Edit File: blockalgo.py
# -*- coding: utf-8 -*- # # Cipher/blockalgo.py # # =================================================================== # 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. # =================================================================== """Module with definitions common to all block ciphers.""" import sys if sys.version_info[0] == 2 and sys.version_info[1] == 1: from Crypto.Util.py21compat import * from Crypto.Util.py3compat import * #: *Electronic Code Book (ECB)*. #: This is the simplest encryption mode. Each of the plaintext blocks #: is directly encrypted into a ciphertext block, independently of #: any other block. This mode exposes frequency of symbols #: in your plaintext. Other modes (e.g. *CBC*) should be used instead. #: #: See `NIST SP800-38A`_ , Section 6.1 . #: #: .. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf MODE_ECB = 1 #: *Cipher-Block Chaining (CBC)*. Each of the ciphertext blocks depends #: on the current and all previous plaintext blocks. An Initialization Vector #: (*IV*) is required. #: #: The *IV* is a data block to be transmitted to the receiver. #: The *IV* can be made public, but it must be authenticated by the receiver and #: it should be picked randomly. #: #: See `NIST SP800-38A`_ , Section 6.2 . #: #: .. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf MODE_CBC = 2 #: *Cipher FeedBack (CFB)*. This mode is similar to CBC, but it transforms #: the underlying block cipher into a stream cipher. Plaintext and ciphertext #: are processed in *segments* of **s** bits. The mode is therefore sometimes #: labelled **s**-bit CFB. An Initialization Vector (*IV*) is required. #: #: When encrypting, each ciphertext segment contributes to the encryption of #: the next plaintext segment. #: #: This *IV* is a data block to be transmitted to the receiver. #: The *IV* can be made public, but it should be picked randomly. #: Reusing the same *IV* for encryptions done with the same key lead to #: catastrophic cryptographic failures. #: #: See `NIST SP800-38A`_ , Section 6.3 . #: #: .. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf MODE_CFB = 3 #: This mode should not be used. MODE_PGP = 4 #: *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 an Initialization Vector (*IV*). #: #: The *IV* is a data block to be transmitted to the receiver. #: The *IV* can be made public, but it should be picked randomly. #: #: Reusing the same *IV* for encryptions done with the same key lead to #: catastrophic cryptograhic failures. #: #: See `NIST SP800-38A`_ , Section 6.4 . #: #: .. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf MODE_OFB = 5 #: *CounTeR (CTR)*. This mode is very similar to ECB, in that #: encryption of one block is done independently of all other blocks. #: Unlike ECB, the block *position* contributes to the encryption and no #: information leaks about symbol frequency. #: #: Each message block is associated to a *counter* which must be unique #: across all messages that get encrypted with the same key (not just within #: the same message). The counter is as big as the block size. #: #: Counters can be generated in several ways. The most straightword one is #: to choose an *initial counter block* (which can be made public, similarly #: to the *IV* for the other modes) and increment its lowest **m** bits by #: one (modulo *2^m*) for each block. In most cases, **m** is chosen to be half #: the block size. #: #: Reusing the same *initial counter block* for encryptions done with the same #: key lead to catastrophic cryptograhic failures. #: #: See `NIST SP800-38A`_ , Section 6.5 (for the mode) and Appendix B (for how #: to manage the *initial counter block*). #: #: .. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf MODE_CTR = 6 #: OpenPGP. This mode is a variant of CFB, and it is only used in PGP and OpenPGP_ applications. #: An Initialization Vector (*IV*) is required. #: #: Unlike CFB, the IV is not transmitted to the receiver. Instead, the *encrypted* IV is. #: The IV is a random data block. Two of its bytes are duplicated to act as a checksum #: for the correctness of the key. The encrypted IV is therefore 2 bytes longer than #: the clean IV. #: #: .. _OpenPGP: http://tools.ietf.org/html/rfc4880 MODE_OPENPGP = 7 def _getParameter(name, index, args, kwargs, default=None): """Find a parameter in tuple and dictionary arguments a function receives""" param = kwargs.get(name) if len(args)>index: if param: raise ValueError("Parameter '%s' is specified twice" % name) param = args[index] return param or default class BlockAlgo: """Class modelling an abstract block cipher.""" def __init__(self, factory, key, *args, **kwargs): self.mode = _getParameter('mode', 0, args, kwargs, default=MODE_ECB) self.block_size = factory.block_size if self.mode != MODE_OPENPGP: