Characterization of Effective Laser Spots during Attacks in the Configuration of a Virtex-II FPGA

SRAM-based FPGAs are an appealing platform to implement many systems, including secure ones. However, secure systems are subject to attacks and one of the main threats is fault-based attacks using lasers. The sensitivity of the configuration memory in a SRAM-based FPGA has to be studied in this context. This paper reports on the characterization of the effective laser spot, or effective sensitive area, with respect to the laser focus and to the attacked configuration bits. The test vehicle is a Virtex II FPGA. It is shown in particular that the initial value of the bit has a strong influence on the effective sensitive area of the spot. Such data can be used to better understand the actual effects of an attack and to design more efficient counter-measures. Also, it is shown that attacks based on single bit flips in the configuration are possible in practice even with relatively large laser spots.