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# HMAC.py - Implements the HMAC algorithm as described by RFC 2104.
#
# ===================================================================
# Portions Copyright (c) 2001, 2002, 2003 Python Software Foundation;
# All Rights Reserved
#
# This file contains code from the Python 2.2 hmac.py module (the
# "Original Code"), with modifications made after it was incorporated
# into PyCrypto (the "Modifications").
#
# To the best of our knowledge, the Python Software Foundation is the
# copyright holder of the Original Code, and has licensed it under the
# Python 2.2 license. See the file LEGAL/copy/LICENSE.python-2.2 for
# details.
#
# The Modifications to 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.
# ===================================================================
"""HMAC (Hash-based Message Authentication Code) algorithm
HMAC is a MAC defined in RFC2104_ and FIPS-198_ and constructed using
a cryptograpic hash algorithm.
It is usually named *HMAC-X*, where *X* is the hash algorithm; for
instance *HMAC-SHA1* or *HMAC-MD5*.
The strength of an HMAC depends on:
- the strength of the hash algorithm
- the length and entropy of the secret key
This is an example showing how to *create* a MAC:
>>> from Crypto.Hash import HMAC
>>>
>>> secret = b'Swordfish'
>>> h = HMAC.new(secret)
>>> h.update(b'Hello')
>>> print h.hexdigest()
This is an example showing how to *check* a MAC:
>>> from Crypto.Hash import HMAC
>>>
>>> # We have received a message 'msg' together
>>> # with its MAC 'mac'
>>>
>>> secret = b'Swordfish'
>>> h = HMAC.new(secret)
>>> h.update(msg)
>>> try:
>>> h.verify(mac)
>>> print "The message '%s' is authentic" % msg
>>> except ValueError:
>>> print "The message or the key is wrong"
.. _RFC2104: http://www.ietf.org/rfc/rfc2104.txt
.. _FIPS-198: http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf
"""
# This is just a copy of the Python 2.2 HMAC module, modified to work when
# used on versions of Python before 2.2.
__revision__ = "$Id$"
__all__ = ['new', 'digest_size', 'HMAC' ]
from binascii import unhexlify
from Crypto.Util.strxor import strxor_c
from Crypto.Util.py3compat import *
#: The size of the authentication tag produced by the MAC.
#: It matches the digest size on the underlying
#: hashing module used.
digest_size = None
class HMAC:
"""Class that implements HMAC"""
#: The size of the authentication tag produced by the MAC.
#: It matches the digest size on the underlying
#: hashing module used.
digest_size = None
def __init__(self, key, msg = None, digestmod = None):
"""Create a new HMAC object.
:Parameters:
key : byte string
secret key for the MAC object.
It must be long enough to match the expected security level of the
MAC. However, there is no benefit in using keys longer than the
`digest_size` of the underlying hash algorithm.
msg : byte string
The very first chunk of the message to authenticate.
It is equivalent to an early call to `update()`. Optional.
:Parameter digestmod:
The hash algorithm the HMAC is based on.
Default is `Crypto.Hash.MD5`.
:Type digestmod:
A hash module or object instantiated from `Crypto.Hash`
"""
if digestmod is None:
import MD5
digestmod = MD5
self.digestmod = digestmod
self.outer = digestmod.new()
self.inner = digestmod.new()
try:
self.digest_size = digestmod.digest_size
except AttributeError:
self.digest_size = len(self.outer.digest())
try:
# The block size is 128 bytes for SHA384 and SHA512 and 64 bytes
# for the others hash function
blocksize = digestmod.block_size
except AttributeError:
blocksize = 64
ipad = 0x36
opad = 0x5C
if len(key) > blocksize:
key = digestmod.new(key).digest()
key = key + bchr(0) * (blocksize - len(key))
self.outer.update(strxor_c(key, opad))
self.inner.update(strxor_c(key, ipad))
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.inner.update(msg)
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: An `HMAC` object
"""
other = HMAC(b(""))
other.digestmod = self.digestmod
other.inner = self.inner.copy()
other.outer = self.outer.copy()
return other
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.
"""
h = self.outer.copy()
h.update(self.inner.digest())
return h.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.
"""
mac = self.digest()
res = 0
# Constant-time comparison
for x,y in zip(mac, mac_tag):
res |= bord(x) ^ bord(y)
if res or len(mac_tag)!=self.digest_size:
raise ValueError("MAC check failed")
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 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, digestmod = None):
"""Create a new HMAC object.
:Parameters:
key : byte string
key for the MAC object.
It must be long enough to match the expected security level of the
MAC. However, there is no benefit in using keys longer than the
`digest_size` of the underlying hash algorithm.
msg : byte string
The very first chunk of the message to authenticate.
It is equivalent to an early call to `HMAC.update()`.
Optional.
:Parameter digestmod:
The hash to use to implement the HMAC. Default is `Crypto.Hash.MD5`.
:Type digestmod:
A hash module or instantiated object from `Crypto.Hash`
:Returns: An `HMAC` object
"""
return HMAC(key, msg, digestmod)