self._cipher = factory.new(key, *args, **kwargs) self.IV = self._cipher.IV else: # OPENPGP mode. For details, see 13.9 in RCC4880. # # A few members are specifically created for this mode: # - _encrypted_iv, set in this constructor # - _done_first_block, set to True after the first encryption # - _done_last_block, set to True after a partial block is processed self._done_first_block = False self._done_last_block = False self.IV = _getParameter('iv', 1, args, kwargs) if not self.IV: raise ValueError("MODE_OPENPGP requires an IV") # Instantiate a temporary cipher to process the IV IV_cipher = factory.new(key, MODE_CFB, b('\x00')*self.block_size, # IV for CFB segment_size=self.block_size*8) # The cipher will be used for... if len(self.IV) == self.block_size: # ... encryption self._encrypted_IV = IV_cipher.encrypt( self.IV + self.IV[-2:] + # Plaintext b('\x00')*(self.block_size-2) # Padding )[:self.block_size+2] elif len(self.IV) == self.block_size+2: # ... decryption self._encrypted_IV = self.IV self.IV = IV_cipher.decrypt(self.IV + # Ciphertext b('\x00')*(self.block_size-2) # Padding )[:self.block_size+2] if self.IV[-2:] != self.IV[-4:-2]: raise ValueError("Failed integrity check for OPENPGP IV") self.IV = self.IV[:-2] else: raise ValueError("Length of IV must be %d or %d bytes for MODE_OPENPGP" % (self.block_size, self.block_size+2)) # Instantiate the cipher for the real PGP data self._cipher = factory.new(key, MODE_CFB, self._encrypted_IV[-self.block_size:], segment_size=self.block_size*8) def encrypt(self, plaintext): """Encrypt data with the key and the parameters set at initialization. The cipher object is stateful; encryption of a long block of data can be broken up in two or more calls to `encrypt()`. That is, the statement: >>> c.encrypt(a) + c.encrypt(b) is always equivalent to: >>> c.encrypt(a+b) That also means that you cannot reuse an object for encrypting or decrypting other data with the same key. This function does not perform any padding. - For `MODE_ECB`, `MODE_CBC`, and `MODE_OFB`, *plaintext* length (in bytes) must be a multiple of *block_size*. - For `MODE_CFB`, *plaintext* length (in bytes) must be a multiple of *segment_size*/8. - For `MODE_CTR`, *plaintext* can be of any length. - For `MODE_OPENPGP`, *plaintext* must be a multiple of *block_size*, unless it is the last chunk of the message. :Parameters: plaintext : byte string 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 with `MODE_OPENPGP`, the encypted IV is prepended to the returned ciphertext. """ if self.mode == MODE_OPENPGP: padding_length = (self.block_size - len(plaintext) % self.block_size) % self.block_size if padding_length>0: # CFB mode requires ciphertext to have length multiple of block size, # but PGP mode allows the last block to be shorter if self._done_last_block: raise ValueError("Only the last chunk is allowed to have length not multiple of %d bytes", self.block_size) self._done_last_block = True padded = plaintext + b('\x00')*padding_length res = self._cipher.encrypt(padded)[:len(plaintext)] else: res = self._cipher.encrypt(plaintext) if not self._done_first_block: res = self._encrypted_IV + res self._done_first_block = True return res return self._cipher.encrypt(plaintext) def decrypt(self, ciphertext): """Decrypt data with the key and the parameters set at initialization. The cipher object is stateful; decryption of a long block of data can be broken up in two or more calls to `decrypt()`. That is, the statement: >>> c.decrypt(a) + c.decrypt(b) is always equivalent to: >>> c.decrypt(a+b) That also means that you cannot reuse an object for encrypting or decrypting other data with the same key. This function does not perform any padding. - For `MODE_ECB`, `MODE_CBC`, and `MODE_OFB`, *ciphertext* length (in bytes) must be a multiple of *block_size*. - For `MODE_CFB`, *ciphertext* length (in bytes) must be a multiple of *segment_size*/8. - For `MODE_CTR`, *ciphertext* can be of any length. - For `MODE_OPENPGP`, *plaintext* must be a multiple of *block_size*, unless it is the last chunk of the message. :Parameters: ciphertext : byte string The piece of data to decrypt. :Return: the decrypted data (byte string, as long as *ciphertext*). """ if self.mode == MODE_OPENPGP: padding_length = (self.block_size - len(ciphertext) % self.block_size) % self.block_size if padding_length>0: # CFB mode requires ciphertext to have length multiple of block size, # but PGP mode allows the last block to be shorter if self._done_last_block: raise ValueError("Only the last chunk is allowed to have length not multiple of %d bytes", self.block_size) self._done_last_block = True padded = ciphertext + b('\x00')*padding_length res = self._cipher.decrypt(padded)[:len(ciphertext)] else: res = self._cipher.decrypt(ciphertext) return res return self._cipher.decrypt(ciphertext)