RSA, one of the public key cryptographies, is the most widely used for a wide variety of information systems. Especially, a compact, high-performance RSA LSI is highly desired for mobile applications, such as a smart card and a cellar phone. This paper describes a RSA Cryptography Co-processor LSI. It can process up to 2048-bit key data, which is requited to guarantee the high security level of RSA. Although a large computational complexity is required to process 2048-bit RSA, our proposed N bit-wise modular multiplier based on Montgomery multiplication algorithm enables to reduce 25% circuit amount compared with the conventional one. A chip capable of operating at 60 MHz was fabricated using 0.18 um TSMC CMOS technology. A total of 98.5 k gates (incl. SRAM and I/Q modules) have been integrated into a 2.2 × 2.2 mm chip. Evaluation result with IC test system shows that power dissipation is 61.5 mW when 20.48-bit RSA processing is operated at 40 MHz. This RSA LSI will make a significant contribution to the development of compact, high-performance secure information systems.