265 lines
8.6 KiB
Python
265 lines
8.6 KiB
Python
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#
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# HMAC.py - Implements the HMAC algorithm as described by RFC 2104.
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#
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# ===================================================================
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#
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# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
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# All rights reserved.
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#
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# Redistribution and use in source and binary forms, with or without
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# modification, are permitted provided that the following conditions
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# are met:
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#
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# 1. Redistributions of source code must retain the above copyright
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# notice, this list of conditions and the following disclaimer.
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# 2. Redistributions in binary form must reproduce the above copyright
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# notice, this list of conditions and the following disclaimer in
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# the documentation and/or other materials provided with the
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# distribution.
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#
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# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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# COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
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# ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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# POSSIBILITY OF SUCH DAMAGE.
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# ===================================================================
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"""HMAC (Hash-based Message Authentication Code) algorithm
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HMAC is a MAC defined in RFC2104_ and FIPS-198_ and constructed using
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a cryptograpic hash algorithm.
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It is usually named *HMAC-X*, where *X* is the hash algorithm; for
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instance *HMAC-SHA1* or *HMAC-MD5*.
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The strength of an HMAC depends on:
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- the strength of the hash algorithm
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- the length and entropy of the secret key
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This is an example showing how to *create* a MAC:
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>>> from Crypto.Hash import HMAC
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>>>
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>>> secret = b'Swordfish'
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>>> h = HMAC.new(secret)
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>>> h.update(b'Hello')
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>>> print h.hexdigest()
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This is an example showing how to *check* a MAC:
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>>> from Crypto.Hash import HMAC
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>>>
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>>> # We have received a message 'msg' together
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>>> # with its MAC 'mac'
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>>>
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>>> secret = b'Swordfish'
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>>> h = HMAC.new(secret)
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>>> h.update(msg)
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>>> try:
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>>> h.verify(mac)
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>>> print "The message '%s' is authentic" % msg
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>>> except ValueError:
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>>> print "The message or the key is wrong"
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.. _RFC2104: http://www.ietf.org/rfc/rfc2104.txt
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.. _FIPS-198: http://csrc.nist.gov/publications/fips/fips198/fips-198-1_final.pdf
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"""
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__all__ = ['new', 'HMAC']
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from Crypto.Util.py3compat import b, bchr, bord, tobytes
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from binascii import unhexlify
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from . import MD5, BLAKE2s
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from Crypto.Util.strxor import strxor
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from Crypto.Random import get_random_bytes
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class HMAC:
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"""Class that implements HMAC"""
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def __init__(self, key, msg=b(""), digestmod=None):
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"""Create a new HMAC object.
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:Parameters:
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key : byte string
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secret key for the MAC object.
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It must be long enough to match the expected security level of the
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MAC. However, there is no benefit in using keys longer than the
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`digest_size` of the underlying hash algorithm.
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msg : byte string
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The very first chunk of the message to authenticate.
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It is equivalent to an early call to `update()`. Optional.
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:Parameter digestmod:
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The hash algorithm the HMAC is based on.
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Default is `Crypto.Hash.MD5`.
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:Type digestmod:
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A hash module or object instantiated from `Crypto.Hash`
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"""
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if digestmod is None:
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digestmod = MD5
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if msg is None:
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msg = b("")
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#: Size of the MAC tag
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self.digest_size = digestmod.digest_size
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self._digestmod = digestmod
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try:
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if len(key) <= digestmod.block_size:
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# Step 1 or 2
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key_0 = key + bchr(0) * (digestmod.block_size - len(key))
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else:
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# Step 3
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hash_k = digestmod.new(key).digest()
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key_0 = hash_k + bchr(0) * (digestmod.block_size - len(hash_k))
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except AttributeError:
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# Not all hash types have "block_size"
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raise ValueError("Hash type incompatible to HMAC")
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# Step 4
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key_0_ipad = strxor(key_0, bchr(0x36) * len(key_0))
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# Start step 5 and 6
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self._inner = digestmod.new(key_0_ipad)
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self._inner.update(msg)
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# Step 7
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key_0_opad = strxor(key_0, bchr(0x5c) * len(key_0))
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# Start step 8 and 9
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self._outer = digestmod.new(key_0_opad)
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def update(self, msg):
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"""Continue authentication of a message by consuming the next
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chunk of data.
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Repeated calls are equivalent to a single call with the concatenation
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of all the arguments. In other words:
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>>> m.update(a); m.update(b)
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is equivalent to:
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>>> m.update(a+b)
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:Parameters:
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msg : byte string
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The next chunk of the message being authenticated
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"""
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self._inner.update(msg)
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def copy(self):
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"""Return a copy ("clone") of the MAC object.
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The copy will have the same internal state as the original MAC
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object.
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This can be used to efficiently compute the MAC of strings that
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share a common initial substring.
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:Returns: An `HMAC` object
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"""
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new_hmac = HMAC(b("fake key"), digestmod=self._digestmod)
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# Syncronize the state
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new_hmac._inner = self._inner.copy()
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new_hmac._outer = self._outer.copy()
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return new_hmac
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def digest(self):
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"""Return the **binary** (non-printable) MAC of the message that has
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been authenticated so far.
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This method does not change the state of the MAC object.
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You can continue updating the object after calling this function.
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:Return: A byte string of `digest_size` bytes. It may contain non-ASCII
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characters, including null bytes.
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"""
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frozen_outer_hash = self._outer.copy()
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frozen_outer_hash.update(self._inner.digest())
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return frozen_outer_hash.digest()
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def verify(self, mac_tag):
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"""Verify that a given **binary** MAC (computed by another party)
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is valid.
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:Parameters:
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mac_tag : byte string
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The expected MAC of the message.
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:Raises ValueError:
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if the MAC does not match. It means that the message
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has been tampered with or that the MAC key is incorrect.
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"""
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secret = get_random_bytes(16)
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mac1 = BLAKE2s.new(digest_bits=160, key=secret, data=mac_tag)
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mac2 = BLAKE2s.new(digest_bits=160, key=secret, data=self.digest())
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if mac1.digest() != mac2.digest():
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raise ValueError("MAC check failed")
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def hexdigest(self):
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"""Return the **printable** MAC of the message that has been
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authenticated so far.
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This method does not change the state of the MAC object.
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:Return: A string of 2* `digest_size` bytes. It contains only
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hexadecimal ASCII digits.
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"""
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return "".join(["%02x" % bord(x)
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for x in tuple(self.digest())])
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def hexverify(self, hex_mac_tag):
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"""Verify that a given **printable** MAC (computed by another party)
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is valid.
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:Parameters:
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hex_mac_tag : string
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The expected MAC of the message, as a hexadecimal string.
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:Raises ValueError:
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if the MAC does not match. It means that the message
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has been tampered with or that the MAC key is incorrect.
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"""
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self.verify(unhexlify(tobytes(hex_mac_tag)))
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def new(key, msg=b(""), digestmod=None):
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"""Create a new HMAC object.
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:Parameters:
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key : byte string
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key for the MAC object.
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It must be long enough to match the expected security level of the
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MAC. However, there is no benefit in using keys longer than the
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*digest_size* of the underlying hash algorithm.
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msg : byte string
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The very first chunk of the message to authenticate.
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It is equivalent to an early call to `HMAC.update()`.
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Optional.
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:Parameter digestmod:
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The hash to use to implement the HMAC. Default is `Crypto.Hash.MD5`.
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:Type digestmod:
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A hash module or instantiated object from `Crypto.Hash`
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:Returns: An `HMAC` object
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"""
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return HMAC(key, msg, digestmod)
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