[PPT] - The art of breaking and designing captchas Elie Bursztein Session PowerPoint Presentation

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The art of breaking and designing captchas

HT02-402 xxxxxxxxxxxx

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SLIDE 6 Elie Bursztein (@elie) https://elie.net 3

SLIDE 7 Elie Bursztein (@elie) https://elie.net 3

SLIDE 8 Elie Bursztein (@elie) https://elie.net 3

SLIDE 9 Elie Bursztein (@elie) https://elie.net 3

SLIDE 10 Elie Bursztein (@elie) https://elie.net 3

SLIDE 11 Elie Bursztein (@elie) https://elie.net

World Most-Popular Captchas

[eBay]

[Baidu] [Captcha.net] [NIH] [Wikipedia] [Digg] [Blizzard] [Google] [Skyrock] [Recaptcha] [Authorize] [CNN] [Megaupload] [Reddit] [Slashdot] 4

SLIDE 12 Elie Bursztein (@elie) https://elie.net

World Most-Popular Captchas

[eBay]

[Baidu] [Captcha.net] [NIH] [Wikipedia] [Digg] [Blizzard] [Google] [Skyrock] [Recaptcha] [Authorize] [CNN] [Megaupload] [Reddit] [Slashdot] 4

SLIDE 13 Elie Bursztein (@elie) https://elie.net

Captcha Design Goal

Hard for computer Hard for human 5

SLIDE 14 Elie Bursztein (@elie) https://elie.net

Captcha Design Goal

Hard for computer Hard for human Human 5

SLIDE 15 Elie Bursztein (@elie) https://elie.net

Captcha Design Goal

Hard for computer Hard for human AI ? Human 5

SLIDE 16 Elie Bursztein (@elie) https://elie.net

Captcha Design Goal

Hard for computer Hard for human AI ? Human sweet spot 5

SLIDE 17 Elie Bursztein (@elie) https://elie.net

Focus of this talk

xw

How to break and design CAPTCHAs 6

SLIDE 18 Elie Bursztein (@elie) https://elie.net Based on the breaking 21 of the most popular schemes and designing the new Wikipedia captcha 7

SLIDE 19 Elie Bursztein (@elie) https://elie.net

Outline

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SLIDE 20 Elie Bursztein (@elie) https://elie.net

Outline

How to break text captcha

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Outline

How to break text captcha How to make captchas easier for human

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Outline

How to break text captcha How to make captchas easier for human How to break audio captcha

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SLIDE 23 Elie Bursztein (@elie) https://elie.net

Outline

How to break text captcha How to make captchas easier for human How to break audio captcha How to break video captcha

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SLIDE 24 Elie Bursztein (@elie) https://elie.net

Evaluation metrics Accuracy

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SLIDE 25 Elie Bursztein (@elie) https://elie.net

Evaluation metrics Accuracy

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Solving time

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Evaluation metrics Accuracy Learnability

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Solving time

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Post-segmentation: inverting rotation 12

