A very simple multiplier cell is developed for use in a linear, purely systolic array forming a digit-serial multiplier for unsigned or 2'complement operands. Each cell produces two digit-product terms and accumulates these into a previous sum of the same weight, developing the product least signiicant digit rst. Grouping two terms per cell, the ratio of active elements to latches is low, and only n 2 cells are needed for a full n by n multiply. A modulo-multiplier is then developed by incorporating a Montgomery type of modulo-reduction. Two such multipliers interconnect to form a purely systolic modulo exponentiator, capable of performing RSA encryption at very high clock frequencies, but with a low gate count and small area. It is also shown how the multiplier, with some simple back-end connections, can compute modular inverses and perform modular division for a power of two as modulus.