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Filename : scrypt.py
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# Copyright 2013 Donald Stufft and individual contributors
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from __future__ import absolute_import
from __future__ import division

import nacl.bindings
import nacl.encoding
from nacl import exceptions as exc
from nacl.exceptions import ensure

_strbytes_plus_one = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_STRBYTES

STRPREFIX = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_STRPREFIX

SALTBYTES = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_SALTBYTES

PASSWD_MIN = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_PASSWD_MIN
PASSWD_MAX = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_PASSWD_MAX

PWHASH_SIZE = _strbytes_plus_one - 1

BYTES_MIN = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_BYTES_MIN
BYTES_MAX = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_BYTES_MAX

MEMLIMIT_MIN = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_MIN
MEMLIMIT_MAX = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_MAX
OPSLIMIT_MIN = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_MIN
OPSLIMIT_MAX = nacl.bindings.crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_MAX

OPSLIMIT_INTERACTIVE = \
    nacl.bindings.crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_INTERACTIVE
MEMLIMIT_INTERACTIVE = \
    nacl.bindings.crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_INTERACTIVE
OPSLIMIT_SENSITIVE = \
    nacl.bindings.crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_SENSITIVE
MEMLIMIT_SENSITIVE = \
    nacl.bindings.crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_SENSITIVE

OPSLIMIT_MODERATE = 8 * OPSLIMIT_INTERACTIVE
MEMLIMIT_MODERATE = 8 * MEMLIMIT_INTERACTIVE


def kdf(size, password, salt,
        opslimit=OPSLIMIT_SENSITIVE,
        memlimit=MEMLIMIT_SENSITIVE,
        encoder=nacl.encoding.RawEncoder):
    """
    Derive a ``size`` bytes long key from a caller-supplied
    ``password`` and ``salt`` pair using the scryptsalsa208sha256
    memory-hard construct.


    the enclosing module provides the constants

        - :py:const:`.OPSLIMIT_INTERACTIVE`
        - :py:const:`.MEMLIMIT_INTERACTIVE`
        - :py:const:`.OPSLIMIT_SENSITIVE`
        - :py:const:`.MEMLIMIT_SENSITIVE`
        - :py:const:`.OPSLIMIT_MODERATE`
        - :py:const:`.MEMLIMIT_MODERATE`

    as a guidance for correct settings respectively for the
    interactive login and the long term key protecting sensitive data
    use cases.

    :param size: derived key size, must be between
                 :py:const:`.BYTES_MIN` and
                 :py:const:`.BYTES_MAX`
    :type size: int
    :param password: password used to seed the key derivation procedure;
                     it length must be between
                     :py:const:`.PASSWD_MIN` and
                     :py:const:`.PASSWD_MAX`
    :type password: bytes
    :param salt: **RANDOM** salt used in the key derivation procedure;
                 its length must be exactly :py:const:`.SALTBYTES`
    :type salt: bytes
    :param opslimit: the time component (operation count)
                     of the key derivation procedure's computational cost;
                     it must be between
                     :py:const:`.OPSLIMIT_MIN` and
                     :py:const:`.OPSLIMIT_MAX`
    :type opslimit: int
    :param memlimit: the memory occupation component
                     of the key derivation procedure's computational cost;
                     it must be between
                     :py:const:`.MEMLIMIT_MIN` and
                     :py:const:`.MEMLIMIT_MAX`
    :type memlimit: int
    :rtype: bytes

    .. versionadded:: 1.2
    """
    ensure(
        len(salt) == SALTBYTES,
        "The salt must be exactly %s, not %s bytes long" % (
            SALTBYTES,
            len(salt)
        ),
        raising=exc.ValueError
    )

    n_log2, r, p = nacl.bindings.nacl_bindings_pick_scrypt_params(opslimit,
                                                                  memlimit)
    maxmem = memlimit + (2 ** 16)

    return encoder.encode(
        nacl.bindings.crypto_pwhash_scryptsalsa208sha256_ll(
            password, salt, 2 ** n_log2, r, p, maxmem=maxmem, dklen=size)
    )


def str(password,
        opslimit=OPSLIMIT_INTERACTIVE,
        memlimit=MEMLIMIT_INTERACTIVE):
    """
    Hashes a password with a random salt, using the memory-hard
    scryptsalsa208sha256 construct and returning an ascii string
    that has all the needed info to check against a future password

    The default settings for opslimit and memlimit are those deemed
    correct for the interactive user login case.

    :param bytes password:
    :param int opslimit:
    :param int memlimit:
    :rtype: bytes

    .. versionadded:: 1.2
    """

    return nacl.bindings.crypto_pwhash_scryptsalsa208sha256_str(password,
                                                                opslimit,
                                                                memlimit)


def verify(password_hash, password):
    """
    Takes the output of scryptsalsa208sha256 and compares it against
    a user provided password to see if they are the same

    :param password_hash: bytes
    :param password: bytes
    :rtype: boolean

    .. versionadded:: 1.2
    """

    ensure(len(password_hash) == PWHASH_SIZE,
           "The password hash must be exactly %s bytes long" %
           nacl.bindings.crypto_pwhash_scryptsalsa208sha256_STRBYTES,
           raising=exc.ValueError)

    return nacl.bindings.crypto_pwhash_scryptsalsa208sha256_str_verify(
        password_hash, password
    )