High Definition Multimedia Interface
(HDMI) Tutorial

The HDMI interface is typically used in conjunction with HDCP. To understand the security of the HDMI interface, we must first understand how HDMI and HDCP work to understand how they make the HDMI interface secure.

HDMI stands for High-Definition Multimedia Interface. HDMI is the newest digital interface standard, supported by the industry, to be used when connecting components of consumer electronics, like high definition television or home theater systems. HDMI allows personal computer manufacturers and audio/visual consumer electronics to bring to the market products that are rich in features and innovative. HDMI utilizes a signal that is all-digital and is not compressed. All the predecessors of the HDMI interface were analog interfaces. Using an analog interface means that a clean digital signal is converted into a “not as precise” analog, and sent out to the T.V, where the signal is translated back again into digital signal to show the screen display. Each time that the signal is converted, the digital signal weakens and loses strength and integrity. This causes some distortion of the quality of the picture. With HDMI the source digital signal is preserved, and there is no conversion of the signal to create the richest, sharpest picture quality available.

HDCP stands for high band width digital-content protection. This is a specification developed by Intel, for the protection of digital entertainment content that uses the DVI interface. HDCP provides encryption for the transmission of digital content between the video source, or transmitter, like a computer, a DVD player, or a set-top box, and the digital display, or the receiver, like a monitor, a television or a projector. HDCP was not designed to prevent the copying or the recording of digital content, but to protect the integrity of the content as it is being transmitted. To implement HDCP, a license is required that may be obtained from the Digital Content Protection, LLC, which will then issue a set of unique secret device keys to all the authorized devices. During the authentication, the receiver will only accept the content once it demonstrates a knowledge of the secret keys. Plus, to prevent any eavesdropping and stealing of the transmitted data, the transmitter and receiver will both generate a shared secret value that is consistently checked through the entire transmission. Once the authentication is established, the transmitter encrypts the data and then sends it to the receiver to be decrypted. In addition to paying the license fees, all licensees agree to limit the capabilities of their products. For example, high-definition digital video content must be restricted to DVD quality on non-HDCP compliant video outputs when requested by the source. DVD-Audio content is restricted to DAT quality on non-HDCP digital audio outputs, analog audio outputs have no quality limits. Licensees may not allow their devices to make copies of content, and must design their products to effectively frustrate attempts to defeat the content .The main purpose of HDCP is to prevent the transmission of non-encrypted high definition content. There are three systems in use that were developed to achieve this goal. The first system is the authentication process, which will not allow non-licensed devices to receive any HD content. The second system in effect is the encryption of the actual data that is sent over the DVI or the HDMI interface, which prevents any eavesdropping of information. The third system in place is the key revocation procedures, which ensure that devices which are manufactured by any vendors who violate the license agreement could be blocked relatively easily from receiving HD data.

Each device model that is HDCP enabled has a unique original set of keys. There are 40 keys, and each key is 56 bits long. These keys are kept strictly confidential, and the failure to keep them a secret could be seen as a violation of the license agreement. For each set of keys, a special key called a Key Selection Vector, or KSV, is created. Each KSV has exactly half the bits, or 20 bits, set to 0 and half the bits, or 20 bits, set to 1. During the authentication process, both parties will exchange their Key Selection Vectors. Then each device adds its own secret keys together, according to a KSV that was received from another device. The specifications for HDCP ensure the constant updating of keys after each encoded frame. If a particular model is considered to be "compromised", its Key Selection Vector is put into the revocation lists, which are written on newly produced disks with HD content. Each revocation list is signed with a digital signature using the DSA algorithm. During the authentication process, if the receiver's KSV is found in the revocation list by a transmitter, then the transmitter will consider the receiver to be compromised, and will refuse to send HD data to it.

There is no security system that is one hundred percent secure. But when HDMI is used along with HDCP, it will provide an audio/video interface that will meet the security requirements of the consumers, the content providers and the systems operators.