pycryptodome/lib/Crypto/Cipher/DES3.py

# -*- coding: utf-8 -*-
#
#  Cipher/DES3.py : DES3
#
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"""Triple DES symmetric cipher

`Triple DES`__ (or TDES or TDEA or 3DES) is a symmetric block cipher standardized by NIST_.
It has a fixed data block size of 8 bytes. Its keys are 128 (*Option 1*) or 192
bits (*Option 2*) long.
However, 1 out of 8 bits is used for redundancy and do not contribute to
security. The effective key length is respectively 112 or 168 bits.

TDES consists of the concatenation of 3 simple `DES` ciphers.

The plaintext is first DES encrypted with *K1*, then decrypted with *K2*,
and finally encrypted again with *K3*.  The ciphertext is decrypted in the reverse manner.

The 192 bit key is a bundle of three 64 bit independent subkeys: *K1*, *K2*, and *K3*.

The 128 bit key is split into *K1* and *K2*, whereas *K1=K3*.

It is important that all subkeys are different, otherwise TDES would degrade to
single `DES`.

TDES is cryptographically secure, even though it is neither as secure nor as fast
as `AES`.

*Use AES, not TDES. This module is provided for legacy purposes only.**

As an example, encryption can be done as follows:

    >>> from Crypto.Cipher import DES3
    >>> from Crypto.Random import get_random_bytes
    >>>
    >>> key = b'Sixteen byte key'
    >>> nonce = get_random_bytes(DES3.block_size/2)
    >>> cipher = DES3.new(key, DES3.MODE_CTR, nonce=nonce)
    >>> plaintext = b'We are no longer the knights who say ni!'
    >>> msg = nonce + cipher.encrypt(plaintext)

.. __: http://en.wikipedia.org/wiki/Triple_DES
.. _NIST: http://csrc.nist.gov/publications/nistpubs/800-67/SP800-67.pdf

:undocumented: __package__
"""

import sys

from Crypto.Cipher import _create_cipher
from Crypto.Util.py3compat import byte_string, b, bchr, bord
from Crypto.Util._raw_api import (load_pycryptodome_raw_lib,
                                  VoidPointer, SmartPointer,
                                  c_size_t, expect_byte_string)

_raw_des3_lib = load_pycryptodome_raw_lib(
                    "Crypto.Cipher._raw_des3",
                    """
                    int DES3_start_operation(const uint8_t key[],
                                             size_t key_len,
                                             void **pResult);
                    int DES3_encrypt(const void *state,
                                     const uint8_t *in,
                                     uint8_t *out,
                                     size_t data_len);
                    int DES3_decrypt(const void *state,
                                     const uint8_t *in,
                                     uint8_t *out,
                                     size_t data_len);
                    int DES3_stop_operation(void *state);
                    """)


def adjust_key_parity(key):
    """Return the TDES key with parity bits correctly set"""

    def parity_byte(key_byte):
        parity = 1
        for i in xrange(1, 8):
            parity ^= (key_byte >> i) & 1
        return (key_byte & 0xFE) | parity

    if len(key) not in (16, 24):
        raise ValueError("Not a valid TDES key")

    result = b("").join([ bchr(parity_byte(bord(x)) )for x in key ])
    return result


def _create_base_cipher(dict_parameters):
    """This method instantiates and returns a handle to a low-level base cipher.
    It will absorb named parameters in the process."""

    try:
        key = dict_parameters.pop("key")
    except KeyError:
        raise TypeError("Missing 'key' parameter")

    expect_byte_string(key)

    if len(key) not in key_size:
        raise ValueError("Incorrect TDES key length (%d bytes)" % len(key))

    key = adjust_key_parity(key)
    if key[:8] == key[8:16] or key[-16:-8] == key[-8:]:
        raise ValueError("Triple DES key degenerates to single DES")

    start_operation = _raw_des3_lib.DES3_start_operation
    stop_operation = _raw_des3_lib.DES3_stop_operation

    cipher = VoidPointer()
    result = start_operation(key,
                             c_size_t(len(key)),
                             cipher.address_of())
    if result:
        raise ValueError("Error %X while instantiating the TDES cipher"
                         % result)
    return SmartPointer(cipher.get(), stop_operation)


def new(key, mode, *args, **kwargs):
    """Create a new TDES cipher

    :Parameters:
      key : byte string
        The secret key to use in the symmetric cipher.
        It must be 16 or 24 bytes long. The parity bits will be ignored.
        The condition K1 != K2 != K3 must hold.

      mode : a *MODE_** constant
        The chaining mode to use for encryption or decryption.

    :Keywords:
      iv : byte string
        (*Only* `MODE_CBC`, `MODE_CFB`, `MODE_OFB`, `MODE_OPENPGP`).

        The initialization vector to use for encryption or decryption.

        For `MODE_OPENPGP`, IV must be 8 bytes long for encryption
        and 10 bytes for decryption (in the latter case, it is
        actually the *encrypted* IV which was prefixed to the ciphertext).

        For all other modes, it must be 8 bytes long.

        If not provided, a random byte string will be generated (you can read
        it back via the ``iv`` attribute).

      nonce : byte string
        (*Only* `MODE_EAX` and `MODE_CTR`)
        A value that must never be reused for any other encryption.

        For `MODE_CTR`, its length must be in the range ``[0..7]``.

        For `MODE_EAX`, there are no restrictions, but it is recommended to
        use at least 16 bytes.

        If not provided for `MODE_EAX`, a random 16 byte string is generated
        (you can read it back via the ``nonce`` attribute).

      mac_len : integer
        (*Only* `MODE_EAX`). Length of the authentication tag, in bytes.
        It must be no larger than 8 (which is the default).

      segment_size : integer
        (*Only* `MODE_CFB`).The number of **bits** the plaintext and ciphertext
        are segmented in. It must be a multiple of 8.
        If not specified, it will be assumed to be 8.

      initial_value : integer
        (*Only* `MODE_CTR`). The initial value for the counter within
        the counter block. By default it is 0.

    :Attention: it is important that all 8 byte subkeys are different,
      otherwise TDES would degrade to single `DES`.

    :Raise ValueError:
      when the key degrades to Single DES.

    :Return: a DES cipher object, of the applicable mode.
    """

    return _create_cipher(sys.modules[__name__], key, mode, *args, **kwargs)

#: Electronic Code Book (ECB). See `Crypto.Cipher._mode_ecb.EcbMode`.
MODE_ECB = 1
#: Cipher-Block Chaining (CBC). See `Crypto.Cipher._mode_cbc.CbcMode`.
MODE_CBC = 2
#: Cipher FeedBack (CFB). See `Crypto.Cipher._mode_cfb.CfbMode`.
MODE_CFB = 3
#: Output FeedBack (OFB). See `Crypto.Cipher._mode_ofb.OfbMode`.
MODE_OFB = 5
#: CounTer Mode (CTR). See `Crypto.Cipher._mode_ctr.CtrMode`.
MODE_CTR = 6
#: OpenPGP Mode. See `Crypto.Cipher._mode_openpgp.OpenPgpMode`.
MODE_OPENPGP = 7
#: EAX Mode. See `Crypto.Cipher._mode_eax.EaxMode`.
MODE_EAX = 9

#: Size of a data block (in bytes)
block_size = 8
#: Size of a key (in bytes)
key_size = (16, 24)