# Point addition equation in projective co ordinates

How can I get point addition equation for elliptic curves in projective co ordinate system? Can I get it by changing $$x = X/Z$$ and $$y =Y/Z$$ in the equation for affine co ordinates' group law?

I have done it using the above narrated step and made the denominator as $Z$ in equation for binary super-singular curves. Unfortunately I am not able to get the correct output. I have used the equation $$x_3 = (y_1 +y_2/x_1 +x_2)^2 + x_1 + x_2$$ $$y_3 = (y_1 +y_2/x_1 +x_2)(x_1 +x_2) + y_1 + c$$ for the curve $y^2 + cy = x^3 + ax+b$.

• I found this, a database of curve equations, with all possible addition formulas. not your curve though. – Willem Hengeveld Aug 3 '14 at 20:16

Your second equation seems a bit off. In the curve of equation: $$y^2 + cy = x^3 + ax + b$$ in a binary field $\mathbb{F}_{2^m}$, to add point $P_1 = (x_1,y_1)$ to point $P_2 = (x_2,y_2)$, resulting in point $P_3 = (x_3,y_3)$, then the two equations are: \begin{eqnarray*} x_3 &=& \lambda^2 + x_1 + x_2\\ y_3 &=& \lambda (x_1+x_3) + y_1 + c \end{eqnarray*} where $$\lambda = \frac{y_1+y_2}{x_1+x_2}$$

In projective coordinates, the point $(x,y)$ is represented by the triplet $(X,Y,Z)$ such that $x = X/Z$ and $y = Y/Z$. It is important to realize that the projective coordinates are not unique; $(X,Y,Z)$ and $(\mu X, \mu Y, \mu Z)$ represent the same point, for any non-zero $\mu$. If we have projective coordinates for $P_1$ and $P_2$, then we get: \begin{eqnarray*} \lambda &=& \frac{\frac{Y_1}{Z_1}+\frac{Y_2}{Z_2}}{\frac{X_1}{Z_1}+\frac{X_2}{Z_2}} \\ &=& \frac{\frac{Y_1 Z_2 + Y_2 Z_1}{Z_1 Z_2}}{\frac{X_1 Z_2 + X_2 Z_1}{Z_1 Z_2}} \\ &=& \frac{Y_1 Z_2 + Y_2 Z_1}{X_1 Z_2 + X_2 Z_1} \end{eqnarray*} Then: \begin{eqnarray*} \frac{X_3}{Z_3} &=& \frac{(Y_1 Z_2 + Y_2 Z_1)^2}{(X_1 Z_2 + X_2 Z_1)^2} + \frac{X_1}{Z_1} + \frac{X_2}{Z_2} \\ \frac{Y_3}{Z_3} &=& \frac{Y_1 Z_2 + Y_2 Z_1}{X_1 Z_2 + X_2 Z_1} \cdot \left(\frac{X_1}{Z_1} + \frac{(Y_1 Z_2 + Y_2 Z_1)^2}{(X_1 Z_2 + X_2 Z_1)^2} + \frac{X_1}{Z_1} + \frac{X_2}{Z_2}\right) + \frac{Y_1}{Z_1} + c \end{eqnarray*}

The goal is to compute without making divisions; thus, we need to define $Z_3$ such that all these inconvenient fractions disappear. One possible solution, given the equations above, is to set: $$Z_3 = Z_1 Z_2 (X_1 Z_2 + X_2 Z_1)^3$$ from which we deduce: \begin{eqnarray*} X_3 &=& (X_1 Z_2 + X_2 Z_1)(Z_1 Z_2 (Y_1 Z_2 + Y_2 Z_1)^2 + (X_1 Z_2 + X_2 Z_1)^2(X_1 Z_2 + X_2 Z_1)) \\ &=& Z_1 Z_2 (X_1 Z_2 + X_2 Z_1) (Y_1 Z_2 + Y_2 Z_1)^2 + (X_1 Z_2 + X_2 Z_1)^4 \end{eqnarray*} and some even more complex -- but without any division -- expression for $Y_3$ (that I am too lazy to compute).

So you get the two main points:

• yes, to get projective coordinates, you "just" replace $x$ with $X/Z$ and so on;
• but ultimately you still get some choice in the expression for $Z_3$, since projective coordinates are not unique. You thus choose an expression which minimizes overall computational cost, which usually means "the simplest expression which avoids divisions".

Since you have a choice in $Z_3$, you do not necessarily end up with the same formulas as other people, and it is not necessarily mean that your formulas are wrong.

• Thank you @Thomas Pornin . I got the result with correct output. And one more doubt, the point I am getting in projective is not unique. My doubt is can I get back same point in affine when I map points from different projective representation of a single point to affine? – vijita Aug 4 '14 at 6:22
• Yes, you get the same point at the end. That's the idea of non-unique coordinates: you have the choice of representation, but they all stand for the same curve point. – Thomas Pornin Aug 4 '14 at 11:02