High-bandwidth Digital Content Protection (HDCP) is a form of digital copy protection developed by Intel Corporation to prevent copying of digital audio and video content as it travels across connections. Types of connections include DisplayPort (DP), Digital Visual Interface (DVI), and High-Definition Multimedia Interface (HDMI), as well as less popular, or now defunct, protocols like Gigabit Video Interface (GVIF) and Unified Display Interface (UDI).
The system is meant to stop HDCP-encrypted content from being played on unauthorized devices or devices which have been modified to copy HDCP content. Before sending data, a transmitting device checks that the receiver is authorized to receive it. If so, the transmitter encrypts the data to prevent eavesdropping as it flows to the receiver.
In order to make a device that plays material protected by HDCP, the manufacturer must obtain a license from Intel subsidiary Digital Content Protection LLC, pay an annual fee, and submit to various conditions. For example, the device cannot be designed to copy; it must "frustrate attempts to defeat the content protection requirements"; it must not transmit high definition protected video to non-HDCP receivers; and DVD-Audio material can be played only at CD-audio quality by non-HDCP digital audio outputs (analog audio outputs have no quality limits).
Cryptanalysis researchers demonstrated flaws in HDCP as early as 2010. In September 2010, an HDCP master key that allows for the generation of valid device keys—rendering the key revocation feature of HDCP useless—was released to the public. Intel has confirmed that the crack is real, and believes the master key was reverse engineered rather than leaked. In practical terms, the impact of the crack has been described as "the digital equivalent of pointing a video camera at the TV", and of limited importance for copyright infringers because the encryption of high-definition discs has been attacked directly, with the loss of interactive features like menus. Intel threatened to sue anyone producing an unlicensed device.
HDCP uses three systems:
- Authentication prevents non-licensed devices from receiving content.
- Encryption of the data sent over DisplayPort, DVI, HDMI, GVIF, or UDI interfaces prevents eavesdropping of information and man-in-the-middle attacks.
- Key revocation prevents devices that have been compromised and cloned from receiving data.
Each HDCP-capable device has a unique set of 40 56-bit keys. Failure to keep them secret violates the license agreement. For each set of values, a special public key called a KSV (Key Selection Vector) is created. Each KSV consists of 40 bits (one bit for each HDCP key), with 20 bits set to 0 and 20 bits set to 1.
During authentication, the parties exchange their KSVs under a procedure called Blom's scheme. Each device adds its own secret keys together (using unsigned addition modulo 256) according to a KSV received from another device. Depending on the order of the bits set to 1 in the KSV, a corresponding secret key is used or ignored in the addition. The generation of keys and KSVs gives both devices the same 56-bit number, which is later used to encrypt data.
Encryption is done by a stream cipher. Each decoded pixel is encrypted by applying an XOR operation with a 24-bit number produced by a generator. The HDCP specifications ensure constant updating of keys after each encoded frame.
If a particular set of keys is compromised, their corresponding KSV is added to a revocation list burned onto new discs in the DVD and Blu-ray formats. (The lists are signed with a DSA digital signature, which is meant to keep malicious users from revoking legitimate devices.) During authentication, the transmitting device looks for the receiver's KSV on the list, and if it is there, will not send protected content to the revoked device.
HDCP devices are generally divided into three categories:
- The source sends the content to be displayed. Examples include set-top boxes, DVD, HD DVD and Blu-ray Disc players, and computer video cards. A source has only an HDCP/HDMI transmitter.
- The sink renders the content for display so it can be viewed. Examples include TVs and digital projectors. A sink has one or more HDCP/HDMI receivers.
- A repeater accepts content, decrypts it, then re-encrypts and retransmits the data. It may perform some signal processing, such as upconverting video into a higher-resolution format, or splitting out the audio portion of the signal. Repeaters have HDMI inputs and outputs. Examples include home theater audio-visual receivers that separate and amplify the audio signal, while re-transmitting the video for display on a TV. A repeater could also simply send the input data stream to multiple outputs for simultaneous display on several screens.
