CAPTCHA 1 Are you Human? (Sorry, I had to ask)

CAPTCHA 2 Agenda What is CAPTCHA? Types of CAPTCHA Where to use CAPTCHAs? Guidelines when making a CAPTCHA Ways to break CAPTCHAs reCAPTCHA Human Computation Games

CAPTCHA 3 Example: Filling out a form Google uses CAPTCHA for Gmail accounts:

CAPTCHA 4 Beginnings C ompletely A utomated P ublic T uring test to tell C omputers and H umans A part Created in 2000 for Yahoo to prevent automated account registration, by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford, Carnegie Mellon University.

CAPTCHA 5 What is CAPTCHA? A program that can tell whether its user is a human or a computer. It uses a type of challenge-response test to determine that the response is not generated by a computer.

CAPTCHA 6 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.

CAPTCHA 7 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.

CAPTCHA 8 So, CAPTCHA is… A program that can generate and grade tests that: Most humans can pass; Current computer programs cannot pass.

CAPTCHA 9 Making a CAPTCHA Pick random string of characters (or words) ifhkfp Renders it into a distorted image

CAPTCHA 10 Making a CAPTCHA … and the program generates a test: Type the characters that appear in the image

CAPTCHA 11 Type: Early CAPTCHAs Generated by the EZ-Gimpy program; Used previously on Yahoo!

CAPTCHA 12 Type: Improved CAPTCHA high contrast for human readability; medium, per-character perturbation; random fonts per character; low background noise;

CAPTCHA 13 Type: A modern CAPTCHA rather than attempting to create a distorted background and high levels of warping on the text; focus on making segmentation difficult by adding an angled line;

CAPTCHA 14 Other Types of CAPTCHA Animated CAPTCHAs 3D CAPTCHA ASCII art Reverse CAPTCHA "Leave this field blank"

CAPTCHA 15 Other: Cognitive Puzzles Distinguish pictures of dogs from cats Choose a word that relates to all the images Trivia questions Math and word problems 3D Object CAPTCHA Solve failed OCR inputs

CAPTCHA 16 Other: Distinguish pictures Microsoft Asirra (Animal Species Image Recognition for Restricting Access); Microsoft Asirra KittenAuth Project. KittenAuth Project

CAPTCHA 17 Other: Tests „Common Sense" questions: „What is 3 + 5?“ „What color is the sky?" Type the word 'orange'; Require a valid to approve; These attempts violate principles: they cannot be automatically generated; they can be easily cracked given the state of AI.

CAPTCHA 18 Where to use CAPTCHAs? Data Collection Worms and Spam Preventing Comment Spam in Blogs Protecting Addresses From Scrapers Online Polls Protecting Website Registration Preventing Dictionary Attacks Search Engine Bots

CAPTCHA 19 Where to use CAPTCHAs? Preventing Comment Spam in Blogs. Protecting Addresses From Scrapers. Mechanism to hide your address, require users to solve a CAPTCHA before showing your address 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.

CAPTCHA 20 Where to use CAPTCHAs? Protecting Website Registration. ( services: Yahoo, Microsoft, Google) 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. Search Engine Bots. It is sometimes desirable to keep webpages unindexed to prevent others from finding them easily.

CAPTCHA 21 Guidelines Image Security. Images of text should be distorted randomly before being presented to the user. Script Level Security. Insecurities: Systems that pass the answer in plain text; Systems where a solution to the same CAPTCHA can be used multiple times ("replay attacks").

CAPTCHA 22 Guidelines: Accesibility Hard to read CAPTCHAs:

CAPTCHA 23 Guidelines: Accesibility Worst CAPTCHAs:

CAPTCHA 24 Ways to break CAPTCHAs Exploiting bugs in the implementation that allow the attacker to completely bypass the CAPTCHA; Improving Character Recognition software (OCR – Optical Character Recognition ); Using cheap human labor to process the tests (sweatshops).

CAPTCHA 25 Break: Character Recognition Programs that have the following functions: Extraction of the image from the web page Removal of background clutter, for example with color filters and detection of thin lines; Segmentation, i.e. splitting the image into regions each containing a single letter; Identifying the letter for each region.

CAPTCHA 26 Attacks – EZ-Gimpy 2000 Yahoo's early CAPTCHA called "EZ-Gimpy“; The program picks a word from a dictionary, and produces a distorted and noisy image of the word; Algorithm for breaking EZ-Gimpy (92%): 1. Locate possible letters at various locations; 2. Construct graph of consistent letters; 3. Look for plausible words in the graph.

CAPTCHA 27 Attacks – EZ-Gimpy 2000 EZ-Gimpy Possible Letters Graph of Letters Plausible Words

CAPTCHA 28 Attacks – Jan/Feb 2008 Google (Jan 17) 20% Hotmail (Feb 6) 30-35% Yahoo (Feb 22) 30-35%

CAPTCHA 29 Break: Human solvers Attacks that uses humans to solve the puzzles; Approaches: relaying the puzzles to a group of human operators who can solve CAPTCHAs; copying the CAPTCHA images and using them as CAPTCHAs for a high-traffic site owned by the attacker.

CAPTCHA 30 Spammer Companies (October 2007) They write a program that fills out the entire registration form (ex Yahoo); When the program gets to the CAPTCHA it can’t solve it; So it copies the CAPTCHA back to the spam page; One person gets the screen saying if you want to see the next picture, you’ve got to tell what word is in the specific CAPTCHA.

CAPTCHA 31 Spammer Companies (October 2007)

CAPTCHA 32 Next CAPTCHA Generation CAPTCHAs can be made stronger, but they are already too advanced for a large percentage of Internet users; CAPTCHA devolves from a simple human reading test into an intelligence test or an acuity test.

CAPTCHA 33 reCAPTCHA 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.

CAPTCHA 34 Time spent Roughly 60 million CAPTCHAs are solved each day; Medium 10 seconds to solve a captcha; People around the world waste more than 150,000 hours on solving CAPTCHAs;

CAPTCHA 35 Human Computation Things that we humans can do and computer cannot, like: Labeling images with words; Picking out a voice in a loudly room; Humans have trouble remembering long, random strings of characters, yet they excel at remembering faces and objects.

CAPTCHA 36 Bibliography Site: Luis von Ahn Website (2006)Luis von Ahn Website Site: reCAPTCHA (2007)reCAPTCHA Site: CAPTCHA (2007)CAPTCHA Site: Gwap (2008)Gwap Interview: „Using “captchas” to digitize books“ (2007)Using “captchas” to digitize books Interview: „For Certain Tasks, the Cortex Still Beats the CPU“ (2007)„For Certain Tasks, the Cortex Still Beats the CPU“

CAPTCHA 37 Thank you!