Implementing public-key cryptography on platforms with limited resources, such as microprocessors, is a challenging task. Hardware/software co-design is often the only answer to implement the computationally intensive operations with limited memory and power at an acceptable speed. This contribution describes such a solution for Hyperelliptic Curve Cryptography (HECC). The proposed hardware/software co-design of the HECC system was implemented and co-simulated using the GEZEL design environment [3]. As a low-cost platform, we chose an 8-bit 8051 microprocessor to which one small hardware co-processor was added for field multiplication. We show that the Jacobian scalar multiplication can be computed in 2.488 sec at 12 MHz on this platform if a minimal hardware module is added i.e. a hardware multiply-add unit. This optimal solution provides a factor of 26 speed-up over a software-only solution.