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Modular exponentiation with large modulus and exponent has been widely used in public key cryptosystems. Montgomery's modular multiplication algorithm is normally used since no trial division is necessary and the critical path is reduced by using carry-save addition (CSA). In this paper, the Montgomery multiplication is greatly optimized and architectures are proposed to perform the Least-Significant-Bit (LSB) first and the Most-Significant-Bit (MSB) first algorithms. The architecture here presented has the following distinctive characteristics: 1) Use of digit-serial approach for Montgomery multiplication. 2) Conversion of the CSA representation of intermediate multiplication using carry-skip addition which reduces the critical path with a small area-speed penalty. 3) Precompute quotient value in Montgomery iteration in order to speed up operation frequency. In this work, implementation results in Xilinx Virtex 5 and Virtex 2 are reported. Experimental results show that the proposed modular exponentiation and modular multiplication design obtains the best delay performance compared with previous published works and outperforms them in terms of area-time complexity.