for months now, images have been increasingly seen in spam, reaching up to 30 to 40 per cent of all spam total. for a while, counter-measures have been in play, developed by many different folks, some we know, some we don’t. from system administrators developing signatures to a team at spamassasin working on an ocr system to break these images and check their text for spamishness.
when first encountered, a friend of mine was as excited as me: “why, it’s exactly like a captcha, only in reverse!”
hence the term i just coined – reverse captcha.
as it’s a cat and mouse game of escalations and counter-measured by bad guys and good guys, the bad guys learn and make our lives more difficult. i will try to explain what a reverse captcha is to me (and no, it’s not a special type of turing test, although we touch on that below).