Captcha

Are you Human? (Sorry, I had to ask) Ecaterina Valică http://students.info.uaic.ro/~evalica/

Agenda

What is CAPTCHA?

Types of CAPTCHA

Where to use CAPTCHAs?

Guidelines when making a CAPTCHA

Ways to break CAPTCHAs

reCAPTCHA

Human Computation Games

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

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 e-mail account registration,

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

Inventor Luis von Ahn ( 1978 - ) Photograph by Mike McGregor

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.

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 .

So, CAPTCHA is…

A program that can generate and grade tests that:

Most humans can pass;

Current computer programs cannot pass.

Making a CAPTCHA

Pick random string of characters

(or words)

ifhkfp

Renders it into a distorted image

Making a CAPTCHA

… and the program generates a test :

Type the characters that appear in the image

Outperform the computers

In many simple tasks, a typical 5-year-old can outperform the most powerful computers

easier for computers:

like medical diagnosis,

playing chess,

hard for computers:

operations requiring vision , hearing , language or motor control .

Type: Early CAPTCHAs

Generated by the EZ-Gimpy program;

Used previously on Yahoo!

Type: Improved CAPTCHA

high contrast for human readability;

medium, per-character perturbation;

random fonts per character;

low background noise ;

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 ;

Type: A modern CAPTCHA

another way to make segmentation difficult is to crowd symbols together ;

this can be read by humans but cannot be segmented by bots;

Other Types of CAPTCHA

Animated CAPTCHAs

3D CAPTCHA

ASCII art

Reverse CAPTCHA "Leave this field blank"

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

Other: Distinguish pictures

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

KittenAuth Project .

Other : Mathematical CAPTCHA

Other: 3D Object CAPTCHA

You must enter them in the exact sequence listed:

The Head of the Walking Man,

The Vase,

The Back of the Chair.

Other: Jumble Game

Other: Drupal Examples

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.

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

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.

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.

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").

Guidelines

Security Even After Wide-Spread Adoption. There are CAPTCHAs that would be insecure if a significant number of sites started using them.

Example: text-based questions;

A parser could easily be written that would allow bots to bypass the test;

Such “CAPTCHAs” rely on the fact that few sites use them, and thus that a bot author has no incentive to program their bot to solve that challenge.

Guidelines

Accessibility.

CAPTCHAs prevent visually impaired users (for example, due to a disability or because it is difficult to read) from accessing the protected resource;

They use screen reader, so when you reached an image, all it can do is to read the caption of that image;

Solution : permitting users to opt for an audio or sound CAPTCHA.

Guidelines: Accesibility Hard to read CAPTCHAs:

Guidelines: Accesibility Worst CAPTCHAs:

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 ).

Break: Insecure implementation

Re-using the session ID of a known CAPTCHA image.

Other CAPTCHA use a hash of the solution as a key passed to the client to validate. Often it is small enough size that it can be cracked .

Other implementations use only a small fixed pool of CAPTCHA images (Asirra – 3 millions).

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.

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% ):

Locate possible letters at various locations;

Construct graph of consistent letters;

Look for plausible words in the graph.

Attacks – EZ-Gimpy 2000

EZ-Gimpy

Possible Letters

Graph of Letters

Plausible Words

Attacks – SimpleOCR Engine 2002

Attacks – Jan/Feb 2008

Google

( Jan 17 ) 20%

Hotmail

(Feb 6) 30-35%

Yahoo

(Feb 22) 30-35%

Attacks – Projects

Several broking CAPTCHAs projects:

http://libcaca.zoy.org/wiki/PWNtcha

http:// www.lafdc.com / captcha /

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 Sweatshops

A computer fills out a form and when it reaches a CAPTCHA, it gives it to the operator to solve.

Weakness for Asirra:

if the database of cat and dog photos can be downloaded,

then paying workers $0.01 to classify each photo,

means that almost the entire database of photos can be deciphered for $30,000.

Once IP has misclassified a challenge, a human needs to just solve two Asirras in a row from the same browser session.

CAPTCHA Sweatshops

Not Economical Viable

A typical spam run of 1 million messages per day would cost $14,000 per day and require 116 people working 24/7.

$2.50 / h for each human 720 CAPTCHAs per hour per human 1/3 cent per account

Porn 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 porn 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.

Porn Companies (October 2007)

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.

reCAPTCHA (2007)

New form of CAPTCHA that also helps digitize books ;

The words displayed to the user come directly from old books that are being digitized;

Words that OCR could not identify ;

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.

reCAPTCHA

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;

Time spent

A fifth of those users giving

30,000 daily man-hours of work;

It would constitute the world's fastest and most accurate character-recognition computer, processing 10 million words a day.

Recreating the books – word by word

Time spent

9 Billion Human-Hours of

Solitaire were played in 2003

Empire State Building

7 million Human-Hours

(6.8 Hours of Solitaire)

Panama Canal

20 Million Human-Hours

(Less than a day of Solitaire)

Wasted human cycles

If the world's computer Solitaire players could be coaxed into enjoying a game that contributed to solving a computing problem, he calculates, it would produce billions of man-hours of labor each year.

„ make all of humanity more efficient by exploiting the human cycles that get wasted“

Wasted human cycles

People will contribute their brainpower, but only if they're given an enjoyable , time-killing experience in exchange.

Most projects that harness human processing power rely on a different motivator: money .

Which produces better results — a small group of experts or a huge mob of amateurs ?

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.

Symbiotic relationship

One in which humans solve some problems, computer solve some other problems;

Image search - A method that every image on the Web could give us accurate textual descriptions of those images;

The ESP Game

Two-player online game;

Partners don’t know each other

and can’t communicate;

Object of the game:

Type the same word ;

The only thing in common

is an image ;

The ESP Game Player 1 Guessing: CAR Guessing : HAT Guessing: KID Success! You agree on CAR Player 2 Guessing : BOY Guessing : CAR Success! You agree on CAR

The ESP Game

The ESP Game has been licensed (2006) by Google in the form of the Google Image Labeler , and is used to improve the accuracy of the Google Image Search .

“ 5000 people playing simultaneously can

label all images on Google in 30 days !”

http://gwap.com/gwap/

http://gwap.com/gwap/ ESP Tag a Tune Matchin

http://gwap.com/gwap/ Squigl Verbosity

Future Games

Language translation. A game could challenge two players who don’t speak the same language to translate text from one language to the other.

Monitoring of security cameras . Players could monitor security cameras and alert authorities about suspected illegal activity.

Future Games

Improving Web search. People have varying degrees of skill at searching for information on the Web. A game could be designed in which the players perform searches for other people.