Remote update of hardware platforms or embedded systems is a convenient service enabled by Field Programmable Gate Array (FPGA)-based systems. This service is often essential in applications like space-based FPGA systems or set-top boxes. However, having the source of the update be remote from the FPGA system opens the door to a set of attacks that may challenge the confidentiality and integrity of the FPGA configuration, the bitstream. Existing schemes propose to encrypt and authenticate the bitstream to thwart these attacks. However, we show that they do not prevent the replay of old bitstream versions, and thus give adversaries an opportunity for downgrading the system. In this article, we propose a new architecture called sarfum that, in addition to ensuring bitstream confidentiality and integrity, precludes the replay of old bitstreams. sarfum also includes a protocol for the system designer to remotely monitor the running configuration of the FPGA. Following our presentation and analysis of the security protocols, we propose an example of implementation with the CCM (Counter with CBC-MAC) authenticated encryption standard. We also evaluate the impact of our architecture on the configuration time for different FPGA devices.

In the context of FPGAs, system downgrade consists in preventing the update of the hardware configuration or in replaying an old bitstream. The objective can be to preclude a system designer from fixing security vulnerabilities in a design. Such an attack can be performed over a network when the FPGA-based system is remotely updated or on the bus between the configuration memory and the FPGA chip at power-up. Several security schemes providing encryption and integrity checking of the bitstream have been proposed in the literature. However, as we show in this paper, they do not detect the replay of old FPGA configurations; hence they provide adversaries with the opportunity to downgrade the system. We thus propose a new architecture that, in addition to ensuring bitstream confidentiality and integrity, precludes replay of old bitstreams. We show that the hardware cost of this architecture is negligible.

International audience

The widening spectrum of applications and services provided by portable and embedded devices brings a new dimension of concerns in security. Most of those embedded systems (pay-TV, PDAs, mobile phones, etc.) make use of external memory. As a result, the main problem is that data and instructions are constantly exchanged between memory (RAM) and CPU in clear form on the bus. This memory may contain confidential data like commercial software or private contents, which either the end-user or the content provider is willing to protect. The paper describes the problem of processor-memory bus communications in this regard and the existing techniques applied to secure the communication channel through encryption. Performance overheads implied by those solutions are discussed extensively.