Edwards normal form

An elliptic curve is in Edwards normal form if it can be described by the equation

$x^{2}+y^{2}=1+dx^{2}y^{2}$

The more general original Edwards form has another parameter c

$x^{2}+y^{2}=c^{2}(1+dx^{2}y^{2})$

but it is very simple to eliminate the scale parameter c and reduce this equation to its normal form for ease of performing algebraic group operations on rational points or finite fields.

The twist

$-x^{2}+y^{2}=1-{\frac {121665}{121666}}x^{2}y^{2}$
This elliptic curve in a so-called twisted Edwards form is the one used in the Ed25519 digital signature scheme .

There is also a “twisted” Edwards form

$ax^{2}+y^{2}=1+dx^{2}y^{2}$

And this brings us back to the ancient Greek word στραγγός with the idea of using something “twisted” for strong cryptography.

Point group arithmetic

The point group operation on an elliptic curve in the twisted Edwards form as defined above is defined as

$(x_{1},y_{1})\oplus (x_{2},y_{2})=\left({\frac {x_{1}y_{2}+y_{1}x_{2}}{1+dx_{1}x_{2}y_{1}y_{2}}},{\frac {y_{1}y_{2}-ax_{1}x_{2}}{1-dx_{1}x_{2}y_{1}y_{2}}}\right)$.

Protestants find it offensive to refer to arithmetic or other operations as of some “law” or as works done by or through some “law” so to speak, although that term does appear as such in the original literature. It is something vaguely felt to be grammatically repulsive, almost as if in Spanish to imply, “bajo la ley” and not “sobre la ley” or above-board.

The term “law” is normally reserved in mathematical contexts for a formal probability measure over an event space.