Abstract: Characterizing EEPROM for usage as a ubiquitous PUF source

Abstract

Physical Unclonable Functions (PUFs) are increasingly being employed as a security primitive in hardware devices. PUFs are used to both empower authentication of individual hardware devices and support derivation of hardware-specific keys. By exposing the process variation in silicon devices as a source of the unpredictable but repeatable unique response to a challenge, PUFs can provide features that typically need dedicated security hardware (e.g., TPMs). A novel PUF source based on the write timing of Electrically Eraseable Programmable ROMs (EEPROMs) is demonstrated in certain process ICs. EEPROM cells are programmed via a process which gradually accrues charge carriers within the gate of a memory cell. Many EEPROMs require relatively long write cycles to establish stable memory contents. If a write operation is not sustained for the requisite duration, the resulting value stored in memory is dependent on physical properties heavily impacted by process variation. As EEPROMs are found on every PC DRAM module and exposed to software via a standardized I2C bus, this PUF source has potential to be both ubiquitous and software-accessible without any modifications to the off-the-shelf hardware. A set of EEPROMs have been tested for suitability for this PUF. Arrays of 64 of each of six device types were constructed and tested for bits of usable entropy, HDinter, and HDintra. The EEPROMs with coarse feature size produced around 90% usable entropy relative to memory size, with HDintra mostly around 45–55% and HDintra mostly under 7%.