reCaptcha's public and private keys are applicant-specific and domain-specific, you can't use reCaptcha by providing arbitrary strings or copy-n-paste wildly for them, they must be got from reCaptcha officially.
Since reCaptcha has been acquired by Google, you will need to sign in Google to get the specific public and private key for youself. Here is the url: https://www.google.com/recaptcha/about/
Different applicants can have different key pairs on the domain: LOCALHOST . You can take this advantage for your local development or test machine. I hope the information helpful.