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

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Agenda

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

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Example: Filling out a form

Google uses CAPTCHA for Gmail accounts:

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Beginnings

Completely Automated Public Turing test to tell Computers and Humans Apart

Created in 2000 for Yahoo to prevent automated e-mail account registration,

by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford, Carnegie Mellon University.

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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.

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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.

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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.

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So, CAPTCHA is…

A program that can generate and grade tests that:

Most humans can pass; Current computer programs cannot pass.

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Making a CAPTCHA

Pick random string of characters (or words)

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Renders it into a distorted image

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Making a CAPTCHA

… and the program generates a test:

Type the characters that appear in the image

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Type: Early CAPTCHAs

Generated by the EZ-Gimpy program; Used previously on Yahoo!

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Type: Improved CAPTCHA

high contrast for human readability; medium, per-character perturbation; random fonts per character; low background noise;

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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;

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Other Types of CAPTCHA

Animated CAPTCHAs

3D CAPTCHA

ASCII art

Reverse CAPTCHA "Leave this field blank"

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Other: Cognitive Puzzles

Distinguish pictures of dogs from catsChoose a word that relates to all the

imagesTrivia questions Math and word problems 3D Object CAPTCHASolve failed OCR inputs

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Other: Distinguish pictures

Microsoft Asirra (Animal Species Image Recognition for Restricting Access);

KittenAuth Project .

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Other: Tests

„Common Sense" questions:• „What is 3 + 5?“• „What color is the sky?"

Type the word 'orange'; Require a valid email to approve;

These attempts violate principles: • they cannot be automatically generated; • they can be easily cracked given the state of AI.

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Where to use CAPTCHAs?

Data Collection Worms and Spam Preventing Comment Spam in Blogs Protecting Email Addresses From Scrapers Online Polls Protecting Website Registration Preventing Dictionary Attacks Search Engine Bots

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Where to use CAPTCHAs?

Preventing Comment Spam in Blogs. Protecting Email Addresses From

Scrapers. Mechanism to hide your email address, require users to solve a CAPTCHA before showing your email 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.

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Where to use CAPTCHAs?

Protecting Website Registration. (E-mail 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.

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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").

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Guidelines: Accesibility

Hard to read CAPTCHAs:

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Guidelines: Accesibility

Worst CAPTCHAs:

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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).

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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.

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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.

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Attacks – EZ-Gimpy 2000

EZ-Gimpy

Possible Letters

Graph of Letters

Plausible Words

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Attacks – Jan/Feb 2008

Google(Jan 17) 20%

Hotmail(Feb 6) 30-35%

Yahoo(Feb 22) 30-35%

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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.

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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.

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Spammer Companies (October 2007)

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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.

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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.

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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;

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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.

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Bibliography

Site: Luis von Ahn Website (2006) Site: reCAPTCHA (2007) Site: CAPTCHA (2007) Site: Gwap (2008) Interview: „Using “captchas” to digitize

books“ (2007) Interview: „For Certain Tasks, the Cortex Still

Beats the CPU“ (2007)

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Thank you!