Hello All,<\/p>\n
There is yet another attack possible against Protected Media Path
process beyond the one involving two global XOR keys [1]. The new
attack may also result in the extraction of a plaintext content key
value.<\/p>\n
The attack has its origin in a white-box crypto [2] implementation.
More specifically, one can devise plaintext content key from white-box
crypto data structures of which goal is to make such a reconstruction
difficult \/ not possible. This alone breaks one of the main security
objective of white-box cryptography which is to protect the secret key
(unbreakability) [3].<\/p>\n
Contrary to the initial (XOR key) attack, the white-box crypto attack
is not limited to the given narrow time window (white-box data
structures need to be present for the time of a movie decryption \/
playback). Fixing it might require a completely new approach \/
implementation (current one is obviously flawed).<\/p>\n
In that context, white-box crypto attack seems to be more severe than
the XOR key one.<\/p>\n
Additionally, a cryptographic check proving that extracted key values
correspond to real keys has been conducted for Canal+ Online, Netflix,
HBO Max, Amazon Prime Video and Sky Showtime.<\/p>\n
The check relies on a digital cryptographic signature verification.
Such a signature is appended at the end of each license issued by
PlayReady license server.<\/p>\n
The crypto check works as following:
– plaintext value of a digital signature key encrypted through ECC is
extracted from a Protected Media Path process
– the extracted signature key is used to calculate the AES-CMAC value
of a binary licence XMR blob
– the calculated signature value is checked against the signature
appended at the end of the issued license
– correct AES-CMAC value implicates correct signature key (and correct
content key)<\/p>\n
The above mechanism is also used by Microsoft to verify the
correctness of decrypted content keys received from a license server.
It relies on the fact that signature key is part of the same encrypted
license blob as content key. Thus, successful extraction of a
signature key implicates successful extraction of a content key.<\/p>\n
In the context of no confirmation \/ denial [4] from the platforms
indicated above as being affected, the crypto check should constitute
sufficient proof to support that claim alone.<\/p>\n
Thank you.<\/p>\n
Best Regards,
Adam Gowdiak<\/p>\n
———————————-
Security Explorations –
AG Security Research Lab
https:\/\/security-explorations.com
———————————-<\/p>\n
References:
[1] Microsoft Warbird and PMP security research
https:\/\/security-explorations.com\/microsoft-warbird-pmp.html
[2] White-box cryptography, Wikipedia
https:\/\/en.wikipedia.org\/wiki\/White-box_cryptography
[3] White-Box Security Notions for Symmetric Encryption Schemes
https:\/\/eprint.iacr.org\/2013\/523.pdf
[4] Microsoft DRM Hack Could Allow Movie Downloads From Popular
Streaming Services
https:\/\/www.securityweek.com\/microsoft-drm-hacking-could-allow-movie-downloads-from-popular-streaming-services\/<\/p>\n","protected":false},"excerpt":{"rendered":"
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