Modified Montgomery modular multiplication and RSA exponentiation techniques

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Modified Montgomery multiplication and associated RSA modular exponentiation algorithms and circuit architectures are presented. These modified multipliers use carry save adders (CSAs) to perform large word length additions. These have the attraction that, when repeatedly used to perform RSA modular exponentiation, the (carry save) format of the output words is compatible with that required by the multiplier inputs. This avoids the repeated interim output/input format conversion, needed when previously reported Montgomery multipliers are used for RSA modular exponentiation. Thus, the lengthy and costly conventional additions required at each stage are avoided. As a consequence, the critical path delay and, hence, the data throughput rate of the resulting Montgomery multiplier architectures are also word length independent. The approach presented is based on a reformulation of the solution to modular multiplication within the context of RSA exponentiation. Two algorithmic variants are presented, one based on a five-to-two CSA and the other on a four-to-two CSA plus multiplexer. The practical application of the approach has been demonstrated by using this to design special purpose RSA processing units with 512-bit and 1024-bit key sizes. The resulting RSA units exhibit the highest data rates reported in the literature to date, reflecting the very low and word length independent critical path delay achieved.