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3 713

Post-segmentation: inverting rotation 12

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3 713

Recognition: 12

SLIDE 45 Elie Bursztein (@elie) https://elie.net Recognition: 3 1 7 3 12

SLIDE 46 Elie Bursztein (@elie) https://elie.net

Breaker 5 Stages Pipeline

Slashdot captcha 13

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Breaker 5 Stages Pipeline

Preprocessing 13

SLIDE 48 Elie Bursztein (@elie) https://elie.net

Breaker 5 Stages Pipeline

Preprocessing 13

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Breaker 5 Stages Pipeline

Preprocessing Segmentation 13

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Breaker 5 Stages Pipeline

Preprocessing Segmentation 13

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Breaker 5 Stages Pipeline

Preprocessing Segmentation Post- segmentation 13

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Breaker 5 Stages Pipeline

Preprocessing Segmentation Post- segmentation 13

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Breaker 5 Stages Pipeline

Preprocessing Segmentation Post- segmentation Recognition 13

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Breaker 5 Stages Pipeline

Preprocessing Segmentation Post- segmentation Recognition

f a e t e s t

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Breaker 5 Stages Pipeline

Preprocessing Segmentation Post- segmentation Recognition

f a e t e s t

Post-recognition 13

SLIDE 56 Elie Bursztein (@elie) https://elie.net

Breaker 5 Stages Pipeline

Preprocessing Segmentation Post- segmentation Recognition

f a e t e s t f a s t e s t

Post-recognition 13

SLIDE 57 From the image to the matrix representation 14

SLIDE 58 From the image to the matrix representation 14

SLIDE 59 From the image to the matrix representation 14

SLIDE 60 From the image to the matrix representation 14

SLIDE 61 From the image to the matrix representation 14

SLIDE 62 From the matrix representation to the vector representation 15

SLIDE 63 From the matrix representation to the vector representation 15

SLIDE 64 From the matrix representation to the vector representation 15

SLIDE 65 L1 L2 From the matrix representation to the vector representation 15

SLIDE 66 L1 L2 L3 From the matrix representation to the vector representation 15

SLIDE 67 L1 L2 L3 L4 L5 L6 vector From the matrix representation to the vector representation 15

SLIDE 68 B B A A C C vector Distance Known vectors From the vector representation to the segment value (classification) 16

SLIDE 69 B B A A C C vector Distance Known vectors From the vector representation to the segment value (classification) 16

SLIDE 70 B B A A C C 42 vector Distance Known vectors From the vector representation to the segment value (classification) 16

SLIDE 71 B B A A C C 42 vector Distance Known vectors From the vector representation to the segment value (classification) 16

SLIDE 72 B B A A C C 42 40 vector Distance Known vectors From the vector representation to the segment value (classification) 16

SLIDE 73 B B A A C C 42 40 vector Distance Known vectors From the vector representation to the segment value (classification) 16

SLIDE 74 B B A A C C 42 40 32 vector Distance Known vectors From the vector representation to the segment value (classification) 16

SLIDE 75 B B A A C C 42 40 32 70 12 18 vector Distance Known vectors From the vector representation to the segment value (classification) 16

SLIDE 76 B B A A C 42 40 32 70 18 vector Distance Known vectors C 12 From the vector representation to the segment value (classification) 16

SLIDE 77 Elie Bursztein (@elie) https://elie.net

Breaker efficiency

Solver accuracy = Coverage * Precision^length Coverage: Segmentation rate Precision: Recognition rate 17

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Anti-recognition techniques

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Anti-recognition techniques

Blurring 18

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Anti-recognition techniques

Blurring Distortion 18

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Anti-recognition techniques

Blurring Distortion Rotation 18

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Anti-recognition techniques

Blurring Distortion Rotation Fonts 18

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Anti-recognition techniques

Blurring Distortion Rotation Fonts Charsets 18

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Anti-recognition techniques

Blurring Distortion Rotation Fonts Charsets 0123456789 18

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SVM learning rate

% success 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Trainning set size 10 20 50 100 200 500 09 AZ09 azAZ09 Distortion 3 fonts 5 fonts Angles 19

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KNN learning rate

% success 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Trainning set size 10 20 50 100 200 500 09 AZ09 azAZ09 Distortion 3 fonts 5 fonts Angles 20

