ERHARD-RNG: A Random Number Generator Built from Repurposed Hardware in Embedded Systems

Abstract: Quality randomness is fundamental to cryptographic operations but on embedded systems good sources are (seemingly) hard to find. Rather than use expensive custom hardware, our ERHARD-RNG Pseudo-Random Number Generator (PRNG) utilizes entropy sources that are already common in a range of low-cost embedded platforms. We empirically evaluate the entropy provided by three sources---SRAM startup state, oscillator jitter, and device temperature---and integrate those sources into a full Pseudo-Random Number Generator implementation based on Fortuna. Our system addresses a number of fundamental challenges affecting random number generation on embedded systems. For instance, we propose SRAM startup state as a means to efficiently generate the initial seed---even for systems that do not have writeable storage. Further, the system's use of oscillator jitter allows for the continuous collection of entropy-generating events---even for systems that do not have the user-generated events that are commonly used in general-purpose systems for entropy, e.g., key presses or network events.