If you’ve ever looked at an open-source development project hosted by Google servers, usually on
http://code.google.com sites, Mailhide will be familiar. It is a less well-known application of the reCAPTCHA detection challenge.
reCAPTCHA Turing test
Mailhide conceals part of an email address
This is how it prevents spammers from accessing email addresses using automated programs. Typically, the first few letters, or numbers, of the username part of the email is visible, followed by an ellipsis i.e. three dots, and then the domain name.
Most Google employees* use Mailhide. Mailhide is offered as an option to developers using Google Code sites.
Mailhide type functionality is also offered by Slashdot for user accounts. Slashdot is not necessarily using Google reCAPTCHA for encryption, however. There are other Turing tests besides reCAPTCHA.
reCAPTCHA is a Google product. It was not developed by Google, though. Google purchased the reCAPTCHA algorithm from Carnegie-Mellon University a few years ago, in 2008.
reCAPTCHA Mailhide API
Are you running a web application that lists users’ email addresses? Do your users a favor by shielding them from spam with reCAPTCHA Mailhide.
Google will give you an API (cryptographic) key. Use it to encrypt user email addresses. Google supplies full documentation for the Mailhide protocol. Everything is free of charge.
I am uncertain whether API restrictions on usage apply. That is a familiar restriction for applications developers relying on the Twitter API. It should not be a binding constraint in this case, as Mailhide is far less transactional that Twitter. Unless one is very, very popular!
reCAPTCHA comes in many flavors!
Libraries are available for PHP, Perl, Ruby and Python programs.
*Google employee accounts in the U.S.A., and many but not all other countries, have the format
firstname.lastname@example.org. Non-employee Google mail accounts are