Practical Analysis of Xilinx FPGAs' Bitstream Encryption

Due to its speed and adaptability, SRAM-based Field Programmable Gate Array (FPGA) is extensively employed in various application fields. Since bitstreams stored in external memory are vulnerable to malicious attacks, most FPGA manufacturers provide bitstream encryption. In this paper, the structural differences between an unencrypted bitstream and an encrypted bitstream for Xilinx's FPGA series are investigated. First, the encryption algorithms used for each series are described, followed by the procedure for generating an encrypted bitstream using Xilinx ISE and Vivado. Lastly, the difference between the unencrypted and encrypted bitstreams for Xilinx FPGAs before 7-Series, 7-Series and after 7-Series is compared. Using the analysis, we demonstrated that it can be possible to verify if encryption is applied or not in real world utilizing encryption information extracted from the bitstream during its transfer from external memory to FPGA.