Slashdot Mirror
Turing Tests to Stop Spam
cexy writes "The Register has a story about how Hotmail and Yahoo! are using Carnegie Mellon developed captcha technology (completely automated public Turing tests to tell computers and humans apart) to stop spammers from automating signups for accounts from which they can send spam. These guys are using captcha too, but to stop incoming spam."
my Spam filter in Yahoo catches way, way, more than the one at hotmail. It is always surprising to me when you open a new hotmail account that it takes only like a week to be flooded with Spam. A week of doing nothing with the account but initially opening it. *sigh*
that is why. all the spammers are targetting hotmail. I hate the anti-ms bias. I use a filter on my hotmail. It is an allow only filter. Those are the best kind because I make the decision of who gets through to me.
For those who dont know, The CMU developed captcha project is great. Check out their work here:
http://www.captcha.net/
Where it shows you a smeared image of a number that you have to type in to register with a site? I think Slashdot has had this for a while now, and I know I have seen it on other sites as far back as a few years ago.
Does Hotmail really think that I have friends named things like ilikeitinthebutt?
I've only had my Yahoo account since last year and my Hotmail account since 1997, so this may not be a fair comparison:
Yahoo spam today:
0
Hotmail spam today:
18
Which is doing a better job at stopping spam you say?
Saskboy's blog is good. 9 out of 10 dentists agree.
I would rather Yahoo stop spam from getting to my mail acocunt before they concentrate on stopping people from signing up automatically. I'm one of the few people who actually pay for Yahoo "additional" services. I thought I would get better anti-spam support. Not so far. I literally have 10 to 20 an hour and I can't block anymore because Yahoo only allows 100 addressed to be blocked. And considering the smammers are using 12374614187641874@optinmail.com along with other numerous addresses, it's impossible to block the majority of them. Hell I would even be happy if they would start allowing people to block entire domains. That would be a good first step.
My sig of choice is Marlboro
I have SpamAssassin at my isp (Verio) and it kicks ass. Probably a false positive per week (and that's often a slashdot Daily Stories email), and a false negative every 3-4 days. Pretty damn good. Cut inbox crapola from 10-20 per day to, well, zero.
sulli
RTFJ.
And I recently noticed that spam, while smaller in quantitiy, are much larger than normal (non-html image bloated crap).
First, I would like to know if there is a server-side daemon I could run that goes through all user accounts and weeds out spam (without knowing their passwords.)
Second, I would like to know if I have any legal recourse against unsolicited email hogging my bandwidth. Could I stockpile a years worth and send the spammers a bill for the used bandwidth?
You can't judge a book by the way it wears its hair.
When someone would send you mail, it would send back a link to a small image, in the image was a 'click here' dot, only a human (or some software that no spammer would take the time to write) can get their email into your mailbox.
.com "troubles".
Kind of offensive though, a lot of people took offence to clicking a link to send me email.
MsgTo.Com dissappeared some time ago during the
Hedley
According to the article, it says that the spammers could pay ppl to signup instead of using scripts. IANAL. but this would seem to be intentional misrepresentaion and "transferrance"(sp?) of the email account. I would think there would be some legal ramifacations of this.
"Completely automated public test to tell males and females apart".
.....
a/s/l?
"18f,Florida"
Do you mind if I ask you to take a quick Captmfa?
"Sure, go ahead"
Test completed. Result = 34m, Detroit.
Instead it's something they hacked up because new programs were getting around the old OCR blockers. Blah.
The truth is accounts like Yahoo and Hotmail only exist to turn a profit for their owners. I know not everyone can get an e-mail address that they can use for personal means in any other way, but you have to accept what you are getting into when you open one of these accounts.
Personally, I have several e-mail accounts and only use my hotmail and yahoo for things like web page registration.
Now if they could just come up with a turing test for slashdot
2 /1 2/30/1740211&mode=thread&tid=111
repeats!
http://developers.slashdot.org/article.pl?sid=0
Granted this is not a direct repeat but the articles are just different sources for the same story.
-- The morphemes of your disquisition are ascertainable, but they have eschewed an ambit of transpicuous exposition.
Don't you think it would be possible to write a program that could handle one of these captcha tests? Has anyone tried this, to validate their claims? Otherwise it's like roll-your-own crypto, worthless if you don't know if it can be defeated.
I turned on my hotmail filters so now only people on my whitelist can send mail directly to my inbox.
0 spam for months now.
The only negative is if someone not on my whitelist sends mail, I have to rummage throught the rest of the junk to find it.
