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Return neutral point for a + -a in weierstrass add #31

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Return neutral point for a + -a in weierstrass add #31

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6 changes: 4 additions & 2 deletions curve/ecgfp5/weierstrass_point.go
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Original file line number Diff line number Diff line change
Expand Up @@ -103,7 +103,9 @@ func (p WeierstrassPoint) Add(q WeierstrassPoint) WeierstrassPoint {

// note: paper has a typo. sx == 1 when x1 != x2, not when x1 == x2
xSame := gFp5.Equals(x1, x2)
yDiff := !gFp5.Equals(y1, y2)
if xSame && !gFp5.Equals(y1, y2) {
return NEUTRAL_WEIERSTRASS
}

var lambda0, lambda1 gFp5.Element
if xSame {
Expand All @@ -118,7 +120,7 @@ func (p WeierstrassPoint) Add(q WeierstrassPoint) WeierstrassPoint {
x3 := gFp5.Sub(gFp5.Sub(gFp5.Square(lambda), x1), x2)
y3 := gFp5.Sub(gFp5.Mul(lambda, gFp5.Sub(x1, x3)), y1)

return WeierstrassPoint{X: x3, Y: y3, IsInf: xSame && yDiff}
return WeierstrassPoint{X: x3, Y: y3, IsInf: false}
}

func (p WeierstrassPoint) Double() WeierstrassPoint {
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