I think that the problem that we will always run into, is that any human-performable task will either be cracked by someone writing a bot, or made trivially ine

I think that the problem that we will always run into, is that any human-performable task will either be cracked by someone writing a bot, or made trivially inexpensive by apps that charge as little as $0.00139 per solved captcha (see [1]). Microsoft has implemented a tagging-type captcha, ASIRRA (see [2]), for which you can hire out the results for $0.004 per solved captcha[3]. I think the only real solution is to make it cost real money (say $0.25 or $0.10) to perform whatever action you are protecting, so that repeated attempts are prohibitively expensive, but one or two by a legitimate user is not too expensive. Otherwise, financially-driven spammers will always find a way to inexpensively circumvent the protection. [1]: http://www.deathbycaptcha.com/user/order http://www.deathbycaptcha.com/user/order [2]: http://research.microsoft.com/en-us/um/redmond/projects/asirra/ http://research.microsoft.com/en-us/um/redmond/projects/asir... [3]: http://de-captcher.com/ http://de-captcher.com/, no direct link to pricing without registering