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eccref.py: handle order-2 points in Montgomery curves.

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If a point doubles to the identity, we should return the identity,
rather than throwing a Python divide-by-zero exception.
This commit is contained in:
Simon Tatham 2020-02-26 19:23:03 +00:00
parent 027f1c67ec
commit 0645824e4d
1 changed files with 5 additions and 1 deletions
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6
test/eccref.py
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@ -316,9 +316,13 @@ class MontgomeryCurve(CurveBase):
xdiff = x2-x1
if xdiff != 0:
slope = (y2-y1) / xdiff
else:
elif y1 != 0:
assert y1 == y2
slope = (3*x1*x1 + 2*self.curve.a*x1 + 1) / (2*self.curve.b*y1)
else:
# If y1 was 0 as well, then we must have found an
# order-2 point that doubles to the identity.
return self.curve.point()
xp = self.curve.b*slope*slope - self.curve.a - x1 - x2
yp = -(y1 + slope * (xp-x1))
return self.curve.point(xp, yp)