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"""This file implements the chaffing algorithm.
Winnowing and chaffing is a technique for enhancing privacy without requiring
strong encryption. In short, the technique takes a set of authenticated
message blocks (the wheat) and adds a number of chaff blocks which have
randomly chosen data and MAC fields. This means that to an adversary, the
chaff blocks look as valid as the wheat blocks, and so the authentication
would have to be performed on every block. By tailoring the number of chaff
blocks added to the message, the sender can make breaking the message
computationally infeasible. There are many other interesting properties of
the winnow/chaff technique.
For example, say Alice is sending a message to Bob. She packetizes the
message and performs an all-or-nothing transformation on the packets. Then
she authenticates each packet with a message authentication code (MAC). The
MAC is a hash of the data packet, and there is a secret key which she must
share with Bob (key distribution is an exercise left to the reader). She then
adds a serial number to each packet, and sends the packets to Bob.
Bob receives the packets, and using the shared secret authentication key,
authenticates the MACs for each packet. Those packets that have bad MACs are
simply discarded. The remainder are sorted by serial number, and passed
through the reverse all-or-nothing transform. The transform means that an
eavesdropper (say Eve) must acquire all the packets before any of the data can
be read. If even one packet is missing, the data is useless.
There's one twist: by adding chaff packets, Alice and Bob can make Eve's job
much harder, since Eve now has to break the shared secret key, or try every
combination of wheat and chaff packet to read any of the message. The cool
thing is that Bob doesn't need to add any additional code; the chaff packets
are already filtered out because their MACs don't match (in all likelihood --
since the data and MACs for the chaff packets are randomly chosen it is
possible, but very unlikely that a chaff MAC will match the chaff data). And
Alice need not even be the party adding the chaff! She could be completely
unaware that a third party, say Charles, is adding chaff packets to her
messages as they are transmitted.
For more information on winnowing and chaffing see this paper:
Ronald L. Rivest, "Chaffing and Winnowing: Confidentiality without Encryption"
http://theory.lcs.mit.edu/~rivest/chaffing.txt
"""
__revision__ = "$Id: Chaffing.py,v 1.7 2003/02/28 15:23:21 akuchling Exp $"
from Crypto.Util.number import bytes_to_long
class Chaff:
"""Class implementing the chaff adding algorithm.
Methods for subclasses:
_randnum(size):
Returns a randomly generated number with a byte-length equal
to size. Subclasses can use this to implement better random
data and MAC generating algorithms. The default algorithm is
probably not very cryptographically secure. It is most
important that the chaff data does not contain any patterns
that can be used to discern it from wheat data without running
the MAC.
"""
def __init__(self, factor=1.0, blocksper=1):
"""Chaff(factor:float, blocksper:int)
factor is the number of message blocks to add chaff to,
expressed as a percentage between 0.0 and 1.0. blocksper is
the number of chaff blocks to include for each block being
chaffed. Thus the defaults add one chaff block to every
message block. By changing the defaults, you can adjust how
computationally difficult it could be for an adversary to
brute-force crack the message. The difficulty is expressed
as:
pow(blocksper, int(factor * number-of-blocks))
For ease of implementation, when factor < 1.0, only the first
int(factor*number-of-blocks) message blocks are chaffed.
"""
if not (0.0<=factor<=1.0):
raise ValueError, "'factor' must be between 0.0 and 1.0"
if blocksper < 0:
raise ValueError, "'blocksper' must be zero or more"
self.__factor = factor
self.__blocksper = blocksper
def chaff(self, blocks):
"""chaff( [(serial-number:int, data:string, MAC:string)] )
: [(int, string, string)]
Add chaff to message blocks. blocks is a list of 3-tuples of the
form (serial-number, data, MAC).
Chaff is created by choosing a random number of the same
byte-length as data, and another random number of the same
byte-length as MAC. The message block's serial number is
placed on the chaff block and all the packet's chaff blocks
are randomly interspersed with the single wheat block. This
method then returns a list of 3-tuples of the same form.
Chaffed blocks will contain multiple instances of 3-tuples
with the same serial number, but the only way to figure out
which blocks are wheat and which are chaff is to perform the
MAC hash and compare values.
"""
chaffedblocks = []
# count is the number of blocks to add chaff to. blocksper is the
# number of chaff blocks to add per message block that is being
# chaffed.
count = len(blocks) * self.__factor
blocksper = range(self.__blocksper)
for i, wheat in map(None, range(len(blocks)), blocks):
# it shouldn't matter which of the n blocks we add chaff to, so for
# ease of implementation, we'll just add them to the first count
# blocks
if i < count:
serial, data, mac = wheat
datasize = len(data)
macsize = len(mac)
addwheat = 1
# add chaff to this block
for j in blocksper:
import sys
chaffdata = self._randnum(datasize)
chaffmac = self._randnum(macsize)
chaff = (serial, chaffdata, chaffmac)
# mix up the order, if the 5th bit is on then put the
# wheat on the list
if addwheat and bytes_to_long(self._randnum(16)) & 0x40:
chaffedblocks.append(wheat)
addwheat = 0
chaffedblocks.append(chaff)
if addwheat:
chaffedblocks.append(wheat)
else:
# just add the wheat
chaffedblocks.append(wheat)
return chaffedblocks
def _randnum(self, size):
# TBD: Not a very secure algorithm.
# TBD: size * 2 to work around possible bug in RandomPool
from Crypto.Util import randpool
import time
pool = randpool.RandomPool(size * 2)
while size > pool.entropy:
pass
# we now have enough entropy in the pool to get size bytes of random
# data... well, probably
return pool.get_bytes(size)
if __name__ == '__main__':
text = """\
We hold these truths to be self-evident, that all men are created equal, that
they are endowed by their Creator with certain unalienable Rights, that among
these are Life, Liberty, and the pursuit of Happiness. That to secure these
rights, Governments are instituted among Men, deriving their just powers from
the consent of the governed. That whenever any Form of Government becomes
destructive of these ends, it is the Right of the People to alter or to
abolish it, and to institute new Government, laying its foundation on such
principles and organizing its powers in such form, as to them shall seem most
likely to effect their Safety and Happiness.
"""
print 'Original text:\n=========='
print text
print '=========='
# first transform the text into packets
blocks = [] ; size = 40
for i in range(0, len(text), size):
blocks.append( text[i:i+size] )
# now get MACs for all the text blocks. The key is obvious...
print 'Calculating MACs...'
from Crypto.Hash import HMAC, SHA
key = 'Jefferson'
macs = [HMAC.new(key, block, digestmod=SHA).digest()
for block in blocks]
assert len(blocks) == len(macs)
# put these into a form acceptable as input to the chaffing procedure
source = []
m = map(None, range(len(blocks)), blocks, macs)
print m
for i, data, mac in m:
source.append((i, data, mac))
# now chaff these
print 'Adding chaff...'
c = Chaff(factor=0.5, blocksper=2)
chaffed = c.chaff(source)
from base64 import encodestring
# print the chaffed message blocks. meanwhile, separate the wheat from
# the chaff
wheat = []
print 'chaffed message blocks:'
for i, data, mac in chaffed:
# do the authentication
h = HMAC.new(key, data, digestmod=SHA)
pmac = h.digest()
if pmac == mac:
tag = '-->'
wheat.append(data)
else:
tag = ' '
# base64 adds a trailing newline
print tag, '%3d' % i, \
repr(data), encodestring(mac)[:-1]
# now decode the message packets and check it against the original text
print 'Undigesting wheat...'
newtext = "".join(wheat)
if newtext == text:
print 'They match!'
else:
print 'They differ!'