A Cryptanalysis of the High-bandwidth Digital Content Protection System

This document describes a critical weakness in the High Bandwidth Digital Content Protection (HDCP) system, which is designed to prevent the unauthorized copying of video data transmitted over the Digital Visual Interface (DVI). The HDCP scheme utilizes an identity-based cryptosystem where each device is assigned a public/private key pair by a trusted authority holding a master secret.

The researchers discovered that if an attacker can obtain the public/private key pairs of just 40 devices, they can recover the authority's master secret in a matter of seconds. With this master secret, the attacker gains significant power, enabling them to:

• Eavesdrop on communications between any two devices in real-time.
• Spoof the identity of any device in real-time.
• Produce new key pairs that bypass any revocation lists.

This essentially allows the attacker to completely usurp the authority's power and compromise the entire system, even if all device keys are signed by the central authority. The document attributes these vulnerabilities to the stringent design requirement of implementing HDCP in under 10,000 gates, which led to the development of custom, insecure algorithms.

The paper recommends that the current HDCP cryptosystem be abandoned and replaced with standard cryptographic primitives. The analysis suggests that the core issue lies in the linear nature of the shared secret generation in HDCP, making it susceptible to linear algebra attacks. The authors also note that the protocol can be broken without fully understanding its internal operations, purely based on the interface and properties of the generated keys.