#! /usr/bin/env python
# -*- coding: utf-8 -*-
#
# Implementation of elliptic curves, for cryptographic applications.
#
# This module doesn't provide any way to choose a random elliptic
# curve, nor to verify that an elliptic curve was chosen randomly,
# because one can simply use NIST's standard curves.
#
# Notes from X9.62-1998 (draft):
#   Nomenclature:
#     - Q is a public key.
#     The "Elliptic Curve Domain Parameters" include:
#     - q is the "field size", which in our case equals p.
#     - p is a big prime.
#     - G is a point of prime order (5.1.1.1).
#     - n is the order of G (5.1.1.1).
#   Public-key validation (5.2.2):
#     - Verify that Q is not the point at infinity.
#     - Verify that X_Q and Y_Q are in [0,p-1].
#     - Verify that Q is on the curve.
#     - Verify that nQ is the point at infinity.
#   Signature generation (5.3):
#     - Pick random k from [1,n-1].
#   Signature checking (5.4.2):
#     - Verify that r and s are in [1,n-1].
#
# Version of 2008.11.25.
#
# Revision history:
#    2005.12.31 - Initial version.
#    2008.11.25 - Change CurveFp.is_on to contains_point.
#
# Written in 2005 by Peter Pearson and placed in the public domain.

from __future__ import division

try:
    from gmpy2 import mpz

    GMPY = True
except ImportError:
    try:
        from gmpy import mpz

        GMPY = True
    except ImportError:
        GMPY = False


from six import python_2_unicode_compatible
from . import numbertheory
from ._rwlock import RWLock


@python_2_unicode_compatible
class CurveFp(object):
    """Elliptic Curve over the field of integers modulo a prime."""

    if GMPY:

        def __init__(self, p, a, b, h=None):
            """
            The curve of points satisfying y^2 = x^3 + a*x + b (mod p).

            h is an integer that is the cofactor of the elliptic curve domain
            parameters; it is the number of points satisfying the elliptic
            curve equation divided by the order of the base point. It is used
            for selection of efficient algorithm for public point verification.
            """
            self.__p = mpz(p)
            self.__a = mpz(a)
            self.__b = mpz(b)
            # h is not used in calculations and it can be None, so don't use
            # gmpy with it
            self.__h = h

    else:

        def __init__(self, p, a, b, h=None):
            """
            The curve of points satisfying y^2 = x^3 + a*x + b (mod p).

            h is an integer that is the cofactor of the elliptic curve domain
            parameters; it is the number of points satisfying the elliptic
            curve equation divided by the order of the base point. It is used
            for selection of efficient algorithm for public point verification.
            """
            self.__p = p
            self.__a = a
            self.__b = b
            self.__h = h

    def __eq__(self, other):
        if isinstance(other, CurveFp):
            """Return True if the curves are identical, False otherwise."""
            return (
                self.__p == other.__p
                and self.__a == other.__a
                and self.__b == other.__b
            )
        return NotImplemented

    def __ne__(self, other):
        return not (self == other)

    def __hash__(self):
        return hash((self.__p, self.__a, self.__b))

    def p(self):
        return self.__p

    def a(self):
        return self.__a

    def b(self):
        return self.__b

    def cofactor(self):
        return self.__h

    def contains_point(self, x, y):
        """Is the point (x,y) on this curve?"""
        return (y * y - ((x * x + self.__a) * x + self.__b)) % self.__p == 0

    def __str__(self):
        return "CurveFp(p=%d, a=%d, b=%d, h=%d)" % (
            self.__p,
            self.__a,
            self.__b,
            self.__h,
        )


class PointJacobi(object):
    """
    Point on an elliptic curve. Uses Jacobi coordinates.

    In Jacobian coordinates, there are three parameters, X, Y and Z.
    They correspond to affine parameters 'x' and 'y' like so:

    x = X / Z²
    y = Y / Z³
    """

    def __init__(self, curve, x, y, z, order=None, generator=False):
        """
        Initialise a point that uses Jacobi representation internally.

