Hollywood and other content owners remain determined to implement content protection of high-definition movies. This is evidenced through the new security standard in Blu-Ray DVDs and the continued improvements in the link protection standard known as the High Definition Content Protection security standard.
HDCP security is administered by the Digital Content Protection LLC which is a subsidiary of Intel. HDCP Rev. 1.x has been in existence since 1999 and will shortly be supplemented by HDCP Rev. 2.0 which was published in October of 2008.
This article will give a brief introduction of HDCP then dive into the recently announced upgrade and how it will be implemented. HDCP 2.0 (as it will be referred to in this article) is a dramatic improvement in security design and will be initially adopted in wireless networks targeted at the distribution of high-definition content in the home.
High-Definition Content Protection ” HDCP
Studios and consumer electronics developers understood that the next generation of entertainment distribution technology such as high definition TV and DVDs would offer pirates the opportunity to steal premiere, high value content.
In 1999 the studios were just beginning to move into an accelerated distribution model which shortened the delay among theatre distribution (where the content is physically protected), premium (pay-TV and captive distribution such as airline entertainment systems) distribution followed shortly thereafter by mass media distribution on DVD.
The metrics for the potential loss of revenue from theft of high-definition content resulting from the new distribution model was astounding and the studios were determined to develop new content protection designs to avoid the massive losses experience in the music industry.
This resulted in four new security designs ” three which are implemented on Blu-Ray DVDs and one ” HDCP, which protects the content as it moves from a source device such as a DVD player or set top box to the display device. The HDCP standard is controlled by an industry licensing authority known as the Digital Content Protection LLC (DCP) which is a subsidiary of Intel Corporation.
The DCP works very closely with the High-Definition-Multimedia-Interface (HDMI) Licensing LLC which controls the overall interface standard that is used ubiquitously by the consumer electronics and PC manufacturers as the de facto connection technology for high-definition content. Figure 1 below illustrates the use model for HDCP as used in HDMI.
Figure 1. Content Protection in HDCP
Security Design in HDCP Rev. 1.3: A review
The vast majority of high-definition products shipping today support a Rev. of HDMI Rev. 1.3. There are three variants shipping today ” 1.3, 1.3a and 1.3b which reflect tweaks to the standard and changes in testing methodology to improve interoperability.
All HDMI Rev. 1.3 interfaces today must implement HDCP Rev. 1.3 in order to comply with the standard. The content protection design in HDCP has three important elements:
1. Encryption and decryption of the digital multi-media content ” both audio and video;
2. Authentication and key derivation; and
As shown in Figure 2 below, under the nomenclature used by the DCP, a device which renders multi-media content is known as a Sink device. A device which sends content over an HDMI cable is known as a Source.
There can also be a repeater in the system which might for example be a surround sound audio system. It would extract the audio from the HDMI cable but allow the video to transition through the repeater to the final video rendering system.
Figure 2. Under HDCP, the cable, known as the Source Device sends content to a Sink device which renders multi-media content.
It is the responsibility of a Source device to cryptographically authenticate that the device on the other end of the cable (i.e. the Sink device) is a valid HDCP device and that it has not been placed on the revocation list by the DCP indicating it has been compromised by hackers. After authenticating a Sink device, the Source device can then encrypt the content and sends it over the cable.
In HDCP Rev. 1.x, content encryption is done through a proprietary cipher that was invented by Intel. As the standard dates all the way back to 1999, the Advanced Encryption Standard (AES) was still under review by the U.S. National Institute of Standard and Technology (NIST) and as such this was not an option.
The HDCP cipher was a reasonable choice as it was very small in terms of silicon area and could easily handle the high speeds required for uncompressed audio/video content. It is likely that the HDCP cipher is subject to attack but there has been no report of it being broken.
This is probably due to the fact that the security design in HDCP has other fundamental flaws that led to the design being broken very quickly so there has been no need for hackers to attack the cipher itself.
