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Re: FORGED CANCELS of posts on n.a.n-a.m

In article <[email protected]>, [email protected] (David Gerard) writes:

>One thing that occurs to me: suppose I go to control, collect cancel messages,
>and build myself a collection of M1's that will work with a given M2?
>That is, I can't actually invert the hashing function. But if a given
>hash function is standard, then I can eventually build up a collection of
>M1s for M2s that will let me cancel quite a few things I may want to.

Good point -- if M1 is known to be small in size (say, a 128-bit MD5 of the
article body + newsgroups + message-id + date + secret passphrase), then an
organization with a lot of $$$$$$$ and computing resources (like Co$ or NSA)
might even try to pre-compute M2 = H(M1) for many possible M1's, sort the
result by M2, and build a (partial) lookup table of inverted H. This would be a
humongous table. Would someone bother to do it just to cancel a few Usenet
messages? Possibly.

But I see an easy fix: change Hujskonen and Franz's original proposal so that
an article posted with message-id X contains the header "Cancel-lock: M2",
where M2 is now H(X + M1), not just H(M1). This way, even if two different
articles happen to share the lock M2, they'll need different keys M1's to be
cancelled because their unique message-id's are different. A pre-computed table
of inverse values of H would be useless. To cancel a given X, a brute force
attacker would have to compute M2 = (X + M1) for all possible M1's. Hopefully,
the article X will expire naturally long before this can be done. :)

And in article <[email protected]>, Rich Salz <[email protected]> writes:

>>If the cancel cannot be authenticated (e.g., because the original article lacks
>>the "Cancel-lock: M2" header, or the cancel lacks the "Cancel-key: M1" header
>>such that H(M1)=M2), then INN should forward the unauthenticated cancel to one
>>or more "collection centers" so the author of the original article may be
>So if 70% of Usenet follows this scheme a handful of forged cancels can easily
>cause melt down.

(Thank you for looking at this!)

If 70% of Usenet followed this convention and refused to honor unauthenticated
cancels and supersedes's, then forged cancels would be much less harmful than
they are now, and there would be less need to notify the victims and to track
down the perpetrators.

Perhaps, not _every site should send out notifications. The purpose of getting
notifications from multiple sites is to compare the Path: header and see where
it was forged. I suppose notifications from just 5--10 well-positions sites
would often suffice. But if these sites are well-known, then an attacker might
put their names in the Path: of the forged cancel, to bypass the notification
and still propagate the cancel to a lot of other sites.

We can start implementing this scheme gradually, first by patching our posting
software to insert the "Cancel-lock:/Cancel-Key:" headers, and by running a few
"watchers" based on Homer Wilson Smith's Lazarus that'll notify the poster when
an article with a Cancel-lock: header is being cancelled without a matching
Cancel-Key: -- in all newsgroups, not just in a.r.s. Notifications about
articles without a "Cancel-Key:" header can be added much later, if ever.

>>Each "collection center" deamon should wake up periodically (say, every hour),
>>group the collected unauthenticated cancels by message-ids of the cancelled
>>articles, and e-mail the (distinct) addresses (other than "[email protected]*" or
>>"[email protected]*") mentioned in the "From:", "Sender:", "Authorized:", and
>>"X-Cancelled-By:" headers, quoting the unauthenticated cancel and the Path's as
>>seen at many different sites that forwarded the cancels. This way, if the
>>unauthenticated cancel is indeed forged, its author will see within hours that
>>it has been fraudulently cancelled _and_ will automatically receive enough
>>"Path:" samples from all over the world to see where it was posted, by
>>comparing the "Path:" headers in several forwarded copies.
>I can post a handful of articles and forge the From line, and create my
>own Cancel-lock headers by "rolling the dice."  I can then get their mailbox
>bombed by forging cancels.  A little more complicated then "sendsys-bombing"
>but not much more so.

Yes -- someone can post an article with random noise in the Cancel-lock:
header, and it would be impossible to cancel except by NoCeM.

As for mailbombing, one can do it much easier by forging a sendsys in the
victim's name, or by e-mailing the victim megabytes of junk from a phoney
"From:" address. This is done, but not too often because the perp is likely to
be tracked down and beaten up. :) Why would someone use this attack and not
straight forged sendsys?

One could address this by limiting the number of notifications e-mailed to one
address or even to one site in a period of time.


<a href="mailto:[email protected]">Dr. Dimitri Vulis</a>
Brighton Beach Boardwalk BBS, Forest Hills, N.Y.: +1-718-261-2013, 14.4Kbps