"No Matter Where You Go.. There You Are." -- Buckaroo Banzai
I recently had to create an e-mail address that I could use for posting to a mailing list where the addresses are all public. I tried Hotmail first, and although I passed part 1 of their Turing test, the captcha test, I think I failed part 2: once I was all done filling in my personal information (retired female homemaker in Antarctica, born in 1891), I got some kind of mystifying error message saying something about my .NET account (which I don't have). I guess if I was human, I'd have been able to figure out what they meant.
Oh well, I passed Yahoo's captcha test, and they didn't have a part 2...
As a recipient of spam, I also don't see this having any benificial effects. I gets lots and lots of spam from hotmail.com and yahoo.com addresses. They're all forged headers, so it doesn't matter that Yahoo and Hotmail have botproofing -- the accounts I'm getting spam from aren't even real Yahoo and Hotmail accounts. It's great that they're trying to make sure they aren't spam havens (and of course it costs them money if spammers use their services), but I really think the whole e-mail infrastructure needs reworking in order to get rid of spam. Sending e-mail should cost some token amount of money, and there should also be some way of tossing out mail with forged headers (e.g., my mail client should be able to tell whether the cryptographic signature on an e-mail indicates that it really came from hotmail.com or yahoo.com).
Find free books.
It's time for my regular rant regarding PopFile and Bayesian excellence and how SPAM WOULD DISAPPEAR IF BAYESIAN TECHNIQUES WERE APPLIED AT THE ISP LEVEL!!!!
And now, back to our regular show.
It's Christmas everyday with BitTorrent.
click mail options:
go to
"Enter email address (or domain) to block:"
enter domain in text baox, such as
whatever.com
click, add block
The Kruger Dunning explains most post on
I was a big Hotmail fan until I found Yahoo to have twice the room for free, and literlly NO SPAM.
The custom filter option in Hotmail now is restricted to just 10 filters. I have 32, and if I edit them once now, I'm sunk. 10 can't possibly keep out all I'm succeeding with now.
Boobs [I wish I had real email with this in the title, but I don't]
Virgins [Once again, wishful thinking]
DVD [Don't own a drive yet]
FREE [Do your friends tell you you are getting something for free?]
And I don't bother reading any "Re:Your Inquiry" emails. I mean, how stupid do you have to be to send an email to someone with the subject "Your Inquiry"?
Saskboy's blog is good. 9 out of 10 dentists agree.
These Turing tests do not stop spam. They discourage spammers from using bogus Hotmail etc accounts to originate spam from. They do this by making it incrementally more expensive to create the accounts; rather than using a bot to create an account a second you have to use a human to create accounts by the minute. So 60 times the effort.
But I don't think that translates into 60 times the cost. The Turing tests are interesting but I don't think that the creation of the accounts ever was a bottleneck in the process in sending spam. You could get a high school kid to create all the accounts you would need for a month in about an hour, and pay him in pr0n.
If the truth were known, Hotmail and Yahoo are just trying to decrease server loads. I bet that when bots create accounts they create hundreds or thousands more than are used, which take up server resources during creation and later as the accounts eat up storage. With Turing tests it is more likely that not too many will be laying around waiting to be used.
=^..^= all your rodent are belong to us
Uh oh, looks like Spam Arrest is inflicted with Patent Priapism, a horrible disease in which you feel you must patent some stupid thing you "invented", when you actually just combined two or more existing things in a most un-original way.
They have patent pending on "calling back to verify a phone number" except it's email.
I would suggest avoiding this company's products and services.
Blocking by address is almost useless, unless you're getting mail from a legitimate spammer (i.e. you didn't read the fine print before signing up for something) and in those cases, you can normally opt-out anyways. The return addresses on regular spam are always forged--even though it says bighairyclit@hotmail.com it's really routed through a server in China and there's no such hotmail account.
It works with Outlook (not Outlook Express).
The coolest part is when you find an email that is spam, which it didn't catch (perhaps about 5% of the time), just click "Block" and it'll record that you blocked it on their servers, so anyone else receiving the same (or nearly similar, I think) email will have it blocked as well.
In other words, it's a community-driven spam blocker which works better the more people use it. And it already works very well.
I feel fantastic, and I'm still alive.
Those images you get in spam are usually bugged, specifically if they have a unique name and are going to a special server, they can confirm that your email address is still good. Also, they may be able to get something out of your browser too as to who you are.
There are only 3 ways spammers can get your email address:
1.) you sign up for something with that email
2.) they randomly generate it
3.) yahoo/hotmail sells/gives it to them or they get hacked
Repeal the DMCA!
