Microsoft PlayReady Cryptography Weakness – Torchsec

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.

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].

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).

In that context, white-box crypto attack seems to be more severe than
the XOR key one.

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.

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.

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)

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.

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.

Thank you.

Best Regards,
Adam Gowdiak

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Security Explorations -
AG Security Research Lab
https://security-explorations.com
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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/