Electromagnetic & Acoustic Side Channels

Besides power, chips emit electromagnetic (EM) radiation and generate acoustic noise. Both channels carry data-dependent signals that can leak cryptographic secrets.

Switching transistors emit EM radiation. A small probe placed near an IC can pick up signals spatially localised to a single functional unit — more precise than a global power measurement.

EM probes can target specific modules (e.g., the AES S-box unit) while ignoring the rest of the chip. This raises signal-to-noise ratio and reduces the number of traces needed for key recovery.

TEMPEST (classified US govt program, later declassified) studies EM emissions from displays, keyboards, and logic circuits. Van Eck phreaking reconstructs CRT/LCD screen content from leaked RF.

In 2013, Shamir and colleagues recovered RSA-4096 keys by recording the high-pitched whine of capacitors in a laptop. The sound changes based on which modular multiplications the CPU performs.

Spectre (2018) exploits speculative execution: secret-dependent memory accesses leave cache traces readable by an attacker process. It breaks process isolation across browsers and VMs.

Flush+Reload: attacker flushes a cache line, victim accesses secret-dependent address, attacker reloads and times the access. Hit = fast = secret bit inferred.

A directional microphone or contact microphone on the chassis picks up capacitor whine. The signal is digitised at 44 kHz–192 kHz and processed with FFT to extract key-dependent frequency patterns.

Researchers have demonstrated EM leakage between co-located cloud VMs on the same physical host, recovering AES keys from a victim VM using a probe attached to shared infrastructure.

CVE-2017-5753 (Spectre), CVE-2017-5754 (Meltdown), and the RSA acoustic attack (Usenix Security 2014) are landmark examples. Browser vendors patched timer resolution to mitigate cache attacks.

What made the 2013 acoustic RSA attack possible?

EM probes give spatial resolution beyond global power. Acoustic attacks exploit capacitor whine. Microarchitectural attacks (Spectre/Meltdown) use cache timing. Countermeasures range from EM shielding to disabling shared timers in browsers.