        :param CurveFp curve: curve on which the point resides
        :param int x: the X parameter of Jacobi representation (equal to x when
          converting from affine coordinates
        :param int y: the Y parameter of Jacobi representation (equal to y when
          converting from affine coordinates
        :param int z: the Z parameter of Jacobi representation (equal to 1 when
          converting from affine coordinates
        :param int order: the point order, must be non zero when using
          generator=True
        :param bool generator: the point provided is a curve generator, as
          such, it will be commonly used with scalar multiplication. This will
          cause to precompute multiplication table for it
        """
        self.__curve = curve
        # since it's generally better (faster) to use scaled points vs unscaled
        # ones, use writer-biased RWLock for locking:
        self._scale_lock = RWLock()
        if GMPY:
            self.__x = mpz(x)
            self.__y = mpz(y)
            self.__z = mpz(z)
            self.__order = order and mpz(order)
        else:
            self.__x = x
            self.__y = y
            self.__z = z
            self.__order = order
        self.__precompute = []
        if generator:
            assert order
            i = 1
            order *= 2
            doubler = PointJacobi(curve, x, y, z, order)
            order *= 2
            self.__precompute.append((doubler.x(), doubler.y()))

            while i < order:
                i *= 2
                doubler = doubler.double().scale()
                self.__precompute.append((doubler.x(), doubler.y()))

    def __getstate__(self):
        try:
            self._scale_lock.reader_acquire()
            state = self.__dict__.copy()
        finally:
            self._scale_lock.reader_release()
        del state["_scale_lock"]
        return state

    def __setstate__(self, state):
        self.__dict__.update(state)
        self._scale_lock = RWLock()

    def __eq__(self, other):
        """Compare two points with each-other."""
        try:
            self._scale_lock.reader_acquire()
            if other is INFINITY:
                return not self.__y or not self.__z
            x1, y1, z1 = self.__x, self.__y, self.__z
        finally:
            self._scale_lock.reader_release()
        if isinstance(other, Point):
            x2, y2, z2 = other.x(), other.y(), 1
        elif isinstance(other, PointJacobi):
            try:
                other._scale_lock.reader_acquire()
                x2, y2, z2 = other.__x, other.__y, other.__z
            finally:
                other._scale_lock.reader_release()
        else:
            return NotImplemented
        if self.__curve != other.curve():
            return False
        p = self.__curve.p()

        zz1 = z1 * z1 % p
        zz2 = z2 * z2 % p

        # compare the fractions by bringing them to the same denominator
        # depend on short-circuit to save 4 multiplications in case of
        # inequality
        return (x1 * zz2 - x2 * zz1) % p == 0 and (
            y1 * zz2 * z2 - y2 * zz1 * z1
        ) % p == 0

    def order(self):
        """Return the order of the point.

        None if it is undefined.
        """
        return self.__order

    def curve(self):
        """Return curve over which the point is defined."""
        return self.__curve

    def x(self):
        """
        Return affine x coordinate.

        This method should be used only when the 'y' coordinate is not needed.
        It's computationally more efficient to use `to_affine()` and then
        call x() and y() on the returned instance. Or call `scale()`
        and then x() and y() on the returned instance.
        """
        try:
            self._scale_lock.reader_acquire()
            if self.__z == 1:
                return self.__x
            x = self.__x
            z = self.__z
        finally:
            self._scale_lock.reader_release()
        p = self.__curve.p()
        z = numbertheory.inverse_mod(z, p)
        return x * z ** 2 % p

    def y(self):
        """
        Return affine y coordinate.

        This method should be used only when the 'x' coordinate is not needed.
        It's computationally more efficient to use `to_affine()` and then
        call x() and y() on the returned instance. Or call `scale()`
        and then x() and y() on the returned instance.
        """
        try:
            self._scale_lock.reader_acquire()
            if self.__z == 1:
                return self.__y
            y = self.__y
            z = self.__z
        finally:
            self._scale_lock.reader_release()
        p = self.__curve.p()
        z = numbertheory.inverse_mod(z, p)
        return y * z ** 3 % p

    def scale(self):
        """
        Return point scaled so that z == 1.