SLIDE 87 Elie Bursztein (@elie) http://elie.im

Anti-recognition taxonomy

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Anti-recognition taxonomy

Background Confusion Background confusion 21

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Anti-recognition taxonomy

Background Confusion Background confusion 21

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Anti-recognition taxonomy

Background Confusion Background confusion 21

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Anti-recognition taxonomy

Background Confusion Background confusion 21

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Anti-recognition taxonomy

Background Confusion Lines Background confusion 21

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Anti-recognition taxonomy

Background Confusion Lines Background confusion 21

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Anti-recognition taxonomy

Background Confusion Lines Background confusion 21

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Anti-recognition taxonomy

Background Confusion Lines Background confusion 21

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Anti-recognition taxonomy

Background Confusion Lines Collapsing Background confusion 21

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Anti-recognition taxonomy

Background Confusion Lines Collapsing Background confusion 21

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Anti-recognition taxonomy

Background Confusion Lines Collapsing Background confusion 21

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Anti-recognition taxonomy

Background Confusion Lines Collapsing Background confusion 21

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Breaking World of Warcraft

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Breaking World of Warcraft

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Breaking World of Warcraft

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Breaking World of Warcraft

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Breaking World of Warcraft

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Breaking Captcha.net

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Breaking Captcha.net

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Breaking Captcha.net

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Breaking Captcha.net

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Breaking Captcha.net

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Breaking Wikipedia

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Breaking Wikipedia

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Breaking Wikipedia

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Breaking Wikipedia

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Breaking Wikipedia

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Breaking Digg

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Breaking Digg

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Breaking Digg

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Breaking Digg

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Breaking Digg

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Breaking Slashdot

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Breaking Slashdot

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Breaking Slashdot

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Breaking Slashdot

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Breaking Slashdot

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Breaking eBay

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Breaking eBay

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Breaking eBay

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Breaking eBay

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Breaking eBay

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Failing to break eBay

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Failing to break eBay

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Failing to break eBay

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Failing to break eBay

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Failing to break eBay

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Breaking Baidu

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Breaking Baidu

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Breaking Baidu

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Breaking Baidu

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Breaking Baidu

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Breaking Baidu

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Overall results

Segmentation rate Solving rate Authorize 84% 66% Baidu 98% 5% Blizzard 75% 70% Captcha.net 96% 73% CNN 50% 16% Digg 86% 20% eBay 95% 43% Google 0% 0% MegaUpload n/a 93% NIH 87% 72% Recaptcha 0% 0% Reddit 71% 42% Skyrock 30% 2% Slashdot 52% 35% Wikipedia 57% 25% 30

SLIDE 142 Elie Bursztein (@elie) https://elie.net

Learning rate for real schemes

% success 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% Trainning set size 10 20 50 100 200 500 Authorize Baidu Blizzard Captcha.net CNN Digg eBay Megaupload NIH Reddit Skyrock Slashdot Wikipedia 31

SLIDE 143 Elie Bursztein (@elie) https://elie.net

Decaptcha main interface

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SLIDE 144 Elie Bursztein (@elie) https://elie.net

Apply design principles

Core design principles

Randomize length Randomize character size Wave the captcha

Use anti-recognition as a means of strengthening captcha security Don’t use a complex charset

Bad for human (see our research on this) Useless for security

Use collapsing or lines

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Designing Better Captchas

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Think Lego again

Decompose in features Analyze

feature in isolation features interaction 35

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Evaluation system

Tasks Web Fronted Test results Monitoring system Captcha image Generator Feedback system Tasks Generator Amazon Mechanical Turk Payement validation 36

SLIDE 148 Elie Bursztein (@elie) https://elie.net

Experiment details

Round Task N possible N sampled N tests per sample Total tests 1 Baseline (“Control”) 1 1 1000 1000 2 Real world captchas 8 8 1000 8000 3 Features in isolation 496 496 200 99200 4 2 feature interactions 60950 60950 5 304750 5 3 feature interactions 1 303 224 25000 10 250000 6 4 feature interactions 113 951 684 25000 10 250000 Total 912150 37

SLIDE 149 Elie Bursztein (@elie) https://elie.net

Some of the features tested

Blurring Text color Collapsing Font Distortion line line shape nb line line size line coverage line position line angle Waving Tilting Distortion Background color Noise 38

SLIDE 150 Elie Bursztein (@elie) https://elie.net

Angle of rotation

solving time (s) 4 5 6 7 8 9 10 11 12 13 14 accuracy 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 rotation angle (°) 50 100 150 200 250 300 350 solving time accuracy 39

SLIDE 151 Elie Bursztein (@elie) https://elie.net

Collapsing

solving time (s) 4 5 6 7 8 9 10 11 12 13 14 accuracy 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 character gap width

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2 4 solving time accuracy 40

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Character size

solving time (s) 4 5 6 7 8 9 10 11 12 13 14 accuracy 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 character size 2 4 6 8 10 12 14 16 18 20 solving time accuracy 41

SLIDE 153 Elie Bursztein (@elie) https://elie.net

Resolution invariant

Accuracy 50 55 60 65 70 75 80 85 90 95 100 captcha length (number of characters) 5 10 15 20 25 30 <= 1024 > 1024 all captchas 42

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2D interactions

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Length vs Angle interaction

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Perception Does Not Match Number

