2017:- TEMPEST Attacks Against AES by Craig Ramsay

### Overview Craig Ramsay presents research conducted at Fox-IT alongside Jasper van Woudenberg (and in collaboration with Riscure) examining TEMPEST (electromagnetic side-channel) attacks against AES implementations from a distance. While past TEMPEST research largely focused on asymmetric cryptography and most symmetric cryptographic side-channel analysis was restricted to "on-package" probing with direct physical access, this project demonstrates that AES-256 keys can be fully recovered from a distance using low-cost, off-the-shelf software-defined radio (SDR) hardware.

### Key Topics - **EM Side-Channel vs. TEMPEST**: Distinguishing near-field, on-chip surface measurements from distance-based TEMPEST/EMSEC attacks on symmetric crypto. - **SDR vs. Oscilloscopes**: Replacing traditional, expensive oscilloscopes (which sample wide frequency bands from DC up and generate massive datasets) with a cheap RTL-SDR dongle tuned down to a narrow band centered around the device clock frequency. - **Signal Capture and Pre-Processing**: Stream-recording analog electromagnetic radiation, filtering and amplifying it, and splitting continuous captures into individual AES execution blocks. - **Correlation Power Analysis (CPA)**: Modeling power consumption and bus switching via Hamming weights on intermediate values (T-table lookups) to brute-force AES key bytes individually. - **Distance Progression**: Scaling the physical attack distance from contact-level EM probes to 30 cm using a log-periodic antenna in a DIY shielded box, and finally to 1 meter in an anechoic chamber.

### Technical Details - **Target Platform**: An ARM development board running an unhardened OpenSSL-style software AES-256 implementation (utilizing standard 32-bit T-table lookups). The researchers initially tested an ARM Cortex-M3 (clocked up to 140 MHz) before moving to an ARM Cortex-A9 to achieve higher clock frequencies, which allowed using smaller, practical antennas matched to the electromagnetic wavelength. - **Hardware & Tooling**: The capture pipeline utilized a 20-euro RTL-SDR USB dongle, Mini-Circuits RF amplifiers, and bandpass filters. Software analysis relied on custom MATLAB scripts for alignment/slicing and Riscure Inspector for correlation analysis. - **Leakage Modeling**: Correlation analysis targeted intermediate values immediately following the initial round key addition and T-table lookup. By analyzing the switching power of the address lines across consecutive load-word instructions (`LDR`), the researchers observed consistent polarity correlation spikes across all bits. - **Attack Milestones**: - *Contact Probe*: Verified the 14-round key schedule and execution rounds with an EM loop probe. - *30 cm Distance*: Using a printed log-periodic antenna and a DIY foil-lined cardboard enclosure to block ambient interference, the attack extracted the AES key in 50 seconds after capturing ~500,000 encryption blocks (total hardware cost ~200 USD). - *1 Meter Distance*: Tested inside an anechoic chamber at OSPL with full electrical isolation (the target was powered via an isolated USB battery pack under foil, encrypting in an autonomous loop without Ethernet/data cables). Using a discone antenna, the full key was recovered from ~2.5 million encryption blocks in roughly 5 minutes of streaming recording.

### Takeaways - Software-defined radios drastically accelerate side-channel attacks compared to oscilloscopes by eliminating arm/trigger/copy latency and reducing data bandwidth via analog down-mixing. - Standard unhardened AES implementations running on general-purpose ARM microprocessors leak sufficient electromagnetic radiation to be vulnerable to TEMPEST attacks up to a meter away using budget hardware. - Moving to longer distances requires directional antennas and careful noise suppression or shielding, but the digital signal analysis methodology remains identical to standard contact-level side-channel attacks.