A very interesting paper, “TEMPEST: A Signal Problem“, was recently declassified by the NSA. It gives a short history of TEMPEST and other side channel attacks, that is ways to discover a secret cryptographic key by monitoring indirect channels like RF emissions, sound, or power. Besides the new information that TEMPEST was discovered by Bell Labs in 1943, there are a number of lessons to learn from this paper.
It’s interesting that RF emissions in the form of oscilloscope perturbation were the first side channel found. After that, the paper covers acoustical, power line, seismics, and flooding. The last two are uncertain since the body of the text is still classified. In modern terminology, the attacks were SPA (simple side-channel analysis) since the plaintext was read directly from distinct “fingerprints” that appeared on the scope. DPA (differential side-channel analysis) involves more complex acquisition and statistical correlation of multiple samples, something this paper does not reveal as known at the time.
The history of attempted countermeasures is a good case study. First, Bell Labs tried to produce a heavily-shielded cipher machine. The army would not purchase it because it wasn’t a simple retrofit to the existing model. The final recommendation was that field personnel control a zone 200 feet in diameter from all cipher machines. There was no study that showed this was the perfect range, only that most field offices could do this without difficulty and it probably helped. This is very similar to countermeasures today, where cost or deployment effort are often more important than achieving the best security.
The categories of countermeasure they identified were:
- Shielding/filtering: reducing the signal strength of emissions
- Masking: adding random noise to the environment
If you think of a side channel attack as a communications problem, it’s obvious this is the classic signal-to-noise ratio. The paper states that they had trouble effectively shielding devices and that masking wasn’t effective either. This fits with the environment today, where addressing side-channel leakage in a modern embedded system is extremely difficult.
As power consumption decreases naturally with shrinking hardware, things improve but similar increases in the sensitivity of monitoring equipment improve as well. Also, processors and FPGAs get faster every day, allowing for more complicated signal processing. As the paper concluded in 1972, side-channel attacks today tend to lead countermeasure sophistication. If you’re concerned about such attacks, be sure to get your design reviewed.