by jithin parakka

CAPTCHA

How it Works, How to create your own CAPTCHA

by jithin parakka

CAPTCHA

Completely Automated Public Turing test to tell Computers and Humans Apart

by jithin parakka

CAPTCHA

• You can see it in almost all signup form• It is used to verify that whether the party

who is filling the form is a human or a computer program instructed to do so

• The most common form of CAPTCHA is an image of several distorted letters

by jithin parakka

Interesting fact

• People who design the tests aren't always upset when their tests fail

• Because every CAPTCHA failure is really an advance in artificial intelligence.

by jithin parakka

CAPTCHA and Turing

• Is a reverse Turing test• it is administered by a machine and targeted

at a human• in contrast to the standard Turing test that is

typically administered by a human and targeted at a machine

by jithin parakka

Things to be taken care

• Your CAPTCHA image should not contain any metadata.

• Make modification to the text.• Don't use same CAPTCHA always.• One possible method to implement CAPTCHA

is to store a set of image and corresponding text in database and compare them.

• Another solution is create image on the fly.

by jithin parakka

How to create CAPTCHA

• Generate a random text– Here I am using md5 function to generate a random

text. I am using php microtime() function as input to md5 to make it random. Store this in session variable.

• Create an image using php ImageCreate() function

• Make the image very complex using functions available in image library and draw the generated text on the image using ImageString() function.

by jithin parakka

How to create CAPTCHA

• Now we have CAPTCHA image and its value stored in session variable.

• Compare the value entered by user and value stored in the session variable. Thats it.

by jithin parakka

THANK you