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Bug in multiplication of point by integer #373

@fedimser

Description

@fedimser

Version: ecdsa==0.19.2

Repro:

from ecdsa.ellipticcurve import INFINITY, CurveFp, Point, PointJacobi

def multiply_naive(point, a):
    ans = point
    for i in range(a-1):
        ans = ans + point
    return ans

curve = CurveFp(p=31, a=2, b=3)
point = Point(curve, 6, 18)
print("point:", point)
print("11*point:", 11*point)
print("11*point (naive):", multiply_naive(point, 11))
print("10*point+point:", 10*point+point)
print("11*point (via Jacobi):", (11*PointJacobi.from_affine(point)).to_affine())

Output:

point: (6,18)
11*point: (6,13)
11*point (naive): (8,29)
10*point+point: (8,29)
11*point (via Jacobi): (8,29)

As I showed in 3 different ways, for P=(6,18), 11*P=(8,29). However, the library returns (6,13).

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