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_mode_ocb.py
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# =================================================================== # # Copyright (c) 2014, Legrandin <helderijs@gmail.com> # All rights reserved. # # Redistribution and use in source and binary forms, with or without # modification, are permitted provided that the following conditions # are met: # # 1. Redistributions of source code must retain the above copyright # notice, this list of conditions and the following disclaimer. # 2. Redistributions in binary form must reproduce the above copyright # notice, this list of conditions and the following disclaimer in # the documentation and/or other materials provided with the # distribution. # # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS # FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, # INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, # BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER # CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE # POSSIBILITY OF SUCH DAMAGE. # =================================================================== """ Offset Codebook (OCB) mode. OCB is Authenticated Encryption with Associated Data (AEAD) cipher mode designed by Prof. Phillip Rogaway and specified in `RFC7253`_. The algorithm provides both authenticity and privacy, it is very efficient, it uses only one key and it can be used in online mode (so that encryption or decryption can start before the end of the message is available). This module implements the third and last variant of OCB (OCB3) and it only works in combination with a 128-bit block symmetric cipher, like AES. OCB is patented in US but `free licenses`_ exist for software implementations meant for non-military purposes. Example: >>> from Crypto.Cipher import AES >>> from Crypto.Random import get_random_bytes >>> >>> key = get_random_bytes(32) >>> cipher = AES.new(key, AES.MODE_OCB) >>> plaintext = b"Attack at dawn" >>> ciphertext, mac = cipher.encrypt_and_digest(plaintext) >>> # Deliver cipher.nonce, ciphertext and mac ... >>> cipher = AES.new(key, AES.MODE_OCB, nonce=nonce) >>> try: >>> plaintext = cipher.decrypt_and_verify(ciphertext, mac) >>> except ValueError: >>> print "Invalid message" >>> else: >>> print plaintext :undocumented: __package__ .. _RFC7253: http://www.rfc-editor.org/info/rfc7253 .. _free licenses: http://web.cs.ucdavis.edu/~rogaway/ocb/license.htm """ from Crypto.Util.py3compat import b, bord, bchr, unhexlify from Crypto.Util.number import long_to_bytes, bytes_to_long from Crypto.Util.strxor import strxor from Crypto.Hash import BLAKE2s from Crypto.Random import get_random_bytes from Crypto.Util._raw_api import (load_pycryptodome_raw_lib, VoidPointer, create_string_buffer, get_raw_buffer, SmartPointer, c_size_t, expect_byte_string, ) _raw_ocb_lib = load_pycryptodome_raw_lib("Crypto.Cipher._raw_ocb", """ int OCB_start_operation(void *cipher, const uint8_t *offset_0, size_t offset_0_len, void **pState); int OCB_encrypt(void *state, const uint8_t *in, uint8_t *out, size_t data_len); int OCB_decrypt(void *state, const uint8_t *in, uint8_t *out, size_t data_len); int OCB_update(void *state, const uint8_t *in, size_t data_len); int OCB_digest(void *state, uint8_t *tag, size_t tag_len); int OCB_stop_operation(void *state); """) class OcbMode(object): """Offset Codebook (OCB) mode. :undocumented: __init__ """ def __init__(self, factory, nonce, mac_len, cipher_params): if factory.block_size != 16: raise ValueError("OCB mode is only available for ciphers" " that operate on 128 bits blocks") self.block_size = 16 """The block size of the underlying cipher, in bytes.""" self.nonce = nonce """Nonce used for this session.""" if len(nonce) not in range(1, 16): raise ValueError("Nonce must be at most 15 bytes long") self._mac_len = mac_len if not 8 <= mac_len <= 16: raise ValueError("MAC tag must be between 8 and 16 bytes long") # Cache for MAC tag self._mac_tag = None # Cache for unaligned associated data self._cache_A = b("") # Cache for unaligned ciphertext/plaintext self._cache_P = b("") # Allowed transitions after initialization self._next = [self.update, self.encrypt, self.decrypt, self.digest, self.verify] # Compute Offset_0 params_without_key = dict(cipher_params) key = params_without_key.pop("key") nonce = (bchr(self._mac_len << 4 & 0xFF) + bchr(0) * (14 - len(self.nonce)) + bchr(1) + self.nonce) bottom = bord(nonce[15]) & 0x3F # 6 bits, 0..63 ktop = factory.new(key, factory.MODE_ECB, **params_without_key)\ .encrypt(nonce[:15] + bchr(bord(nonce[15]) & 0xC0)) stretch = ktop + strxor(ktop[:8], ktop[1:9]) # 192 bits offset_0 = long_to_bytes(bytes_to_long(stretch) >> (64 - bottom), 24)[8:] # Create low-level cipher instance raw_cipher = factory._create_base_cipher(cipher_params) if cipher_params: raise TypeError("Unknown keywords: " + str(cipher_params)) self._state = VoidPointer() result = _raw_ocb_lib.OCB_start_operation(raw_cipher.get(), offset_0, c_size_t(len(offset_0)), self._state.address_of()) if result: raise ValueError("Error %d while instantiating the OCB 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_ocb_lib.OCB_stop_operation) # Memory allocated for the underlying block cipher is now owed # by the cipher mode raw_cipher.release() def _update(self, assoc_data, assoc_data_len): expect_byte_string(assoc_data) result = _raw_ocb_lib.OCB_update(self._state.get(), assoc_data, c_size_t(assoc_data_len)) if result: raise ValueError("Error %d while MAC-ing in OCB mode" % result) def update(self, assoc_data): """Process the associated data. If there is any associated data, the caller has to invoke this method one or more times, before using ``decrypt`` or ``encrypt``. By *associated data* it is meant any data (e.g. packet headers) that will not be encrypted and will be transmitted in the clear. However, the receiver shall still able to detect modifications. If there is no associated data, this method must not be called. The caller may split associated data in segments of any size, and invoke this method multiple times, each time with the next segment. :Parameters: assoc_data : byte string A piece of associated data. """ if self.update not in self._next: raise TypeError("update() can only be called" " immediately after initialization") self._next = [self.encrypt, self.decrypt, self.digest, self.verify, self.update] if len(self._cache_A) > 0: filler = min(16 - len(self._cache_A), len(assoc_data)) self._cache_A += assoc_data[:filler] assoc_data = assoc_data[filler:] if len(self._cache_A) < 16: return self # Clear the cache, and proceeding with any other aligned data self._cache_A, seg = b(""), self._cache_A self.update(seg) update_len = len(assoc_data) // 16 * 16 self._cache_A = assoc_data[update_len:] self._update(assoc_data, update_len) return self def _transcrypt_aligned(self, in_data, in_data_len, trans_func, trans_desc): out_data = create_string_buffer(in_data_len) result = trans_func(self._state.get(), in_data, out_data, c_size_t(in_data_len)) if result: raise ValueError("Error %d while %sing in OCB mode" % (result, trans_desc)) return get_raw_buffer(out_data) def _transcrypt(self, in_data, trans_func, trans_desc): # Last piece to encrypt/decrypt if in_data is None: out_data = self._transcrypt_aligned(self._cache_P, len(self._cache_P), trans_func, trans_desc) self._cache_P = b("") return out_data # Try to fill up the cache, if it already contains something expect_byte_string(in_data) prefix = b("") if len(self._cache_P) > 0: filler = min(16 - len(self._cache_P), len(in_data)) self._cache_P += in_data[:filler] in_data = in_data[filler:] if len(self._cache_P) < 16: # We could not manage to fill the cache, so there is certainly # no output yet. return b("") # Clear the cache, and proceeding with any other aligned data prefix = self._transcrypt_aligned(self._cache_P, len(self._cache_P), trans_func, trans_desc) self._cache_P = b("") # Process data in multiples of the block size trans_len = len(in_data) // 16 * 16 result = self._transcrypt_aligned(in_data, trans_len, trans_func, trans_desc) if prefix: result = prefix + result # Left-over self._cache_P = in_data[trans_len:] return result def encrypt(self, plaintext=None): """Encrypt the next piece of plaintext. After the entire plaintext has been passed (but before `digest`), you **must** call this method one last time with no arguments to collect the final piece of ciphertext. If possible, use the method `encrypt_and_digest` instead. :Parameters: plaintext : byte string The next piece of data to encrypt or ``None`` to signify that encryption has finished and that any remaining ciphertext has to be produced. :Return: the ciphertext, as a byte string. Its length may not match the length of the *plaintext*. """ if self.encrypt not in self._next: raise TypeError("encrypt() can only be called after" " initialization or an update()") if plaintext is None: self._next = [self.digest] else: self._next = [self.encrypt] return self._transcrypt(plaintext, _raw_ocb_lib.OCB_encrypt, "encrypt") def decrypt(self, ciphertext=None): """Decrypt the next piece of ciphertext. After the entire ciphertext has been passed (but before `verify`), you **must** call this method one