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_mode_ofb.py
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# -*- coding: utf-8 -*- # # Cipher/mode_ofb.py : OFB mode # # =================================================================== # The contents of this file are dedicated to the public domain. To # the extent that dedication to the public domain is not available, # everyone is granted a worldwide, perpetual, royalty-free, # non-exclusive license to exercise all rights associated with the # contents of this file for any purpose whatsoever. # No rights are reserved. # # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, # EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF # MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND # NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS # BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN # ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE # SOFTWARE. # =================================================================== """ Output Feedback (CFB) mode. """ __all__ = ['OfbMode'] from Crypto.Util.py3compat import _copy_bytes from Crypto.Util._raw_api import (load_pycryptodome_raw_lib, VoidPointer, create_string_buffer, get_raw_buffer, SmartPointer, c_size_t, c_uint8_ptr, is_writeable_buffer) from Crypto.Random import get_random_bytes raw_ofb_lib = load_pycryptodome_raw_lib("Crypto.Cipher._raw_ofb", """ int OFB_start_operation(void *cipher, const uint8_t iv[], size_t iv_len, void **pResult); int OFB_encrypt(void *ofbState, const uint8_t *in, uint8_t *out, size_t data_len); int OFB_decrypt(void *ofbState, const uint8_t *in, uint8_t *out, size_t data_len); int OFB_stop_operation(void *state); """ ) class OfbMode(object): """*Output FeedBack (OFB)*. This mode is very similar to CBC, but it transforms the underlying block cipher into a stream cipher. The keystream is the iterated block encryption of the previous ciphertext block. An Initialization Vector (*IV*) is required. See `NIST SP800-38A`_ , Section 6.4. .. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf :undocumented: __init__ """ def __init__(self, block_cipher, iv): """Create a new block cipher, configured in OFB mode. :Parameters: block_cipher : C pointer A smart pointer to the low-level block cipher instance. iv : bytes/bytearray/memoryview The initialization vector to use for encryption or decryption. It is as long as the cipher block. **The IV must be a nonce, to to be reused for any other message**. It shall be a nonce or a random value. Reusing the *IV* for encryptions performed with the same key compromises confidentiality. """ self._state = VoidPointer() result = raw_ofb_lib.OFB_start_operation(block_cipher.get(), c_uint8_ptr(iv), c_size_t(len(iv)), self._state.address_of()) if result: raise ValueError("Error %d while instantiating the OFB mode" % result) # Ensure that object disposal of this Python object will (eventually) # free the memory allocated by the raw library for the cipher mode self._state = SmartPointer(self._state.get(), raw_ofb_lib.OFB_stop_operation) # Memory allocated for the underlying block cipher is now owed # by the cipher mode block_cipher.release() self.block_size = len(iv) """The block size of the underlying cipher, in bytes.""" self.iv = _copy_bytes(None, None, iv) """The Initialization Vector originally used to create the object. The value does not change.""" self.IV = self.iv """Alias for `iv`""" self._next = ["encrypt", "decrypt"] def encrypt(self, plaintext, output=None): """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. It can be of any length. :Keywords: output : bytearray/memoryview The location where the ciphertext must be written to. If ``None``, the ciphertext is returned. :Return: If ``output`` is ``None``, the ciphertext is returned as ``bytes``. Otherwise, ``None``. """ if "encrypt" not in self._next: raise TypeError("encrypt() cannot be called after decrypt()") self._next = ["encrypt"] if output is None: ciphertext = create_string_buffer(len(plaintext)) else: ciphertext = output if not is_writeable_buffer(output): raise TypeError("output must be a bytearray or a writeable memoryview") if len(plaintext) != len(output): raise ValueError("output must have the same length as the input" " (%d bytes)" % len(plaintext)) result = raw_ofb_lib.OFB_encrypt(self._state.get(), c_uint8_ptr(plaintext), c_uint8_ptr(ciphertext), c_size_t(len(plaintext))) if result: raise ValueError("Error %d while encrypting in OFB mode" % result) if output is None: return get_raw_buffer(ciphertext) else: return None def decrypt(self, ciphertext, output=None): """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. It can be of any length. :Keywords: output : bytearray/memoryview The location where the plaintext is written to. If ``None``, the plaintext is returned. :Return: If ``output`` is ``None``, the plaintext is returned as ``bytes``. Otherwise, ``None``. """ if "decrypt" not in self._next: raise TypeError("decrypt() cannot be called after encrypt()") self._next = ["decrypt"] if output is None: plaintext = create_string_buffer(len(ciphertext)) else: plaintext = output if not is_writeable_buffer(output): raise TypeError("output must be a bytearray or a writeable memoryview") if len(ciphertext) != len(output): raise ValueError("output must have the same length as the input" " (%d bytes)" % len(plaintext)) result = raw_ofb_lib.OFB_decrypt(self._state.get(), c_uint8_ptr(ciphertext), c_uint8_ptr(plaintext), c_size_t(len(ciphertext))) if result: raise ValueError("Error %d while decrypting in OFB mode" % result) if output is None: return get_raw_buffer(plaintext) else: return None def _create_ofb_cipher(factory, **kwargs): """Instantiate a cipher object that performs OFB encryption/decryption. :Parameters: factory : module The underlying block cipher, a module from ``Crypto.Cipher``. :Keywords: iv : bytes/bytearray/memoryview The IV to use for OFB. IV : bytes/bytearray/memoryview Alias for ``iv``. Any other keyword will be passed to the underlying block cipher. See the relevant documentation for details (at least ``key`` will need to be present). """ cipher_state = factory._create_base_cipher(kwargs) 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 if len(iv) != factory.block_size: raise ValueError("Incorrect IV length (it must be %d bytes long)" % factory.block_size) if kwargs: raise TypeError("Unknown parameters for OFB: %s" % str(kwargs)) return OfbMode(cipher_state, iv)