Twisted Hessian curves


In mathematics, twisted Hessian curves are a generalization of Hessian curves; they were introduced in elliptic curve cryptography to speed up the addition and doubling formulas and to have strongly unified arithmetic. In some operations, it is close in speed to Edwards curves. Twisted Hessian curves were introduced by Bernstein, Lange, and Kohel.

Definition

Let be a field. The twisted Hessian form in affine coordinates is given by:
and in projective coordinates by
where and and. These curves are birationally equivalent to Hessian curves, and Hessian curves are just the special case of twisted Hessian curves in which.
Considering the equation, note that, if has a cube root in, then there exists a unique such that ; otherwise, it is necessary to consider an extension field of, such as. Then, since, defining, the following equation is needed to do the transformation:
This means that twisted Hessian curves are birationally equivalent to elliptic curves in Weierstrass form.

Group law

It is interesting to analyze the group law of the elliptic curve, defining the addition and doubling formulas. In general, the group law is defined in the following way: if three points lies in the same line then they sum up to zero. So, by this property, the explicit formulas for the group law depend on the curve shape.
Let be a point; its inverse is then in the plane. In projective coordinates, let be a point; then is its inverse. Furthermore, the neutral element in affine plane is, and in projective coordinates it is.
In some applications of elliptic curves for cryptography and integer factorization, it is necessary to compute scalar multiples of, say for some integer, and they are based on the double-and-add method, so the addition and doubling formulas are needed. Using affine coordinates, the addition and doubling formulas for this elliptic curve are as follows.

Addition formulas

Let and ; then,, where

Doubling formulas

Let ; then, where

Algorithms and examples

Here some efficient algorithms of the addition and doubling law are given; they can be important in cryptographic computations, and the projective coordinates are used to this purpose.

Addition

The cost of this algorithm is 12 multiplications, one multiplication by a constant, and 3 additions.
Example:
Let and be points over a twisted Hessian curve with. Then is given by:
That is,.

Doubling

The cost of this algorithm is 3 multiplications, one multiplication by a constant, 3 additions, and 3 cubings. This is the best result obtained for this curve.
Example:
Let be a point over the curve defined by as above; then, is given by:
That is,.