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for some niche reasons I often find that a text-only captcha would be better than the traditional image/audio solutions. Some of the reasons I can think of off the top of my head:

  1. Text-based browser support(lynx, links, etc)
  2. To provide write-capable public APIs, but prevent massive amount of spam
  3. For accessibility reasons (I've heard that the audio versions are very hard to understand to prevent voice-recognition software attacks)

Are there any text-only CAPTCHA systems available out there?

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http://textcaptcha.com/ is a logic based, but also purely text based, captcha system., and is free too.

I don't think you have any control over the type of questions that are passed to you, but worth reading the documentation. Whilst it may pose problems for users with cognitive disabilities, it's on the right track, and an improvement on the standard recaptcha type captchas, which create difficulties for blind and vision impaired users alike.

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All of the CAPTCHAs I've seen that don't use images or audio involve some sort of cognition:

  • What day of the week is tomorrow?
  • Who was the host of last week's SNL?
  • What is 2 + 3 plus four?
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  • I use to implement a simple math problem captcha for my blog. I didn't even have it change (something like what is 2 plus 4) per request. It did eventually get spammed, but it hasn't since I changed it to what is two raised to the power of one. But a changing problem is definitely the way to go for anything mainstream
    – Earlz
    Commented Feb 9, 2011 at 4:53
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    You could ask e to the power of (pi times i), and reduce both spam and real users. Two birds with one stone!
    – Justin Morgan
    Commented Feb 9, 2011 at 5:40

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