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<H2><A NAME="SECTION00052000000000000000">
Crypto.Protocol.Chaffing</A>
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<P>
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.
<P>
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.
<P>
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.
<P>
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.
<P>
<BR>
21#21
<BR>
<P>
Chaff instances have the following methods:
<P>
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22#22
<BR>
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