Thread: How CAPTCHAs Usually Work?

Could anyone explain how captchas work?
Is there any particular format of captcha or we can use any type of captcha to our website?

They can work in completely different ways, but generally the process is this:

When the page is loaded a dynamic image is created with noise in the background (stops certain types of bots from "reading" the image) and some text on it. The GD library in PHP is good to do this.
When an image is created store the text in a cookie or session variable (session variable would be more secure) - make sure to overwrite this if the page reloads.
Ask the user to input the correct text.
After the form is submitted check that the string entered matches the string in the cookie/session variable.

Thats how I make them anyway. I should probably re-vamp my script for that actually, it's not been updated in about a year.

Originally Posted by tsdesigns

They can work in completely different ways, but generally the process is this:

When the page is loaded a dynamic image is created with noise in the background (stops certain types of bots from "reading" the image) and some text on it. The GD library in PHP is good to do this.
When an image is created store the text in a cookie or session variable (session variable would be more secure) - make sure to overwrite this if the page reloads.
Ask the user to input the correct text.
After the form is submitted check that the string entered matches the string in the cookie/session variable.

Thats how I make them anyway. I should probably re-vamp my script for that actually, it's not been updated in about a year.

well explained about the working of captcha thank you