From the captcha site:
"[...] humans can read distorted text as the one shown below but current computer programs can't:"
I think they mean "non-blind humans". How exactly will they ever solve that problem? If a blind
man's OCR program can read the text, so can the spammer's.
I see a lot of posts here comparing the relative merits of different spam filters, based on how little spam gets through. The thing I worry about a lot more with spam filters is how much of my non-spam mail gets blocked. And yes, I've had this happen with every spam filtering mechanism some sysadmin has inflicted on me. This is the main reason I like spam filtering at the user level, not the ISP or system level -- at least you have some control over the imperfections.
Tired of flames?
- Use the emacs psychologist to determine the mood of people sending you email!
Any sufficiently advanced libertarian utopia is indistinguishable from government.
This idea means licensing them so that they are properly registered, Meaning we know who they are and where they live.
Meaning that they can be billed for use of service, etc. and jail those not properly licensed.
Meaning that we can send bill collectors and tax collectors hunting after them.
The bottom line is that IF we can make it profitable to go after these guys, someone will make a business of it. We just go to figure a way how.
Then we get to use the scum of society, such as bill collectors and tax collectors, and turn them to some good, going after spammers.
And we can use the money collected to subsidise the cost of something useful.
Now Lessig has also proposed something similar to this:
http://www.cioinsight.com/article2/0,3959,533225,0 0.asp
Which essentially means that there are more eyeballs to track the scum down. And a financial reward to do so.
The twist in my proposal is to mach spam have a cost even if sent "legally" - [lots of states have finance problems], and make the penalties truly painful if done illegally. I want to set my own fees for receiving spam
"It is a greater offense to steal men's labor, than their clothes"
Damn Slashvertisements. I don't care if it is to block spam, it doesn't belong.
On the other hand, the banners are just fine and for those of you who have their banners turned off, Blizzard has an opening for a Unix Admin and a great ad. I'd link to it here but you should really turn banners on. I know they are annoying, but banners bring in money for slashdot. That $49.99 or $9.99 or whatever you pay for your ISP is NOT giving that money to slashdot, and for them to remain free, they need you to download those damn ads.
Now, turning off pop-ups, that's accetable. But think of all the porn you're missing!
Every time you want to send an e-mail to someone, their ISP (or even their own mail server) quickly replies to you with a challenge (image for you to decipher), when you decipher the image, and reply ("as in confirm you're a human") your original message appears in the in-box of the person to whom you've sent it. Anyone can define their own tests if they're not happy with default ones, and you never see an e-mail which hasn't passed YOUR tests.
And since these tests are interactive (ie: you're asking the PERSON who e-mailed you a question, they can be quite hard to fool with a computer).
Non-challenging e-mail addresses (or mailings) can still exist, and will be clearly marked as haven't bee 'verified'... ie: streated as bulk e-mail.
"If anything can go wrong, it will." - Murphy
It is amazing how much spam you can block by filtering out all mail with a "%" or "$" sign in the subject line. Another good one is filtering subject lines ending with "?". Although the question mark filter doesn't work if you are on mailing lists. These are far from foolproof, but could be used to determine the spamness of an email. Hotmail/Yahoo could work on a method for rating/filtering email based on a series of spamness tests.
Having said that, I believe that prevention is better than the cure. Especially from a bandwidth point of view.
Well, it's not, but you know...
Mozilla now comes with it's own Spam Filter starting with 1.3Alpha. Anyone know how well it works? I haven't had a chance to try it.
Think this is off topic? Read the last line of the slashdot story and click the link, where you can take a "Free 30-Day Trial!!"
=)
Mail.app's filtering is fantastic. I only look at around one spam message every two weeks, and I've only had one false positive (which was adveritising something, as it was) in the year and a half that I've been using it. The filter is probably too CPU intensive to use on any large scale, though.
I get advertisements for spamarrest on the bottom of my spam quite often.
This has got to be a spammer that runs it.
The man who trades freedom for security does not deserve nor will he ever receive either. - Benjamin Franklin
well, probably because the spammers already found a way to get by that. Spam nowadays come in different packages. Different subject lines everytime, different email addresses everytime (some are illegal like penis@enlarge.it, I have even seen some from another user who had no idea a spam was sent through their account. Two things also to consider: the amount of CPU power needed to do content filters, and service objective. Like you said, filtering through email address. What about those that use illegal + dynamic addresses? Content, the content is roughly the same. But account for the number of people using hotmail, and account for # of emails per user, and account for the power needed to read through all messages doing an greedy search for matching keywords and phrases. As for service objective, Hotmail is a email provider, and they can't really afford to be wrong in their filtering. Some people use hotmail for professional reasons, and hotmail can't afford to miss
Will I retire or break 10K?