        Modifies point in place, returns self.
        """
        try:
            self._scale_lock.reader_acquire()
            if self.__z == 1:
                return self
        finally:
            self._scale_lock.reader_release()

        try:
            self._scale_lock.writer_acquire()
            # scaling already scaled point is safe (as inverse of 1 is 1) and
            # quick so we don't need to optimise for the unlikely event when
            # two threads hit the lock at the same time
            p = self.__curve.p()
            z_inv = numbertheory.inverse_mod(self.__z, p)
            zz_inv = z_inv * z_inv % p
            self.__x = self.__x * zz_inv % p
            self.__y = self.__y * zz_inv * z_inv % p
            # we are setting the z last so that the check above will return
            # true only after all values were already updated
            self.__z = 1
        finally:
            self._scale_lock.writer_release()
        return self

    def to_affine(self):
        """Return point in affine form."""
        if not self.__y or not self.__z:
            return INFINITY
        self.scale()
        # after point is scaled, it's immutable, so no need to perform locking
        return Point(self.__curve, self.__x, self.__y, self.__order)

    @staticmethod
    def from_affine(point, generator=False):
        """Create from an affine point.

        :param bool generator: set to True to make the point to precalculate
          multiplication table - useful for public point when verifying many
          signatures (around 100 or so) or for generator points of a curve.
        """
        return PointJacobi(
            point.curve(), point.x(), point.y(), 1, point.order(), generator
        )

    # plese note that all the methods that use the equations from hyperelliptic
    # are formatted in a way to maximise performance.
    # Things that make code faster: multiplying instead of taking to the power
    # (`xx = x * x; xxxx = xx * xx % p` is faster than `xxxx = x**4 % p` and
    # `pow(x, 4, p)`),
    # multiple assignments at the same time (`x1, x2 = self.x1, self.x2` is
    # faster than `x1 = self.x1; x2 = self.x2`),
    # similarly, sometimes the `% p` is skipped if it makes the calculation
    # faster and the result of calculation is later reduced modulo `p`

    def _double_with_z_1(self, X1, Y1, p, a):
        """Add a point to itself with z == 1."""
        # after:
        # http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian.html#doubling-mdbl-2007-bl
        XX, YY = X1 * X1 % p, Y1 * Y1 % p
        if not YY:
            return 0, 0, 1
        YYYY = YY * YY % p
        S = 2 * ((X1 + YY) ** 2 - XX - YYYY) % p
        M = 3 * XX + a
        T = (M * M - 2 * S) % p
        # X3 = T
        Y3 = (M * (S - T) - 8 * YYYY) % p
        Z3 = 2 * Y1 % p
        return T, Y3, Z3

    def _double(self, X1, Y1, Z1, p, a):
        """Add a point to itself, arbitrary z."""
        if Z1 == 1:
            return self._double_with_z_1(X1, Y1, p, a)
        if not Z1:
            return 0, 0, 1
        # after:
        # http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian.html#doubling-dbl-2007-bl
        XX, YY = X1 * X1 % p, Y1 * Y1 % p
        if not YY:
            return 0, 0, 1
        YYYY = YY * YY % p
        ZZ = Z1 * Z1 % p
        S = 2 * ((X1 + YY) ** 2 - XX - YYYY) % p
        M = (3 * XX + a * ZZ * ZZ) % p
        T = (M * M - 2 * S) % p
        # X3 = T
        Y3 = (M * (S - T) - 8 * YYYY) % p
        Z3 = ((Y1 + Z1) ** 2 - YY - ZZ) % p

        return T, Y3, Z3

    def double(self):
        """Add a point to itself."""
        if not self.__y:
            return INFINITY

        p, a = self.__curve.p(), self.__curve.a()

        try:
            self._scale_lock.reader_acquire()
            X1, Y1, Z1 = self.__x, self.__y, self.__z
        finally:
            self._scale_lock.reader_release()

        X3, Y3, Z3 = self._double(X1, Y1, Z1, p, a)

        if not Y3 or not Z3:
            return INFINITY
        return PointJacobi(self.__curve, X3, Y3, Z3, self.__order)

    def _add_with_z_1(self, X1, Y1, X2, Y2, p):
        """add points when both Z1 and Z2 equal 1"""
        # after:
        # http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian.html#addition-mmadd-2007-bl
        H = X2 - X1
        HH = H * H
        I = 4 * HH % p
        J = H * I
        r = 2 * (Y2 - Y1)
        if not H and not r:
            return self._double_with_z_1(X1, Y1, p, self.__curve.a())
        V = X1 * I
        X3 = (r ** 2 - J - 2 * V) % p
        Y3 = (r * (V - X3) - 2 * Y1 * J) % p
        Z3 = 2 * H % p
        return X3, Y3, Z3

