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The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:16:19 ADT
mgraffam@mhv.net
Fri, 27 Mar 1998 00:58:01 -0500 (EST)
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On Thu, 26 Mar 1998, Douglas Dike wrote:
> I think I've got the general idea, but I seem to missing the key (pun
> inetended). How is Alice letting Bob know which packets are good, without
> letting everyone else know? What secret do they share?
They share a bit of data, that I'll call S, the plaintext is P. If
H() is a one-way function, then M = H(S+P) where + is concatenation.
M is the MAC. It's just a hash. Alice sends a packet, composed of
a serial number, P and M to Bob.
Bob takes his copy of S, tacks P on the end and runs it through H,
yielding M'. If M' and M match, then P is wheat.
The revised paper is at http://theory.lcs.mit.edu/~rivest/chaffing.txt
In an actual software implementation, we wouldn't necessarily need to copy
S and P around all the time to physically concatenate them. Just hash
S once and keep that around for the initial values. Other speed hacks
could be put into place too.
A few weeks ago I uploaded some code to ftp.funet.fi
(/pub/crypt/hash/sha/sha_crypto.tgz) that uses SHA to generate private
digests, symmetric signatures, MACs, or whatever you want to call them.
I was thinking repudiation at the time, which is one reason I like
Rivest's idea. It is nice and simple.
Michael J. Graffam (mgraffam@mhv.net)
http://www.mhv.net/~mgraffam -- Philosophy, Religion, Computers, Crypto, etc
"Act only according to that maxim by which you can at the same time will that
it should become a universal law.." - Immanuel Kant "Metaphysics of Morals"
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The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:16:19 ADT