What is CAPTCHA :

CAPTCHA stands for Completely Automated Public Turing test . CAPTCHA is implemented to differentiate between humans and machines ,it is generally used to keep machine or bots or automated scripts away from the website page or content protected by it . .

Where we use Captcha :
CAPTCHA is generally used to protect following cases :
1. Website Registration form
2. Online Polls
3. Comment functionality in Blogs
4. login after failed attempts
5. E-ticketing
6. Google uses CAPTCHA for search queries when there have been large numbers of queries from a single IP address

Types of CAPTCHA :
Mainly CAPTCHA can be divided in following categories :
1. Audio CAPTCHA
2. Distorted word CAPTCHA
3. Math solving CAPTCHA
4. video CAPTCHA
5. Image based CAPTCHA

Method to Bypass CAPTCHA :
CAPTCHA can protect the web page in certain extent or u can say that it can protect when less professional attack is carried out . But when advance method is carried out by bots then the CAPTCHA protection can be break out .Below are the techniques which is used generally to Bypass CAPTCHA .

1. OCR(Optical Character Recognition)
   
2. Audio to text converters .
   
3. By Reusing of session ID .
   
4. Breaking MD5 of CAPTCHA solutions
   

Captcha solving service provider :
Below is the list of the service provider who provide services for solving CAPTCHA :
1. De-captcher
2. GSA Captcha Breaker
3. Imagetyperz
4. Deathbycaptcha
5. 2captcha

Service provider for CAPTCHA:
There are various companies which provide service that protects your site from spam and abuse ,one of these is Google reCAPTCHA. Which is a free service provided by the google.

Add ons for CAPTCHA :
There are many add ons available in the market which you can be Install in your browser and you can get rigid of entering the CAPTCHA by your self .Generally these add ons are for the user who are suffering from dyslexia and/or sight problems .Below is the list of extension available in the market :

1. Captcha Monster for Firefox .
   
2. Rumola for Firefox , Chrome , Safari
   

Checklist while performing Captcha testing :
1. Entry of CAPTCHA should be case sensitive .
2. Audio support should be availabe if the website is supported for blind end-user .
3. Refresh option should be available to refresh the text in CAPTCHA as it will help the user to change the CAPTCHA characters if they are not comfortable in understanding it .
4. Check the background noise while testing Audio CAPTCHA .
5. If third party services is used for CAPTCHA then its is important to see whether CAPTCHA is rendering smoothly in sync with the other components of the webpage .
6. Make it sure that after each wrong entry CAPTCHA will get refresh automatically As it will protect the CAPTCHA field against Brute force attack .