curve.py

from collections import namedtuple

FIELD_SIZE = 0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f
GROUP_ORDER = 0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
TONELLI_SHANKS_CONSTANT = (FIELD_SIZE + 1) // 4
BASE_POINT_PARAMETERS = (
    0x79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798,
    0x483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8
)

Point = namedtuple('Point', ('x', 'y'))


def parity(num):
    return num & 1


def x_to_y(x, y_parity):

    y = pow(x ** 3 + 7, TONELLI_SHANKS_CONSTANT, FIELD_SIZE)

    if parity(y) != y_parity:
        y = FIELD_SIZE - y

    return y