Hardware-assisted code obfuscation for FPGA soft microprocessors

Soft microprocessors are vital components of many embedded FPGA systems. As the application domain for FPGAs expands, the security of the software used by soft processors increases in importance. Although software confidentiality approaches (e.g. encryption) are effective, code obfuscation is known to be an effective enhancement that further deters code understanding for attackers. The availability of specialization in FPGAs provides a unique opportunity for code obfuscation on a per-application basis with minimal hardware overhead. In this paper we describe a new technique to obfuscate soft microprocessor code which is located outside the FPGA chip in an unprotected area. Our approach provides customizable, data-dependent control flow modification to make it difficult for attackers to easily understand program behavior. The application of the approach to three benchmarks illustrates a control flow cyclomatic complexity increase of about 7x with a modest logic overhead for the soft processor.