A low energy modulo-multiplier is proposed for elliptic curve cryptography (ECC) processor, especially for authentication in mobile device or key encryption in embedded health-care system. The multiplier uses only two 40-bit multipliers to execute 160-bit operation based on the Montgomery modulo-multiplication algorithm. Partial products of multiplication are accumulated with shift registers to get final 160-bit MSB of output value. One modulo-multiplication is executed with 20 clock cycles at 40 MHz operating frequency. It consumes 6.3 nJ for each modulo-multiplication at IV supply voltage. It is implemented by using 0.18-mum CMOS process and has 0.7 mm x 1.0 mm area.