The present paper proposes a systematic design approach to provide the optimal high-radix Montgomery multipliers for an RSA processor satisfying user requirements. We introduces three multiplier-based architectures using different intermediate-data forms ((i) single form, (ii) semi carry-save form, and (iii) carry-save form), and combined them with a wide variety of arithmetic components. Their radices are also parameterized from 28 to 2 64. A total of 202 designs for 1,024- bit RSA processors were obtained for each radix, and were synthesized using a 90-nm CMOS standard cell library. The smallest design of 0.9 Kgates with 137.8 ms/RSA to the fastest design of 1.8 ms/RSA at 74.7 Kgates were then obtained. In addition, the optimal design to meet the user requirements can be easily obtained from all the combinations. In addition to choosing the datapath architecture, the arithmetic component, and the radix parameters, the proposed systematic approach can also adopt other process technologies.