I like this idea with some modifications...
I want to be whitelisted for x number of days. Or maybe a setup similar to DHCP where I've got a lease for x number of days that doesn't expire until I haven't used it for y number of days.
This would allow email to remain FREE like it should be and solve the problem at the same time.
The man who trades freedom for security does not deserve nor will he ever receive either. - Benjamin Franklin
I've watched Spamarrest movie. The exactly same system (you have to read a word, obscured to defeat OCR programs) is beeing used by one of Polish mobile phone operators. If you want to send SMS from www->sms gate you also have to read a word. You can see it here.
:wq
AFAIK, /. doesn't do business with the government.
Are you sure? I'd figure that Congress has set out a pretty broad definition of "doing business with the government", just like the government tries and usually succeeds to classify virtually all commerce as "interstate commerce".
Besides, a blind person could always get a sighted person to help them with the one-time account signup.
Likewise, a person using a wheelchair could always get a walking person to help them with climbing the landlord's stairway to sign up to rent an apartment.
Will I retire or break 10K?
What do you get if you eliminate the human from the above? Why, a protocol link. Might as well require me to type in TCP/IP packets and consider me human if I make too many erorrs :-)
Welcome to the net of 1000 lies. Upgrades are scheduled soon that should bring us to the 10,000 lies mark.
was all done filling in my personal information (retired female homemaker in Antarctica, born in 1891
I'm a 70 year old Afghan woman who is the head of a major multimedia corporation, making less than $20,000 per year. At least, that's what the New York Times thinks...
General Relativity: Space-time tells matter where to go; Matter tells space-time what shape to be.
If you happen to be in the fortunate position of ISP, you can play at racketeering and generally get away with it: offer your subscribers' email addresses for a fee, then offer them spam blocking for a fee. Repeat until your customers are all gone.
Don't think that'll work? Your phone company is already doing it with telemarketers.
An "autonated Turing test" is an oxymoron.
The Turing test is where a human talks to a computer and tries to decide if the backend that's answering him is a human or a computer program.
This is more of a reverse turing test, where the computer asks questions to try and find out if it's interacting with a person or a program.
It would be possible to write a program to beat this system, but it would not qualify as having passed the Turing test, because it would have only fooled another computer program, not a real person. Of course maybe said program could go on to pass the Turing test.
Wouldn't it be weird if spam was the driving force behind the creation of the first real AI?
Skynet began learning at a geometric rate.......by 1800 hours every mailbox in the world was jammed with unfilterable spam.
Life is too short to proofread.
Comment removed based on user account deletion
The way SPAMmers seem to outsmart anti-SPAM filters with every new advancement, they just might make a big leap in AI to get past these new filtering techniques.
I was thinking that a technique that might help is to set up two accounts - something like a hotmail account in addition to your normal email account. One account is the valid one you use for whatever, the other address you don't give out to anyone you expect mail from.
Then, when you get mail at your "real" account that mail is examined to see if it matches any of the mail received at the "fake" account.
This is sort of like the digital camera technique of taking a "picture" of the CCD image with the shutter closed after a long exposure, to get an idea of what just the noise from the CCD looks like so it can be subtracted from the image data collected.
Of course, I'm not sure how well it would work in practice or if you'd really get the same spam very opten in both accounts...
"There is more worth loving than we have strength to love." - Brian Jay Stanley
This is the typical case of spam on ./, just a profit free one.
Less is more !
Is to make it a crime to send email from a bogus account. I'm thinking this crime would be called.. oh I dunno maybe fraud. If I have a real email address then I can request to be removed and am not, then it should be just like telemarketing and I could sue for $500.
As long as you spam me from a legitmate email address I can request that the ISP delete your account. If the ISP chooses not to do so, then I can block the whole damn domain guilt-free. If the ISP has a decent EULA they could sue their subscriber for breaking the terms of their agreement and use that money to pay their various postmasters to take care of spam complaints.
You had me all excited when you said we should be able to "hunt" spammers. I though you meant really hunt them. I was all ready to go get my hunting license and a buy a gun. ;)? What better way is there to 'opt-out'?
They definatly seem to have overpopulated, given the volume of spam I've been getting. Don't you think it's time we thinned the herd
Makes you wonder.....do spammers taste like spam?
