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# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
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# notice, this list of conditions and the following disclaimer.
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# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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"""
OpenPGP mode.
"""
__all__ = ['OpenPgpMode']
from Crypto.Util.py3compat import _copy_bytes
from Crypto.Random import get_random_bytes
class OpenPgpMode(object):
"""OpenPGP mode.
This mode is a variant of CFB, and it is only used in PGP and
OpenPGP_ applications. If in doubt, use another mode.
An Initialization Vector (*IV*) is required.
Unlike CFB, the *encrypted* IV (not the IV itself) is
transmitted to the receiver.
The IV is a random data block. For legacy reasons, two of its bytes are
duplicated to act as a checksum for the correctness of the key, which is now
known to be insecure and is ignored. The encrypted IV is therefore 2 bytes
longer than the clean IV.
.. _OpenPGP: http://tools.ietf.org/html/rfc4880
:undocumented: __init__
"""
def __init__(self, factory, key, iv, cipher_params):
#: The block size of the underlying cipher, in bytes.
self.block_size = factory.block_size
self._done_first_block = False # True after the first encryption
# Instantiate a temporary cipher to process the IV
IV_cipher = factory.new(
key,
factory.MODE_CFB,
IV=b'\x00' * self.block_size,
segment_size=self.block_size * 8,
**cipher_params)
iv = _copy_bytes(None, None, iv)
# The cipher will be used for...
if len(iv) == self.block_size:
# ... encryption
self._encrypted_IV = IV_cipher.encrypt(iv + iv[-2:])
elif len(iv) == self.block_size + 2:
# ... decryption
self._encrypted_IV = iv
# Last two bytes are for a deprecated "quick check" feature that
# should not be used. (https://eprint.iacr.org/2005/033)
iv = IV_cipher.decrypt(iv)[:-2]
else:
raise ValueError("Length of IV must be %d or %d bytes"
" for MODE_OPENPGP"
% (self.block_size, self.block_size + 2))
self.iv = self.IV = iv
# Instantiate the cipher for the real PGP data
self._cipher = factory.new(
key,
factory.MODE_CFB,
IV=self._encrypted_IV[-self.block_size:],
segment_size=self.block_size * 8,
**cipher_params)
def encrypt(self, plaintext):
"""Encrypt data with the key and the parameters set at initialization.
A cipher object is stateful: once you have encrypted a message
you cannot encrypt (or decrypt) another message using the same
object.
The data to encrypt can be broken up in two or
more pieces and `encrypt` can be called multiple times.
That is, the statement:
>>> c.encrypt(a) + c.encrypt(b)
is equivalent to:
>>> c.encrypt(a+b)
This function does not add any padding to the plaintext.
:Parameters:
plaintext : bytes/bytearray/memoryview
The piece of data to encrypt.
:Return:
the encrypted data, as a byte string.
It is as long as *plaintext* with one exception:
when encrypting the first message chunk,
the encypted IV is prepended to the returned ciphertext.
"""
res = self._cipher.encrypt(plaintext)
if not self._done_first_block:
res = self._encrypted_IV + res
self._done_first_block = True
return res
def decrypt(self, ciphertext):
"""Decrypt data with the key and the parameters set at initialization.
A cipher object is stateful: once you have decrypted a message
you cannot decrypt (or encrypt) another message with the same
object.
The data to decrypt can be broken up in two or
more pieces and `decrypt` can be called multiple times.
That is, the statement:
>>> c.decrypt(a) + c.decrypt(b)
is equivalent to:
>>> c.decrypt(a+b)
This function does not remove any padding from the plaintext.
:Parameters:
ciphertext : bytes/bytearray/memoryview
The piece of data to decrypt.
:Return: the decrypted data (byte string).
"""
return self._cipher.decrypt(ciphertext)
def _create_openpgp_cipher(factory, **kwargs):
"""Create a new block cipher, configured in OpenPGP mode.
:Parameters:
factory : module
The module.
:Keywords:
key : bytes/bytearray/memoryview
The secret key to use in the symmetric cipher.
IV : bytes/bytearray/memoryview
The initialization vector to use for encryption or decryption.
For encryption, the IV must be as long as the cipher block size.
For decryption, it must be 2 bytes longer (it is actually the
*encrypted* IV which was prefixed to the ciphertext).
"""
iv = kwargs.pop("IV", None)
IV = kwargs.pop("iv", None)
if (None, None) == (iv, IV):
iv = get_random_bytes(factory.block_size)
if iv is not None:
if IV is not None:
raise TypeError("You must either use 'iv' or 'IV', not both")
else:
iv = IV
try:
key = kwargs.pop("key")
except KeyError as e:
raise TypeError("Missing component: " + str(e))
return OpenPgpMode(factory, key, iv, kwargs)
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