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Filename : modes.py
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# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.

from __future__ import absolute_import, division, print_function

import abc

import six

from cryptography import utils


@six.add_metaclass(abc.ABCMeta)
class Mode(object):
    @abc.abstractproperty
    def name(self):
        """
        A string naming this mode (e.g. "ECB", "CBC").
        """

    @abc.abstractmethod
    def validate_for_algorithm(self, algorithm):
        """
        Checks that all the necessary invariants of this (mode, algorithm)
        combination are met.
        """


@six.add_metaclass(abc.ABCMeta)
class ModeWithInitializationVector(object):
    @abc.abstractproperty
    def initialization_vector(self):
        """
        The value of the initialization vector for this mode as bytes.
        """


@six.add_metaclass(abc.ABCMeta)
class ModeWithTweak(object):
    @abc.abstractproperty
    def tweak(self):
        """
        The value of the tweak for this mode as bytes.
        """


@six.add_metaclass(abc.ABCMeta)
class ModeWithNonce(object):
    @abc.abstractproperty
    def nonce(self):
        """
        The value of the nonce for this mode as bytes.
        """


@six.add_metaclass(abc.ABCMeta)
class ModeWithAuthenticationTag(object):
    @abc.abstractproperty
    def tag(self):
        """
        The value of the tag supplied to the constructor of this mode.
        """


def _check_aes_key_length(self, algorithm):
    if algorithm.key_size > 256 and algorithm.name == "AES":
        raise ValueError(
            "Only 128, 192, and 256 bit keys are allowed for this AES mode"
        )


def _check_iv_length(self, algorithm):
    if len(self.initialization_vector) * 8 != algorithm.block_size:
        raise ValueError("Invalid IV size ({0}) for {1}.".format(
            len(self.initialization_vector), self.name
        ))


def _check_iv_and_key_length(self, algorithm):
    _check_aes_key_length(self, algorithm)
    _check_iv_length(self, algorithm)


@utils.register_interface(Mode)
@utils.register_interface(ModeWithInitializationVector)
class CBC(object):
    name = "CBC"

    def __init__(self, initialization_vector):
        if not isinstance(initialization_vector, bytes):
            raise TypeError("initialization_vector must be bytes")

        self._initialization_vector = initialization_vector

    initialization_vector = utils.read_only_property("_initialization_vector")
    validate_for_algorithm = _check_iv_and_key_length


@utils.register_interface(Mode)
@utils.register_interface(ModeWithTweak)
class XTS(object):
    name = "XTS"

    def __init__(self, tweak):
        if not isinstance(tweak, bytes):
            raise TypeError("tweak must be bytes")

        if len(tweak) != 16:
            raise ValueError("tweak must be 128-bits (16 bytes)")

        self._tweak = tweak

    tweak = utils.read_only_property("_tweak")

    def validate_for_algorithm(self, algorithm):
        if algorithm.key_size not in (256, 512):
            raise ValueError(
                "The XTS specification requires a 256-bit key for AES-128-XTS"
                " and 512-bit key for AES-256-XTS"
            )


@utils.register_interface(Mode)
class ECB(object):
    name = "ECB"

    validate_for_algorithm = _check_aes_key_length


@utils.register_interface(Mode)
@utils.register_interface(ModeWithInitializationVector)
class OFB(object):
    name = "OFB"

    def __init__(self, initialization_vector):
        if not isinstance(initialization_vector, bytes):
            raise TypeError("initialization_vector must be bytes")

        self._initialization_vector = initialization_vector

    initialization_vector = utils.read_only_property("_initialization_vector")
    validate_for_algorithm = _check_iv_and_key_length


@utils.register_interface(Mode)
@utils.register_interface(ModeWithInitializationVector)
class CFB(object):
    name = "CFB"

    def __init__(self, initialization_vector):
        if not isinstance(initialization_vector, bytes):
            raise TypeError("initialization_vector must be bytes")

        self._initialization_vector = initialization_vector

    initialization_vector = utils.read_only_property("_initialization_vector")
    validate_for_algorithm = _check_iv_and_key_length


@utils.register_interface(Mode)
@utils.register_interface(ModeWithInitializationVector)
class CFB8(object):
    name = "CFB8"

    def __init__(self, initialization_vector):
        if not isinstance(initialization_vector, bytes):
            raise TypeError("initialization_vector must be bytes")

        self._initialization_vector = initialization_vector

    initialization_vector = utils.read_only_property("_initialization_vector")
    validate_for_algorithm = _check_iv_and_key_length


@utils.register_interface(Mode)
@utils.register_interface(ModeWithNonce)
class CTR(object):
    name = "CTR"

    def __init__(self, nonce):
        if not isinstance(nonce, bytes):
            raise TypeError("nonce must be bytes")

        self._nonce = nonce

    nonce = utils.read_only_property("_nonce")

    def validate_for_algorithm(self, algorithm):
        _check_aes_key_length(self, algorithm)
        if len(self.nonce) * 8 != algorithm.block_size:
            raise ValueError("Invalid nonce size ({0}) for {1}.".format(
                len(self.nonce), self.name
            ))


@utils.register_interface(Mode)
@utils.register_interface(ModeWithInitializationVector)
@utils.register_interface(ModeWithAuthenticationTag)
class GCM(object):
    name = "GCM"
    _MAX_ENCRYPTED_BYTES = (2 ** 39 - 256) // 8
    _MAX_AAD_BYTES = (2 ** 64) // 8

    def __init__(self, initialization_vector, tag=None, min_tag_length=16):
        # len(initialization_vector) must in [1, 2 ** 64), but it's impossible
        # to actually construct a bytes object that large, so we don't check
        # for it
        if not isinstance(initialization_vector, bytes):
            raise TypeError("initialization_vector must be bytes")
        if len(initialization_vector) == 0:
            raise ValueError("initialization_vector must be at least 1 byte")
        self._initialization_vector = initialization_vector
        if tag is not None:
            if not isinstance(tag, bytes):
                raise TypeError("tag must be bytes or None")
            if min_tag_length < 4:
                raise ValueError("min_tag_length must be >= 4")
            if len(tag) < min_tag_length:
                raise ValueError(
                    "Authentication tag must be {0} bytes or longer.".format(
                        min_tag_length)
                )
        self._tag = tag
        self._min_tag_length = min_tag_length

    tag = utils.read_only_property("_tag")
    initialization_vector = utils.read_only_property("_initialization_vector")

    def validate_for_algorithm(self, algorithm):
        _check_aes_key_length(self, algorithm)