Re: Chaffing & winnowing without overhead

Bill Stewart (bill.stewart@pobox.com)
Tue, 12 May 1998 18:02:31 -0700

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By the way, instead of transmitting the first bit for which
MAC(sequence,0) differs from MAC(sequence,1), as Jesús suggests,
you can get the same effect by transmitting a 0 or 1 depending on
    MAC(sequence,0) < MAC(sequence,1)
(This assumes a big-endian system and unsigned comparisons;
little-endians will have to calculate it the hard way.)
If you're willing to be wrong 1 time out of 2**33,
you can just use the top 32 bits.

Earlier in this discussion:
>> In the Rivest's paper you transmit, indeed, all the 2^n plaintexts for a
>> n bit length };-).
>Not so. In his paper (before the package tranform stuff), he had the following expansion.
>Assuming a 32 bit serial number and a 160 bit MAC, n bits would expand to 388n.
>>To make this clearer with an example, note that the adversary
>>will see triples of the form:
>> (1,0,351216)
>> (1,1,895634)
>> (2,0,452412)
>> (2,1,534981)

But that _does send the 2^n plaintexts, which are 0 and 1, and n=1.
                                Thanks!
                                        Bill
Bill Stewart, bill.stewart@pobox.com
PGP Fingerprint D454 E202 CBC8 40BF 3C85 B884 0ABE 4639

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