Scalar multiplication by -1 is crashing

[AurelienCha]

Hi,

I have just discovered that scalar multiplication by -1 on an EC point raises an unexpected error. I am using the Montgormery Curve 25519. However it seems to fail on other curves as well (but I haven't try all the curves yet).

I can multiply an EC point by any scalar (even negative scalar) except -1. I can for example multiply a two points P by 2 and -3 respectively and then add them together to get -1*P without any error. However, doing (-1)*G crashes. Note that the simple negation -G works.

Here is a code example:

`

from ecpy.curves import Curve, Point
...
... curve = Curve.get_curve('Curve25519')
... G = curve.generator
... P = 2*G + (-3)*G
... assert P == -G
... Q = (-1)*G
...
Traceback (most recent call last):
File "", line 7, in
Q = (-1)*G
~~~~^~
File "/home/garuda/.local/lib/python3.13/site-packages/ecpy/curves.py", line 1109, in rmul
return self.mul(scal)
~~~~~~~~~~~~^^^^^^
File "/home/garuda/.local/lib/python3.13/site-packages/ecpy/curves.py", line 1105, in mul
return self.curve._mul_point(scal,self)
~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
File "/home/garuda/.local/lib/python3.13/site-packages/ecpy/curves.py", line 879, in _mul_point
return Point (kx, ky, self)
File "/home/garuda/.local/lib/python3.13/site-packages/ecpy/curves.py", line 981, in init
raise ECPyException("Point not on curve")
ecpy.curves.ECPyException: <exception str() failed>
`

Python Version:
Python 3.13.11 (main, Dec 7 2025, 13:01:45) [GCC 15.2.1 20251112] on linux
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