5 10 15 20 25 30 35 az 09 AZ az09 AZ09 azAZ azAZ09 pretty HF+ cutest LF + guilty HF - molest LF - %fast %easy %like 45

SLIDE 157 Elie Bursztein (@elie) https://elie.net

The New Wikipedia

Use digit Wave the captcha Use random length (5-7) Use random size (34-50) Rotate letter (-25/ 25) Add a line for a super secure version

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End result

Accuracy Solving time 47

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End result

Accuracy Solving time 84.8% 47

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End result

Accuracy Solving time 84.8% 7.8s 47

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End result

Accuracy Solving time 84.8% 7.8s 47

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End result

Accuracy Solving time 84.8% 7.8s 89.2% 82.6% 47

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End result

Accuracy Solving time 84.8% 7.8s 89.2% 82.6% 4.9s 5.3s 47

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End result

confusing Accuracy Solving time 84.8% 7.8s 89.2% 82.6% 4.9s 5.3s 47

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End result

confusing Accuracy Solving time 84.8% 7.8s 89.2% 82.6% 4.9s 5.3s 47

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End result

confusing Accuracy Solving time 84.8% 7.8s 89.2% 82.6% 4.9s 5.3s 97% 92.2% 47

SLIDE 167 Elie Bursztein (@elie) https://elie.net

End result

confusing Accuracy Solving time 84.8% 7.8s 89.2% 82.6% 4.9s 5.3s 97% 92.2% 4.9s 5.2s 47

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How to Break Audio-Captcha

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Audio Captchas

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Audio Captchas

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SLIDE 171 Elie Bursztein (@elie) https://elie.net Super secure captcha Captcha Maker

Creating Audio Captcha

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SLIDE 172 Elie Bursztein (@elie) https://elie.net Super secure captcha Captcha Maker

Creating Audio Captcha

Voices 50

SLIDE 173 Elie Bursztein (@elie) https://elie.net Super secure captcha Captcha Maker

Creating Audio Captcha

Noises 50

SLIDE 174 Elie Bursztein (@elie) https://elie.net Super secure captcha

Creating Audio Captcha

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Noise intensity (RMS/SNR)

2 9 Micros J Dig A K Authori K J 5 H 51

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Sound representation

WAV DFT Cep TFR TCR TDC 52

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Solving an audio captcha

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Solving an audio captcha

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Solving an audio captcha

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Solving an audio captcha

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Solving an audio captcha

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Solving an audio captcha

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Solving an audio captcha

C T T A R A F R S 2 53

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Dealing with random noise

Statistical learning Supervised learning RLS (Regularized least square) classifier

Authorize eBay Recaptcha Authorize Digg

5: J:

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Semantic noise

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Results

Length Coverage Digit Captcha Authorize 5 100 97 89.2% Digg 5 100 76 41.4% eBay 6 85.6 92.5 82.9% Microsoft 10 80.6 89.6 48.9% Recaptcha 8 99.9 40.5 1.5% Yahoo 7 99.1 74.7 45.4% 56

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Recaptcha semantic noise

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20 40 60 80 100 120 140 160 180 200 3 7 N 2 1 4 9 N 5 D B Time in seconds 57

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Recaptcha semantic noise

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20 40 60 80 100 120 140 160 180 200 3 7 N 2 1 4 9 N 5 D B Time in seconds 57

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How many captchas do you need ?

10 2 10 3 10 4 10 20 30 40 50 60 70 80 90 100 Per−Captcha Precision (%) Corpus Size (in Digits) Authorize Digg Ebay MSLive Recaptcha Yahoo 58

SLIDE 190 Elie Bursztein (@elie) https://elie.net

Video captcha

Interesting direction -> more design space Good for human Good for computer :(

Working on it 59

See blog post for more information: http://elie.im/blog

SLIDE 191 Elie Bursztein (@elie) https://elie.net

Apply

Within 3 months

Make sure you have a strong captcha scheme (use mine if you want) Ensure that your site is accessible

Within 6 months

Log your captchas failure rate and monitor them Have a backup captcha scheme in case your scheme is broken 60

SLIDE 192 Elie Bursztein (@elie) https://elie.net

Thank you !

Questions ? Follow-me ! Thank you Twitter: @elie 61 Captcha research: http://elie.im/tag/captcha