A method of obtaining C=(c0, . . . , cn-1) of a product of two elements A and B of a finite field GF(2n). The method includes permuting the last d coefficients (an-1, . . . , an-d) of a multiplier A with predetermined variables (sn-1, . . . , sn-d); operating C:C=⊕(bi+j●A) for (I+j)th coefficient of a multiplicand B to update coefficients of C, where i and j are integers, and A:=(sn-1-j,α0, . . . ,αn-2)⊕(0,sn-1-j●f1, . . . ,sn-1-j●fn-d,0, . . . ,0) repeatedly for j=0 to (d−1) to update coefficients of A, where ⊕ represents an XOR operation and ● represents an AND operation; and repeatedly performing the permuting and operating by increasing i from 0 to (n-1) by d.