Markus G. Kuhn

Radiated electromagnetic emissions from video cables can leak information about the displayed image content to radio receivers in the vicinity. This confidentiality risk has long been demonstrated for analog video interfaces, such as VGA, as well as certain digital interfaces, such as DVI, HDMI, and the LVDS video connections often used inside devices with display panels. In all these cases, rasterizing amplitude-demodulated radio signals received in a quiet part of the spectrum can lead to recognizable screen content and readable text. However, DisplayPort, another commonly used computer video interface, has so far resisted such eavesdropping attacks. DisplayPort's significantly more complex signal structure and line encoding, and in particular its use of a linear-feedback shift register to scramble the bit stream, make its rasterized emissions appear like noise after amplitude demodulation. Here, we demonstrate more sophisticated video eavesdropping techniques for DisplayPort emissions, based on software-defined radio technology. These allow us to identify the video-mode parameters used by the target and synchronize the receiver with its timing structure, to then reconstruct not only readable text but also recognize different colours used. Our techniques work even if the receiver's bandwidth is an order of magnitude lower than the bitrate used on the video link.

Template attacks recover data values processed by tamper-resistant devices from side-channel waveforms, such as supply-current fluctuations (power analysis) or electromagnetic emissions. They first profile a device to generate multivariate statistics of the waveforms emitted for each of a set of known processed values, which then identify maximum-likelihood candidates of unknown processed values during an attack. We identify several practical obstacles arising in the implementation of template attacks, ranging from numerical errors to the incompatibility of templates across different devices, and propose and compare several solutions. We identify pooled covariance matrices and prior dimensionality reduction through Fisher’s linear discriminant analysis as particularly efficient and effective, especially where many attack traces can be acquired. We evaluate alternative algorithms not only for the task of recovering key bytes from a hardware implementation of the Advanced Encryption Standard; we even reconstruct the value transferred by an individual byte-load instruction, with success rates reaching 85% (or a guessing entropy of less than a quarter bit remaining) after 1000 attack traces, thereby demonstrating direct eavesdropping of eight-bit parallel data lines. Using different devices during the profiling and attack phase can substantially reduce the effectiveness of template attacks. We demonstrate that the same problem can also occur across different measurement campaigns with the same device and that DC offsets (e.g., due to temperature drift) are a significant cause. We improve the portability of template parameters across devices by manipulating the DC content of the eigenvectors that form the projection matrix used for dimensionality reduction of the waveforms.