# Copyright (C) 2018  Lars Wirzenius
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.


import codecs


from Cryptodome.Random import get_random_bytes
from Cryptodome.Protocol.KDF import scrypt


class SecretHasher:

    # The following have been chosen based on:
    # https://www.pycryptodome.org/en/latest/src/protocol/kdf.html
    # Corrections, with references, welcome.
    _salt_size = 16
    _key_len = 128
    _N = 16384
    _r = 8
    _p = 1

    _version = 1

    _ok_args = ['salt', 'key_len', 'N', 'r', 'p']

    def get_salt(self):
        return get_random_bytes(self._salt_size)

    def set_n(self, n):
        self._N = n

    def _full_hash(self, cleartext, **kwargs):
        assert kwargs == self._filter_kwargs(kwargs)

        hashed_binary = scrypt(cleartext, **kwargs)
        params = {
            'version': self._version,
            'hash': self.hex_encode(hashed_binary),
            'salt': self.hex_encode(kwargs['salt'])
        }
        for key in kwargs:
            if key not in params:
                params[key] = kwargs[key]

        return params

    def hex_encode(self, byte_string):
        return codecs.encode(byte_string, 'hex').decode('ascii')

    def hex_decode(self, hex_string):
        return codecs.decode(bytes(hex_string, 'ascii'), 'hex')

    def _filter_kwargs(self, kwargs):
        return {
            key: kwargs[key]
            for key in self._ok_args
        }

    def hash(self, cleartext, salt=None):
        if salt is None:
            salt = self.get_salt()
        assert isinstance(salt, bytes)

        kwargs = {
            'salt': salt,
            'key_len': self._key_len,
            'N': self._N,
            'r': self._r,
            'p': self._p,
        }
        return self._full_hash(cleartext, **kwargs)

    def is_correct(self, hashed, cleartext):
        kwargs = self._filter_kwargs(hashed)
        salt = self.hex_decode(kwargs.pop('salt'))
        h2 = self._full_hash(cleartext, salt=salt, **kwargs)
        return hashed == h2