last time with no arguments to collect the remaining piece of plaintext. If possible, use the method `decrypt_and_verify` instead. :Parameters: ciphertext : byte string The next piece of data to decrypt or ``None`` to signify that decryption has finished and that any remaining plaintext has to be produced. :Return: the plaintext, as a byte string. Its length may not match the length of the *ciphertext*. """ if self.decrypt not in self._next: raise TypeError("decrypt() can only be called after" " initialization or an update()") if ciphertext is None: self._next = [self.verify] else: self._next = [self.decrypt] return self._transcrypt(ciphertext, _raw_ocb_lib.OCB_decrypt, "decrypt") def _compute_mac_tag(self): if self._mac_tag is not None: return if self._cache_A: self._update(self._cache_A, len(self._cache_A)) self._cache_A = b("") mac_tag = create_string_buffer(16) result = _raw_ocb_lib.OCB_digest(self._state.get(), mac_tag, c_size_t(len(mac_tag)) ) if result: raise ValueError("Error %d while computing digest in OCB mode" % result) self._mac_tag = get_raw_buffer(mac_tag)[:self._mac_len] def digest(self): """Compute the *binary* MAC tag. Call this method after the final `encrypt` (the one with no arguments) to obtain the MAC tag. The MAC tag is needed by the receiver to determine authenticity of the message. :Return: the MAC, as a byte string. """ if self.digest not in self._next: raise TypeError("digest() cannot be called now for this cipher") assert(len(self._cache_P) == 0) self._next = [self.digest] if self._mac_tag is None: self._compute_mac_tag() return self._mac_tag def hexdigest(self): """Compute the *printable* MAC tag. This method is like `digest`. :Return: the MAC, as a hexadecimal string. """ return "".join(["%02x" % bord(x) for x in self.digest()]) def verify(self, received_mac_tag): """Validate the *binary* MAC tag. Call this method after the final `decrypt` (the one with no arguments) to check if the message is authentic and valid. :Parameters: received_mac_tag : byte string This is the *binary* MAC, as received from the sender. :Raises ValueError: if the MAC does not match. The message has been tampered with or the key is incorrect. """ if self.verify not in self._next: raise TypeError("verify() cannot be called now for this cipher") assert(len(self._cache_P) == 0) self._next = [self.verify] if self._mac_tag is None: self._compute_mac_tag() secret = get_random_bytes(16) mac1 = BLAKE2s.new(digest_bits=160, key=secret, data=self._mac_tag) mac2 = BLAKE2s.new(digest_bits=160, key=secret, data=received_mac_tag) if mac1.digest() != mac2.digest(): raise ValueError("MAC check failed") def hexverify(self, hex_mac_tag): """Validate the *printable* MAC tag. This method is like `verify`. :Parameters: hex_mac_tag : string This is the *printable* MAC, as received from the sender. :Raises ValueError: if the MAC does not match. The message has been tampered with or the key is incorrect. """ self.verify(unhexlify(hex_mac_tag)) def encrypt_and_digest(self, plaintext): """Encrypt the message and create the MAC tag in one step. :Parameters: plaintext : byte string The entire message to encrypt. :Return: a tuple with two byte strings: - the encrypted data - the MAC """ return self.encrypt(plaintext) + self.encrypt(), self.digest() def decrypt_and_verify(self, ciphertext, received_mac_tag): """Decrypted the message and verify its authenticity in one step. :Parameters: ciphertext : byte string The entire message to decrypt. received_mac_tag : byte string This is the *binary* MAC, as received from the sender. :Return: the decrypted data (byte string). :Raises ValueError: if the MAC does not match. The message has been tampered with or the key is incorrect. """ plaintext = self.decrypt(ciphertext) + self.decrypt() self.verify(received_mac_tag) return plaintext def _create_ocb_cipher(factory, **kwargs): """Create a new block cipher, configured in OCB mode. :Parameters: factory : module A symmetric cipher module from `Crypto.Cipher` (like `Crypto.Cipher.AES`). :Keywords: nonce : byte string A value that must never be reused for any other encryption. Its length can vary from 1 to 15 bytes. If not specified, a random 15 bytes long nonce is generated. mac_len : integer Length of the MAC, in bytes. It must be in the range ``[8..16]``. The default is 16 (128 bits). 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). """ try: nonce = kwargs.pop("nonce", None) if nonce is None: nonce = get_random_bytes(15) mac_len = kwargs.pop("mac_len", 16) except KeyError, e: raise TypeError("Keyword missing: " + str(e)) return OcbMode(factory, nonce, mac_len, kwargs)