April 23, 2014


A Challenging Response to Challenge-Response

One of the trendy ideas these days is challenge-response (CR) anti-spam technologies. The idea is simple: incoming email is intercepted before you see it, and a “challenge” email is returned to the sender. If the sender replies to the challenge message, then the original message is forwarded on to you; otherwise it is discarded. The idea is to require some kind of human involvement in the sending of each message. Sometimes the sender has to answer some kind of puzzle that is supposed to be easy for people but hard for computers.

Whenever we analyze a security technology – and that is what CR is – we need to look not only at the immediate effect of the technology, but also at how people will adapt to it. We need to look especially at how the bad guys will adapt. Will they adjust their attack strategy to defeat the new defense? Will the new defense create new opportunities for malicious attacks? Will the technology lead to an arms race between defenders and attackers? If so, can we predict the outcome of the arms race?

CR stands up poorly to this kind of analysis. To see why, suppose that Alice sends an email to Bob, and Bob is using CR. Bob’s computer sends a challenge message back to Alice and awaits her response. This challenge message had better get through to Alice; if it doesn’t, the whole scheme breaks down. If Alice is using anti-spam technology that blocks the challenge message, then she’ll never see the challenge – her original message won’t get through to Bob, and she won’t know what went wrong.

We can fix this problem by making sure that Alice’s anti-spam technology has a loophole for challenge messages, to make sure they are never blocked. (Note that although Bob is the one using CR, it is Alice who has to create the loophole.) If CR is going to succeed, most of the Alices out there will have to open the loophole. Messages with certain “challenge-ish” attributes will be mostly immune from spam controls.

At this point, the bad guys’ response is obvious: create spam that can exploit the loophole, spam that looks like a challenge message. If they can do this, then CR will have made things worse – spam will pour in through the loophole.

We might try to solve this problem by narrowing the loophole, requiring the challenge messages to be so narrowly stylized that they cannot carry a spam. This too creates an opportunity for the spammers. If the challenges are so predictable, then the spammers will be able to develop computer programs that spot the challenges and auto-send the required responses. If they can do this, then the spammers can just add automated CR responses to their automated email-sending software, and continue to pollute our inboxes.

Given all of this, I’m skeptical of CR as a response to email. If you’re the first on your block to adopt CR, and if nobody else uses anti-spam technology, then CR might provide you some modest benefit. But it’s hard to see how CR can be widely successful in a world where most people use some kind of spam defense.


  1. Mt. Molelog says:

    Felton on Spam Challenge/Response

    Edward Felton has some comments on email challenge/response systems. He’s got some valid points, but I think he’s missing two important things.

    First, the ‘hole’ that needs to be opened in the sender’s Spam blocker doesn&#8

  2. arcticzoo says:

    Spam and Email Filtering

    I’ve been using a program called qconfirm for over a year now to help stem the deluge of spam that choked my inbox on a daily basis. If you send a message to one of my publicly available email addresses…

  3. all feet on deck says:

    Challenge response(s)…

    Ed Felten makes some interesting observations on the use of challenge-response mechanisms as a possible antidote for spam. I agree with the problems he cites, yet I see another significant practical ‘challenge’. The example that comes to mind is the…