DTV Content Protection

[Anonymous]

DTV Content Protection

Two content protection systems are in use to protect digital television
(DTV) signals on the wires of American home video systems: HDCP and DTCP.
HDCP is used for the most common digital cable connection to HD monitors,
HDMI, which is a variant of DVI.  DTCP is used for digital connections
to video equipment, especially digital VCRs.  It was originally designed
for Firewire (aka iLink, aka IEEE-1394) but has been extended to USB-2
and Bluetooth, with IP in the works.  Apparently monitors with both HDMI
and Firewire connections would have to implement both.

HDCP is described at http://www.digital-cp.com/ and DTCP at
http://www.dtcp.com/.  The full DTCP spec is still secret unless you
are a licensee and the site has only limited information.

The two systems are very different cryptographically.  HDCP uses a
56-bit keyed stream cipher based on LFSRs.  DTCP uses block ciphers,
either a 56-bit key proprietary block cipher from Hitachi called M6,
or AES with 128-bit keys.  M6 is the default that all devices must
implement.  M6 uses an odd chaining mode called "converted CBC" which
seems to chain the ciphertext into the next block's key material rather
than the plaintext, possibly with an abbreviated key schedule.

SKDH

Here I want to focus on the key agreement protocol.  Both systems use a
similar approach which has never been formally presented or documented.
For convenience I will call it SKDH, for Symmetric Key Diffie Hellman.
SKDH has some properties of Diffie Hellman key exchange, but it uses
simple addition operations rather than public key functions.  It also has
some properties of identity-based encryption, in that there is a master
key center that issues the private keys to each device.  However it
is not secure against collusion by users who know their private keys,
so would not be suitable for a true IBE system.

DTCP has two key agreement protocols. There is a full protocol which is
EC-DH (elliptic curve Diffie Hellman) and is mandatory for "copy never"
content, ie. pay per view content.  It also specifies a restricted
protocol which is acceptable for "copy once" and "copy no more" content,
that uses the SKDH technique described below.  This will be much cheaper
to implement for manufacturers and is probably used by typical recording
devices.

DHCP has just one key agreement protocol and it is of this new type
as well.

SKDH key agreement has not been published but it is presumed that it
works as follows.  There is a secret matrix which is known only to
the agency that issues keys.  Let us call this the Master Matrix, MM.
The system is based on matrix algebra as follows:

Pub1 * MM * Pub2 = shared key.

Pub1 and Pub2 are vectors of 1's and 0's which are the "public keys"
of the two devices, called "key selection vectors" or KSVs.  Each device
is issued such a vector, along with its private keys, which are defined
as follows:

Priv1 = Pub1 * MM

Priv2 = MM * Pub2

Priv1 and Priv2 are vectors of numbers whose size depends on the values
in MM.  Details for the two known implementations are described below.

By associativity, we have:

Pub1 * MM * Pub2 = Priv1 * Pub2 = Pub1 * Priv1 = shared key.

The two parties do a key exchange by giving each other their KSVs,
the public Pub1 and Pub2 values.  Each one then multiples the vector
of 1's and 0's they received from the other side times their vector of
Priv values.  This amounts to simply adding the Priv values selected
by the 1's received from the other side.  Because of the relationship
between the public and private values, this insures that both sides
receive the same shared key.

The analogy to Diffie Hellman which motivated the name SKDH should now
be clear.  Each side receives a public value from the other, combines
it with its own private data, and creates a shared secret.

In HDCP, the MM matrix is 40 by 40, and entries are 56 bits long.  In
DTCP, the MM matrix is 12 by 12, and entries are 64 bits long.

The weakness of this system is that if the the private key vectors are
published, they leak information about the MM matrix.  In principle as
few as 40 private/public key pairs could fully reveal MM in the case of
HDCP, and as few as 12 in the case of DTCP.  This makes the cryptographic
scheme unsuitable for any widespread identity based encryption scheme;
it will only work in a closed system like these, where manufacturers
must take great pains to keep their private keys secret.