Each device may contain one or more HDCP transmitters and/or receivers. (A single transmitter or receiver chip may combine HDCP and HDMI functionality.)
In the United States, the Federal Communications Commission (FCC) approved HDCP as a "Digital Output Protection Technology" on August 4, 2004. The FCC's Broadcast flag regulations, which were struck down by the United States Court of Appeals for the District of Columbia Circuit, would have required DRM technologies on all digital outputs from HDTV signal demodulators. Congress is still considering legislation that would implement something similar to the Broadcast Flag. The HDCP standard is more restrictive than the FCC's Digital Output Protection Technology requirement. HDCP bans compliant products from converting HDCP-restricted content to full-resolution analog form, presumably in an attempt to reduce the size of the analog hole.
On January 19, 2005, the European Information, Communications, and Consumer Electronics Technology Industry Associations (EICTA) announced that HDCP is a required component of the European "HD ready" label.
HDCP strippers remove HDCP information from the video signal in order to allow the data to flow freely to a non-HDCP display. It is currently unclear whether such devices would remain working if the HDCP licensing body issued key-revocation lists, which may be installed via new media (e.g. newer Blu-ray Discs) played-back by another device (e.g. a Blu-ray Disc player) connected to it.
In 2001, Scott Crosby of Carnegie Mellon University wrote a paper with Ian Goldberg, Robert Johnson, Dawn Song, and David Wagner called "A Cryptanalysis of the High-bandwidth Digital Content Protection System", and presented it at ACM-CCS8 DRM Workshop on November 5.
The authors concluded that HDCP's linear key exchange is a fundamental weakness, and discussed ways to:
- Eavesdrop on any data.
- Clone any device with only its public key.
- Avoid any blacklist on devices.
- Create new device key vectors.
- In aggregate, usurp the authority completely.
They also said the Blom's scheme key swap could be broken by a so-called conspiracy attack: obtaining the keys of at least 40 devices and reconstructing the secret symmetrical master matrix that was used to compute them.
Around the same time, Niels Ferguson independently claimed to have broken the HDCP scheme, but he did not publish his research, citing legal concerns arising from the controversial Digital Millennium Copyright Act.
Master key release
On September 14, 2010, Engadget reported the release of a possible genuine HDCP master key which can create device keys that can authenticate with other HDCP compliant devices without obtaining valid keys from The Digital Content Protection LLC. This master key would neutralize the key revocation feature of HDCP, because new keys can be created when old ones are revoked. Since the master key is known, it follows that an unlicensed HDCP decoding device could simply use the master key to dynamically generate new keys on the fly, making revocation impossible. It was not immediately clear who discovered the key or how they discovered it, though the discovery was announced via a Twitter update which linked to a Pastebin snippet containing the key and instructions on how to use it. Engadget said the attacker may have used the method proposed by Crosby in 2001 to retrieve the master key, although they cited a different researcher. On September 16, Intel confirmed that the code had been cracked. Intel has threatened legal action against anyone producing hardware to circumvent the HDCP, possibly under the Digital Millennium Copyright Act.
HDCP v2.2, v2.1 and v2.0 Breach
On August 2012 version 2.1 was proved broken. The attack used the fact that the pairing process sends the Km key obfuscated with an XOR. That makes the encryptor (receiver) unaware of whether it encrypts or decrypts the key. Further, the input parameters for the XOR and the AES above it are fixed from the receiver side, meaning the transmitter can enforce repeating exactly the same operation. Such a setting allows an attacker to monitor the pairing protocol, repeat it with a small change and extract the Km key. The small change is to pick the "random" key to be the encrypted key from the previous flow. Now, the attacker runs the protocol and in its pairing message it gets E(E(Km)). Since E() is based on XOR it undoes itself, thus exposing the Km of the legitimate device.
V2.2 was released to fix that weakness by adding randomness provided by the receiver side. However the transmitter in V2.2 must not support receivers of V2.1 or V2.0 in order to avoid this attack. Hence a new erratum was released to redefine the field called "Type" to prevent backward compatibility with versions below 2.2. The "Type" flag should be requested by the content's usage rules (i.e. via the DRM or CAS that opened the content).
