Practical non-image based CAPTCHA approaches?

Answer 1

I know that no one will read this, but what about the dog or cat CAPTCHA?

You need to say which one is a cat or a dog, machines can't do this.. http://research.microsoft.com/asirra/

Is a cool one..

Answer 2

So, CAPTCHA is mandatory for all users except moderators. [1]

That's incredibly stupid. So there will be users who can edit any post on the site but not post without CAPTCHA? If you have enough rep to downvote posts, you have enough rep to post without CAPTCHA. Make it higher if you have to. Plus there are plenty of spam detection methods you can employ without image recognition, so that it even for unregistered users it would never be necessary to fill out those god-forsaken CAPTCHA forms.

Answer 3

Be sure it isn't something Google can answer though. Which also shows an issue with that --order of operations!

Answer 4

Although we all should know basic maths, the math puzzle could cause some confusion. In your example I'm sure some people would answer with "8" instead of "1".

Would a simple string of text with random characters highlighted in bold or italics be suitable? The user just needs to enter the bold/italic letters as the CAPTCHA.

E.g. ssdfatwerweajhcsadkoghvefdhrffghlfgdhowfgh

In this case "stack" would be the CAPTCHA. There are obviously numerous variations on this idea.

Edit: Example variations to address some of the potential problems identified with this idea:

• using randomly coloured letters instead of bold/italic.
• using every second red letter for the CAPTCHA (reduces the possibility of bots identifying differently formatted letters to guess the CAPTCHA)

Answer 5

Although this similar discussion was started:

We are trying this solution on one of our frequently data mined applications:

A Better CAPTCHA Control (Look Ma - NO IMAGE!)

You can see it in action on our Building Inspections Search.

You can view Source and see that the CAPTCHA is just HTML.

Answer 6

Very simple arithmetic is good. Blind people will be able to answer. (But as Jarod said, beware of operator precedence.) I gather someone could write a parser, but it makes the spamming more costly.

Sufficiently simple, and it will be not difficult to code around it. I see two threats here:

1. random spambots and the human spambots that might back them up; and
2. bots created to game Stack Overflow

With simple arithmetics, you might beat off threat #1, but not threat #2.