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The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:16:15 ADT
Mordechai Ovits (movits@syndata.com)
Wed, 25 Mar 1998 13:03:31 -0500
Ron Rivest's paper on C/W was very thought provoking. It is especially
notable for being very easy to implement, even if it is not all that
practical. While I was reading it, I had a few questions.
My first question was this:
Why must the sequence number be doubled and used with the complement
bit?
Ron has it like this -
(sn,bit,mac)
------------
(1,0,234632)
(1,1,457733)
(2,0,578848)
(2,1,479797)
(3,0,734373)
(3,1,874869)
...
Why not just use plain old sequence numbers, and discard those with bad
MACs? The advantage of doing it this way is that you can alter the
ratio of chaff to wheat below 1:1. This could help alleviate the size
issue. It would look like this -
(1,0,335683) // good bit
(2,1,484653) // bad mac, discard
(3,1,373636) // good bit
(4,0,345732) // good bit
(5,0,237345) // bad mac discard
The message would be 010
Assuming you didn't do a 50/50 decision whether the bit you about to
send
should be chaff or drawn from the wheat, you WOULD be leaking some
information about your message that could possibly be recreated
statistically. Perhaps the message was plain ASCII, with all the
letters bunched into one spot of the 256. I'm no statistician, but
I think that it can be done. Lemme know if you know better.
My second question is:
Ron notes that you can add more than one subsequence of chaff to the
stream, and make those chaff with another key. The "extra" key that
created the chaff could be given over to authorities, which would
extract a readable, but incorrect message. He compares this to
deniable encryption. If you are doing this, why bother making it
a subsequence? Let ALL the chaff come from some innocuous message!
This would work especially well with my first idea, since the two
messages would not have to be the same size.
Please lemme know what you think,
movits@syndata.com
The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:16:15 ADT