# -*- coding: utf-8 -*- # # Hash/CMAC.py - Implements the CMAC algorithm # # =================================================================== # 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. # =================================================================== """CMAC (Cipher-based Message Authentication Code) algorithm CMAC is a MAC defined in `NIST SP 800-38B`_ and in RFC4493_ (for AES only) and constructed using a block cipher. It was originally known as `OMAC1`_. The algorithm is sometimes named *X-CMAC* where *X* is the name of the cipher (e.g. AES-CMAC). This is an example showing how to *create* an AES-CMAC: >>> from Crypto.Hash import CMAC >>> from Crypto.Cipher import AES >>> >>> secret = b'Sixteen byte key' >>> cobj = CMAC.new(secret, ciphermod=AES) >>> cobj.update(b'Hello') >>> print cobj.hexdigest() And this is an example showing how to *check* an AES-CMAC: >>> from Crypto.Hash import CMAC >>> from Crypto.Cipher import AES >>> >>> # We have received a message 'msg' together >>> # with its MAC 'mac' >>> >>> secret = b'Sixteen byte key' >>> cobj = CMAC.new(secret, ciphermod=AES) >>> cobj.update(msg) >>> try: >>> cobj.verify(mac) >>> print "The message '%s' is authentic" % msg >>> except ValueError: >>> print "The message or the key is wrong" A cipher block size of 128 bits (like for AES) guarantees that the risk of MAC collisions remains negligeable even when the same CMAC key is used to authenticate a large amount of data (2^22 Gbytes). This implementation allows also usage of ciphers with a 64 bits block size (like TDES) for legacy purposes only. However, the risk is much higher and one CMAC key should be rotated after as little as 16 MBytes (in total) have been authenticated. .. _`NIST SP 800-38B`: http://csrc.nist.gov/publications/nistpubs/800-38B/SP_800-38B.pdf .. _RFC4493: http://www.ietf.org/rfc/rfc4493.txt .. _OMAC1: http://www.nuee.nagoya-u.ac.jp/labs/tiwata/omac/omac.html """ from Crypto.Util.py3compat import b, bchr, bord, tobytes from binascii import unhexlify from Crypto.Hash import BLAKE2s from Crypto.Util.strxor import strxor from Crypto.Util.number import long_to_bytes, bytes_to_long from Crypto.Random import get_random_bytes #: The size of the authentication tag produced by the MAC. digest_size = None def _shift_bytes(bs, xor_lsb=0): num = (bytes_to_long(bs) << 1) ^ xor_lsb return long_to_bytes(num, len(bs))[-len(bs):] class CMAC(object): """Class that implements CMAC""" #: The size of the authentication tag produced by the MAC. digest_size = None def __init__(self, key, msg=None, ciphermod=None, cipher_params=None): """Create a new CMAC object. :Parameters: key : byte string secret key for the CMAC object. The key must be valid for the underlying cipher algorithm. For instance, it must be 16 bytes long for AES-128. msg : byte string The very first chunk of the message to authenticate. It is equivalent to an early call to `update`. Optional. ciphermod : module A cipher module from `Crypto.Cipher`. The cipher's block size has to be 128 bits. It is recommended to use `Crypto.Cipher.AES`. cipher_params : dictionary Extra keywords to use when creating a new cipher. """ if ciphermod is None: raise TypeError("ciphermod must be specified (try AES)") self._key = key self._factory = ciphermod if cipher_params is None: self._cipher_params = {} else: self._cipher_params = dict(cipher_params) # Section 5.3 of NIST SP 800 38B and Appendix B if ciphermod.block_size == 8: const_Rb = 0x1B self._max_size = 8 * (2 ** 21) elif ciphermod.block_size == 16: const_Rb = 0x87 self._max_size = 16 * (2 ** 48) else: raise TypeError("CMAC requires a cipher with a block size" "of 8 or 16 bytes, not %d" % (ciphermod.block_size,)) # Size of the final MAC tag, in bytes self.digest_size = ciphermod.block_size self._mac_tag = None # Compute sub-keys zero_block = bchr(0) * ciphermod.block_size cipher = ciphermod.new(key, ciphermod.MODE_ECB, **self._cipher_params) l = cipher.encrypt(zero_block) if bord(l[0]) & 0x80: self._k1 = _shift_bytes(l, const_Rb) else: self._k1 = _shift_bytes(l) if bord(self._k1[0]) & 0x80: self._k2 = _shift_bytes(self._k1, const_Rb) else: self._k2 = _shift_bytes(self._k1) # Initialize CBC cipher with zero IV self._cbc = ciphermod.new(key, ciphermod.MODE_CBC, zero_block, **self._cipher_params) # Cache for outstanding data to authenticate self._cache = b("") # Last two pieces of ciphertext produced self._last_ct = self._last_pt = zero_block self._before_last_ct = None # Counter for total message size self._data_size = 0 if msg: self.update(msg) def update(self, msg): """Continue authentication of a message by consuming