    def _add_with_z_eq(self, X1, Y1, Z1, X2, Y2, p):
        """add points when Z1 == Z2"""
        # after:
        # http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian.html#addition-zadd-2007-m
        A = (X2 - X1) ** 2 % p
        B = X1 * A % p
        C = X2 * A
        D = (Y2 - Y1) ** 2 % p
        if not A and not D:
            return self._double(X1, Y1, Z1, p, self.__curve.a())
        X3 = (D - B - C) % p
        Y3 = ((Y2 - Y1) * (B - X3) - Y1 * (C - B)) % p
        Z3 = Z1 * (X2 - X1) % p
        return X3, Y3, Z3

    def _add_with_z2_1(self, X1, Y1, Z1, X2, Y2, p):
        """add points when Z2 == 1"""
        # after:
        # http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian.html#addition-madd-2007-bl
        Z1Z1 = Z1 * Z1 % p
        U2, S2 = X2 * Z1Z1 % p, Y2 * Z1 * Z1Z1 % p
        H = (U2 - X1) % p
        HH = H * H % p
        I = 4 * HH % p
        J = H * I
        r = 2 * (S2 - Y1) % p
        if not r and not H:
            return self._double_with_z_1(X2, Y2, p, self.__curve.a())
        V = X1 * I
        X3 = (r * r - J - 2 * V) % p
        Y3 = (r * (V - X3) - 2 * Y1 * J) % p
        Z3 = ((Z1 + H) ** 2 - Z1Z1 - HH) % p
        return X3, Y3, Z3

    def _add_with_z_ne(self, X1, Y1, Z1, X2, Y2, Z2, p):
        """add points with arbitrary z"""
        # after:
        # http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian.html#addition-add-2007-bl
        Z1Z1 = Z1 * Z1 % p
        Z2Z2 = Z2 * Z2 % p
        U1 = X1 * Z2Z2 % p
        U2 = X2 * Z1Z1 % p
        S1 = Y1 * Z2 * Z2Z2 % p
        S2 = Y2 * Z1 * Z1Z1 % p
        H = U2 - U1
        I = 4 * H * H % p
        J = H * I % p
        r = 2 * (S2 - S1) % p
        if not H and not r:
            return self._double(X1, Y1, Z1, p, self.__curve.a())
        V = U1 * I
        X3 = (r * r - J - 2 * V) % p
        Y3 = (r * (V - X3) - 2 * S1 * J) % p
        Z3 = ((Z1 + Z2) ** 2 - Z1Z1 - Z2Z2) * H % p

        return X3, Y3, Z3

    def __radd__(self, other):
        """Add other to self."""
        return self + other

    def _add(self, X1, Y1, Z1, X2, Y2, Z2, p):
        """add two points, select fastest method."""
        if not Y1 or not Z1:
            return X2, Y2, Z2
        if not Y2 or not Z2:
            return X1, Y1, Z1
        if Z1 == Z2:
            if Z1 == 1:
                return self._add_with_z_1(X1, Y1, X2, Y2, p)
            return self._add_with_z_eq(X1, Y1, Z1, X2, Y2, p)
        if Z1 == 1:
            return self._add_with_z2_1(X2, Y2, Z2, X1, Y1, p)
        if Z2 == 1:
            return self._add_with_z2_1(X1, Y1, Z1, X2, Y2, p)
        return self._add_with_z_ne(X1, Y1, Z1, X2, Y2, Z2, p)

    def __add__(self, other):
        """Add two points on elliptic curve."""
        if self == INFINITY:
            return other
        if other == INFINITY:
            return self
        if isinstance(other, Point):
            other = PointJacobi.from_affine(other)
        if self.__curve != other.__curve:
            raise ValueError("The other point is on different curve")

        p = self.__curve.p()
        try:
            self._scale_lock.reader_acquire()
            X1, Y1, Z1 = self.__x, self.__y, self.__z
        finally:
            self._scale_lock.reader_release()
        try:
            other._scale_lock.reader_acquire()
            X2, Y2, Z2 = other.__x, other.__y, other.__z
        finally:
            other._scale_lock.reader_release()
        X3, Y3, Z3 = self._add(X1, Y1, Z1, X2, Y2, Z2, p)

        if not Y3 or not Z3:
            return INFINITY
        return PointJacobi(self.__curve, X3, Y3, Z3, self.__order)