It is in the authentication and key derivation process that the vulnerability of the security design is most evident. When an HDCP device is manufactured, the DCP provides a set of highly confidential keys and other secrets to the manufacturer.
These are then stored in the silicon device in non-volatile memory or in encrypted form in off-chip memory. These keys are known as Device Private Keys and are secret values that must not be revealed to hackers. In addition, there is a Key Selection Vector (KSV) which is a 40 bit binary value that uniquely identifies the HDCP device.
During the authentication process, the KSVs are exchanged. Each device does a mathematical transform involving the other device’s KSV with the Device Private Keys (i.e. the Sink KSV with the Source Device Private Keys and vice-versa) to derive a symmetric key to use with the HDCP cipher.
This process is much simpler than a traditional key exchange algorithm used for example in banking transactions but it was chosen as it fits the cost model for consumer electronics. It has however proven to be the Achilles heel of the security design and was quickly broken by experts in cryptanalysis.
Despite knowing that this flaw existed, the DCP continued to use the design over the last ten years so as to provide continuity to consumers and manufacturers. Finally however, the DCP has moved to change the design as we’ll see later in this article.
Lastly, all HDCP based consumer electronic devices must maintain a revocation list that is administered by the DCP. Each time a new Blu-Ray movie is created for example, the most recent revocation list is included on the disk.
The Source device checks the revocation list on the media to see if it is more recent than the one stored locally on the device and if so copies it into local memory.
Each time a Sink device is attached to the Source, the Source checks to determine if the Sink device KSV is on the revocation list. If the Sink device KSV is on the revocation list, the Source device cannot transmit high definition content to the revoked rendering device.
What’s new in HDCP 2.0
Table 1 below outlines the changes that are coming in HDCP 2.0. The following description of how HDCP 2.0 works is slightly simplified. The actual protocol permits the authentication of both repeaters and rendering devices as this is the accepted topology for HDCP 2.0 based networks.
Table 1. Key improvements in Version 2.0 of HDCP
It is also possible to encounter situations where legacy HDMI devices supporting HDCP Rev. 1.x are present which add some additional complexity to the security model. This basic description however covers off the cryptographic principles that are used in HDCP 2.0. The entire specification can be found on the DCP’s web site at www.digital-cp.com.
This article will now cover the two of the most important aspects of the requirements for HDCP 2.0: 1) AKE ” Authentication and 2) Key Exchange.
The DCP elected to make a change to the nomenclature used in their new standard. Instead of a Source Device, the standard now references an HDCP Transmitter. References to Sink Devices have now been updated to use the term HDCP Receiver and lastly there are HDCP Repeaters in the standard.
The Authentication and Key Exchange process relies on certificates which are widely used in the Internet and IPsec. There are secrets and other security parameters provided to manufacturers by the DCP after the manufacturer signs the adoption agreement and indicates that they will comply with the robustness rules which govern how secrets are protected.
A manufacturer licensed to build an HDCP Transmitter is provided with the 3072 bit DCP public key which is used in cryptographic verification of the HDCP Receiver certificate.
A manufacturer licensed to build an HDCP Receiver is provided a certificate by the DCP along with a public and private key pair specific to the manufacturer. The certificate takes the form shown below in Table 2 and in Figure 3.
Table 2. Specifics of an HDCP receiver Certificate
Figure 3. Transactions between an HDCP transmitter and receiver under Version 2.0.
There are several important cryptographic and procedural steps in the authentication protocol. The obvious one is the verification of the HDCP Receiver certificate by the HDCP Transmitter. The certificate is signed off-line using the 3072-bit DCP private key and as such it cannot be forged. A failure of the cryptographic verification of the HDCP certificate aborts the authentication process.
The System Renewability Message (SRM) is a list of revoked Receiver IDs which is signed by the DCP and verified by the HDCP Transmitter before being stored in system memory. If the HDCP Receiver ID is on the revocation list, then the HDCP Receiver has been compromised and cannot receive high-definition content. The authentication process is aborted if this is the case.