the next chunk of data. Repeated calls are equivalent to a single call with the concatenation of all the arguments. In other words: >>> m.update(a); m.update(b) is equivalent to: >>> m.update(a+b) :Parameters: msg : byte string The next chunk of the message being authenticated """ self._data_size += len(msg) if len(self._cache) > 0: filler = min(self.digest_size - len(self._cache), len(msg)) self._cache += msg[:filler] if len(self._cache) < self.digest_size: return self msg = msg[filler:] self._update(self._cache) self._cache = b("") update_len, remain = divmod(len(msg), self.digest_size) update_len *= self.digest_size if remain > 0: self._update(msg[:update_len]) self._cache = msg[update_len:] else: self._update(msg) self._cache = b("") return self def _update(self, data_block): """Update a block aligned to the block boundary""" if len(data_block) == 0: return assert len(data_block) % self.digest_size == 0 ct = self._cbc.encrypt(data_block) if len(data_block) == self.digest_size: self._before_last_ct = self._last_ct else: self._before_last_ct = ct[-self.digest_size * 2:-self.digest_size] self._last_ct = ct[-self.digest_size:] self._last_pt = data_block[-self.digest_size:] def copy(self): """Return a copy ("clone") of the MAC object. The copy will have the same internal state as the original MAC object. This can be used to efficiently compute the MAC of strings that share a common initial substring. :Returns: A `CMAC` object """ obj = CMAC(self._key, ciphermod=self._factory, cipher_params=self._cipher_params) obj._cbc = self._factory.new(self._key, self._factory.MODE_CBC, self._last_ct, **self._cipher_params) for m in ['_mac_tag', '_last_ct', '_before_last_ct', '_cache', '_data_size', '_max_size']: setattr(obj, m, getattr(self, m)) return obj def digest(self): """Return the **binary** (non-printable) MAC of the message that has been authenticated so far. This method does not change the state of the MAC object. You can continue updating the object after calling this function. :Return: A byte string of `digest_size` bytes. It may contain non-ASCII characters, including null bytes. """ if self._mac_tag is not None: return self._mac_tag if self._data_size > self._max_size: raise ValueError("MAC is unsafe for this message") if len(self._cache) == 0 and self._before_last_ct is not None: ## Last block was full pt = strxor(strxor(self._before_last_ct, self._k1), self._last_pt) else: ## Last block is partial (or message length is zero) ext = self._cache + bchr(0x80) +\ bchr(0) * (self.digest_size - len(self._cache) - 1) pt = strxor(strxor(self._last_ct, self._k2), ext) cipher = self._factory.new(self._key, self._factory.MODE_ECB, **self._cipher_params) self._mac_tag = cipher.encrypt(pt) return self._mac_tag def hexdigest(self): """Return the **printable** MAC of the message that has been authenticated so far. This method does not change the state of the MAC object. :Return: A string of 2* `digest_size` bytes. It contains only hexadecimal ASCII digits. """ return "".join(["%02x" % bord(x) for x in tuple(self.digest())]) def verify(self, mac_tag): """Verify that a given **binary** MAC (computed by another party) is valid. :Parameters: mac_tag : byte string The expected MAC of the message. :Raises ValueError: if the MAC does not match. It means that the message has been tampered with or that the MAC key is incorrect. """ secret = get_random_bytes(16) mac1 = BLAKE2s.new(digest_bits=160, key=secret, data=mac_tag) mac2 = BLAKE2s.new(digest_bits=160, key=secret, data=self.digest()) if mac1.digest() != mac2.digest(): raise ValueError("MAC check failed") def hexverify(self, hex_mac_tag): """Verify that a given **printable** MAC (computed by another party) is valid. :Parameters: hex_mac_tag : string The expected MAC of the message, as a hexadecimal string. :Raises ValueError: if the MAC does not match. It means that the message has been tampered with or that the MAC key is incorrect. """ self.verify(unhexlify(tobytes(hex_mac_tag))) def new(key, msg=None, ciphermod=None, cipher_params=None): """Create a new CMAC object. :Parameters: key : byte string secret key for the CMAC object. The key must be valid for the underlying cipher algorithm. For instance, it must be 16 bytes long for AES-128. msg : byte string The very first chunk of the message to authenticate. It is equivalent to an early call to `CMAC.update`. Optional. ciphermod : module A cipher module from `Crypto.Cipher`. The cipher's block size has to be 128 bits, like `Crypto.Cipher.AES`, to reduce the probability of collisions. :Returns: A `CMAC` object """ return CMAC(key, msg, ciphermod, cipher_params)