Attacks on HDCP

Several attacks have been published and unpublished on HDCP.  The most
famous is from Niels Ferguson, who has announced an attack but will not
publish it for the reasons described at
http://www.macfergus.com/niels/dmca/cia.html.  According to Ferguson:

"HDCP is fatally flawed. My results show that an experienced IT person
can recover the HDCP master key in about 2 weeks using four computers
and 50 HDCP displays. Once you know the master key, you can decrypt any
movie, impersonate any HDCP device, and even create new HDCP devices
that will work with the 'official' ones. This is really, really bad
news for a security system. If this master key is ever published, HDCP
will provide no protection whatsoever. The flaws in HDCP are not hard
to find. As I like to say: 'I was just reading it and it broke.'"

Keith Irwin has published four (actually five) attacks on HDCP at
http://www.angelfire.com/realm/keithirwin/HDCPAttacks.html.  His last one
is perhaps along the lines that Ferguson envisioned, and I will say more
about it below.

Similar concepts are presented in
http://apache.dataloss.nl/~fred/www.nunce.org/hdcp/hdcp111901.htm by
Scott Crosby, Ian Goldberg, Robert Johnson, Dawn Song and David Wagner.
This paper assumes (unlike Irwin) that attackers have access to the
private keys of chosen devices.  This is a questionable assumption as
it will take very expensive laboratory equipment to extract sensitive
key material from today's protected chips.

Irwin's fifth attack is presented in an addendum.  It involves presenting
random KSVs to a display and using a birthday trick to find the "shared
key" values relatively quickly, which could lead to a break in about
a month.  However Irwin is more pessimistic about the number of displays
and assumes that 74 will be needed to have a good chance of spanning the
whole matrix.  If 50 properly chosen displays are enough then Irwin's
attack would be pretty close to Ferguson's two week time frame.

All of these attacks focus on finding the master secret MM value; once
that is found, the security of the system collapses.  Given a KSV it is
immediately possible to deduce the corresponding private key if you know
the MM.  Although both HDCP and DTCP have mechanisms for revocations of
cracked keys, a total break like this cannot be rescued by revocation.

Attacks on DTCP

DTCP seems not have gotten as much attention from cryptographers, in
part because the details are secret.  But its restricted authentication
handshake uses the same SKDH algorithm as HDCP.  DTCP requires the two
sides to have certificates, but it appears that the receiver of the
data does not check them, so Irwin's random-KSV birthday attack should
still work.  It may take a little longer due to the key size being 64
rather than 56 bits, but this will be somewhat compensated by the fact
that the matrix size is 12 rather than 40.

The time will depend crucially on how quickly a handshake can be
completed with a device, and the DTCP spec is somewhat vague about this.
It's possible that the handshake could be much faster, and then the attack
could complete even more quickly than HDCP.  Since DTCP has only a 12 bit
public key there are only 2^12 possibilities, so these keys will not be
distinct between different devices, but rather will be common across a
model or even across a manufacturer.  This could make it harder to find
12-15 devices with different keys that will span the MM matrix.

The M6 cipher used in DTCP is actually a family of ciphers, and some of
them are attacked in http://www.schneier.com/paper-mod3.html by John
Kelsey, Bruce Schneier and David Wagner.  However that attack depends
crucially on the use of addition in a certain phase of the algorithm, and
M6 is also documented to use XOR there in some variants.  The specific
version of M6 used in DTCP is not known, and even if the attackable
version is used it's not clear if this would weaken the cipher enough
to make it the weakest link in the chain.

Summary

Both HDCP and DTCP use cryptographic mechanisms for which published
attacks have existed for several years, yet deployment proceeds unabated.
Almost all new HDTV equipment in the U.S. implements one or both of
these cryptographic protocols.  As discussed above, if enough private
keys leak or are scraped out, or if Irwin's attack works, the security
of these systems will collapse.

DTCP has a fallback in is its certificate protected, EC-DH based
full authentication mode, using AES-128 to avoid any weaknesses in M6.
It would take years though before existing equipment could be obsoleted
by ceasing to support the restricted authentication mode that uses SKDH.
HDCP has no fallback and it would be necessary to redesign the handshake,
again with a several year lag time for deployment of the updated system.

It will be interesting to see whether these theoretical attacks can
be successfully mounted now that HDTV equipment is widely available.
As the cost continues to drop over the next couple of years it should
soon be practical for hobbyists to begin experimenting with Irwin's
attack and start collecting KSV + private key pairs.  DTCP would be an
even more attractive target as it would allow easy computer recording
of protected data via Firewire, USB2, or IP.  However its reliance on
the much-maligned principle of security through obscurity (keeping the
details secret) may in practice give it a greater degree of protection.