ó õùPc@scdZdZddlmZddd„ƒYZedkr_dZdGHeGHd GHgZd Zx8e d e eƒeƒD]Z ej ee e e!ƒquWd GHdd l mZmZdZgeD]$Zejeedeƒjƒ^q¿Ze eƒe eƒkst‚gZee e eƒƒeeƒZeGHx-eD]%\Z ZZej e eefƒq7WdGHeddddƒZejeƒZddlmZgZ dGHxˆeD]€\Z ZZejeedeƒZ!e!jƒZ"e"ekrdZ#e j eƒndZ#e#Gde Ge$eƒGeeƒd GHq«WdGHdj%e ƒZ&e&ekrWdGHq_dGHndS( s£ 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 s$Id$iÿÿÿÿ(t bytes_to_longtChaffcBs/eZdZddd„Zd„Zd„ZRS(sRClass 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. gð?icCsVd|kodkns(td‚n|dkr@td‚n||_||_dS(sÐChaff(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. ggð?s$'factor' must be between 0.0 and 1.0is 'blocksper' must be zero or moreN(t ValueErrort_Chaff__factort_Chaff__blocksper(tselftfactort blocksper((s3..\python\site-packages\Crypto\Protocol\Chaffing.pyt__init__Zs     cCsCg}t|ƒ|j}t|jƒ}xttt|ƒƒ|ƒD]÷\}}||kr.|\}}} t|ƒ} t| ƒ} d} x‰|D]} ddl}|j| ƒ}|j| ƒ}|||f}| rt|jdƒƒd@r|j|ƒd} n|j|ƒqW| r;|j|ƒq;qD|j|ƒqDW|S(sXchaff( [(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. iiÿÿÿÿNii@i( tlenRtrangeRtziptsyst_randnumRtappend(Rtblockst chaffedblockstcountRtitwheattserialtdatatmactdatasizetmacsizetaddwheattjR t chaffdatatchaffmactchaff((s3..\python\site-packages\Crypto\Protocol\Chaffing.pyRus,(       cCs#ddlm}|jƒj|ƒS(Niÿÿÿÿ(tRandom(tCryptoRtnewtread(RtsizeR((s3..\python\site-packages\Crypto\Protocol\Chaffing.pyR «s(t__name__t __module__t__doc__RRR (((s3..\python\site-packages\Crypto\Protocol\Chaffing.pyRJs 6t__main__sŽWe 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. sOriginal text: ==========s ==========i(isCalculating MACs...(tHMACtSHAt Jeffersont digestmodsAdding chaff...Rgà?Ri(t encodestringschaffed message blocks:s-->s s%3dsUndigesting wheat...ts They match!s They differ!N(('R%t __revision__tCrypto.Util.numberRRR#ttextRR"R R RRt Crypto.HashR'R(tkeytblockR tdigesttmacstAssertionErrortsourceR tmRRtcRtchaffedtbase64R+Rthtpmacttagtreprtjointnewtext(((s3..\python\site-packages\Crypto\Protocol\Chaffing.pytDsVf .