The tripling-oriented Doche–Icart–Kohel curve is a form of an elliptic curve that has been used lately in cryptography; it is a particular type of Weierstrass curve. At certain conditions some operations, as adding, doubling or tripling points, are faster to compute using this form. The tripling-oriented Doche–Icart–Kohel curve, often called with the abbreviation 3DIK was introduced by Christophe Doche, Thomas Icart, and David R. Kohel in.
Definition
Let K {\displaystyle K} be a field of characteristic different form 2 and 3. An elliptic curve in tripling oriented Doche–Icart–Kohel form is defined by the equation:
T a : y 2 = x 3 + 3 a ( x + 1 ) 2 {\displaystyle T_{a}\ :\ y^{2}=x^{3}+3a(x+1)^{2}}
with a ∈ K {\displaystyle a\in K} . A general point P on T a {\displaystyle T_{a}} has affine coordinates ( x , y ) {\displaystyle (x,y)} . The "point at infinity" represents the neutral element for the group law and it is written in projective coordinates as O = (0:1:0). The negation of a point P = (x, y) with respect to this neutral element is −P = (x, −y).
The group law Consider an elliptic curve in the Tripling-oriented Doche-Icart-Kohel form in affine coordinates:
T a : y 2 = x 3 + 3 a ( x + 1 ) 2 , a ≠ 0 , 9 4 . {\displaystyle T_{a}:\quad y^{2}=x^{3}+3a(x+1)^{2},\qquad a\neq 0,{\tfrac {9}{4}}.}
As in other forms of elliptic curves, it is possible to define some "operations" between points, such as adding points, or doubling (See also The group law). In the following sections formulas to add, negate and doubling points are given. The addition and doubling formulas are often used for other operations: given a point P on an elliptic curve it is possible to compute [n]P, where n is an integer, using addition and doubling; computing multiples of points is important in elliptic curve cryptography and in Lenstra elliptic curve factorization.
Addition Given P 1 = ( x 1 , y 1 ) {\displaystyle P_{1}=(x_{1},y_{1})} and P 2 = ( x 2 , y 2 ) {\displaystyle P_{2}=(x_{2},y_{2})} on T a {\displaystyle T_{a}} , the point P 3 = ( x 3 , y 3 ) = P 1 + P 2 {\displaystyle P_{3}=(x_{3},y_{3})=P_{1}+P_{2}} has coordinates:
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