Programmable Feedback Shift Register

Abstract

Conventionally, a linear feedback shift register (LFSR) constitutes linear sets of sequences with predictable periods, which are considered vulnerable to intruders. Besides, an LFSR limits the sequences of scan-for-test patterns. This work introduces a private key, which is a collection of keywords, to program the feedback coefficients and initial states of the LFSR, where each keyword modulates the LFSR with a different polynomial of the same degree. That is, each polynomial generates a linear set of sequences. Consequently, the aggregate polynomial has a set of unpredictable and nonlinear sequences with high statistical randomness. The keyword, which holds the coefficients and initial states of the LFSR, can be managed in a few bits and stored in first-in first-out memory array. HSPICE simulations for 90 nm CMOS technology verify the functionality and speed of the proposed programmable feedback shift register (PFSR) of size 16-bit with 64 keywords. Results show a clock speed of 500 MHz with a power consumption of 73 µW and transistor count of 37,593, wherein the overall period has 4,194,240 unpredictable nonlinear sequences that surpass most LFSR structures. The PFSR can be suited for ASIC and reconfigurable HDL synthesis for efficient stream cipher and scan-for-test applications.