    def __rmul__(self, other):
        """Multiply point by an integer."""
        return self * other

    def _mul_precompute(self, other):
        """Multiply point by integer with precomputation table."""
        X3, Y3, Z3, p = 0, 0, 1, self.__curve.p()
        _add = self._add
        for X2, Y2 in self.__precompute:
            if other % 2:
                if other % 4 >= 2:
                    other = (other + 1) // 2
                    X3, Y3, Z3 = _add(X3, Y3, Z3, X2, -Y2, 1, p)
                else:
                    other = (other - 1) // 2
                    X3, Y3, Z3 = _add(X3, Y3, Z3, X2, Y2, 1, p)
            else:
                other //= 2

        if not Y3 or not Z3:
            return INFINITY
        return PointJacobi(self.__curve, X3, Y3, Z3, self.__order)

    @staticmethod
    def _naf(mult):
        """Calculate non-adjacent form of number."""
        ret = []
        while mult:
            if mult % 2:
                nd = mult % 4
                if nd >= 2:
                    nd = nd - 4
                ret += [nd]
                mult -= nd
            else:
                ret += [0]
            mult //= 2
        return ret

    def __mul__(self, other):
        """Multiply point by an integer."""
        if not self.__y or not other:
            return INFINITY
        if other == 1:
            return self
        if self.__order:
            # order*2 as a protection for Minerva
            other = other % (self.__order * 2)
        if self.__precompute:
            return self._mul_precompute(other)

        self = self.scale()
        # once scaled, point is immutable, not need to lock
        X2, Y2 = self.__x, self.__y
        X3, Y3, Z3 = 0, 0, 1
        p, a = self.__curve.p(), self.__curve.a()
        _double = self._double
        _add = self._add
        # since adding points when at least one of them is scaled
        # is quicker, reverse the NAF order
        for i in reversed(self._naf(other)):
            X3, Y3, Z3 = _double(X3, Y3, Z3, p, a)
            if i < 0:
                X3, Y3, Z3 = _add(X3, Y3, Z3, X2, -Y2, 1, p)
            elif i > 0:
                X3, Y3, Z3 = _add(X3, Y3, Z3, X2, Y2, 1, p)

        if not Y3 or not Z3:
            return INFINITY

        return PointJacobi(self.__curve, X3, Y3, Z3, self.__order)

    @staticmethod
    def _leftmost_bit(x):
        """Return integer with the same magnitude as x but only one bit set"""
        assert x > 0
        result = 1
        while result <= x:
            result = 2 * result
        return result // 2

    def mul_add(self, self_mul, other, other_mul):
        """
      Do two multiplications at the same time, add results.

      calculates self*self_mul + other*other_mul
      """
        if other is INFINITY or other_mul == 0:
            return self * self_mul
        if self_mul == 0:
            return other * other_mul
        if not isinstance(other, PointJacobi):
            other = PointJacobi.from_affine(other)
        # when the points have precomputed answers, then multiplying them alone
        # is faster (as it uses NAF)
        if self.__precompute and other.__precompute:
            return self * self_mul + other * other_mul

        if self.__order:
            self_mul = self_mul % self.__order
            other_mul = other_mul % self.__order

        i = self._leftmost_bit(max(self_mul, other_mul)) * 2
        X3, Y3, Z3 = 0, 0, 1
        p, a = self.__curve.p(), self.__curve.a()
        self = self.scale()
        # after scaling, point is immutable, no need for locking
        X1, Y1 = self.__x, self.__y
        other = other.scale()
        X2, Y2 = other.__x, other.__y
        both = self + other
        if both is INFINITY:
            X4, Y4 = 0, 0
        else:
            both.scale()
            X4, Y4 = both.__x, both.__y
        _double = self._double
        _add = self._add
        while i > 1:
            X3, Y3, Z3 = _double(X3, Y3, Z3, p, a)
            i = i // 2

            if self_mul & i and other_mul & i:
                X3, Y3, Z3 = _add(X3, Y3, Z3, X4, Y4, 1, p)
            elif self_mul & i:
                X3, Y3, Z3 = _add(X3, Y3, Z3, X1, Y1, 1, p)
            elif other_mul & i:
                X3, Y3, Z3 = _add(X3, Y3, Z3, X2, Y2, 1, p)

        if not Y3 or not Z3:
            return INFINITY

        return PointJacobi(self.__curve, X3, Y3, Z3, self.__order)

    def __neg__(self):
        """Return negated point."""
        try:
            self._scale_lock.reader_acquire()
            return PointJacobi(
                self.__curve, self.__x, -self.__y, self.__z, self.__order
            )
        finally:
            self._scale_lock.reader_release()


class Point(object):
    """A point on an elliptic curve. Altering x and y is forbidding,
     but they can be read by the x() and y() methods."""

