MS's new antispam idea

I'm sure I've heard this one before, so it's not even a new idea...
hope whoever came up with it originally patented it. :sunglasses: Then again,
maybe it was MS that I heard about the first time, and the Beeb is
simply late to the game here.

Yes, puzzles have been suggested before as defenses against SYN floods
and SSL DoS attacks, and many other things as well.

Has anyone calculated the increased server load, the extra storage
needed for the now lengthened outgoing mail queue, and the extra
bandwidth required to handle all this extra back and forth puzzle
thing? YahooGroups and the like would definitely be impacted. It would
be interesting to see what protections will be built into the puzzle
thing as well... I can see some joker setting up his server to require
that the sending computer calculate PI to some ridiculous number of
decimals... although that might make a good honeypot. Or, if the puzzle
is open source (which would be a good thing), how soon before the
spammers (or even legit MTA authors) hardcode the answers into the
server software? I suppose there would have to be some random elements.

The usual way this is done is to pick a puzzle that's hard to compute
but easy to verify. For example, the server could pick a random number,
take the top N bits, and challenge the
client to find *any* number whose SHA1 hash has the same high-order N
bits *and* includes some other random string as the high-order bits of
the answer. There are no known short cuts; the only feasible strategy is
to calculate lots of SHA1 hashes for different input values. (The
server sends some other random number to avoid precomputation attacks.)

Bandwidth is probably not an issue; it's one extra round trip, and it's
not very much text. Mail sender queues are more of an issue, as is the
load on the sender; if I were doing this, I'd make it adaptive, with a
high cost being required for unknown senders, or those that have sent
suspected spam. For example, start with a 12-bit puzzle, i.e., one of
client difficulty 4096. For each piece of non-spam, subtract some
small value from the difficulty. For each piece of spam, double the
difficulty rating for that client. There are lots of ways to do things
like this; it will take more than back-of-the-envelope calculcations to
understand all the knobs, let alone what countermeasures the spammers
will deploy.

For an introduction to schemes like this, see Stubblefield, A.., and
D. Dean, "Using Client Puzzles to Protect TLS," Proceedings of the
Tenth USENIX Security Symposium, Washington, DC, August 2001, available
at .

It is interesting.... as an extension it might be nice to be able to
set up a "whitelist" of trusted servers that don't have to go through
the computational gyrations to send you mail - that way it would,
hopefully, eventually impact the spammers more than it would impact
legitimate e-mail servers.

According to the article, that is indeed part of the scheme.

    --Steve Bellovin,

It's too easy to introduce a worm that gives a spammer access to many
teraflops of unwittingly collaborative computing resources. I can't
imagine a compute-intensive puzzle scheme is going to do much more than
the average iteration of a rule-based anti-spam filter. They'll just
provide a temporary dent in the total spam flow.

A reliance on new puzzles to provide obstacles to such spammers will
end up being very close to homomorphic to rule-based filter
iterations. Perhaps even a little less useful, as the spammers will
not need to analyze and change each individual bit of spam, but merely
need to reload the distributed sending cluster with the new solvers.

Microsoft could indeed wipe out spam, in the short and long run. And
they can do so without schemes that are likely to end up building upon
the substantial plaque that already clogs the arteries of the net.