Life is too short to proofread.
Looks like it should be very effective in keeping blind people out, though.
Warning: this article may contain humor, sarcasm, parody, and perhaps even irony. Read at your own risk.
(Moreover, there also isn't yet any standard mechanism for users to report spamminess or nonspamminess back to normal IMAP or POP mail hosts -- and Bayesian algorithms require sampling both spam and non-spam mail, not just spam reported to an abuse address.)
Over IMAP this is exeedingly simple. Create two standard folders for the user, say 'Valid' and 'Spam.' Have them sort the mail manually into those folders.
Since IMAP orginizes the mail on the server, reading the contents of those folders after the user sorts his mesages and using it as a base for filtering would be no problem at all. Additionally, it has the benifit that the user could simply chose to ignore or delete those folders, side stepping privacy issues.
I'm well aware that an admin *can* read my mail... Fuck, I'm the admin after all... But I don't think running a filtering program over the contents of two folders is anymore invasive than running SpamAssasin or SpamBouncer over their incoming mail. It does become an issue if the admin pokes his nose into a person's scoring system... But then again, an admin could just as easily collect the same statistics using a simple shell/perl script.
The biggest problem is that this type of filtering is that it's an administration/support nightmare. I wouldn't be willing to hand hold a few hundred users through the conversion to IMAP, explain how they are supposed to sort their mail, or deal with the inevitable issues accompanying a complex system like this.
Plus, are filteres of this type even available as a semi stable product?
P.S.: IMAP rocks. ^_^
the project itself is pretty interesting, but something rubs me the wrong way about the term "automated turing test". The turing test is based on the idea that sentience can not be defined in any simple mechanizable way.
maybe it's just my cognitive science degree making me touchy, but i'd prefer the term "automated coherence filter" or something(even "automated intelligence test" would be an improvement).
lysergically yours
From my understanding, the use of image recognition in the captcha test would make it nearly impossible for blind people to pass the test.
How, praytell, would you get e-mail addresses or domains to put in the block list without first getting spammed by them? Consult Yahoo's new Magic 8 Ball service?
FTC Consumer Complaint form
It's that simple. Once the federal government starts to get half a million reports of spam a day, may be someone will realize that it's costing a lot of money to a lot of people and maybe Congress will act.
there's no place like ~
you could just include 4.) and 5.) in the category 1.) of "giving out your email address"
I guess I was too specific.
Repeal the DMCA!
I just apt-getted it and I'm trying it out now. mailfilter works well, but it has to be added to each users cron scripts (blech!)
You can't judge a book by the way it wears its hair.
I have gotten spam on my fastmail account, but I'm not using their spam filters. The thing that fastmail does that I haven't seen is that in addition to allowing the usual (for recent email systems) tagged login format like username+tag@fastmail.fm , which lets you give everybody email addresses with a different tag value, it also automagically translates between this and tag@username.fastmail.fm - this not only avoids confusing web forms and avoids confusing your mother, it also reduces the risk that spammers will guess that simply using the untagged "username@domain.com" will reach you.
Bill Stewart
New Fast-Compression-only CPR http://preview.tinyurl.com/dy575ks
"Make something idiot proof, and they will design a better idiot."
I Browse at +4 Flamebait
Open Source Sysadmin
Turing test is a bit of an exaggeration. They have you look at some garbled text and type what you see. And it's been going on for a very long time.
The Register article had absolutely nothing of value to add. As you were.
Donate background CPU time to fight cancer.
1. Decide which hotmail/yahoo/whatever account you want to sign up.
2. Send most of the (fake) registration info until it sends you a "turing test" image.
3. Display the image in the next webhit on your popular porn site saying "to get free porn, type these characters"
4. Send whatever they type to hotmail/yahoo/whatever & complete your registration.
5. Profit?
Some people have already produced excellent results in breaking visual CAPTCHAs.
... why don't they use the Voight-Kampf test to tell them apart?
I fuse with Mercer every single day...
Is Paul Graham's statistical filtering of spam applied anywhere?
:)
To me, it seems as an obvious step forward in spam filtering and achieves amazing results.
Somehow though, most good ideas get ignored
What are you wittering on about? MS doesn't sell addresses to spammers, it's against the privacy policy and EVEN MS wouldn't be stupid enough to break their OWN privacy policy. The short/dictionary names are simply being bruteforced - anyone doing mail admin on a decent sized domain sees the same thing all the time.
