Faster Scalar Multiplication on Koblitz Curves Combining Point Halving with the Frobenius Endomorphism

Let E be an elliptic curve defined over F2n . The inverse operation of point doubling, called point halving, can be done up to three times as fast as doubling. Some authors have therefore proposed to perform a scalar multiplication by an “halve-and-add” algorithm, which is faster than the classical double-and-add method. If the coefficients of the equation defining the curve lie in a small subfield of F2n , one can use the Frobenius endomorphism τ of the field extension to replace doublings. Since the cost of τ is negligible if normal bases are used, the scalar multiplication is written in “base τ” and the resulting “τ -and-add” algorithm gives very good performance. For elliptic Koblitz curves, this work combines the two ideas for the first time to achieve a novel decomposition of the scalar. This gives a new scalar multiplication algorithm which is up to 14.29% faster than the Frobenius method, without any additional precomputation.