German computer scientists have taken advantage of the powerful number-crunching abilities of graphics chips to demonstrate a practical attack on the encryption scheme in programmable chips.

Field Programmable Gate Array (FPGA) chips of the type used in embedded systems belonging to the military and the aerospace industry are vulnerable to attacks based on analysing power usage during the power up sequence of the chip. The side-channel attack against the bitstream encryption mechanism used by Virtex 4 and Virtex 5 chips from Xilinx allowed researchers from the Ruhr University to extract a key used to decrypt configuration instruction files. The technique uncovered secret keys by analysing fluctuations in power consumption during the decryption process.

Only one power up sequence needs to be monitored. The subsequent number crunching took up to nine hours, in the case of the more advanced Virtex 5 chip, or six hours in the case of the earlier Virtex 4.

The approach is akin to listening to the clicks coming from the tumblers of a safe to work out a combination, but using variations in power consumption rather than sound.

Identical decryption keys are normally used in all FPGAs in a mass-produced product, so the attack opens the way up towards the creation of counterfeit kit that would be difficult to tell apart from the real thing, or even the surreptitious introduction of hardware Trojans, the researchers warn. It might be possible to introduce revised instructions that compromised devices would accept as genuine because they had been signed with the correct digital key.

Access to the key allows cloning and manipulating a design, which has been encrypted to protect the intellectual property and to prevent fraud. As a consequence, the target product faces serious threats like IP theft and more advanced attacks such as reverse engineering or the introduction of hardware Trojans. To the best of our knowledge, this is the ﬁrst successful attack against the bitstream encryption of Xilinx Virtex 4 and Virtex 5 reported in the open literature.

The attack could be carried out with off-the-shelf hardware at moderate effort.

Countermeasures against such side-channel attacks have already been applied in some high-security devices, such as smartcards for banking or pay-TV applications, but not previously though necessary for complex semiconductor circuits, such as FPGAs cracked by the German team.

More details on the research by Amir Moradi, Markus Kasper and Christof Paa from the Horst Gortz Institute for IT-Security at Ruhr University can be found in a paper, On the Portability of Side-Channel Attacks, here. ®