PURPOSE: To provide the power arithmetic unit of high speed by reducing not only a maximum frequency of residue multiplication but also an average frequency by residue inverse element A-1modN with respect to power residue operation ABmodN. ;CONSTITUTION: Power residue operation ABmodN is performed where A and N are integers which have relations 0≤A≤N-1 and are expressed by plural bits and B is an integer expressed by plural bits. Integers A, B, and N are stored in storage means AR, BR, and NR. Residue inverse element AI= A-1modN of the integer A is stored in a storage means AIR, and an integer X as the result on the way of the operation is stored in a storage means XR. A selecting means GEL selects one integer Y from A, X, and AI. A residue multiplication means SM performs residue multiplication XxYmodN. The storage means XR is updated by its output. A means CC for code conversion of data in the storage means BR is provided to reduce the probability that a bit which is not 0 is generated. A control means CNT controls the selecting means SET. in accordance with the value of 0, 1, or -1 of the bit obtained by code conversion of B.;COPYRIGHT: (C)1995,JPO
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