Re: Chaffing & winnowing without overhead

Bill Stewart (bill.stewart@pobox.com)
Mon, 11 May 1998 11:51:53 -0700

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>Jesús Cea Avión wrote:
>> You can have chaffing & winnowing without bandwidth overhead, but the
>> resulting scheme hasn't the original "elegance" anymore. In particular,
>> you don't send the plaintext on the clear.
...
>> b) Calculate the signature for:
>> [sequence]0 -> MAC0
>> [sequence]1 -> MAC1
>> c) Compare both MACs and locate the first "different" bit,
>> from high to low bit or viceversa.
>> d) Send that bit from MAC0 if you want to send a "0" or from
>> MAC1 if you want to send a "1".

So why not _send_ the plaintext in the clear?
Send the 0 bit, and the bit from the MAC0, and the 1 and the MAC1 bit
        0 0, 1 1, 0 1, 1 0,
Yes, it's expanding the data 4:1, but that's much better than before.

At 12:04 PM 5/11/98 -0400, Mordechai Ovits wrote:
>On the contrary, it has an elegance all it's own :-).

I strongly agree. I had proposed using a short checksum,
e.g. 8 bits of the MAC, which does leave collisions every ~256 sets,
but this is almost as short a checksum as you can get,
and eliminates the collision except every ~2**64 pairs.

>However clever this technique is (and it *is* clever),
>it defeats the original purpose of Ron's idea.

If you do include the data bits, you maintain (very marginally)
the letter of the requirement here. What you do lose
with this method is the ability to mix traffic from different people;
1 bit of MAC just isn't enough to pick out your own bits.
Any short MAC limits the amount of mixing you can do;
an 8-bit MAC lets you mix a bit without too many collisions,
and a 64-bit MAC should be enough for any mixture you'd
ever bother with (probably 16 or 32 would as well,
though especially for 16 you'd still need a longitudinal
checksum or some method of handling rare collisions.)

Is it close enough for government work? Probably.
                                Thanks!
                                        Bill
Bill Stewart, bill.stewart@pobox.com
PGP Fingerprint D454 E202 CBC8 40BF 3C85 B884 0ABE 4639

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