An improved Montgomery modular inversion targeted for efficient implementation on FPGA

Modular multiplication and inversion/division are the most common primitives in today's public key cryptography. Elliptic Curve Public Key Cryptosystems (ECPKC) are becoming increasingly popular for use in mobile appliances where bandwidth and chip area are strongly constrained. For the same level of security, ECPKC use much smaller key length than the commonly used RSA but need modular inversion/division. This paper presents an improved algorithm for prime field Montgomery modular inversion. The first important contribution lies in the reduction of the number of operations needed. Resource sharing is also used to lighten the control part of the algorithm. The second contribution is the minimization of the set of different instructions to enable powerful FPGA implementations. Resulting 256-bit circuit achieves a ratio throughput/area improved by at least 70% compared to the only known Montgomery inverse design in FPGA technology. Though the implementations are first oriented towards FPGA, some improvements are generic. So, they could prove to be also efficient for ASIC designs in terms of area and power consumption.