Patent ID: 7970131

Claim:
An elliptic curve cryptosystem apparatus, comprising: an obtaining unit obtaining coordinate of a point P on an elliptic curve over an finite field and an n-bit natural number d; and a processor configured to execute an arithmetic by concurrently performing an elliptic curve addition ECADD and an elliptic curve doubling ECDBL when obtaining d×P by repeatedly performing arithmetics (1) through (3) listed below a predetermined number of times based on the coordinates of the point P and the natural number d obtained by said obtaining unit Q[ 2 ]=ECADD ( Q[ 0 ], Q[ 1]) (1) Q[ 0 ]=ECDBL ( Q[ 0]) (2) Q[ 1 ]=Q[ 1 +d[i]] (3) where an initial value of a variable Q[ 0 ] is P, an initial value of a variable Q[ 1 ] is 0, and a coefficient of a binary expression of a natural number d obtained by said obtaining unit is d[i] (i=0˜n−1, d[i]=0, 1).