  The hard mathematical problems that assure the security of our current
public-key cryptography (RSA, ECC) are broken if and when a quantum computer
appears rendering them ineffective for use in the quantum era. Lattice based
cryptography is a novel approach to public key cryptography, of which the
mathematical investigation (so far) resists attacks from quantum computers. By
choosing a module learning with errors (MLWE) algorithm as the next standard,
National Institute of Standard & Technology (NIST) follows this approach. The
multiplication of polynomials is the central bottleneck in the computation of
lattice based cryptography. Because public key cryptography is mostly used to
establish common secret keys, focus is on compact area, power and energy budget
and to a lesser extent on throughput or latency. While most other work focuses
on optimizing number theoretic transform (NTT) based multiplications, in this
paper we highly optimize a Toom-Cook based multiplier. We demonstrate that a
memory-efficient striding Toom-Cook with lazy interpolation, results in a
highly compact, low power implementation, which on top enables a very regular
memory access scheme. To demonstrate the efficiency, we integrate this
multiplier into a Saber post-quantum accelerator, one of the four NIST
finalists. Algorithmic innovation to reduce active memory, timely clock gating
and shift-add multiplier has helped to achieve 38% less power than state-of-the
art PQC core, 4x less memory, 36.8% reduction in multiplier energy and 118x
reduction in active power with respect to state-of-the-art Saber accelerator
(not silicon verified). This accelerator consumes 0.158mm2 active area which is
lowest reported till date despite process disadvantages of the state-of-the-art
designs.
