Are you Human?

What is CAPTCHA? Completely Automated Public Turing test to tell Computers and Humans Apart It uses a type of challenge-response test to determine that the response is not generated by a computer. The goal: To create an automated test that is easy for a human to pass but difficult for a computer

Turing Test Reverse Turing Test „Standard Interpretation" player C, the interrogator, is tasked with trying to determine which player - A or B - is a computer and which is a human. Reverse Turing Test A CAPTCHA is sometimes described as a reverse Turing test, because it is administered by a machine and targeted to a human.

Background CAPTCHAs are based on open AI problems Breaking CAPTCHAs help advance AI by solving these open problems Improving CAPTCHAs help telling computers and human apart Win-win situation

Applications Preventing Comment Spam in Blogs. Online Polls. You cannot trust the results of an online roll because anybody could just write a program to vote for their favorite option thousands of times. Protecting Website/E-mail Registration Preventing Dictionary Attacks (in password systems). Prevent a computer to iterate through the entire space of passwords by requiring it to solve a CAPTCHA after a certain number of unsuccessful logins.

Types of CAPTCHAs Text based Graphic based Audio based Gimpy, ez-gimpy Gimpy-r, Google CAPTCHA Simard’s HIP (MSN) Graphic based Bongo Pix Audio based Simple Math based

Breaking CAPTCHAs Most text based CAPTCHAs have been broken by software (OCR) Using cheap human labor to process the tests (sweatshops). Insecure implementation that allow the attacker to completely bypass the CAPTCHA

reCAPTCHA New form of CAPTCHA that also helps digitize books The words displayed to the user come directly from old books that are being digitized and OCR could not identify Pairs an unknown word with a known one Distorts them both and puts a line through them and then sent them to be proofread Respondent answers both elements: half of effort validates the challenge the other half is captured as work

No CAPTCHA reCAPTCHA (by Google)

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