captcha....

1. CAPTCHA
Something That Human Can Do But
Computers Can’t………..
Presented By : Shreya

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3. Definition
CAPTCHA stands for Completely Automated

Public Turing test to tell Computers and Humans
Apart
A program that can tell whether its user is a
human or a computer.
The challenge: develop a software program that
can create and grade challenges most humans can
pass but computers cannot
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4. Background
 First used by Altavista in1997
Reduced SPAM add-url by over 95%
 CMU/Yahoo!
Automated the creating and grading of challenges
 PARC
Relies on document image degradation to prevent
successful OCR
Conducted user-focused studies to assess the
effectiveness of CAPTCHAs
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5. Background
CAPTCHAs are based on open AI problems
Breaking CAPTCHAs help advance AI by
solving these open problems
Improving CAPTCHAs help telling
computers and human apart
Win-win situation
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6. Applications
Free email services
Online polls
Dictionary attacks
Newsgroups, Blogs, etc…
SPAM
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7. Types of CAPTCHAs
Text based
Gimpy, ez-gimpy
Gimpy-r, Google CAPTCHA
Simard’s HIP (MSN)
Graphic based
Bongo
Pix
Audio based
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8. Text Based CAPTCHAs
Gimpy, ez-gimpy
Pick a word or words from a small dictionary
Distort them and add noise and background
Gimpy-r, Google’s CAPTCHA
Pick random letters
Distort them, add noise and background
Simard’s HIP
Pick random letters and numbers
Distort them and add arcs
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9. Text Based CAPTCHAs
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10. Graphic Based CAPTCHAs
Bongo
Display two series of blocks
User must find the characteristic that sets the two
series apart
User is asked to determine which series each of four
single blocks belongs to
Difference? thick vs. thin lines
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11. Graphic Based CAPTCHAs
PIX
Create a large database of labeled images
Pick a concrete object
Pick four images of the object from the images database
Distort the images
Ask the user to pick the object for a list of words
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12. Graphic Based CAPTCHAs
Dog
Pool
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13. Audio Based CAPTCHAs
Pick a word or a sequence of numbers at random
Render them into an audio clip using a TTS software
Distort the audio clip
Ask the user to identify and type the word or
numbers
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14. Breaking CAPTCHAs
Most text based CAPTCHAs have been broken by
software
OCR
Segmentation
Other CAPTCHAs were broken by streaming the tests
for unsuspecting users to solve.
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15. Properties

CAPTCHA should be automatically generated
and graded
 Test can be taken quickly and easily by human
users
 Test will accept virtually all human users and
reject software agents
 Test will resist automatic attack for many years
despite the technology advances and prior
knowledge of algorithms

16. Final Thoughts
They are crucial to preventing boot attacks
Hopefully, they will become more user-friendly to
people with disabilities (visual, mental)
CAPTCHA’s are mainly produced from AJAX and
PHP technology
Various algorithms are present
Use of XML

17. Bibliography……
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