What is the most common problem that devices with HDMI connections have with capability?

What is the most common problem that devices with HDMI connections have with capability?

What is the most common problem that devices with HDMI connections have with capability? Most of the common compatibility problems with HDMI connections on devices are related to HDCP. First we need to understand what HDCP is and how it works, then examine why HDCP causes these compatibility problems.

HDCP is short for high band width digital-content protection. This is a specification that was developed by Intel for the express 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, who 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, for the prevention of 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 the licensees agree to limit the capabilities of their manufactured products. For example, any 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 target of HDCP is to prevent the transmission of non-encrypted high definition content. There are three systems which were developed to achieve this goal. The first system is the authentication process which disallows 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. It also prevents any "man in the middle" attacks. 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 capable has a unique set of keys. There there are 40 keys, and each key is 56 bits long. These keys are kept 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, without overflow, its own secret keys together, according to a KSV that was received from another device. If a particular bit in the vector is set to 1, then the corresponding secret key is used in the addition, otherwise the bit is ignored. Keys and KSVs are created in such a way that during this process both devices get the identical 56 bit number as a result. That number is used later in the encryption process. The encryption process is done by a stream cipher. Each pixel, that has been decoded, is encrypted by using an XOR operation with a 24-bit number produced by a generator. The specifications for HDCP help ensure the constant updating of keys after each encoded frame. If a particular model is considered to have been "compromised", its Key Selection Vector is put into the revocation lists, which are then written on newly produced disks with HD content. Each revocation list is signed with a digital signature that uses the DSA algorithm. This digital signature is used to prevent any malicious users from revoking legitimate devices. 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 have been compromised, and will refuse to send HD data to the receiver.

The most common problem with compatibility among devices that have HDMI connections has to do with the HDCP. The problem that is most common is the failure of the HDCP repeater function , or the lack of the HDCP repeater to reliably perform the authentication in all types of usage scenarios.