificate
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return self.service_certificate
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except DecodeError:
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pass
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try: # SignedDrmCertificate input
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signed_drm_certificate.ParseFromString(certificate)
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if not signed_drm_certificate.drm_certificate:
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raise DecodeError()
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drm_certificate.ParseFromString(signed_drm_certificate.drm_certificate)
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self.service_certificate = drm_certificate
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return self.service_certificate
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except DecodeError:
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pass
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try: # DrmCertificate input
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drm_certificate.ParseFromString(certificate)
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self.service_certificate = drm_certificate
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return self.service_certificate
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except DecodeError:
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pass
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raise DecodeError("Could not parse certificate as a Service Certificate")
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def get_license_challenge(self, type_: LicenseType = LicenseType.STREAMING, privacy_mode: bool = True) -> bytes:
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"""
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