AbstractThis paper proposes an LSI‐oriented multiple‐valued hardware algorithm based on the signed‐digit number (SD number) for highspeed RSA public key encryption processor. A very long word‐length arithmetic is required in RSA encryption processing. The proposed algorithm realizes a high‐speed processing by iterative additions based on the radix‐A SD number system. Microprogram control is adopted in the encryption processor. To verify the basic operation of the proposed processor architecture, a model processor for 16‐bit encryption is implemented using binary TTL logic IC's. A comprehensive evaluation of the 2 μm CMOS encryption LSI is made by a simulation using the electronic circuit analysis program SPICE2. The encryption rate for the message of 512‐bit word length is 60 kbit/s, which is a factor of eight times faster than that of the corresponding binary LSI. The developed processor contains about 100 k transistors and it can be implemented on a single single chip.