In August 2015 version 2.2 was rumored to be broken. An episode of Netflix's UHD version of series Breaking Bad was leaked to internet in UHD format and information was attached as HDMI cap which means it was captured through HDMI interface via removing HDCP 2.2 protection.
On November 4, 2015, HDFury, a company already known for their other HDCP rippers/splitters, released the HDFury Integral, a device that can remove HDCP 2.2. On December 31, 2015, Warner Bros and Digital Content Protection, LLC (the owners of HDCP) filed a lawsuit against LegendSky, the owner of HDFury. Warner Bros and DCP confirmed in the lawsuit complaint document that the HDFury Integral is able to strip HDCP 2.2 from protected 4k content.
HDCP can cause problems for users who want to connect multiple screens to a device; for example, a bar with several televisions connected to one satellite receiver or when a user has a closed laptop and uses an external display as the only monitor. HDCP devices can create multiple keys, allowing each screen to operate, but the number varies from device to device; e.g., a Dish or Sky satellite receiver can generate 16 keys. The technology sometimes causes handshaking problems where devices cannot establish a connection, especially with older high-definition displays.
Edward Felten wrote "the main practical effect of HDCP has been to create one more way in which your electronics could fail to work properly with your TV," and concluded in the aftermath of the master key fiasco that HDCP has been "less a security system than a tool for shaping the consumer electronics market."
Additional issues arise when interactive media (i.e. video games) suffer from control latency due to the additional processing (encoding/decoding) required. Various everyday usage situation, such as live streaming or capture of game play, are also adversely affected.
There is also the problem that all Apple laptop products, presumably in order to reduce switching time, when confronted with an HDCP compliant device, automatically switch all output from the DVI / Mini DisplayPort / Thunderbolt connector port to HDCP compliant. This is a problem if the user wishes to record or use videoconferencing facilities further down the chain, because these are inherently forbidden by HDCP. This applies even if the output is not HDCP material, like a PowerPoint presentation.
When connecting a HDCP 2.2 source device through compatible distribution to a video wall made of multiple legacy displays the ability to display an image can not be guaranteed.
|HDCP revision||Release Date||Supported interfaces|
|1.0||Feb 17, 2000||DVI|
|1.1||Jun 9, 2003||DVI, HDMI|
|1.2||Jun 13, 2006||DVI, HDMI|
|1.3||Dec 21, 2006||DVI, HDMI, DP, GVIF, UDI|
|1.4||Jul 8, 2009|
|2.0 IIA||Oct 23 2008|
|2.1 IIA||Jul 18, 2011|
|2.2 IIA||Oct 16, 2012|
|2.2 for HDMI||Feb 13, 2013|
|2.2 for MHL||Sep 11, 2013|
The 2.x version of HDCP is not a continuation of HDCPv1, and is rather a completely different link protection. Version 2.x employs industry-standard encryption algorithms, such as 128-bit AES with 3072 or 1024-bit RSA public key and 256-bit HMAC-SHA256 hash function. While all of the HDCP v1.x specifications support backwards compatibility to previous versions of the specification, HDCPv2 devices may interface with HDCPv1 hardware only by natively supporting HDCPv1, or by using a dedicated converter device. As a result, there is currently no deployment plan for v2 to replace v1 in existing systems. This means that HDCPv2 is only applicable to new technologies. It has been selected for the WirelessHD and Miracast (formerly WiFi Display) standards.
HDCP 2.x features a new authentication protocol, and a locality check to ensure the receiver is relatively close (it must respond to the locality check within 20 ms on a normal DVI/HDMI link ). Version 2.1 of the specification was recently cryptanalyzed and found to have several flaws, including the ability to recover the session key.
There are still few commonalities between HDCP v2 and v1
- Both are under DCP LLC authority
- Both share same license agreement, compliance rules and robustness rules
- Both share same revocation system and same device ID formats