# -*- coding: utf-8 -*-
"""The AES decrypter implementation."""
import pycaes
from dfvfs.encryption import decrypter
from dfvfs.encryption import manager
from dfvfs.lib import definitions
[docs]
class AESDecrypter(decrypter.Decrypter):
"""AES decrypter using pycaes."""
ENCRYPTION_METHOD = definitions.ENCRYPTION_METHOD_AES
_BLOCK_SIZE = 16
# TODO: add CFB and OFB support
_ENCRYPTION_MODES = frozenset([
definitions.ENCRYPTION_MODE_CBC,
definitions.ENCRYPTION_MODE_ECB])
[docs]
def __init__(
self, cipher_mode=None, initialization_vector=None, key=None, **kwargs):
"""Initializes a decrypter.
Args:
cipher_mode (Optional[str]): cipher mode.
initialization_vector (Optional[bytes]): initialization vector.
key (Optional[bytes]): key.
kwargs (dict): keyword arguments depending on the decrypter.
Raises:
ValueError: when key is not set or the cipher mode is not supported.
"""
if not key:
raise ValueError('Missing key.')
if cipher_mode not in self._ENCRYPTION_MODES:
raise ValueError(f'Unsupported cipher mode: {cipher_mode!s}')
if (cipher_mode != definitions.ENCRYPTION_MODE_ECB and
not initialization_vector):
raise ValueError('Missing initialization vector.')
if initialization_vector and not isinstance(initialization_vector, bytes):
raise TypeError('Unsupported initialization vector type.')
if initialization_vector and len(initialization_vector) != self._BLOCK_SIZE:
raise ValueError('Unsupported initialization vector size.')
super(AESDecrypter, self).__init__()
self._aes_context = pycaes.context()
self._cipher_mode = cipher_mode
self._initialization_vector = initialization_vector
self._aes_context.set_key(pycaes.crypt_modes.DECRYPT, key)
[docs]
def Decrypt(self, encrypted_data, finalize=False):
"""Decrypts the encrypted data.
Args:
encrypted_data (bytes): encrypted data.
finalize (Optional[bool]): True if the end of data has been reached and
the cipher context should be finalized.
Returns:
tuple[bytes, bytes]: decrypted data and remaining encrypted data.
"""
encrypted_data_size = len(encrypted_data)
_, block_offset = divmod(encrypted_data_size, self._BLOCK_SIZE)
remaining_encrypted_data = b''
if block_offset > 0:
if finalize:
block_padding_size = self._BLOCK_SIZE - block_offset
encrypted_data = b''.join([
encrypted_data, b'\x00' * block_padding_size])
else:
remaining_encrypted_data = encrypted_data[-block_offset:]
encrypted_data = encrypted_data[:-block_offset]
if self._cipher_mode == definitions.ENCRYPTION_MODE_CBC:
decrypted_data = pycaes.crypt_cbc(
self._aes_context, pycaes.crypt_modes.DECRYPT,
self._initialization_vector, encrypted_data)
self._initialization_vector = encrypted_data[-self._BLOCK_SIZE:]
elif self._cipher_mode == definitions.ENCRYPTION_MODE_ECB:
decrypted_data = pycaes.crypt_ecb(
self._aes_context, pycaes.crypt_modes.DECRYPT, encrypted_data)
decrypted_data_size = encrypted_data_size - block_offset
return decrypted_data[:decrypted_data_size], remaining_encrypted_data
manager.EncryptionManager.RegisterDecrypter(AESDecrypter)