Lastly, the HDCP Receiver runs an HMAC;SHA-256 cryptographic hash on the HDCP Transmitter Nonce XOR’d with the derived key which is referred to as H’. This is then send back to the HDCP Transmitter and the HDCP Transmitter will do the same XOR operation with its derived key AND the original Nonce it generated. H’ must equal H or the authentication process fails.
It is worth noting that the Nonce and session key are both generated as random numbers. The DCP has specified two entropy (measure of randomness) requirements for the TRNG ” one for nonces and one for keys.
The minimum entropy requirement for nonces is 40 random bits out of 64-bits. This means that a reasonable level of variability or entropy is established if out of 1,000,000 random numbers generated the probability of there being any duplicates in this list is less than 50%.
For the session key mentioned above, a cryptographic grade TRNG will be required similar to the hardware random number generators offered by Elliptic. The DCP has indicated that the randomness required for this is 128-bits of entropy i.e. the probability of there being any duplicates in the list of 2^64 secret values and first authentication attempt on the HDCP Transmitter logic is less than 50%.
There are two other important elements to the authentication process. In HDCP 2.0 there is the concept of locality which has existed in other link content protection schemes such as DTCP.
This process ensures that the HDCP Transmitter and HDCP Receiver are co-located. The time-out is specified as 7 ms which is the roundtrip delay that would be encountered over a wireless network link. The protocol for this is shown in Figure 4 below.
|Figure 4. The timeout process between an HDCP transmitter and receiver under Revision 2.0.
Lastly, as in HDCP 1.x, there is a limitation on the number of elements and depth in the topology of the network. Figure 5 below illustrates a potential, valid network configuration consisting of HDCP Repeaters, Transmitters and Receivers.
Figure 5. A valid HDCP network configuration
An HDCP Receiver such as a surround sound system can also have Repeater functionality as illustrated in the diagram. An HDCP Repeater must gather the topology information for all HDCP devices underneath it and report that to the HDCP Transmitter which has the ultimate responsibility to enforce the overall network topology rules. The initial release of HDCP specifies that the device count must not exceed 31 and the depth of the network must not exceed 4.
The changes required to support HDCP 2.0 will mean that both the silicon and embedded software be upgraded. The television chips and Blu-Ray player devices will have to be re-designed to replace the HDCP cipher with an AES-CTR mode cipher.
The embedded processor will have to be enhanced to a higher performance design as the RSA signature verification algorithm is based on 3072 bits which is a challenging implementation even for 32 bit processors.
When combined with the RSA encrypt or decrypt required for key exchange, the software load will increase dramatically. This means that the commercial silicon designs in current use ” some of which are off-the-shelf processors, while others are custom ASICs, will have to be upgraded to support the new standard.
Benefits of the Update
HDCP Rev. 2.0 is initially targeted at wireless designs using technologies such as WHDI and WirelessHD emerging which will allow devices to transmit high-definition content over wireless links using uncompressed video. It is also possible to consider transmission of compressed video over lower bandwidth links such as 801.11n or wired IP networks such as Ethernet.
The DCP used to support the ART (Alternative Retransmission Technology) process whereby a company could submit a non-standard physical interface and target implementation for review to permit HD designs in automobiles and airplanes for example.
This one off process has now been rendered obsolete through the availability of HDCP Rev. 2.0 which is media interdependent. It is therefore very likely that design incorporating HDCP Rev. 2.0 will begin to emerge as early as 2010.
With the new standard, there are therefore significant benefits to the consumer which come along with the incremental costs to implement. The much enhanced security model frees up HDCP from a point to point cable allowing high-definition content to be rendered anywhere in the house on PCs, mobile phones, and media players.
This is a significant upgrade to the capability offered by HDCP and should prove very popular with consumers by allowing them to use their high-definition content when and where they want it as opposed to being tied down to a single attached television.