pycryptodome/lib/Crypto/PublicKey/ECC.py

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# Copyright (c) 2015, Legrandin <helderijs@gmail.com>
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from Crypto.Math.Numbers import Integer

class _Curve(object):
    pass

_curve = _Curve()
_curve.p = Integer(115792089210356248762697446949407573530086143415290314195533631308867097853951)
_curve.n = Integer(115792089210356248762697446949407573529996955224135760342422259061068512044369)
_curve.Gx = Integer(0x6b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296)
_curve.Gy = Integer(0x4fe342e2fe1a7f9b8ee7eb4a7c0f9e162bce33576b315ececbb6406837bf51f5)


# https://www.nsa.gov/ia/_files/nist-routines.pdf
# http://point-at-infinity.org/ecc/nisttv
# http://www.hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-3.html
# https://en.wikibooks.org/wiki/Cryptography/Prime_Curve/Jacobian_Coordinates
# https://eprint.iacr.org/2013/816.pdf

class ECPoint(object):

    def __init__(self, x, y):
        self._x = Integer(x)
        self._y = Integer(y)

    def __eq__(self, point):
        return self._x == point._x and self._y == point._y

    def __neg__(self):
        if self.is_point_at_infinity():
            return self.point_at_infinity()
        return ECPoint(self._x, _curve.p - self._y)

    def copy(self):
        return ECPoint(self._x, self._y)

    def is_point_at_infinity(self):
        return self._x == 0 and self._y == 0

    @staticmethod
    def point_at_infinity():
        return ECPoint(0, 0)

    @property
    def x(self):
        if self.is_point_at_infinity():
            raise ValueError("Point at infinity")
        return self._x

    @property
    def y(self):
        if self.is_point_at_infinity():
            raise ValueError("Point at infinity")
        return self._y

    def double(self):
        """Return a new point, doubling this one"""

        if self._y == 0:
            return self.point_at_infinity()

        common = (pow(self._x, 2, _curve.p) * 3 - 3) * (self._y << 1).inverse(_curve.p) % _curve.p
        x3 = pow(common, 2, _curve.p) - self._x - self._x
        while x3 < 0:
            x3 += _curve.p
        y3 = ((self._x - x3) * common - self._y) % _curve.p

        return ECPoint(x3, y3)

    def add(self, point):
        """Return a new point, the addition of this one and another"""

        if self.is_point_at_infinity():
            return point.copy()

        if point.is_point_at_infinity():
            return self.copy()

        if self == point:
            return self.double()

        if self._x == point._x:
            return self.point_at_infinity()

        common = (point._y - self._y) * (point._x - self._x).inverse(_curve.p) % _curve.p
        x3 = pow(common, 2, _curve.p) - self._x - point._x
        while x3 < 0:
            x3 += _curve.p
        y3 = ((self._x - x3) * common - self._y) % _curve.p

        return ECPoint(x3, y3)

    def multiply(self, scalar):
        """Return a new point, the scalar product of this one"""

        assert(scalar >= 0)

        # Trivial results
        if scalar == 0 or self.is_point_at_infinity():
            return self.point_at_infinity()
        elif scalar == 1:
            return self.copy()

        # Convert to NAF
        WINDOW_BITS = 3
        window_high = 1 << WINDOW_BITS
        window_low = 1 << (WINDOW_BITS - 1)
        window_mask = window_high - 1

        scalar_int = int(scalar)
        naf = []
        while scalar_int > 0:
            if scalar_int & 1:
                di = scalar_int & window_mask
                if di >= window_low:
                    di -= window_high
                scalar_int -= di
            else:
                di = 0
            naf.append(di)
            scalar_int >>= 1
        naf.reverse()

        # naf contains d_(i-1), d_(i-2), .. d_1, d_0

        # import pdb; pdb.set_trace()
        # Precompute 1P, 3P, 5P, .. (2**(W-1) - 1)P
        # which is only 1P, 3P for W=3 (we also add negatives)
        precomp = [0, self, 0, self.add(self.double()) ]
        precomp += [ -x for x in precomp[:0:-1]]

        result = self.point_at_infinity()
        for x in naf:
            result = result.double()
            if x != 0:
                result = result.add(precomp[x])

        return result