How Captchas work?

[RonaldA]

When we submit a form or register online on yahoo or google and other secure connections, we asked to type the word in the image (Captchas) given on registration page. whats the use of that and how it works?

[Techno01]

A CAPTCHA image shows a random string which the user has to type to submit a form. This is a simple problem for (seeing) humans, but a very hard problem for computers which have to use character recognition, especially, because the displayed string is alienated in a way, which makes it very hard for a computer to decode.

To use this technology, your web pages have to be generated dynamically in any programming language.

[johnson22]

Both webserver and CAPTCHA server need a common secret key. The picture is integrated quite similar to page counters. As a parameter you have to send a random string within the link. The secret key and the random string are used to generate the password. Both strings are needed to compute the password. The password is computed by the CAPTCHA server to generate the image and by your server to validate the users input. A random string should not be used multiple times.

[Trio]

Captchas are those images you see on many sites with letters and numbers, where the site asks you to type them in. The purpose of this is to prove that you are a human.

The web has always been plagued by automated software, robots, that go around various sites and create fake usernames to send spam from. The idea was that by requiring text from an image to be typed in, the robot would not be able to do it.

Captchas doesn't work. Bcoz the problem is that text recognition technology is pretty advanced. Spammers have started including such technology into their software, and can keep up with the advancements in captchas. To this day, the majority of captchas used on the web can be beaten by some of those bots.

[Techguru01]

Some sites, especially banks and financial sites, use images of things instead of words. They ask you to memorize the look of a bird, and then confirm that you see the bird as you login.

This is fundamentally flawed, as a true captcha requires the images to be automatically generated. If the images have to be manually made by humans, such as pictures of birds, that means there is a finite number of such images. If the site has, let's say, 100 different bird pictures, then all the bot maker has to do is go through the 100 images and code them inside his program.

[Jackson2]

CAPTCHA stands for “Completely Automated Public Turing test to tell Computers and Humans Apart” which to the rest of us means those images of text that you have to read and enter into a text field in order to identify yourself as a human, and not spam, when registering for a website.

There one new concept recently started i.e. Recaptcha. The concept is to use 2 words instead of 1, side by side in the image. The user is asked to type in both words. The first word would be known by the site, and be used in the typical captcha manner. The second word is taken from a book scanning project as one of the words the text recognition software can't make out.

This way by using recaptcha, you both ensure that the image will be harder for bots to decipher, and you help organizations scan books.

It really seems to me that there is no catch all accessible alternative to CAPTCHA that can be secured from spammers. As we’ve seen some sites make efforts to incorporate an audio CAPTCHA but this isn’t sufficient, even if a logic question were thrown into the mix, (putting aside the fact that this places a lot of development work on the website owner to provide all three options). In addition to this it seems that it is possible to break CAPTCHA’s either using automated techniques or humans. Despite all that however it certainly seems that website owners are choosing security over accessibility.