CALCULATING APPARATUS AND METHOD OF ENCRYPTING/DECRYPTING COMMUNICATION DATA BY USING THE SAME

A modular multiplication of integers of large digits can be calculated at high speed although the size of the circuit can be reduced. Furthermore, encryption/decryption for a communication by using cryptograph is performed by using it. In order to achieve this, when the modular multiplication is executed by alternately repeating a partial sum of product calculation and the residue calculation, a portion of the result of the previous calculation larger than the maximum digit of a number which is the modulo of this residue calculation is subjected to the residue calculation in the intermediate stage. Furthermore, the residue is directly obtained from the result of the previous calculation and the partial sum of product calculation is performed in such a manner that an obtained residue is added in place of a portion of the result of the previous calculation larger than the maximum digit of the modulo of this residue calculation. Furthermore, the additions are performed by a plurality of adders in parallel and a carry generated in each adder is added at the next addition performed by each upper adder. This circuit is formed into a systolic array composed by the same processing elements to realize the aforesaid calculation by a pipeline process.