  Physical layer security (PLS) encompasses techniques proposed at the physical
layer to achieve information security objectives while requiring a minimal
resource footprint. The channel coding-based secrecy and signal
modulation-based encryption approaches are reliant on certain channel
conditions or a certain communications protocol stack to operate on, which
prevents them from being a generic solution. This paper presents Grain-128PLE,
a lightweight physical layer encryption (PLE) scheme that is derived from the
Grain-128AEAD v2 stream cipher. The Grain-128PLE stream cipher performs
encryption and decryption at the physical layer, in between the channel coding
and signal modulation processes. This placement, like that of the A5 stream
cipher that had been used in the GSM communications standard, makes it a
generic solution for providing data confidentiality in IoT networks. The design
of Grain-128PLE maintains the structure of the main building blocks of the
original Grain-128AEAD v2 stream cipher, evaluated for its security strength
during NIST's recent Lightweight Cryptography competition, and is therefore
expected to achieve similar levels of security.