---- Den ene knappen er powerknapp, den andre er Bender voice knapp "Bite My Shiny Metal Ass"
A lot of spammers like to guess names, like datacommarketing.
:)
On the mail servers I manage, they just keep sending mails to all kinds of addresses. like this:
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: damien@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: bart@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: agustin@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: hans@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: stan@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: adolfo@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: murray@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: curt@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: russel@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: erwin@DOMAIN.com... User unknown
Dec 22 07:18:14 www sendmail[50726]: gBM6IAcC050726: from=joe@nowhere.com, size=0, class=0, nrcpts=0, proto=SMTP, daemon=MTA, relay=mx01.datacommarketing.com [65.242.117.50]
After a while it get's annoying and you block their entire subnet in the firewall.
I can't figure out why Worldcom wants to provide them with traffic. Maybe they need the money.
I visited their site once where they claimed that all their emails where opt-in. So is it opt-in as in "the email-adr exists so they must want spam"?
my sig
I only use SpamAssassin to tag suspect emails. I have a filter rule in KMail that sends tagged mail directly to it's Trash folder. A quick scan of the subjects and froms suffices to weed out the (rare) false positives. Note that I don't have to read the spam bodies to verify them and I've already been spared the trouble of weeding them from my legitimate mail.
Use a little imagination; it isn't necessary for a spam filter to immediately trash suspect mails. By default, all SpamAssassin does is TAG the emails in their subject lines and add a scoring report to the body. It suffices for me to have probable spams all collected together so that it is only one quick scan and a button click away from destruction.
Come to think of it, if my quick from/subject scan method doesn't suffice, that attached scoring report does. A mail with a score of 33 with a web bug is certainly bogus. I'll cheerfully trash that without reading the rest of the body and those reports can be quickly parsed as well. Not that I usually bother. Simply having your signal not interleaved with the probable noise is useful and SpamAssassin can certainly be trusted for that.
I alerted Slashteam to the existence of CMU's Captcha project six months before they began development on their own system, and even outlined a manner in which it could be used to stop all scripted posting to Slashdot. I will do so again here, for reference:
Logged-in posters don't need CAPTCHA, because their account is their authentication. Accounts are already rate-limited per day. So the problem is with AC posting. If AC comments posted logged-in do not already count towards an account's comment total, that should be corrected.
When an AC poster wants to post to Slashdot, Slashdot should request a cookie (we'll call this cookie "A"). If the client does not have "A", then he is presented with a CAPTCHA dialog. If he passes the dialog, "A" is set to a random hash. The value of cookie "A" represents a "license to post", and the server will allow it to be presented by an AC a certain number of times (admin variable "X") to post comments. After the cookie value (license) has been used X times, that hash value (license) is expired, and is no longer valid to post, and the client must pass CAPTCHA again.
The storage structure required would be a perl hash keyed by the license, with the data value being equal to the number of posts remaining per license. A process to "age" this table would have to run on an interrupt. No DB interaction is required, and the entire system could be managed in resident memory. Not rocket science by a long shot.
X could be set initially to 5, and tweaked per system. This isn't hard to implement (it's no harder than formkeys) and it was given to Slashteam along with a reference to CMU's CAPTCHA project long before they ever started writing their own CAPTCHA code.
Sadly, the automated posting problem on Slashdot is still alive and well as I write this comment, despite the fact that I have documented the solution.
So, based on that information, you might be able to take your evaluation further.
--
What happens when you outlaw guns
I think this method could very easily be used to create an almost spam-proof email client.
The idea is to have a buddy list in your email client, which is a list of all the people authorized to send you email. If one of those people sends you an email you simply get it.
If someone not on your list sends you an email, the mail client automagically sends them a reply explaining that they need to pass a test. That test could be one with a scrambled text image or whatever. Once they pass the test (replying to the email with the right answer) the email client tells you that a new buddy sent you an email, and if you want to permanently add them to your list.
The list could also contain wildcards to use when you expect to get an automated email (like a bill from a credit card company) but you don't know the exact email ahead of time.
It sounds like a good idea to me, I was wondering if anyone could think of reasons why this wouldn't work
For instance, I'm currently looking for a job and one of the newsletters I get had been marked as spam by someone. I unblocked it, and from then on those newsletters weren't marked as spam. So it's got some smarts to it, and even if it makes a wrong decision, it's reversible.
I believe it also evaluates members of the community's decisions, so that people who block things that are later unblocked become weighted less, and thus irrelevant.
YMMV but I've had a great experience with SpamNet.
I feel fantastic, and I'm still alive.