    def __init__(self, curve, x, y, order=None):
        """curve, x, y, order; order (optional) is the order of this point."""
        self.__curve = curve
        if GMPY:
            self.__x = x and mpz(x)
            self.__y = y and mpz(y)
            self.__order = order and mpz(order)
        else:
            self.__x = x
            self.__y = y
            self.__order = order
        # self.curve is allowed to be None only for INFINITY:
        if self.__curve:
            assert self.__curve.contains_point(x, y)
        # for curves with cofactor 1, all points that are on the curve are
        # scalar multiples of the base point, so performing multiplication is
        # not necessary to verify that. See Section 3.2.2.1 of SEC 1 v2
        if curve and curve.cofactor() != 1 and order:
            assert self * order == INFINITY

    def __eq__(self, other):
        """Return True if the points are identical, False otherwise."""
        if isinstance(other, Point):
            return (
                self.__curve == other.__curve
                and self.__x == other.__x
                and self.__y == other.__y
            )
        return NotImplemented

    def __neg__(self):
        return Point(self.__curve, self.__x, self.__curve.p() - self.__y)

    def __add__(self, other):
        """Add one point to another point."""

        # X9.62 B.3:

        if not isinstance(other, Point):
            return NotImplemented
        if other == INFINITY:
            return self
        if self == INFINITY:
            return other
        assert self.__curve == other.__curve
        if self.__x == other.__x:
            if (self.__y + other.__y) % self.__curve.p() == 0:
                return INFINITY
            else:
                return self.double()

        p = self.__curve.p()

        l = (
            (other.__y - self.__y)
            * numbertheory.inverse_mod(other.__x - self.__x, p)
        ) % p

        x3 = (l * l - self.__x - other.__x) % p
        y3 = (l * (self.__x - x3) - self.__y) % p

        return Point(self.__curve, x3, y3)

    def __mul__(self, other):
        """Multiply a point by an integer."""

        def leftmost_bit(x):
            assert x > 0
            result = 1
            while result <= x:
                result = 2 * result
            return result // 2

        e = other
        if e == 0 or (self.__order and e % self.__order == 0):
            return INFINITY
        if self == INFINITY:
            return INFINITY
        if e < 0:
            return (-self) * (-e)

        # From X9.62 D.3.2:

        e3 = 3 * e
        negative_self = Point(self.__curve, self.__x, -self.__y, self.__order)
        i = leftmost_bit(e3) // 2
        result = self
        # print_("Multiplying %s by %d (e3 = %d):" % (self, other, e3))
        while i > 1:
            result = result.double()
            if (e3 & i) != 0 and (e & i) == 0:
                result = result + self
            if (e3 & i) == 0 and (e & i) != 0:
                result = result + negative_self
            # print_(". . . i = %d, result = %s" % ( i, result ))
            i = i // 2

        return result

    def __rmul__(self, other):
        """Multiply a point by an integer."""

        return self * other

    def __str__(self):
        if self == INFINITY:
            return "infinity"
        return "(%d,%d)" % (self.__x, self.__y)

    def double(self):
        """Return a new point that is twice the old."""

        if self == INFINITY:
            return INFINITY

        # X9.62 B.3:

        p = self.__curve.p()
        a = self.__curve.a()

        l = (
            (3 * self.__x * self.__x + a)
            * numbertheory.inverse_mod(2 * self.__y, p)
        ) % p

        x3 = (l * l - 2 * self.__x) % p
        y3 = (l * (self.__x - x3) - self.__y) % p

        return Point(self.__curve, x3, y3)

    def x(self):
        return self.__x

    def y(self):
        return self.__y

    def curve(self):
        return self.__curve

    def order(self):
        return self.__order


# This one point is the Point At Infinity for all purposes:
INFINITY = Point(None, None, None)
