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Accessibility and Usability Forum

Is < CAPTCHA > accessible and usable?

 10:18 am on Mar 2, 2006 (gmt 0)

CAPTCHA - Completely Automated Public Turing Tests to Tell Computers and Humans Apart

I'm currently working with a classic ASP text generated CAPTCHA system that Birdman [webmasterworld.com] turned me on to. While working with this particular CAPTCHA system I realized that it may present an issue with those who have visual disabilities.

How do Screen Readers react to CAPTCHA?

In our quest to battle form abuse, are we implementing features that put up major roadblocks for those with disabilities?

Receiving garbled online form submissions



 10:49 am on Mar 2, 2006 (gmt 0)

How do Screen Readers react to CAPTCHA?

They don't - the screen reader user is completely blocked at this point, unless you offer him/her an audio file alternative, which is the only option that I've ever seen.

In our quest to battle form abuse, are we implementing features that put up major roadblocks for those with disabilities?

Probably. But I think that's more lack of discussion about usability than anything else. The technology to achieve security and accessibility is certainly in existence. Certainly the W3C formally expressed their dislike:-


If you want to use Captcha but maintain accessibility, you need to have an audio file download available telling the hard of sight user what text to type in. If accessibility is important to your site, it shouldn't be too tricky to organise.



 11:16 am on Mar 2, 2006 (gmt 0)

Great link TJ!

The widespread use of CAPTCHA in low-volume, low-resource sites, on the other hand, is unnecessarily damaging to the experience of users with disabilities. An explicitly inaccessible access control mechanism should not be promoted as a solution, especially when other systems exist that are not only more accessible, but may be more effective, as well. It is strongly recommended that smaller sites adopt spam filtering and/or heuristic checks in place of CAPTCHA.

Well, it was at least fun testing the CAPTCHA solution. I'm going to put it away in the attic for now. ;)


 11:19 am on Mar 2, 2006 (gmt 0)

I've come across a number of visual captchas that I have had some trouble distinguishing, and my eyesight is not impaired. They are just increasingly garbled to foil form spammers, but the end result is they're getting more frustrating for real people to use.


 11:42 am on Mar 2, 2006 (gmt 0)

I've been exploring some of the alternatives in this thread:-


With the particular site in question, by the nature of the subject matter (radio), I have a large proportion of blind and hard of sight users.

Ultimately, I may need to go Captcha as a last line of defence, but I'll be serving the Captcha only to anything or anyone hitting the script multiple times inside a certain time frame.

Well, it was at least fun testing the CAPTCHA solution. I'm going to put it away in the attic for now

Don't write it off just yet.

One of the things I've been exploring is serving an audio file along with the traditional Captcha image. I found a free (GPL) Java based speech synthesiser on Source Forge:-


The general idea with audio is to replicate the complexity of the Captcha graphic image. Voice recognition systems have got pretty accurate in recent years, but they're still not "intelligent". It's the lack of intelligence that needs to be taken advantage of. Just like the graphic equivilent, you can mix up elements, for example:-

1. "The numbers are one, three and thirty-five"
2. "Your three numbers are thirty-five, one and three"
3. "Please enter three in the first box, thirty-five in the last box and one in the second box".

It's generally accepted that combinations like the above, served in random order, would make life very hard for an automated process to decode.

Audio equivilents of the Captcha type system are completely accessible to the blind. Combinations of audio and graphic cover both hard of hearing and hard of sight users.

So I can see a future in this system, without the destruction of accessibility.



 5:25 pm on Mar 2, 2006 (gmt 0)

I have implemented several CAPTCHA with audio files (never without audio). What I typically do is record a wav sound of all possible characters. The raw text string is stored securely on the server and retrieved by some form of session cookie or similar information. Then there's a link to a script which merges the applicable sound files before sending them back to the user. Has worked quite well.

Along with TJ's suggestion of merging that with additional information, I think it can be both accessible, usable, and nifty.

As far as the graphical representation goes... For those with no mobile impairments, I also offer the option of clicking on the image to generate a new graphic (same code, though).


 5:36 pm on Mar 2, 2006 (gmt 0)

The presumption is that CAPTCHA requires text in distorted image format so machines cannot automatically read it.

This is incorrect.

I have developed CAPTCHA solutions where a random image of an animal appears. The animal's name has to be selected from list. The image can be zoomed. Additionally the animal's name is attached in a small MP3 file.

Two dozen images and two dozen sounds files are sufficient.

This is much simpler then creating random number and letter strings then converting them to image then applying distortion.


 6:00 pm on Mar 2, 2006 (gmt 0)

Using that approach, however, assumes that you are a semi fluent speaker of the language. For English sites that is a dangerous assumption.

Otherwise, yes, that approach works very well for low traffic sites. The randomness is, however, insufficient on large sites.


 12:54 am on Mar 7, 2006 (gmt 0)

Actually I used it on a tri-lingual web site, and it worked well.

As for quantity of images, it can be increased.


 6:23 am on Mar 7, 2006 (gmt 0)

For someone the size of Yahoo, for example, you would need thousands of words in hundreds of languages. The randomness and flexibility is simply not enough on truly large sites.
And, most smaller sites don't need CAPTCHA anyway.

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