Novel CAPTCHA Schemes - Presentation

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Novel CAPTCHA schemes

Ville Saalo

9.12.2010

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Agenda

What are CAPTCHAs?

Attack on the Microsoft CAPTCHA

The transparent letter scheme

Wrap up

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Agenda

What are CAPTCHAs?

Attack on the Microsoft CAPTCHA

The transparent letter scheme

Wrap up

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What are CAPTCHAs? (1/3)

Prevents bots from

automatically accessingvarious servicesVarious types:

Text-based

Sound-basedImage-based

General or domain specific Source: CAPTCHA.net

CAPTCHA: Completely Automated Public Turingtest to tell Computers and Humans Apart

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What are CAPTCHAs? (2/3)

Example of an image-based CAPTCHA:

Asirra

Source:http://research.microsoft.com/en-us/um/redmond/projects/asirra/

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What are CAPTCHAs? (3/3)

Example of a domain-

specific CAPTCHA:

CAPNTHLAfor

NetHackplayers.Source:http://nethackwiki.com/wiki/CAPNTHLA

Source: http://nethackwiki.com/wiki/File:Nh343-vt-ibmgraphics.png

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Agenda

What are CAPTCHAs?

Attack on the Microsoft CAPTCHA

The transparent letter scheme

Wrap up

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Attack on the Microsoft CAPTCHA (1/2)

A paper calledA Low-cost Attack on a MicrosoftCAPTCHA by Yan & El Ahmad, 2008.

Focuses on the segmentation challenge

Six stages

Generally two phases:

segmentation (hard!)recognition (easy!)

Source: Yan & El Ahmad, 2008

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Attack on the Microsoft CAPTCHA (2/2)

1. Pre-processing

2. Vertical segmentation

3. Color filling segmentation

4. Thick arc removal

5. Locating connectedcharacters

6. Segmenting connectedcharacters

Source: Yan & El Ahmad, 2008

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Agenda

What are CAPTCHAs?

Attack on the Microsoft CAPTCHA

The transparent letter scheme

Wrap up

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Relies on heavily overlapping letters and largeclutter objects

The transparent letter scheme

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The transparent letter scheme

How do you create a CAPTCHA scheme like this?Start with random text, random fonts:

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Push the letters together:

Try doing the vertical segmentation phase now!Not very readable anymore? Wait for it...

The transparent letter scheme

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Make the letters transparent:

The transparent letter scheme

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Add large clutter objects:

The transparent letter scheme

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Add holes to make the objects more like letters:

The transparent letter scheme

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Add fine clutter:

The transparent letter scheme

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Complement with some distortions:

The transparent letter scheme

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Agenda

What are CAPTCHAs?

Attack on the Microsoft CAPTCHA

The transparent letter scheme

Wrap up

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Wrap up

Many kinds of CAPTCHAs exist

Computers are excellent at recognizing characters

Therefore, text-based CAPTCHAs should rely onmaking segmentation hard

The Transparent letter scheme is a new attempt onthat