What is Side Channel Analysis and why should I worry about it?

by Steve Weingart

Side Channel Analysis (SCA) has its roots in the TEMPEST work that goes back to the World War II era. TEMPEST was the study of electrical, mechanical and/or acoustical emissions from devices. These emissions could contain information that was supposed to remain hidden and TEMPEST methods were used in attempts to learn the hidden secrets. Side Channel Analysis is more recent work, started by Paul Kocher and others at the end of the 1980’s that examines emissions from electronic devices to learn secrets that are not supposed to be leaked.

The best known methods of SCA are Simple Power Analysis and Differential Power Analysis (SPA and DPA). SPA and DPA are extremely effective ways to extract information from small computing devices, such as smart cards and tokens. SPA and DPA work by sampling and examining the power supply current (Icc) of these devices. By simple inspection, in the case of SPA, or by mathematical processing in the case of DPA, it is often possible to determine the data, and the secrets, that were processed by the device.

When SPA/DPA was first put into use, it was often possible to take a single oscilloscope trace of the Icc as a Smart Card performed an encryption operation and then, with a little practice, read the encryption key from the screen directly. It was pretty scary! Especially since it used no special or exotic equipment and the command that invoked the cryptographic operation was a normal identification command.

Once people understood the risk, the race was on. Developers would create mechanisms to make it harder and harder to find any information in the available signals, and the attackers would create more and more sophisticated methods of extracting that data.

In the 1990’s and 2000’s several important things happened in SCA. The attack and defense mechanisms have both become so exotic that it now takes very specialized equipment to mount an attack that is likely to be effective. But it is certain that both sides are working harder than ever, and the risk is still there, bigger than ever.

In addition, other avenues of SCA have been explored, such as Electro Magnetic Analysis (EMA). EMA examines the radio frequency emanations from these same electronic devices and can be significantly more effective than SPA/DPA at extracting secrets — despite prevention mechanisms.

What this means to developers of cryptographic devices and tokens is that SCA is an important risk to assess. SPA/DPA risk analysis is becoming required for Smart Cards and tokens used in the credit card and Personal Identity Verification (PIV) industries, and that list of industries is growing. Some Common Criteria Protection Profiles now require SPA/DPA analysis, and FIPS 140-3 is very likely to require SPA/DPA analysis. In fact, it might be added to FIPS 140-2 or to a companion standard, to become a requirement even sooner, rather than waiting for the release of FIPS 140-3.