A fast radix-4 modular multiplication hardware algorithm is proposed. It is efficient especially in applications, such as encryption/decryption in the RSA cryptosystem, where modular multiplications are carried out iteratively. Each subtraction for the division for residue calculation is embedded in the repeated multiply-addition. Numbers are represented in a redundant representation and addition/subtractions are performed without carry propagation. A serial-parallel modular multiplier based on the algorithm has a regular cellular array structure with a bit slice feature suitable for VLSI implementation.
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