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What equation suitable for generation of a public key is used for Edwards curve Ed25519?

The problem is the following – info on Edwards Elliptic Curve on Wikipedia page presents an equation y^2 + x^2 = 1 + d * y^2 * x^2 that doesn't allow us to plot a curve depicted on the first picture.

So, what equation is used for Ed25519? This one?

y^2 + x^2 = 1 - d * y^2 * x^2

enter image description here

Or this one?

y^2 + x^2 = 1 + d * y^2 * x^2

enter image description here

I haven't found any documentation covering this topic.

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The general equation of the Edwards curve is

ax^2 + y^2 = 1 + dx^2y^2

for Ed25519 a = -1, d = -121665/121666.

The curve is also living in the prime field. The equation is enter image description here

I could not plot this equation in geograba, I guess due to the nature of the prime field over which the equation is defined.(?)

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  • I think IOHK used different a and d values for Ed25519 equation. As far as we can easily build a Bitcoin curve (secp256k1) over definite field using its native equation y^2 = x^3 + 7, we'll also be able to build a Cardano curve.
    – Andy Jazz
    Sep 10, 2021 at 12:33

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