Capcha stands for 'Completely Automated Public Turing test to tell Computers and Humans Apart'
Google reCaptcha usually contains two ciphered words as shown in above image.
Among the two words one word is 'Control word' while other is 'Unknown word' i.e. 'Control word' is the word for which reCaptcha software knows the correct result where for the 'unknown word' Google reCaptcha software itself don't know the exact result.
Q : So, the question is if i write a invalid word in place of 'unknown word' in the reCapcha response will it pass.
A : Yes, it will pass as Google itself does not know the correct result :)
For example :
If two words are displayed as a challenge, then the word which seems more difficult to be readable by humans and OCR is the 'Unknown word.' (In above picture "ynnonp" looks like the unknown word.)
So if you type the first word(ynnonp) incorrectly and second word (execellent) correctly, you will be authenticated successfully.
To get more info about 'Control word' and 'Unknown word' and what is the story behind this go to "http://www.google.com/recaptcha/learnmore".
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