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# mac.py - module for checking value signatures
# coding: utf-8
#
# Copyright (C) 2014-2016 Arthur de Jong
#
# This library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
# License as published by the Free Software Foundation; either
# version 2.1 of the License, or (at your option) any later version.
#
# This library is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with this library; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
# 02110-1301 USA
"""Module that provides message authentication for PSKC values.
This module provides a MAC class that is used to store information about
how the MAC should be calculated (including the MAC key) and a ValueMAC
class that provides (H)MAC checking for PSKC key data.
The MAC key is generated specifically for each PSKC file and encrypted
with the PSKC encryption key.
"""
import re
_hmac_url_re = re.compile(r'^.*#hmac-(?P<hash>[a-z0-9]+)$')
def get_hash(algorithm):
"""Return the hash function for the specifies HMAC algorithm."""
import hashlib
match = _hmac_url_re.search(algorithm)
if match:
return getattr(hashlib, match.group('hash'), None)
def get_hmac(algorithm):
"""Return an HMAC function that takes a secret and a value and returns a
digest."""
import hmac
digestmod = get_hash(algorithm)
if digestmod is not None:
return lambda key, value: hmac.new(key, value, digestmod).digest()
def get_mac(algorithm, key, value):
"""Generate the MAC value over the specified value."""
from pskc.exceptions import DecryptionError
if algorithm is None:
raise DecryptionError('No MAC algorithm set')
hmacfn = get_hmac(algorithm)
if hmacfn is None:
raise DecryptionError(
'Unsupported MAC algorithm: %r' % algorithm)
return hmacfn(key, value)
class MAC(object):
"""Class describing the MAC algorithm to use and how to get the key.
Instances of this class provide the following attributes:
algorithm: the name of the HMAC to use (currently only HMAC_SHA1)
key: the binary value of the MAC key if it can be decrypted
"""
def __init__(self, pskc):
self.pskc = pskc
self._algorithm = None
self.key_plain_value = None
self.key_cipher_value = None
self.key_algorithm = None
@property
def key(self):
"""Provides access to the MAC key binary value if available."""
if self.key_plain_value:
return self.key_plain_value
elif self.key_cipher_value:
return self.pskc.encryption.decrypt_value(
self.key_cipher_value, self.key_algorithm)
# fall back to encryption key
return self.pskc.encryption.key
@key.setter
def key(self, value):
self.key_plain_value = value
self.key_cipher_value = None
@property
def algorithm(self):
"""Provide the MAC algorithm used."""
if self._algorithm:
return self._algorithm
@algorithm.setter
def algorithm(self, value):
from pskc.algorithms import normalise_algorithm
self._algorithm = normalise_algorithm(value)
@property
def algorithm_key_length(self):
"""Recommended minimal key length in bytes for the set algorithm."""
# https://tools.ietf.org/html/rfc2104#section-3
# an HMAC key should be at least as long as the hash output length
hashfn = get_hash(self.algorithm)
if hashfn is not None:
return int(hashfn().digest_size)
else:
return 16
def generate_mac(self, value):
"""Generate the MAC over the specified value."""
return get_mac(self.algorithm, self.key, value)
def setup(self, key=None, algorithm=None):
"""Configure an encrypted MAC key.
The following arguments may be supplied:
key: the MAC key to use
algorithm: MAC algorithm
None of the arguments are required, reasonable defaults will be
chosen for missing arguments.
"""
if key:
self.key = key
if algorithm:
self.algorithm = algorithm
# default to HMAC-SHA1
if not self.algorithm:
self.algorithm = 'hmac-sha1'
# generate an HMAC key
if not self.key:
from Crypto import Random
self.key = Random.get_random_bytes(self.algorithm_key_length)
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