Friday, August 20, 2010

Lessons of two attacks on allegedly tamper proof chips.

Today, at CHES 2010, two interesting talks shed new light on the myth of tamper resistant hardware: Di-Battista showed in his talk "When Failure Analysis Meets Side-Channel Attacks" (paper written by J. Di-Battista, J.-C. Courrège, B. Rouzeyre, L. Torres and P. Perdu) how they used photon emissions of transistors as side channel in a DPA attack to recover keys stored inside an Actel A3PE FPGA which claims to be highly tamper-proof. (The photon emission basically occurs in all chips, no matter whether they're ASICs or FPGAs.)

In the data sheet Actel claims: "The flash cells are located beneath seven metal layers, and many device design and layout techniques have been used to make invasive attacks extremely difficult." The light emissions do not come from the flash cells but from switching transistors; this however is no big difference: While it is indeed difficult to probe through all those layers of metal interconnect (and probably no photon would ever get through it) it is much easier to flip the chip upside down and look at its bottom where it has no interconnect. In the first attack they only use 'natural' photon emissions from the transistors, in the second attack they use laser stimulation to increase the photon emissions. From the observed photons they are able to construct traces for DPA attacks.

One might argue that the attacks of Di-Battista et al. are extremely expensive compared to normal DPA attacks: The equipment needed for their weaker attack costs 500 K$ while the equipment for the stronger attack costs 2 M$ but can be reused and should be available in any semiconductor lab. However, the price argument does not apply to a weaker class of attacks that S. Skorobogatov introduced in an earlier talk today, "Flash Memory 'Bumping' Attacks". Skorobogatov attacks three chips including an Actel A3P250 with relatively low cost laboratory hardware and a little bit of cryptanalysis. (That's the 'bumping'.) He is able to break the security claims of the chips regarding configuration extraction; the configuration of an FPGA is equal to the designer's intellectual property (IP) and therefor a major target whenever FPGA based devices are reverse engineered. However, in the Actel case he only attacked one of multiple options available to secure the IP - to break the other options the attack of Di-Battista et al. is needed.

In summary, both talks showed the dangers of relying on FPGA based security claims to protect IP and, at least in case of the light based DPA attack, any cryptographic operation. It may be assumed, that non-Actel FPGAs and many ASICs suffer similar problems. Very impressive results.

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