A scalable dual mode arithmetic unit for public key cryptosystems

Elliptic Curve Cryptosystems (ECC) have become popular in recent years due to their smaller key sizes than traditional public key schemes such as RSA. However the gap between the size of these systems is increasing as security requirements become more demanding due to cryptanalytic advances. At current security levels of 80 bits, the corresponding key sizes for ECC and RSA are 160 and 1, 024 bits respectively. Although the ECC key size is attractive for embedded applications, the popularity of RSA means that it will remain in legacy applications for the foreseeable future. This paper proposes a dual mode arithmetic unit capable of supporting the underlying field operations performed by both the ECC and RSA public key schemes. A hardware optimised version of the Montgomery algorithm is employed to perform modular multiplication efficiently. The disparity in key sizes is addressed by combining the dual processors to operate in parallel for ECC or in a pipelined series for RSA.