Trusted computing is an emerging technology to improve the trustworthiness of computing platforms. The Trusted Computing Group has proposed specifications for a Trusted Platform Module and a Mobile Trusted Module. One of the key problems when integrating these trusted modules into an embedded system-on-chip design, is the lack of on-chip multiple-time-programmable non-volatile memory. In this paper, we describe a solution to protect the trusted module's persistent state in external memory against non-invasive attacks. We introduce a minimal cryptographic protocol to achieve an authenticated channel between the trusted module and the external non-volatile memory. A MAC algorithm has to be added to the external memory to ensure authenticity. As a case study, we discuss trusted computing on reconfigurable hardware. In order to make our solution applicable to the low-end FPGA series which has no security measures on board, we present a solution that only relies on the reverse engineering complexity of the undocumented bitstream encoding and uses a physically unclonable function for one-time-programmable key storage. Clearly, this solution is also applicable to high-end series with special security measures on board. Our solution also supports field updates of the trusted module.
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