  Secret Unknown Ciphers (SUC) have been proposed recently as digital
clone-resistant functions overcoming some of Physical(ly) Unclonable Functions
(PUF) downsides, mainly their inconsistency because of PUFs analog nature. In
this paper, we propose a new practical mechanism for creating internally random
ciphers in modern volatile and non-volatile SoC FPGAs, coined as SRAM-SUC. Each
created random cipher inside a SoC FPGA constitutes a robust digital PUF. This
work also presents a class of involutive SUCs, optimized for the targeted SoC
FPGA architecture, as sample realization of the concept; it deploys a generated
class of involutive 8-bit S-Boxes, that are selected randomly from a defined
large set through an internal process inside the SoC FPGA. Hardware and
software implementations show that the resulting SRAM-SUC has ultra-low latency
compared to well-known PUF-based authentication mechanisms. SRAM-SUC requires
only $2.88/0.72 \mu s$ to generate a response for a challenge at 50/200 MHz
respectively. This makes SRAM-SUC a promising and appealing solution for
Ultra-Reliable Low Latency Communication (URLLC).
