  The qualities of Physical Unclonable Functions (PUFs) suffer from several
noticeable degradations due to silicon aging. In this paper, we investigate the
long-term effects of silicon aging on PUFs derived from the start-up behavior
of Static Random Access Memories (SRAM). Previous research on SRAM aging is
based on transistor-level simulation or accelerated aging test at high
temperature and voltage to observe aging effects within a short period of time.
In contrast, we have run a long-term continuous power-up test on 16 Arduino
Leonardo boards under nominal conditions for two years. In total, we collected
around 175 million measurements for reliability, uniqueness and randomness
evaluations. Analysis shows that the number of bits that flip with respect to
the reference increased by 19.3% while min-entropy of SRAM PUF noise improves
by 19.3% on average after two years of aging. The impact of aging on
reliability is smaller under nominal conditions than was previously assessed by
the accelerated aging test. The test we conduct in this work more closely
resembles the conditions of a device in the field, and therefore we more
accurately evaluate how silicon aging affects SRAM PUFs.
