CAPTCHA is a type of challenge-response test used to determine if the user is human. It displays distorted letters as an image that must be correctly interpreted. The purpose is to tell computers and humans apart, by having computers fail the test. When designing CAPTCHAs, the text should be modified and different images should be used each time, rather than always using the same one. To create a CAPTCHA, random text is generated and an image is created showing that text in a complex, distorted way. The text value is stored, then on form submission it is compared to what the user entered to verify they are human.

1. CAPTCHA
How it Works, How to create your
own CAPTCHA
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2. CAPTCHA
Completely Automated Public Turing test to
tell Computers and Humans Apart
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3. 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
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4. 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.
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5. 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
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6. 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.
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7. 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.
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8. 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.
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9. THANK you
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